PORTABLE TUNGSTEN COIL ATOMIC EMISSION AND ATOMIC FLUORESCENCE SPECTROMETRY JIYAN GU. A Dissertation Submitted to the Graduate Faculty of

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1 PORTABLE TUNGSTEN COIL ATOMIC EMISSION AND ATOMIC FLUORESCENCE SPECTROMETRY BY JIYAN GU A Dissertation Submitted to the Graduate Faculty of WAKE FOREST UNIVERSITY GRADUATE SCHOOL OF ARTS AND SCIENCES in Partial Fulfillment of the Requirements for the Degree of DOCTOR OF PHILOSOPHY Chemistry August 2010 Winston Salem, North Carolina Copyright Jiyan Gu 2010 Examining Committee: Bradley T. Jones, Ph.D., Advisor Eric D. Carlson, Ph.D., Chair Willie L. Hinze, Ph.D. Akbar Salam, Ph.D. Christa L. Colyer, Ph.D.

2 AKNOWLEDGEMENTS I remember when I was a little kid in kindergarten, my teacher always liked to ask every kids what do they want to do when they grow up, when it came to me, I usually will say, I want to be a scientist. It is actually a pretty silly scenario. A kid like my age probably did not know what a scientist really does, or what scientist really meant, but certainly, at that time, I know it must be something very cool, and to be proud of. After 4 years pursuit of my Ph.D. degree, I almost fulfill the dream I had as a kid. Even though; I have never planned for it. I think first I have to say thank you to my life. My life takes me here. Looking back, I was offered so many opportunities, challenges, loves and pains, and all those things in my life added up, took me here; made me who I am today. I am the lucky one compared to others, and I am really grateful at this moment. I would like to appreciate chemistry department for accepting me as a graduate student here. Because of this, I was able to come from my home town, China, another end of the earth, to here. With such an opportunity, I was able to see such a different world and meet so many wonderful people. From day one, I felt the warm welcome from this department, from everybody here. I really appreciate it. The person I need to acknowledge most is my Ph.D. advisor, Dr Bradley T Jones. There is an old saying in china that describes the relationship between advisors and students, one day as teacher, forever as father. I have been always respecting Dr Jones, just like the way I respect my father. I remember the first time I was asking to join his lab, he accepted me as his student without any hesitation. He just looked at me and say, Sure! For four years, he taught me knowledge, helped me set up the experiments, II

3 showed me how to solve difficult problems, and eventually made me understand the real meaning of research. I appreciate so much what he has given to me and I am really proud to be his student. I would also like to acknowledge my committee members, Professor Eric D. Carlson, Professor Willie L. Hinze, Professor Akbar Salam and Professor Christa Colyer. Thank you all for being my committee member and devote so much of your time to me. Thanks for all the corrections and advices you have given to me for my dissertation. Also I would like to express my gratitude to Professor Clifton Calloway, Professor Joaquim Nobrega, Porfessor Ulrich Bierbach and Professor Al Rives for their help and advice during my 4 years study. My friends are very important to me for those four years. Without them, I would not have such a wonderful time at Wake Forest. I want to say thank you to Zhidong Ma, Lu Rao, Maben Ying, Xiao Xu, Donghui, Chen, Yan Ma, Han Wang, Tianjia, Dai, Xiaochun, Chen, Ye Yuan, Yue Zhao, Carl Young, George Donati, Summer Hanna, Kathryn Pharr, Benjamin Vaughan, Ranjan Banerjee, Saurav Sarma, Elizabeth Carlson, Professor Miaohua Jiang and everyone that I have spent value time with. I will not be able to list every one here, because it might be pages after pages. Thanks you all for sharing wonderful time with me. At last, I will thank my family including my wife, my parents, my parents in law and my cousins. They gave me the support and strength to overcome all the difficulties. Specially my wife, even though we have to be apart because my pursuit of Ph.D., she has always been so understanding and supportive. She is the best. III

4 TABLE OF CONTENTS LIST OF TABLES AND FIGURES... V Abstract... IX Chapter I. Introduction... 1 II. A Portable Tungsten Coil Atomic Emission Spectrometer for the Simultaneous Determination of Metals in Water and Soil Samples... 8 III. A Portable Tungsten Coil Atomic Emission Spectrometer With Two Coils IV. Continuum Source Tungsten Coil Atomic Fluorescence Spectrometry V. A Rugged, Portable Tungsten Coil Atomic Emission Spectrometer VI. Conclusion Appendix Simultaneously Determination of the Lanthanides by Tungsten Coil Atomic Emission Spectrometry Scholastic Vita IV

5 LIST OF TABLES AND FIGURES TABLES Page CHAPTER II 1. WCAES atomization cycles for two different tungsten coils Analytical figures of merit for WCAES Determination of Ca, Ba, Sr, Cs and Rb in the soil sample (NIST SRM#2711) with WCAES. 26 CHAPTER III 1. Tungsten coil heating cycles Analytical figures of merit for One Coil WCAES and Two Coil WCAES Accuracy of Two Coil WCAES system (Water Pollution Standard 1, Product Number WPS1-100, VHG). 52 CHAPTER IV 1. W-coil heating cycle for CSWCAFS Analytical figures of merit for CSWCAFS. 69 CHAPTER V 1. Heating program of WCAES Analytical figures of merit for portable WCAES and reference materials 85 V

6 LIST OF FIGURES Page CHAPTER II 1. Photograph of WCAES instrument Simultaneous multi-element WCAES determination of Yb (1 mg L -1 ), Ca (2 mg L -1 ), Eu (2 mg L -1 ), Sr (2 mg L -1 ), Ba (1 mg L -1 ), Na (0.5 mg L -1 ). Background is subtracted Simultaneous multi-element WCAES determination of K (0.3 mg L -1 ), Rb (0.3 mg L -1 ) and Cs (0.8 mg L -1 ) in Near-IR 30 region. Background is subtracted. 4. Relationship between sample volume and emission signal Intensity (Cr nm, 1 mg L -1 ) Relationship between W-Coil power (V i) and the emission signal intensity ( Cr nm, 1 mg L -1 ) WCAES spectrum for Mg, Cu, Ag, Ni and Yb (10 mg L -1 ), with 400 W power supply WCAES spectrum for Al (10 mg L -1 ), Cr (2 mg L -1 ), Ga (20 mg L -1 ), Mn (20 mg L -1 ), Li (100 mg L -1 ) and V (5 mg L -1 ), with 400 W power supply. 35 CHAPTER III 1. Photograph of the WCAES instrument (system setup for both one coil and two coil system) Photograph of WCAES atomization cell with two coils. 46 VI

7 3. Simultaneous multi-element WCAES (two coil) determination of Yb (1.0 mg/l), Cr (0.2 mg/l), Ag (200 mg/l), Cu (200 mg/l), Fe (150 mg/l), Co (150 mg/l) Simultaneous multi-element WCAES (two coils) determination of Yb (1.4 mg/l), Cr (0.3 mg/l), Ag (280 mg/l), Cu (280 mg/l) Temperature effect on the Ocean Optics USB 4000 detector (Cr hollow cathode lamp, nm). 51 CHAPTER IV 1. Photograph of the continuum source tungsten coil atomic fluorescence spectrometer Simultaneous multi-element CSWCAFS spectrum of a solution containing 50 µg/ml Bi, Cr, Cu, Ga, In, Pb, Ag, Tl, Mg, and Mn Relationship between CSWCAFS signal and vaporization current for 100 µg/ml Bi Tungsten coil atomic emission spectrum for 10 µg/ml Ca and Yb. 67 CHAPTER V 1. Picture of portable WCAES Picture of Aluminum Atomization Cell Simultaneous multi-element WCAES determination of VII

8 Yb, Sr, Ba, Eu and Cr (1 mg L -1 ) Simultaneous multi-element WCAES determination of Al (10 mg L -1 ), Cr (2 mg L -1 ), Ga (20 mg L -1 ), Mn (20 mg L -1 ) and V (5 mg L -1 ) 84 VIII

9 ABSTRACT Portable Tungsten Coil Atomic Emission and Atomic Fluorescence Spectrometry By Jiyan Gu Dissertation under the direction of Bradley T Jones, Ph. D. Professor of Chemistry & Associate Dean of Graduate School Elemental Analysis is considered to be a mature science. However, continued interest remains for the development of portable devices for analyzing liquids or solids in the field. This dissertation is focused on developing various portable W-Coil spectroscopy devices for field applications. Improvement of the devices sensitivity, accuracy and overall performance are also the major part of this work. In this study, four novel portable W-Coil techniques have been developed. First, a portable fast heating rate tungsten coil atomic emission spectrometer (WCAES) device was built. A W Coil was used as both atomization and excitation source. A portable CCD spectrometer was coupled with the W Coil device for the first time. The heating rate and coil temperature was dramatically increased by a 400 W power supply. Sensitivity for most elements was much improved by this set up. Second, a portable double tungsten coil atomic emission spectrometer (DWCAES) device was developed. One tungsten coil was used as the atomization source and the second one was employed as the excitation source. This device increased the sensitivity for elements like Cu and Ag up to 100 times IX

10 compared to normal WCAES. Third, continuum source tungsten coil atomic fluorescence spectrometry (CSWCAES) was observed for the first time. By using a continuum xenon arc lamp, 10 elements were determined simultaneously in the UV region after the sample was atomized from a tungsten coil. Lastly, a novel design of WCAES was developed and the whole device including atomization cell, lens and detector was fixed on a ceramic rail (1 6 30cm). This is the smallest WCAES instrument constructed to date. The whole device can be carried to the field for elemental analysis. The determination of 15 elements was accomplished with this system. Tungsten coil devices are ideal for field applications because of their low energy requirements, no need for a cooling system, and low cost. All four techniques are simple and ready for further field applications. Improvement of the sensitivity will be the next important step for portable W Coil devices. X

11 CHAPTER 1 Introduction Portable systems are the new trend in analytical instrument development. Various samples such as liquids, solids and biologicals are routinely analyzed in the lab nowadays by all types of analytical techniques. Converting those laboratory-based analytical instruments into small portable devices for field applications will be the next important step. These techniques will open the door for many potential applications which are considered to be impossible at this moment. Commercially available portable analytical instruments such as Raman devices, X-ray devices, mass spectrometers, and laser induced breakdown spectrometers (LIBS) have been extensively used in different areas. For example, X-ray devices can detect most elements in the periodic table in solid samples with a dynamic range from 100% down to the µg/g level [1]. Recent innovations such as miniature X-ray tubes enable the conversion of the X-ray device down to the size of a price gun used in the supermarket. Because the technique is nondestructive, rapid, and multi-element in character, it has been used for the analysis of paint, metals, alloys, minerals, forensic samples, waters, and works of art [1]. Another promising technique involves the portable Raman device. The recent developments of miniature lasers along with low cost CCD detectors and optical filter technology have led to the modern portable Raman technique. Portable Raman can be used for material identification of plastics, minerals, rocks, narcotics and explosives at crime scenes [2]. Portable LIBS also benefits from these new lasers. These devices can produce an extremely hot plasma by focusing the laser on the surface of the sample. This method can be used for analyzing soils, liquids, metal and even polymers [3]. These techniques have similar functions, but each tends to excel in particular applications. In each case, various samples can be analyzed and identified immediately without sending samples to the laboratory and 1

12 waiting for the results. This result in a tremendous time saving, also, sending samples to a laboratory can result in sample loss, degradation, etc. With further instrumental developments, more scientists are expected to perform field based analytical research in the near future. For elemental analysis, techniques such as inductively coupled plasma (ICP), flame atomic emission spectroscopy (FAES), flame atomic absorption spectroscopy (FAAS), graphite furnace atomic absorption spectroscopy (GFAAS) and X-Ray fluorescence (XRF) are commonly used in the laboratory [4-6]. Among all techniques, three of them have the most potential for field application: portable X-ray fluorescence spectroscopy, portable LIBS and portable tungsten coil (W Coil) spectroscopy. Both portable X-ray and LIBS devices have been introduced above and are commercially available. Compared with those two techniques, the W Coil device has unique characteristics [7]. The W Coil, along with many other electrothermal devices, has been an important part of atomic spectroscopy for a long time [8-28]. Tungsten has the highest melting point (3410 ) of all metals and has a very low specific heat (0.133 Jg -1 K -1 ). Also, tungsten was found to be more resistant to high concentrations of acid when compared to other high melting point metals. These characteristics make tungsten a perfect candidate for electrothermal atomizers [29, 30]. In the early 1970 s, Williams and Piepmeier first used a tungsten coil filament extracted from a light bulb as the atomization source [10]. Since then, tungsten devices have been used for various purposes in analytical atomic spectroscopy. 1n 1988 Berndt and Schaldach utilized a simple low cost tungsten coil as the atomizer for tungsten coil atomic absorption spectrometry (WCAAS). Research showed that WCAAS is an extremely sensitive technique for elemental analysis. The sensitivity of a group of elements determined by the system was comparable to GFAAS [24]. The first portable WCAAS was introduced in The whole system was only 19 in. x 8 in. x 3 in. and powered by a 12V car battery. The absolute detection limit was 20 pg Pb in a 20 µl sample 2

13 volume. The accuracy of the system was tested with NIST SRM #1579a ''Powdered Lead- Based Paint'' and NIST SRM #955a ''Lead in Blood''. Recovery for both samples was around 95% with the standard addition method [31]. Two years later, a similar portable WCAAS system was used to determine Cd in urine, soil, and stream water with a glass atomization cell [32]. Typical results included 90% recovery with 10% relative standard deviation for the analysis of those samples. Portable WCAAS had the lowest detection limits among three portable techniques for liquid samples. However, WCAAS is limited by its single element technique. Even though multi-element hollow cathode lamps can be coupled with WCAAS to determine up to 4 elements at one time, portable LIBS and portable X-ray devices are capable of the simultaneous determination of more than 10 elements. Recent development of new W Coil devices has been focused on improving its multielement capability. Fraunhofer effect atomic absorption spectrometry (FE-AAS) was reported recently [33]. A 25µL aliquot of sample was placed on the tungsten coil and dried by applying low currents. After the liquid was dried, 10 A was applied to the coil for atomization of the sample. During this step, the blackbody radiation emitted by the coil was absorbed by the metals present in the sample at their characteristic wavelengths. Detection limits were reported for Ca, Co, Cr, Sr, Yb, Mn and K. Later a continuum source tungsten coil atomic absorption spectrometer (CSWCAAS) was developed by the same lab to overcome the single element character of tungsten coil devices [34, 35]. The system employed a xenon or deuterium arc lamp as the light source. Elements like Pb, Al, V, Ni, Cd, Cu, Co, Yb and Zn were determined simultaneously by the same system without the need to switch lamps. Unfortunately, both of these techniques required a high resolution spectrometer; thus neither device was portable. Around the same time, tungsten coil atomic emission spectrometry (WCAES) was reported for the first time [36]. The tungsten coil was extracted from a 150 W, 15 V 3

14 commercial slide projector light bulb. A simple, laboratory constructed, computer-controlled power supply provided a constant current to the coil. A high-resolution Czerny-Turner monochromator with a charge coupled device detector completed the system. Simultaneous, multi-element analyses were possible for eleven test elements: Al (396.1 nm), Co (353.0 nm), Cr (427.1 nm), Dy (404.6 nm), Ga (403.3 nm), K (404.4 nm), Mn (403.1 nm), Pb (405.8 nm), Rb (420.2 nm), Sc (404.8 nm), and Yb (398.7 nm). Tungsten coil atomic emission detection limits were reported for these elements for the first time: 0.02 ng Al, 0.7 ng Co, ng Cr, 0.01 ng Dy, 0.7 ng Ga, 0.3 ng K, 0.04 ng Mn, 10 ng Pb, 0.07 ng Rb, I ng Sc, and ng Yb. The elimination of the external radiation source needed for atomic absorption measurements made the whole system quite portable. Also, no high resolution spectrometer was required for WCAES. Later, the same system was used to determine 14 lanthanides in soil samples. With the same experimental set up, the WCAES was found to be more sensitive than FAES for most lanthanides. The simplicity, low cost, and multielement character of WCAES make it an ideal candidate for a portable elemental analysis device. However, further research is required to study this technique. The techniques discussed in this dissertation are focused on improving the portability and sensitivity of WCAES devices. Also, other possible portable tungsten coil techniques are investigated. 4

15 References 1. X.D. Hou, Y.H. He, and B.T. Jones, Recent Advances in Portable X-ray Fluorescence Spectrometry. Applied Spectroscopy Reviews, (1): p K. Carron, and R. Cox, Qualitative Analysis and the Answer Box: A Perspective on Portable Raman Spectroscopy. Analytical Chemistry, (9): p D.A. Cremers and R.C. Chinni, Laser-Induced Breakdown Spectroscopy-Capabilities and Limitations. Applied Spectroscopy Reviews, (6): p D. Beauchemin, Inductively Coupled Plasma Mass Spectrometry. Analytical Chemistry, (12): p G. Horlick, Flame Emission, Atomic-Absorption and Fluorescence Spectrometry. Analytical Chemistry, (5): p. R290-R K.W. Jackson and S.J. Lu, Atomic Absorption, Atomic Emission, and Flame Emission Spectrometry. Analytical Chemistry, (12): p. 363r-383r. 7. X.D. Hou and B.T. Jones, Field Instrumentation in Atomic Spectroscopy. Microchemical Journal, (1-3): p M.P. Bratzel, R.M. Dagnall, and J.D. Winefordner, A New, Simple Atom Reservoir for Atomic Fluorescence Spectrometry Analytica Chimica Acta, : p M.P. Bratzel, R.M. Dagnall, and J.D. Winefordner, A Hot Wire Loop Atomizer for Atomic Fluorescence Spectrometry. Applied Spectroscopy, (5): p M. Williams and E.H. Piepmeier, Commercial Tungsten Filament Atomizer for Analytical Atomic Spectrometry. Analytical Chemistry, (7): p K. Ohta, and M. Suzuki, Determination of Selenium in Metallurgical Samples by Flameless Atomic-Absorption Spectrometry. Analytica Chimica Acta, (Jul): p K. Ohta, and M. Suzuki, Trace-Metal Analysis of Rocks by Flameless Atomic- Absorption Spectrometry with a Metal Micro-Tube Atomizer. Talanta, (4-5): p R.D. Reid, and E.H. Piepmeier, Horizontal Tungsten Coil Atomizer and Integrated Absorbance Readout for Atomic-Absorption Spectrometry. Analytical Chemistry, (2): p M. Suzuki, and K. Ohta, Electrothermal Atomization of Calcium and Strontium in a Molybdenum Micro-Tube. Talanta, (3): p M., K. Suzuki, Ohta, and T. Yamakita, Atomic Emission-Spectrometry of Barium with a Metal Electrothermal Atomizer. Analytical Chemistry, (12): p

16 16. M. Suzuki, K. Ohta, and T. Yamakita, Elimination of Alkali Chloride Interference with Thiourea in Electrothermal Atomic-Absorption Spectrometry of Copper and Manganese. Analytical Chemistry, (1): p M. Suzuki, K. Ohta, and T. Yamakita, Improved Sensitivity Using a Microcomputer for Electrothermal Atomic-Absorption Spectrometry with a Metal Microtube. Analytica Chimica Acta-Computer Techniques and Optimization, (2): p M. Suzuki, et al., Electrothermal Atomization with a Metal Micro-Tube in Atomic- Absorption Spectrometry. Spectrochimica Acta Part B-Atomic Spectroscopy, (7): p M. Suzuki, and K. Ohta, Determination of Strontium in Biological Samples by Atomic Emission-Spectrometry with Electrothermal Atomization. Fresenius Zeitschrift Fur Analytische Chemie, (1): p M. Suzuki, and K. Ohta, Atomic Emission-Spectrometry with Metal Microtube Atomization. Analytical Chemistry, (1): p M. Suzuki, and K. Ohta, Determination of Thallium by Electrothermal Atomic- Absorption Spectrometry with a Metal Atomizer. Fresenius Zeitschrift Fur Analytische Chemie, (5): p M. Suzuki, K. Ohta, and K. Isobe, Mechanism of Interference Elimination by Thiourea in Electrothermal Atomic-Absorption Spectrometry. Analytica Chimica Acta, (Jul): p H. Berndt, G. Schaldach, and R. Klockenkamper, Improvement of the Detection Power in Electrothermal Atomic-Absorption Spectrometry by Summation of Signals - Determination of Traces of Metals in Drinking-Water and Urine. Analytica Chimica Acta, (1): p H. Berndt and G. Schaldach, Simple Low-Cost Tungsten-Coil Atomizer for Electrothermal Atomic-Absorption Spectrometry. Journal of Analytical Atomic Spectrometry, (5): p K. Dittrich, et al., Comparative-Study of Injection into a Pneumatic Nebulizer and Tungsten Coil Electrothermal Vaporization for the Determination of Rare-Earth Elements by Inductively Coupled Plasma Optical-Emission Spectrometry. Journal of Analytical Atomic Spectrometry, (8): p K. Ohta, S.I. Itoh, and T. Mizuno, Electrothermal Atomization Atomic-Absorption Spectrometry of Cadmium with a Platinum Tube Atomizer. Talanta, (8): p C.G. Bruhn, et al., Analytical Evaluation of a Tungsten Coil Atomizer for Cadmium, Lead, Chromium, Manganese, Nickel and Cobalt Determination by Electrothermal 6

17 Atomic-Absorption Spectrometry. Analytica Chimica Acta, (2-3): p M. Ezer, et al., Evaluation of a Tungsten Coil Atomization-Laser-Induced Fluorescence Detection Approach for Ttrace Elemental Analysis. Analytica Chimica Acta, (1): p X.D. Hou, et al., Tungsten Coil Devices in Atomic Spectrometry: Absorption, Fluorescence, and Emission. Analytical Sciences, (1): p J.A. Rust, et al., An Overview of Electrothermal Excitation Sources for Atomic Emission Spectrometry. Spectrochimica Acta Part B-Atomic Spectroscopy, (3): p C.L. Sanford, S.E. Thomas, and B.T. Jones, Portable, Battery-Powered, Tungsten Coil Atomic Absorption Spectrometer for Lead Determinations. Applied Spectroscopy, (2): p K.A. Wagner, K.E. Levine, and B.T. Jones, A Simple, Low Cost, Multielement Atomic Absorption Spectrometer with a Ttungsten Coil Atomizer. Spectrochimica Acta Part B-Atomic Spectroscopy, (11): p J.A. Rust, et al., Fraunhofer Effect Atomic Absorption Spectrometry. Analytical Chemistry, (4): p J.A. Rust, et al., Advances with Tungsten Coil Atomizers: Continuum Source Atomic Absorption and Emission Spectrometry. Spectrochimica Acta Part B-Atomic Spectroscopy, (5): p J.A. Rust, et al., Analytical Characteristics of a Continuum-Source Tungsten Coil Atomic Absorption Spectrometer. Analytical Sciences, (8): p J.A. Rust, et al., Tungsten Coil Atomic Emission Spectrometry. Spectrochimica Acta Part B-Atomic Spectroscopy, (2): p

18 CHAPTER 2 A Portable Tungsten Coil Atomic Emission Spectrometer for the Simultaneous Determination of Metals in Water and Soil Samples Jiyan Gu, Summer Hanna and Bradley T. Jones The following manuscript was written to submit to Analytical Science. Stylistic variations are due to the requirements of the journal. All of the presented research was conducted by Jiyan Gu. Summer Hanna assisted with editing. The final manuscript was prepared by Jiyan Gu and edited by Bradley T. Jones. 8

19 Abstract Tungsten Coil Atomic Emission Spectrometry (WCAES) has been evaluated as a potentially portable technique for field applications. The tungsten coil (W-Coil) was extracted from a commercially available slide projector bulb and used as both the atomizer and the excitation source. The coil was powered by a small solid-state power supply. A hand-held CCD spectrometer, powered from a laptop computer, collected the signal. Fifteen elements were used to evaluate the portable system. For elements in the UV region, LODs were inproved by a factor of 2000 for Cu; 200 for Ag; and 25 for Co through a 400 W Solid state power supply. Signals for Al, Cr, Ga, Mn, Li and V in the near UV region also increased significantly. Therefore, the WCAES device could be used for elements in both the visible and UV regions, and the system could be taken into the field to measure elements in various samples. Key words: Atomic Emission, Trace Metals, Tungsten Coil, Electrothermal Atomizer, Portable, Signal Improvement 9

20 Introduction Tungsten coil (W-Coil) devices have been used in analytical atomic spectrometry for nearly 40 years. In early 1970s, W-Coil atomizers were employed in electrothermal atomic absorption spectrometry, and atomic fluorescence spectrometry. 1-3 Since then, the W-Coil has been used as an electrothermal vaporizer for sample introduction into inductively coupled plasmas (ICP), microwave induced plasmas (MIP) and mass spectrometers (MS). 4-6 Recently, W-Coil devices have gained popularity due to their potential for portable devices in field applications. Though flames, arcs, plasmas and graphite furnaces have been widely used to determine metals in water and soil samples, these techniques are seldom portable due to their large size and high demands for power and gas. The small size, minimal power requirements, and low cost make W-Coil devices attractive for field applications. 7-9 A portable tungsten coil atomic absorption spectrometry (WCAAS) device was developed in the early 1990s. The device measured inches and was powered by a car battery. Two test elements, Pb and Cd, produced limits of detection at the pg level using a 20 µl sample aliquot. 10, 11 Compared to the WCAAS, W-Coil atomic emission spectrometry (WCAES) is even more portable because no hollow cathode lamp is needed, and multielement determinations are possible, and optical alignment is simplified. Molybdenum and tungsten tubes have been used as metal atomizers for atomic emission spectrometry, but few papers describe W-Coil atomic emission spectrometers WCAES was initially observed during W-Coil self absorption experiments. 16 Thirteen elements were detected by WCAES using a high resolution spectrometer. Limits of detection for most of the Lanthanides were lower than those reported for flame emission spectrometry In the current work, a small, portable CCD detector is used to improve the portability of the WCAES instrument. Fifteen elements are used to evaluate the system, and optimize method parameters such as sample volume, atomization power, and heating rate. Mg, Ni, Co, 10

21 Cu, Ag, Cr, Ga, Mn, and V signals are observed in the UV region at high atomization power. An aqueous NIST standard, a water pollution standard, and a NIST soil standard are used to test the accuracy and reliability of the system for field applications. 11

22 Experimental WCAES Instrumentation The WCAES system has been previously described in the literature (Fig. 1). 18 The W- Coil filament was extracted from a 15 V 150 W commercially available slide projector light bulb (Osram Xenophot HXL Pullach Germany) and secured in a laboratory designed aluminum mount. The Al mount contained a gas inlet for the flow of purge gas, electrical connections to the solid-state power supply, and a ceramic bulb socket to hold the filament. The mounted coil was housed in a glass vaporization cell (Ace Glass, product No. D131703, Vineland, NJ, USA) which had two fused-silica windows and one sample injection port. A continuous flow of 10% H 2 /Ar gas was flushed through the cell at a rate of 1.1 L/min to avoid oxidation of the coil. The purge gas also served as a cooling gas, and it escaped through the sample introduction port. The W-Coil filament was controlled by a solid state, constant current power supply (Victor, VI-LU1-EU-BM, Andover, MA, USA). A simple digital-to-analog board coupled with a home-written Visual Basic program allowed for current control in the range 0-10 amps. The maximum output voltage of the power supply was 15 V, 200W. The W-Coil was heated using a previously reported atomization cycle which optimized the drying time and increased emission signal. 18 The radiation emitted by the coil during the high-temperature atomization step was collected by a 50 mm diameter focal length fused silica lens. A 1:1 image of the coil was formed by the lens on a light blocking aperture (4 mm diameter). The aperture was used to block the black body emission from the coil, while allowing the atomic emission signal above the coil surface to pass through. After passing the aperture, the radiation was refocused by a 50 mm diameter focal length fused silica lens, onto the entrance aperture of the spectrometer. 12

23 To enhance portability, a miniature CCD spectrometer (USB4000, Ocean Optics, Dunedin, FL, USA) was used to collect the atomic emission signal. The size of USB4000 was only 89.1 mm x 63.3 mm x 34.4 mm and the mass was 190 grams. The detector was connected to a computer via a USB cable, and the detector power originated from the PC. The spectrometer had an entrance slit width of 25 µm. The spectrometer observed a spectral window 200 nm in width, adjustable across the range nm using a set screw. The CCD detector was a Toshiba TCD1304AP 3648-element linear array. The individual pixel size was 8 µm wide by 200 µm high. The USB4000 had a typical signal-to-noise ratio of 300:1 and a theoretical spectral bandpass of 0.05 nm per pixel. In practice, emission peaks had half widths (FWHM) of roughly 3-4 pixels, so the practical spectral bandpass was 0.2 nm. WCAES for Elements in UV Region In an effort to increase the signal intensity for the W-Coil in the UV region, the typical 15 V, 150 W bulb filament was replaced with a 24 V, 250 W version (Osram Xenophot HXL Pullach Germany). The new W-Coil was larger, and thus was able to hold triple the volume of sample relative to the smaller filament. The light-blocking aperture was removed with this design in order to simplify the instrumentation and to increase portability. The background signal increased without the aperture, but this was counter-acted by using a shorter detector integration time (50 ms per spectrum, 40 spectra total). The 24 V W-Coil was heated with a 24 V, 400 W power supply (Victor, VI-MU3-CQ-BM, Andover MA, USA). Procedure For the 150 W W-Coil system, a 25 µl aliquot of solution was deposited directly onto the W-Coil with an Eppendorf pipette. The solution was dried with a simple heating program (Table 1). As the liquid dried, the resistance of the W-Coil increased, so progressively smaller currents were used to gradually dry the sample. Once the coil was dry, a 10 s cooling 13

24 interval (0 current) was employed to allow the W-Coil return to room temperature. This ensured that the high temperature heating step was most reproducible. The high temperature step was achieved 7.7 A applied for 5 s. During this step, the Visual Basic program triggered the detector to begin data collection. Data were collected for the duration of the 5 s atomization step. A 15 s cooling step was employed at the end, to ensure that the W-Coil was ready for the next injection of sample. For the 250 W W-Coil system, 50 µl sample aliquots were used for all experiments. The 250 W W-Coil heating cycle differed slightly (Table 1). The limits of detection (LOD) were determined by the IUPAC method: 3σ/m, where σ is the standard deviation in the blank signal and m is the slope of the analytical calibration curve. The precision of the method was calculated as the relative standard deviation (RSD, n=11) for each element (Table 2). The precision was measured at the following analyte concentrations: 1 mg L -1 Li, 1 mg L -1 Yb, 0.05 mg L -1 Ca, 1 mg L -1 Eu, 0.1 mg L -1 Sr, 0.05 mg L -1 Ba, 0.05 mg L -1 Na, 0.02 mg L -1 K, 0.05 mg L -1 Rb, 0.02 mg L -1 Cs, 20 mg L -1 Co, 20 mg L -1 Mg, 20 mg L -1 Ag, 20 mg L -1 Cu, and 20 mg L -1 Ni. Reference Solutions and Sample Preparation All reference solutions were prepared from the dilution of single element stock solutions (1000 mg L -1, SPEX CerPrep, Metuchen, NJ, USA) with distilled-deionized water (Milli-Q, Millipore Corp., Bedford, MA, USA). A soil standard reference material from the National Institute of Standards and Technology (Montana soil, SRM # 2711, NIST, Gaithersburg, MD, USA) was digested as follows. Approximately 1 g of soil (accurately weighed) was transferred to a plastic container with a snap top lid. A 2 ml aliquot of concentrated HNO 3 was added, the lid was closed, and the container was placed in an aluminum hot block at 100 for two hours. Two ml aliquots of distilled-deionized water were added as needed during the heating process to prevent evaporation. The acid extract was filtered using coarse filter paper. The total filtrate was diluted to 50 ml with distilled- 14

25 deionized water in a volumetric flask. Two aqueous standard reference materials were also analyzed: Water Pollution Standard 1 (Product Number WPS1-100, VHG Labs Inc., Manchester, NH, USA) and Trace Elements in Water (SRM #1634e, NIST, Gaithersburg, MD, USA). 15

26 Results and Discussion Portable WCAES Fifteen elements were used to characterize the portable WCAES instrument. The system was extremely sensitive for elements that had emission lines in the visible to Near-IR regions (Figs. 2 and 3). The observed LODs ranged from 0.5 ug L -1 (Na nm) to 9 ug L -1 (Yb nm) (Table 2). Elements with emission lines below 400 nm (Mg, Cu, Ag, Co, Ni) were difficult to be detect using the portable detector. This indicated that the Ocean Optics USB4000 spectrometer was less sensitive compared with the benchtop spectrometer and chilled CCD detector described previously However, advantages associated with the portable spectrometer included its relatively large spectral window (200 nm compared to 50 nm reported previously), its low power requirement (provided by the laptop computer), and its operation at ambient temperature (requiring no circulating water). WCAES emission signals for Li, Na, K, Rb, and Cs were observed for the first time using the portable device. In addition, the Yb nm line was observed with this system. Compared with the ICP-AES, WCAES had no argon interference in the near-ir region for elements like Rb and Cs. LODs were 3 ug L -1 for Rb and 9 ug L -1 for Cs. Optimization of Instrumental Parameters The following empirical equation describes the relationship of WCAES signal (S) and instrumental variables. S = KPV/m W [1] The total emission signal intensity (S) is integrated throughout the atomization step. A proportionality constant (K) includes fixed parameters such as the sensitivity of the detector, the solid angle light collected by the spectrometer, the transmittance of all of the optics, and other small instrumental factors that remain predominantly unchanged. The power applied to 16

27 the coil, P in Watts, is given by the constant current applied during the atomization step multiplied by the potential across the coil. This potential is measured with a voltmeter and is dependent upon the resistance of the filament. For example, an older filament will be thinner than a new one, thus the resistance is higher for the old filament and the signal at constant current is higher. The emission signal is directly proportional to the sample volume (V) injected onto the coil. Finally, the mass, m W, of the W filament is inversely proportional to the signal (m W slowly decreases with use). While the signal intensity increases linearly with sample volume, the maximum volume is limited in practice by W-Coil size and shape. The 150 W W-Coil can hold a maximum volume of 25 µl. The sample droplet is held on the coil by surface tension, so higher volumes run the risk of falling off due to gravity. The 250 W W-Coil is larger, and hence may hold a larger sample volume (up to 75 µl). The relationship between sample volume and signal intensity with the 250 W Coil is shown in Figure 4. Emission signal doubled when the sample volume doubled from 25 µl to 50 µl. As mentioned above, signal intensity is directly proportional to applied power. For example, by applying specified constant atomization currents, and measuring the voltage applied during that step, the relationship in Figure 5 was observed (Cr nm 1 mg L -1 ). When the power applied to the coil increased, two separate factors produced a signal increase. First of all, the temperature of the surface of the coil increases. 20 Secondly the heating rate of the coil increases. At higher power, the atomization current is reached more quickly. As the sample is vaporized, it quickly leaves the surface of the coil. When the heating rate is low, the atoms may leave the vicinity of the coil prior to its reaching the maximum temperature. At higher heating rates (higher power), more energy can be transferred to the analyte atoms before they leave the high temperature region. This results in an increased signal. In addition, the high power signal appears in a shorter time period ( s at 400 W compared to

28 s at 200 W). Blackbody emission from the coil acts as a major interference in detection, so the quicker time frame improves the signal to noise ratio. The physical properties of the W-Coil also affect the emission signal. These properties include the coil mass (m W ), the W wire diameter, and size and number of loops in the coil. A coil with smaller mass is heated at a faster rate, so signal increases with decreasing mass. If the mass of the coil is intentionally lowered, by conditioning the filament at high temperature in air, higher signals are observed. 18 The mass of a W-Coil measured before and after this conditioning showed a 20% reduction in mass and a 50% increase in signal. As a consequence of this reduction in mass, the W filament must have a smaller diameter. Therefore, the resistance increases, and the power increases at constant current. Of course, at very high power, temperatures in excess of the melting point of W are approached, and the filament breaks (producing very strong atomic emission lines for W). Improved Sensitivity for WCAES in the UV Region Based on Equation 1, a high power WCAES device was tested with the 24 V, 250 W W-Coil and a 400 W power supply. The new coils were conditioned by heating for 5 s at 5 A in air. As mentioned above, this reduced the coil mass, and resulted in faster heating during the atomization step. Sample aliquots of 50 µl were analyzed in all cases. With the traditional 150 W W-Coil, the optimal sample size was 25 µl, the best atomization current was 7.7 A, and the signal integration time was 5 s (Table 1). The atomic emission signal actually appeared during the first 2-3 s of this period, depending upon the analyte. With the 250 W W-Coil, the optimal sample size was 50 µl, the best atomization current was 12.5 A, and the signal integration time was 2 s. In this case, emission signals appeared during the s. The coil was heated more rapidly and emission signals increased dramatically. All elements gave a much stronger emission signal in the new system. Even elements such as Pt, Zn and Cd, which were not detectable at 200 W, gave rise to emission signals with 18

29 100 ppm solutions. Five elements with different characteristics were used to compare the two systems. Emission lines from Mg, Ni, Ag, Cu, and Yb were observed in the same spectral window as Figure 6. The LODs were 200 mg L -1 for Cu, 400 mg L -1 for Ag, and 25 mg L -1 for Co with the 150 W W-Coil. With the 250 W W-Coil, the LOD was improved by a factor of 2000 for Cu, 200 for Ag, and 50 for Co (Table II). Using the 150 W W-Coil, Ni and Mg were not detectable, while the 250 W device resulted in LODs less than 1 mg L -1. The lifetime of the 24 V, 250 W W-Coil was approximately atomization cycles, similar as the 15 V, 150 W W-Coil. Lifetimes were less for samples containing high salt or acid concentrations. Six additional elements were detected with 250 W W-Coil system at wavelengths below 430 nm: Al, Cr, Ga, Mn, Li and V (Fig. 7). For Li and V these were secondary emission wavelengths. Limits of detection were 70 ng ml -1 Al, 20 ng ml -1 Cr, 500 ng ml -1 Ga, 20 ng ml -1 Mn, 600 ng ml -1 V and 11 µg ml -1 Li. Clearly the portable WCAES device can detect at least 11 elements below 430 nm using the 250 W W-Coil. Accuracy The accuracy of the system was tested with two reference materials. For the NIST standard reference material 1643e, matrix effects were minimized by employing the standard addition method. Calcium (422.6 nm), Sr (460.7 nm), Ba (553.5 nm), and Na (588.9 nm) were determined in a single spectral window, while K (766.4 nm) and Rb (780.8 nm) were determined in another. Each analyte was determined with an accuracy in the % range except for Rb. The found value for Rb was 183 % of the expected value of mg/l. This error was due to the low level of Rb present in the diluted sample, which approached the LOD. A second reference material (a water pollution standard, Product Number WPS1-100, VHG Labs) was employed to test the accuracy of the 250 W W-Coil system. Cobalt, Ni and Cu were determined in this sample. Recovery was 95 % for Co, 108 % for Ni, and 77 % for 19

30 Cu. Each of these elements was present at levels near the detection limit after dilution for the standard addition method. Finally, a soil sample was analyzed (Montana Soil, NIST SRM #2711, Table III). Among the five elements detected by WCAES, the values for Ca, Sr, and Ba were certified in the soil sample, while the values for Cs and Rb were not. A mild sample preparation method was used to extract the elements from the soil (no HF was employed). As demonstrated in Figures 2 and 3, Ca, Sr, Ba were determined in one spectral window, while Cs and Rb were determined in another. The recovery results for the five elements were similar to those reported for EPA sample preparation method (EPA method 3050B, ACID DIGESTION OF SEDIMENTS, SLUDGES, AND SOILS)

31 Conclusions The traditionally used 150 W WCAES instrument is excellent for elements in the visible to near IR region. Emission signals for elements in the UV region were substantially increased largely by the new portable 250 W WCAES instrument. At least 11 elements in the UV and Near UV regions can be simultaneously determined in one simple spectrum. LODs for 20 elements were all below the 1 mg L -1 level. The multi-element capabilities of WCAES make it an attractive alternative for field applications. On-site analyses of polluted water and soil samples could be performed. 21

32 References 1. M. Williams and E. H. Piepmeier, Anal. Chem., 1972, 44, R. D. Reid and E. H. Piepmeier, Anal. Chem., 1976, 48, M. P. Bratzel, R. M. Dagnall, and J.D. Winefordner, Appl. Spectrosc., 1970, 24, K. Levine, K. A. Wagner, and B. T. Jones, Appl. Spectrosc., 1998, 52, K. Dittrich, H. Berndt, J. A. C. Broekaert, G. Schaldach, and G. Tolg, J. Anal. At. Spectrom., 1988, 3, K. Dittrich, H. Fuchs, H. Berndt, J. A. C. Broekaert, and G. Schaldach, Fresen. J. Anal. Chem., 1990, 336, X. D. Hou and B. T. Jones, Microchem. J., 2000, 66, X. D. Hou and B.T. Jones, Spectrochim. Acta, Part B, 2002, 57, X. D. Hou, K. E. Levine, A. Salido, B. T. Jones, M. Ezer, S. Elwood, and J. B. Simeonsson, Anal. Sci., 2001, 17, C. L. Sanford, S. E. Thomas, and B. T. Jones, Appl. Spectrosc., 1996, 50, J. D. Batchelor, S. E. Thomas, and B. T. Jones, Appl. Spectrosc., 1998, 52, M. Suzuki and K. Ohta, Talanta., 1981, 28, M. Suzuki, K. Ohta, and T. Yamakita, Anal. Chem., 1981, 53, M. Suzuki and K. Ohta, Anal. Chem., 1985, 57, K. Ohta, S. I. Itoh, and T. Mizuno, Talanta., 1991, 38, J. A. Rust, J. A. Nobrega, C. P. Calloway, and B. T. Jones, Spectrochim. Acta, Part B, 2005, 60, J. A. Rust, J. A. Nobrega, C. P. Calloway, and B. T. Jones, Spectrochim. Acta, Part B, 2006, 61, G. L. Donati, J. Gu, J. A. Nobrega, C. P. Calloway, and B. T. Jones, J. Anal. At. Spectrom., 2008, 23,

33 19. G.. L. Donati, B. E. Kron, and B. T. Jones, Spectrochim. Acta, Part B, 2009, 64, J. A. Rust, G. L. Donati, M. T. Afonso, J. A. Nobrega, and B. T. Jones, Spectrochim. Acta, Part B., 2009, 64, EPA 3050b 23

34 Table 1 WCAES atomization cycles for two different tungsten coils. WCAES ( 15V,150W ) WCAES( 24V,250W) Step Current (A) Time (s) Current (A) Time (s)

35 Table 2 Analytical figures of merit for WCAES. Elements Wavelength Limit of Detection LDR Precision Reported Found (nm) (mg L -1 ) (%) 400W Power Supply, Max Coil Current 12.5 A VHG Water Pollution Standard(µg ml -1 ) Mg Cu ±4 Ag Co ±14 Ni ±25 200W Power Supply, Max Coil Current 7.7 A Reference 1634e (µg L -1 ) Yb Ca ± ±1800 Eu Sr ± ±14 Ba ± ±9 Na ± ±1500 Li K ± ±300 Rb ± ±3 Cs

36 Table 3 Determination of Ca, Ba, Sr, Cs and Rb in the soil sample (NIST SRM#2711) with WCAES. Values Found in Reference Certified Values Lab Method EPA Method 3050 Element Mass Fraction Mass Fraction Leach Recovery leach Recovery Calcium 2.8±0.5% 2.88±0.08% 97% 73% Barium 274.5± 8.0 ug/g 726ug/g 38% 28% Strontium 35.3±3.8 ug/g 245.3ug/g 14% 20% Cesium 8.3±6.1 ug/g 6.1ug/g* 140% not reported Rubidium 29.5±0.8 ug/g 110ug/g* 27% not reported 26

37 Figure Captions Fig. 1 Photograph of the WCAES instrument. Fig. 2 Simultaneous multi-element WCAES determination of Yb (1 mg L -1 ), Ca (2 mg L -1 ), Eu(2 mg L -1 ), Sr(2 mg L -1 ), Ba (1 mg L -1 ), Na (0.5 mg L -1 ), background is subtracted. Fig. 3 Simultaneous multi-element WCAES determination of K (0.3 mg L -1 ), Rb (0.3 mg L -1 ) and Cs (0.8 mg L -1 ) in Near-IR region, background is subtracted. Fig. 4 Relationship between sample volume and emission signal intensity (Cr nm 1 mg L -1 ) Fig. 5 Relationship between W-Coil Power (V i) and the emission signal intensity (Cr nm 1 mg L -1 ) Fig. 6 WCAES spectrum for Mg, Cu, Ag, Ni and Yb (10 mg L -1 ), with 400 W power supply. Fig. 7 WCAES spectrum for Al (10 mg L -1 ), Cr (2 mg L -1 ), Ga (20 mg L -1 ), Mn (20 mg L -1 ), Li (100 mg L -1 ) and V (5 mg L -1 ), with 400 W power supply. 27

38 Fig. 1 Photograph of the WCAES instrument. 28

39 50000 Ba Relative Emission Signal (0.5 s) Ca Sr Eu Eu Eu Yb Ca Na Wavelength (nm) Fig. 2 Simultaneous multi-element WCAES determination of Yb (1 mg L -1 ), Ca (2 mg L -1 ), Eu (2 mg L -1 ), Sr (2 mg L -1 ), Ba (1mg L -1 ), Na (0.5 mg L -1 ). Background is subtracted. 29

40 55000 K Relative Emission Signal (0.5 s) K Rb Rb Cs Wavelength (nm) Fig. 3 Simultaneous multi-element WCAES determination of K (0.3 mg L -1 ), Rb (0.3 mg L -1 ) and Cs (0.8 mg L -1 ) in Near-IR region. Background is subtracted. 30

41 Relative Emission Intensity Sample Volumn (µl) Fig. 4 Relationship between sample volume and emission signal intensity (Cr nm, 1 mg L -1 ) 31

42 Relative Emission Intensity Vi (Watts,Energy Dissipated on the Coil) Fig. 5 Relationship between W-Coil power (V i) and the emission signal intensity (Cr nm, 1 mg L -1 ) 32

43 Relative Emission Intensity Mg Ni Ni Cu Yb Ag Ni Ni Ni CuAg Ni Ni Ni Yb Ni Ni Ni Wavelength (nm) Fig. 6 WCAES spectrum for Mg, Cu, Ag, Ni and Yb (10 mg L -1 ), with 400 W power supply. 33

44 Relative Emission Intensity Al Cr Mn Al Cr Cr Cr Cr Ga 5000 Al Al V Li V Ga V V V V Wavelength (nm) Fig. 7 WCAES spectrum for Al (10 mg L -1 ), Cr (2 mg L -1 ), Ga (20 mg L -1 ), Mn (20 mg L -1 ), Li (100 mg L -1 ) and V (5 mg L -1 ), with 400 W power supply. 34

45 CHAPTER 3 A Portable Tungsten Coil Atomic Emission Spectrometer With Two Coils Jiyan Gu, Clifton P. Calloway and Bradley T. Jones The following manuscript was written to submit to Talanta. Stylistic variations are due to the requirements of the journal. All of the presented research was conducted by Jiyan Gu. Clifton P. Calloway assisted in computer programming and instrumentation design. The manuscript was prepared by Jiyan Gu and edited by Bradley T. Jones. 35

46 Abstract Tungsten coils have been employed recently in instruments that may be used for field applications. Intense emission signals and µg/l limits of detection (LOD) have been observed for alkali metals, alkaline earth metals and some lanthanides (such as Yb and Eu). However, for many transition elements, relatively high LODs are observed due to the insufficient excitation energy provided by the coil at high temperature. In this study, two tungsten coils are employed in an attempt to increase the emission signal and lower the LOD. A lower coil is heated to vaporize the sample and then a second upper coil is heated at high temperature to excite the sample atoms. The emission signal is viewed above the upper coil. Six test elements (Co, Fe, Ag, Cu, Cr and Yb) are used to compare the performance with one and two coils. Double tungsten coil atomic emission is reported for the first time for all six elements. Addition of the second coil improved the LOD by a factor of 100 for Cu and Ag; a factor of 40 for Co; a factor of 12 for Fe; and a factor of 2 for Cr and Yb. All six elements can be determined in one simple spectrum with the two coil system. Cobalt, Fe, Cr and Cu were determined in a certified polluted water reference sample to evaluate the accuracy of the system. Recoveries were in the range of 93%-102% for all elements. Key words: Atomic Emission, Trace Metals, Tungsten Coil, Electrothermal Atomizer, Portable, Signal Improvement 36

47 1 Introduction Tungsten coil atomic absorption spectrometry (WCAAS) has been employed as a portable device to determine Pb and Cd for field applications [1-2]. However, WCAAS is a single element technique. A device that can determine more elements at one time will be more attractive compared to WCAAS. Metal atomizers, such as molybdenum and tungsten tubes, have been used previously in atomic emission spectrometry [3-7]. More recently, several papers have been published describing a tungsten coil atomic emission spectrometer (WCAES) [8-10]. Numerous elements (alkali metals, alkali earth metals, most lanthanides, and many transition metals) have been determined with the device. Compared to WCAAS, WCAES is a multi element technique and much simpler in terms of instrumentation. All of these results show that WCAES is a promising technique for field applications. One tungsten coil atomic emission device employs a portable hand-held spectrometer with a built in CCD detector [11]. The limit of detection for ten different elements in visble region was in the range from ng/ml. The portable spectrometer was powered by a laptop, and the entire system could be converted into a portable device using a car battery for atomization power. Unfortunately, the portable WCAES device lacked sensitivity in the UV region for elements Cu, Ag and Co. These elements were detected, however, with a laboratory-based CCD detector that was more sensitive at lower wavelengths or with a 400 W power supply. Therefore, the goal of the current work was to improve sensitivity in this important region of the spectrum. Since the portable device will employ the same hand-held spectrometer, avenues for increasing the WCAES emission levels will be explored. The new WCAES system described here will employ a second tungsten coil (W-Coil). The Sample will be deposited onto a single coil, which will be used to vaporize the analytes into the vicinity of the second coil which will be operated at full power. Six elements have been 37

48 selected to compare the double coil system with the traditional single coil arrangement. Copper, Ag and Fe are detected by WCAES for the first time, and all six elements are determined simultaneously in a single spectral window with two coils. A reference material (polluted water) is used to evaluate the reliability of the new double coil WCAES system. While a similar hand-held spectrometer is employed, a slightly higher resolution model is employed. This helps to isolate the atomic emission signal from the blackbody radiation rising from the coils. The double coil WCAES system significantly improves sensitivity, and therefore could be applied for both field and clinical applications [12-16]. A similar double coil system is also reported recently [17]. In that system, signal is viewed between two coils instead of above the upper coil. However, in our study, we found that the signal above the upper coil is much stronger. 38

49 2 Experimental 2.1 Instrumentation The photograph of the WCAES instrument (one coil and two coil system) is given Fig.1. For one coil system, a 15 V, 150 W W-Coil is housed in a glass atomization cell purged with a 10% H 2 in Ar gas mixture. The atomization cell, W-Coil atomizer and optical system have been described elsewhere [10]. An aperture is placed between the two emission collection lenses to block the background emission from the coil. The sample is injected onto the coil and then atomized with a heating program. The emission signal above the coil is collected by a relatively low resolution but portable CCD spectrometer (USB4000, Ocean Optics, Dunedin, FL, USA). For the current study, a second W-Coil (24 V, 250 W) is placed in the system (Fig. 2). The rest of the apparatus remained unchanged. The second coil is positioned just 1 mm above the original coil. The aperture was adjusted to block the background image of each coil. The sample is injected onto the original W-Coil, and the emission signal is collected above the upper W-Coil. The W-Coils were powered by two separate solid state, constant current power supplies (Victor, VI-LU1-EU-BM and VI-MU3- CQ-BM, Andover, MA, USA ) The heating cycle (Table I) for both devices is controlled by a single Visual Basic program. 2.2 Procedure Six elements with different atomic emission characters were selected to compare the single and dual coil systems: Co, nm; Fe, nm; Ag, nm; Cu, nm; Cr, nm; and Yb, nm. A 25 µl aliquot of solution was injected onto the lower coil using a micropipette. Then the heating cycle was applied, and the sample was atomized in the last step with a maximum 10 A current. With the two coil system, the drying step was the same for the lower coil, and the upper coil was heated to maximum current 1 second prior to 39

50 the lower coil s vaporization step. This resulted in the highest emission signal for all elements. The integration time for the single coil system was 0.5 s per spectrum, and 6 continuous spectra were collected (3 s total). For the dual coil system, due to the higher background from the two coils, a smaller (0.1 s per spectrum) integration time was employed. Thirty successive spectra were collected, again resulting in a total collection time of 3 s. Limits of detection (LODs) were determined using the IUPAC (Table II). Figure 3 demonstrates that all 6 elements may be detected in the same spectral window ( nm): Co (345.3 nm); Fe (374.6 nm); Ag (328.1 nm); Cu (324.8 nm); Cr (425.4 nm); and Yb (398.8 nm). Since Fe and Co have multiple emission lines in this wavelength region, a spectrum without these two elements is much less complicated (Fig. 4). A certified reference material was used to test the accuracy of the dual coil system for Co, Fe, Cr and Cu. Reagents: All reference solutions were prepared by dilution of single element stock solutions (1000 mg/l, SPEX CertPrep, Metuchen, NJ) with distilled-deionized water (Milli-Q, Millipore Corp, Bedford, MA, USA). A polluted water standard reference material from VHG lab (Water Pollution Standard 1, WPS1-100, VHG Labs Inc., Manchester, NH, USA) was used to test the reliability of the system. 40

51 3 Results and Discussion 3.1 Spectrometer Characteristics The portability of the WCAES device is made possible by the hand-held spectrometer. The disadvantage of the spectrometer is its relatively poor sensitivity in the UV region: Yb (398.8 nm) has a literature LOD of 0.1 ng/ml, and the hand-held spectrometer gives a 9 ng/ml LOD [10]. The reason for decreased sensitivity was uncertain. The two most probable reasons for this increase in LOD are the poorer spectral resolution of the hand-held system, and the fact that the literature CCD detector was chilled to -44, while the hand-held device operates at ambient temperature. A simple experiment was performed to test the effect of detector temperature. The output from a Cr hollow cathode lamp (HCL) was collected by the hand-held detector using a fiber optic coupling. A constant signal from the lamp was maintained by applying a constant lamp current. Next the spectrometer was placed in a cooler with a block of dry ice. Spectra were collected along with detector temperature (given by the spectrometer software). Figure 5 demonstrates S/N increases linearly as detector temperature decreases from 26 to 14. The LOD will decrease with increasing S/N. However, the advantages of this hand-held spectrometer are obvious. The detector is powered through the USB connection to a laptop computer. The lower resolution also results in a larger spectral window, so more analytes can be determined simultaneously with the handheld device. 3.2 Dual W-Coil analyses Analyses were carried out on the single W-Coil system as follows. The sample was injected onto the coil and the heating cycle was applied. The emission signal was observed at a height of approximately 1 mm above the top surface of the coil. Full current was applied during the atomization step (10 A) resulting in a coil temperature of approximately 3400 C 41

52 (just below the melting point of W). Once the maximum current was applied, approximately 2 s were required to reach the highest temperature. This delay allowed a portion of the analyte to vaporize and leave the vicinity of the coil surface prior to thermal excitation. The addition of the second coil eliminated this problem. The second coil was placed approximately 1 mm above the top of the original coil. The sample was still injected onto the lower coil and vaporized. The upper coil was heated at full power one second prior to the vaporization. The atomized sample, therefore, was produced in the vicinity of the high temperature upper coil, thus thermal excitation was more efficient. The emission signal was collected 1 mm above the surface of the upper coil. The emission signal increased with decreasing distance between the two coils, but the 1 mm gap ensured that they did not touch. The upper coil was also extracted from a higher power light bulb (24 V, 250 W) and powered by a 400 W power supply. This provided faster heating of that coil. Six different test elements were selected. Two of them (Yb and Cr) were known to be relatively strong emitters, two were intermediate emitters (Co, Fe), and two were poor emitters with the single coil (Cu, Ag). The sensitivity with two W-Coils was higher in every case (Table II). These improvements are mostly likely due to the analyte atoms being vaporized into a hotter environment with the second W-Coil. Also, the atoms have less opportunity to escape prior to the attainment of maximum temperature. Emission signals were especially improved in the UV region of the spectrum. The LOD improvement factor using the dual coil arrangement fell in the range For Cr and Yb, the single coil sensitivity was fairly high, so the improvement factor was low. All six elements can be detected simultaneously with the dual coil WCAES system (Figs 3 and 4). These spectra were collected with 0.1 s integration times, and the background was subtracted by storing a blank spectrum. The polluted water reference sample was analyzed for Fe, Co, Cr and Cu (Table III). The accuracy of the found value was 93% for Cu, 42

53 94% for Fe, 97% for Cr and 102% for Co. Yb and Ag were not detected because their concentrations were not reported for the sample. 43

54 Conclusion WCAES emission signals increase dramatically with a dual coil arrangement. Detection limits are in the ng/ml range, and elements with emission lines in the UV region of the spectrum may be determined. A small hand-held spectrometer allows a broader spectral window to be observed, while adding portability. The resulting instrument should be easily portable for field analyses. 44

55 Figure 1. Photograph of the WCAES instrument (system setup for both one coil and two coil system). 45

56 Figure 2. Photograph of WCAES atomization cell with two coils. 46

57 Table I. Tungsten coil heating cycles. One Coil WCAES Two Coil WCAES Step Time(s) Current(A) Time(s) Current(Under coil) Current(Upper Coil)

58 Table II. Analytical figures of merit for One Coil WCAES and Two Coil WCAES. One Coil WCAES Two Coil WCAES Element Wavelength(nm) LOD(mg/L) LDR Precision (%) LOD(mg/L) LDR Precision (%) Co Fe Ag Cu Cr Yb

59 Figure 3. Simultaneous multi-element WCAES (two coil) determination of Yb (1.0 mg/l), Cr (0.2 mg/l), Ag (200 mg/l), Cu (200 mg/l),fe (150 mg/l), Co (150 mg/l) Emission Signal (Counts) Yb Co Ca Fe Fe Cr 7500 Co Fe CuCu Ag Ag Wavelength (nm) 49

60 Figure 4. Simultaneous multi-element WCAES (two coils) determination of Yb (1.4 mg/l), Cr (0.3 mg/l), Ag (280 mg/l), Cu (280 mg/l) Emission Signal (Counts) Cu Cu Ag Ag Yb Cr Yb Ca Cr Wavelength (nm) 50

61 Figure 5. Temperature effect on Ocean Optics USB400 detector (Cr hollow cathode lamp, nm). S/N S/N temprature 51

62 Table III. Accuracy of Two Coil WCAES system (Water Pollution Standard 1, Product Number WPS1-100, VHG) Found (mg/l) Certified (mg/l) Accuracy Cu 93± % Fe 94± % Co 102± % Cr 97± % 52

63 References [1] C.L. Sanford, S.E. Thomas, B.T. Jones, Appl. Spectrosc. 50(2) (1996) [2] J.D. Batchelor, S.E. Thomas, B.T. Jones, Appl. Spectrosc. 52(8) (1998) [3] K. Ohta, S. Itoh, T. Mizuno, Talanta. 38(3) (1991) [4] M. Suzuki, K. Ohta, Talanta. 28(3) (1981) [5] M. Suzuki, K. Ohta, Fresenius J. Anal. Chem. 313(1) (1982) [6] M. Suzuki, K. Ohta, Anal. Chem. 57(1) (1985) [7] M. Suzuki, K. Ohta, T. Yamakita, Anal. Chem. 53(12) (1981) [8] G.L. Donati, J. Gu, J.A. Nobrega, C.P. Calloway, B.T. Jones, J. Anal. At. Spectrom. 23(3) (2008) [9] J.A. Rust, J.A. Nobrega, C.P. Calloway, B.T. Jones, Spectrochim. Acta Part B 60(5) (2005) [10] J.A. Rust, J.A. Nobrega, C.P. Calloway, B.T. Jones, Spectrochim. Acta Part B 61(2) (2006) [11] J. Gu, S. Hanna, B.T. Jones (Chapter 2, paper is submitted to Analytical Science) [12] X.D. Hou, B.T. Jones, Spectrochim. Acta Part B 57(4) (2002) [13] A. Salido, C.L. Sanford, B.T. Jones, Spectrochim. Acta Part B 54(8) (1999) [14] X.D. Hou, K.E. Levine, A. Salido, B.T. Jones, M. Ezer, S. Elwood, J.B. Simeonsson, Anal. Sci. 17(1) (2001) [15] X.D. Hou, B.T. Jones, Microchem. J. 66(1-3) (2000) [16] X.D. Hou, W. Chen, Y.H. He, B.T. Jones, Appl. Spectrosc. Rev. 40(3) (2005) [17] G.L. Donati, C.P. Collway, B.T. Jones, J. Anal. At. Spectrom. 24(8) (2009)

64 CHAPTER 4 Continuum Source Tungsten Coil Atomic Fluorescence Spectrometry Jiyan Gu, George L.Donati, Carl G.Young and Bradley T. Jones The following manuscript was written to submit to Applied Spectroscopy. Stylistic variations are due to the requirements of the journal. All of the presented research was conducted by Jiyan Gu. George L. Donati assisted in instrumentation design. The manuscript was prepared by Jiyan Gu and edited by Bradley T. Jones. 54

65 ABSTRACT A simple continuum source tungsten coil atomic fluorescence spectrometer is constructed and evaluated. The heart of the system is the atomizer: a low-cost tungsten filament extracted from a 150 W light bulb. The filament is resistively heated with a small, solid state, constant current power supply. The atomizer is housed in a glass chamber and purged with a 1 L/min flow of a conventional welding gas mixture: 10% H 2 /Ar. A 25 µl sample aliquot is pipetted onto the tungsten coil, the liquid is dried at low current, and then the atomic vapor is produced by applying a current in the range A. The atomization current does not produce temperatures high enough to excite atomic emission. Radiation from a 300 W Xenon lamp is focused through the atomic vapor, exciting atomic fluorescence. Fluorescence signals are collected using a hand-held charge coupled device spectrometer. Simultaneous determination of 10 elements (Ag, Bi, Cr, Cu, Ga, In, Mg, Mn, and Tl) results in detection limits in the range 0.3 to 10 ng. The application of higher atomization currents (10 A) leads to straightforward detection of atomic emission signals with no modifications to the instrument. Index Headings: Electrothermal vaporizer; Tungsten coil; Atomic fluorescence spectrometry; Continuum source; Atomic emission spectrometry; Portable 55

66 INTRODUCTION Electrothermal atomizers find broad applications in elemental analysis. 1 The ideal atomizer should be chemically inert and physically stable. The device should have a high melting point and a rapid heating rate. Finally, the atomizer should be readily available at a reasonable cost. The tungsten coil (W-Coil) atomizer meets these criteria in many respects. Tungsten has one of the highest melting points (3422 ) of all the elements, second only to carbon. The W-coil heating rate has been measured as high as 30 K ms -1, and the filament is resistant to attack even from harsh chemicals such as hydrochloric, sulfuric and nitric acid. 2 In the early 1970s, the W-Coil was employed in electrothermal atomization atomic absorption spectrometry. 3 Subsequently, the W-Coil has been reported as an electrothermal vaporizer for inductively coupled plasma (ICP) emission spectrometry, microwave induced plasma (MIP) emission spectrometry, mass spectrometry (MS), ICP-MS, and flame-furnace atomic absorption spectrometry (FFAAS). 4,5,6,7,8,9 A small, portable tungsten coil atomic absorption spectrometer (WCAAS) was used for the determination of Pb and Cd in the 1990s. 10,11 For this device, the W-Coil was extracted from a commercially available light bulb. The entire spectrometer measured cm and it was completely powered by a 12 V car battery. The system was simplified later by the development of tungsten coil atomic emission spectrometry (WCAES), which obviated the need for the external light source common with AAS devices. Furthermore, WCAES was capable of simultaneous, multielement determinations. High sensitivity was reported for a broad range of elements including Al, Co, Cr, Ga, K, Mn, Pb, Rb, Sc, Cr, Ga, In, V and the lanthanides. 12,13,14,15 Absolute detection limits are near or below the ng level for most of these elements. Unfortunately, many elements do not emit strongly at the highest temperatures achieved with the W-coil (3300 K). For example, WCAES reported with the CCD detector used in the present work was capable of detecting the emission of all but 16 of the metals and semi- 56

67 metals with atomic number less than Those elements with excitation energies above 350 kj/mole were not detected with this system. Some particularly interesting analytes are included in this category: Ag, Bi, Cu, and Mg for example. Atomic fluorescence spectrometry (AFS) has been employed for chemical analysis for almost 50 years. 17 For many elements, AFS provides the lowest detection limit reported to date. Very few of these reports, however, use metal filaments to produce the atomic vapor. In the 1970s a tungsten wire loop and an electrodeless discharge lamp excitation source provided sub-ppb level detection limits for many elements. 18,19 More recently, a W-coil similar to the one described previously, 11 was used for sample vaporization in a flame atomic fluorescence instrument. 20 Cadmium and eight hydride-forming elements were determined with this system. The same vaporizer also has been employed in a laser-induced fluorescence device (W-Coil LIF). 21 This system provided ng/l detection limits. The author suggested that a portable device could be developed. In the current work, continuum source tungsten coil atomic fluorescence spectrometry (CS-WCAFS) is reported for the first time. The radiant flux from a xenon arc lamp with no wavelength selection filter is focused near the surface of the W-coil vaporizer. This provides simultaneous excitation of multiple elements, and the resulting fluorescence is captured with a charge coupled device detector. 57

68 EXPERIMENTAL Instrumentation. The laboratory-constructed W-Coil vaporizer has been described previously. 12 The tungsten coil filament was extracted from a 15 V, 150 W commercially available slide projector light bulb (Osram Xenophot HXL Pullach, Germany) and was housed in a glass vaporization cell (Ace Glass, product No. D131703, Vineland, NJ, USA) which had three fused-silica windows arranged in a T-shape configuration, and one sample injection port (Figure 1). A continuous flow of 10% H 2 /Ar purged the cell at a rate of 1.0 L/min. This prevented oxidation of the coil, provided a reducing atmosphere for the analyte atoms, and cooled the vaporizer between heating cycles. The purge gas escaped through the sample introduction port. The W-coil was heated resistively by applying a constant current from a solid-state, computer-controlled power supply (Vicor BatMod, Andover, MA, USA). The power supply provided a specified current in the range 0-10 A (at up to 15 V DC). The current was controlled using a simple Visual Basic program that supplied a user-selectable 1-5 V reference signal. The instrumental arrangement was straightforward (Fig. 1). The collimated radiant flux from a 300 W compact xenon arc lamp (Luxtel CL 300 BUV, Danvers, MA USA) passed through an electronically activated shutter. The lamp was operated at maximum power (300 W) so closing the shutter prevented bright reflections during sample introduction. The collimated beam was focused through one window on the atomization cell to a point near the surface of the W-coil using a 5 cm diameter, 15 cm focal length fused silica lens. The diverging beam leaving the W-coil exited the cell through a second window. The fluorescence signal was viewed 90 with respect to the xenon lamp path, through the third fused silica window. A 2.5 cm diameter, 7.5 cm focal length fused silica lens was placed 15 cm away from the W-coil. The atomic fluorescence collected with this lens was focused as a 58

69 1:1 image on the entrance aperture of a hand-held CCD spectrometer (Ocean Optics USB4000, Dunedin, FL USA). The CCD spectrometer was powered by a computer via a USB connection and responded to wavelengths in the range 200 nm-1100 nm. The spectrometer was fitted with a 10 µm fixed entrance slit and a 1800 grooves/mm holographic grating. The resulting spectral image was focused on a 3648 pixel CCD detector. The system covered a spectral window of nm. Figure 2 presents a portion of this window. The spectral resolution is 0.4 nm FWHM at the Tl nm line. The detector software had user-selectable inputs for integration time and number of successive spectra to collect and save. The detector was triggered at the beginning of the vaporization step by the Visual Basic W-coil control program mentioned above. Precise optical alignment was critical. Fluorescence was viewed at 90 with respect to the excitation source to prevent direct detection of the xenon lamp emission. Even at this angle the W-coil reflected any Xe lamp radiation that struck the surface. Fine adjustment of the focusing lens height assured that the lamp radiation was focused at a spot 2 mm above the upper surface of the W-coil without striking the filament directly. The fluorescence collection lens was also positioned to focus this same point on the entrance slit of the CCD spectrometer. Solutions. All reference solutions were prepared by serial dilution of single element stock solutions (1000 mg/l, SPEX CerPrep, Metuchen, NJ, USA) with distilled-deionized water (Milli-Q, Millipore Corp., Bedford, MA, USA). A standard reference material (Water Pollution Standard 1, Product Number WPS1-100, VHG Labs Inc., Manchester, NH, USA) and an instrumentation calibration standard (Perkin Elmer # N , Waltham, MA, USA) containing Pb and Tl were used to test the accuracy of the system. Precision of the instrument was tested at concentrations fifty times greater than the detection limit. 59

70 Procedure. A 25 µl sample aliquot was pipetted onto the W-coil and heated with the program in Table 1. The first three steps served to dry the liquid drop at progressively lower currents. This prevented premature analyte loss due to overheating the coil during the dry step. While the liquid drop remained on the filament, the constant current could pass through the drop rather than the coil wire, thus the amount of resistive heating was diminished. As the drop dried and became smaller, the same constant current caused a higher temperature, so the current was reduced. The dry coil finally reached a maximum drying temperature at a coil current of 2 A (step 3). Step 4 allowed the completely dry coil to return to room temperature. Then during the vaporization step 5, a current in the range A was applied and the analyte atoms were driven into the vapor phase. The detector was triggered at the beginning of this step. Five successive spectra, each with a 3 s integration time, were collected during this 15 s period. Atomic fluorescence appeared beginning at 3 s into the vaporization step and ending at 15 s. Maximum signals were observed at either 9 or 12 s depending upon the element, so all signals collected during the period 3-15 s were summed. At the end of the vaporization step, a 3 s cleaning step at 8 A was applied to remove any sample residue. The final step allowed the coil to return to room temperature prior to the next sample cycle. The entire vaporization program took less than 3 minutes. The optimal vaporization current for each element was determined independently. The results for Bi are shown in Figure 3. At currents lower than 3 A, no fluorescence was observed for any element. Peak vaporization currents ranged between 3.5 A for the more volatile elements like Bi (Fig. 3), to 5.5 A for less volatile elements like Cu. At currents of 5.5 A and higher, the blackbody emission from the W-coil was observed at the detector. This effect increased with increasing current. A compromise current of 5.0 A was chosen for 60

71 simultaneous multi-element determinations. At this current the fluorescence was reduced by no more than 40% from the maximum current of any element. RESULTS AND DISCUSSION Multi-element determinations. Ten test elements were determined simultaneously by CS-WCAFS (Table 2, Fig. 2). Limits of detection were similar for all elements, ranging from 10 to 400 ng/ml. Limits of detection were calculated by the IUPAC method: three times the standard deviation of the blank signal divided by the slope of the calibration curve. These results are compared with those observed for the single-element determination using a flame as the atomization cell, a photomultiplier tube detector, and a 300 W Xe lamp light source. 17 While the WCAFS LODs are a bit higher, the elements are determined simultaneously. Among the 10 test elements, only Ga, In, and Tl could be detected by WCAES. Mn (279.8 nm) had the lowest fluorescence wavelength observed in this window. Elements with lower wavelength were even more difficult to detect because the Xe lamp intensity dropped off at their excitation wavelengths, and the detector sensitivity also was much lower in the UV region. The precision for the 10 elements ranged from 3% to 7% RSD using concentrations fifty times greater than the detection limits. For some elements (Cu, Ag, Mg) the strongest fluorescence wavelength corresponded with strong absorption wavelengths. Self-absorption was observed in these cases. When the fluorescence was viewed near the surface of the coil, 61

72 self-absorption was very severe, degrading linearity. The linear range for most elements was 3-4 orders of magnitude by using atomic fluorescence with a strong line source such as pulsed EDLs or lasers. The detection limits in these cases can be as low as 1 µg/l and the calibration curve starts to bend over when the concentration reaches mg/l. But for CS- WCAFS, the linear range was only 1-2 orders of magnitude. This was mainly because the xenon lamp was weaker and CCD detector is less sensitive, so LODs are around 100 µg/l. The calibration curves bend over at concentration around 50 mg/l. The LODs can be lowered by using a pulsed xenon lamp or a cooled CCD detector. The accuracy of the method was evaluated for the determination of Pb and Tl in the water reference materials. For the polluted water sample, a 20-fold dilution with distilleddeionized water was the only sample preparation. The resulting Tl level was below the detection limit, and the Pb recovery was 128% (100 mg/l known, and 128 mg/l found). This sample contains high concentrations of Al and V (500 and 250 mg/l) so matrix effects may have caused the inaccuracy. The instrument calibration standard contained 5 elements including Pb and Tl, with none above a concentration of 100 mg/l. The accuracy for Pb was 98% (50 mg/l present, and 49 mg/l found), while that for Tl was 106% (100 mg/l present, and 106 mg/l found). The CS-WCAFS system may also be used for WCAES measurements. No additional alignment or instrumental changes are necessary. Simply switching off the xenon lamp, and using a current of 10A for the atomization step, the system can be used to determine Al, Co, Cr, Ga, K, Mn, Pb, Rb, Sc, Cr, Ga, In, V, and lanthanides at the ppb level Yb and Ca were used to demonstrate this dual ability of the system (Figure 4). All emission signals were collected in only three seconds: the integration time was 0.5 s per spectrum, and 6 successive spectra were collected. The highest emission signals appeared on the 3rd or 4th spectrum. Both atomic and ionic emission lines were observed: Ca nm, Yb nm, Yb 346.4, 62

73 Ca (II) nm (Fig. 4). By using both atomic fluorescence and emission analysis, a large group of elements can be determined simultaneously by this portable tungsten coil system. 63

74 Figure 1. Photograph of the continuum source tungsten coil atomic fluorescence spectrometer. 64

75 20 18 Relative Fluorescence Signal Mn Mg Bi Cu In Cu Ag Ag Cr Pb Tl In Ga Pb Ga Wavelength (nm) Figure 2. Simultaneous multi-element CSWCAFS spectrum of a solution containing 50 µg/ml Bi, Cr, Cu, Ga, In, Pb, Ag, Tl, Mg, and Mn. 65

76 6 Relative Fluorescence Signal Vaporization Current (A) Figure 3. Relationship between CSWCAFS signal and vaporization current for 100 µg/ml Bi. 66

77 50 45 Ca Relative Emission Signal Yb Ca II Yb Wavelength (nm) Figure 4. Tungsten coil atomic emission spectrum for 10 µg/ml Ca and Yb. 67

78 Table I. W-coil heating cycle for CSWCAFS. Step Current (A) Time (s)

79 Table II. Analytical figures of merit for CSWCAFS. Element Fluorescence Limit of Detection (ng/ml) Wavelength (nm) CS-WCAFS Flame a Linear Dynamic Range (decades) Precision (% RSD) Mn Mg Bi Cu Ag Cr Tl Pb In Ga

80 References Cited 1. Jackson, K. W. Electrothermal Atomization for Analytical Atomic Spectrometry; John Wiley: Chichester, Hou, X. D.; Jones, B. T. Spectrochimica Acta Part B-Atomic Spectroscopy 2002, 57, Reid, R. D.; Piepmeier, E. H. Analytical Chemistry 1976, 48, Dittrich, K.; Berndt, H.; Broekaert, J. A. C.; Schaldach, G.; Tolg, G. Journal of Analytical Atomic Spectrometry 1988, 3, Dittrich, K.; Fuchs, H.; Berndt, H.; Broekaert, J. A. C.; Schaldach, G. Fresenius Journal of Analytical Chemistry 1990, 336, Hayashi, H.; Hara, Y.; Tanaka, T.; Hiraide, M. Bunseki Kagaku 2001, 50, Hayashi, H.; Tanaka, T.; Hiraide, M. Analytical Sciences 2001, 17, Levine, K.; Wagner, K. A.; Jones, B. T. Applied Spectroscopy 1998, 52, Wu, P.; Zhang, Y. C.; Liu, R.; Lv, Y.; Hou, X. D. Talanta 2009, 77, Batchelor, J. D.; Thomas, S. E.; Jones, B. T. Applied Spectroscopy 1998, 52, Sanford, C. L.; Thomas, S. E.; Jones, B. T. Applied Spectroscopy 1996, 50, Donati, G. L.; Gu, J.; Nobrega, J. A.; Calloway, C. P.; Jones, B. T. Journal of Analytical Atomic Spectrometry 2008, 23, Rust, J. A.; Nobrega, J. A.; Calloway, C. P.; Jones, B. T. Spectrochimica Acta Part B- Atomic Spectroscopy 2005, 60, Rust, J. A.; Nobrega, J. A.; Calloway, C. P.; Jones, B. T. Spectrochimica Acta Part B- Atomic Spectroscopy 2006, 61, Donati, G.L.; B.E. Kron; Jones, B. T. Simultaneous determination of Cr, Ga, In and V in soil and water samples by tungsten coil atomic emission spectrometry, Spectrochimica Acta Part B, 2009, 64,

81 16. Rust, J.A.; Donati, G.L.; Afonso, M.T.; Nobrega, J.A.;Jones, B.T. An Overview of Electrothermal Excitation Sources for Atomic Emission Spectrometry, Spectrochimica Acta Part B, 2009, 64, Smith, B. W.; Glick, M. R.; Spears, K. N.; Winefordner, J. D. Applied Spectroscopy 1989, 43, Bratzel, M. P.; Dagnall, R. M.; Winefordner, J.D Applied Spectroscopy 1970, 24, Bratzel, M. P.; Dagnall, R. M.; Winefordner, J.D Analytica Chimica Acta 1969, 48, Wu, P.; Wen, X. D.; He, L.; He, Y. H.; Chen, M. Z.; Hou, X. D. Talanta 2008, 74, Ezer, M.; Elwood, S. A.; Jones, B. T.; Simeonsson, J. B. Analytica Chimica Acta 2006, 571,

82 CHAPTER 5 A Rugged, Portable Tungsten Coil Atomic Emission Spectrometer Jiyan Gu and Bradley T. Jones The following manuscript was written to submit to Analytical Chemistry. Stylistic variations are due to the requirements of the journal. All of the presented research was conducted by Jiyan Gu. The manuscript was prepared by Jiyan Gu and edited by Bradley T. Jones. 72

83 Abstract Tungsten Coil Atomic Emission Spectrometry is an ideal technique for field applications because of its simplicity, low cost, low power requirement and independence from cooling systems. A new, portable, compact design is reported here. The tungsten coil is extracted from a 24 V, 250 W commercial projector light bulb. The coil is housed in a small, aluminum cell. The emission signal exits from a small aperture in the cell, while the bulk of the blackbody emission from the tungsten coil is blocked. The resulting spectra exhibit extremely low background signals. The atomization cell, a single lens, and a small CCDbased spectrometer are mounted on a cm ceramic base. The resulting system is robust and easily transported. A 400 W solid state power supply is employed to control the current through the coil. The whole system can be powered by a car battery and controlled with a laptop computer. Fifteen elements are determined with the system (Ba, Cs, Li, Rb, Cr, Sr, Eu, Yb, Mn, Fe, Cu, Mg, V, Al and Ga). The precision ranged from 4.3% to 8.4% relative standard deviation for repetitive measurements of the same solution. Detection limits were in the µg/l range. Accuracy was tested using standard reference materials for polluted water, peach leaves and tomato leaves. Keywords: Electrothermal atomizer; Tungsten coil; Atomic emission; Metal analysis, Portable device; 73

84 Introduction Bench-top atomic spectrometry is a mature technique. Atomic emission spectrometry (AES), atomic absorption spectrometry (AAS), and atomic fluorescence spectrometry (AFS) have been employed for many years, with detection power allowing the determination of most elements at the sub ng/ml level. 1 These techniques produce free analyte atoms in the gas state using a high temperature atomization source. For example, the detection of metal vapors in the flame was reported in the mid-1800s by Kirchhoff and Bunsen. Since then, flame atomic emission spectrometry (FAES), flame atomic absorption spectrometry (FAAS) and flame atomic fluorescence spectrometry (FAFS) have become well established methods. 2 Depending upon its gaseous composition, the flame provides a temperature in the K range. At these temperatures, atoms with low ionization energies will be easily analyzed by FAES, while those with medium to high ionization energies may be detected by FAAS and FAFS. Modern laboratory instruments use more efficient atomization sources such as the inductively coupled plasma (ICP) or the graphite furnace. The ICP can reach temperatures as high as 10,000 K, so atoms may be detected by atomic emission, and ions may be detected by either atomic emission or mass spectrometry. The ICP allows simultaneous multi-element determinations at the ng/ml level, while ICP- MS can go even lower. 3 Graphite furnace AAS is usually a single element technique, but its sensitivity allows the analysis of low analyte concentrations even with limited sample volume. Modern laboratories are capable of performing fast, simultaneous and multi-element analyses for a multitude of sample types and elements. Transferring this technology for field analysis is not a simple endeavor. A successful field instrument would allow a screening process, so that only the most important samples would be transported back to the central laboratory. This would result in a significant savings of time, effort and would preserve sample integrity. Currently, the three most popular techniques for portable elemental field 74

85 analyses are portable X-ray fluorescence (XRF) spectrometry, 4 laser induced breakdown spectrometry (LIBS) 5 and tungsten coil atomic absorption spectrometry (WCAAS). 6-9 Both portable XRF and LIBS are commercially available. The recent miniaturization of X-ray tubes has resulted in the application of portable XRF in fields such as positive material identification (PMI), mining, and environmental testing. Both XRF and LIBS have their drawbacks. Portable XRF can seldom be employed when analyte concentrations are below ppm levels. The LIBS instrument is often more expensive than XRF. Both techniques suffer from the need for solid standards to produce calibration curves. WCAAS is a technique developed with a more traditional electrothermal atomization device WCAAS usually has LODs (limit of detection) lower than LIBS or XRF, and the instrumentation cost is less. The tungsten filament may be extracted from an inexpensive commercially available light bulb. This may produce analyte atoms with a simple solid state, miniature 200 W power supply. Emission spectra may be collected with a miniature CCDbased detector and processed with a laptop computer. The whole system may be very small, and no cooling device is needed. The power source for the device can be as simple as a 12 V car battery. Unfortunately, WCAAS is a single element technique, requiring a different hollow cathode lamp for each element determined. Also for solids, a sample digestion or extraction method is necessary, since WCAAS requires liquid analytical samples. More recently, a tungsten coil atomic emission spectrometry (WCAES) technique has been reported This method can perform simultaneous multi-element determinations without the need for a light source. Elements including Al, Co, Cr, Ga, K, Mn, Pb, Rb, Sc, Cr, Ga, In, V, and lanthanides have been determined by the technique The WCAES method is very sensitive when a high resolution spectrometer is employed. This allows effective separation of atomic emission signals from the blackbody emission of the W-coil, and absolute detection limits are at or below the ng level for most elements. Conversion of such a 75

86 WCAES device into a small robust package will enable field analyses. A new rugged, portable design is reported here. The device is taken to the field for the analysis of a natural water sample. 76

87 Experimental Instrumentation. Figure 1 is a photograph of the rugged spectrometer. An aluminum tungsten coil housing, an emission signal collection lens, and a small CCD-based spectrometer are mounted on a ceramic rail ( cm). The housing is fashioned from a 2.54 cm diameter, 7.5 cm long Al rod (Fig. 2). The bottom of the rod is tapped for a 1/4-20 mounting screw. The rod is hollowed out from the top to a depth of 6.5 cm with an internal diameter of 1.75 cm. Near the bottom of the hollow section, a 1/16 NPT port is placed to fit the fitting for the purge gas. The view port for atomic emission is a 1.5 mm hole drilled at a height of 3.3 cm from the base of the housing. A second view port is drilled at a 180 angle to the first one. This allows for easy line of sight alignment, and also provides a path for atomic absorption measurements. The sample introduction port (a 5 mm hole placed 4 cm from the bottom of the cell) is placed at a 90 angle for the view port. The sample introduction port is not visible in Figure 2. The W-coil is the filament from a 24 V, 250 W light bulb (Osram Xenophot HLX 64655, Augsburg, Germany). The fused silica envelope was removed from the bulb leaving the filament and glass base intact (Fig. 2). The base fits in a standard, 2-pronged, ceramic bulb socket. The socket is cemented to the end of an Al mount. The mount is 2.2 cm long. The lower 2 cm of the mount is machined to an outer diameter to match the internal diameter of the housing (1.75 cm). The upper portion of the mount has a diameter of 2.54 cm, so when the mount is inserted in the housing it acts as a cap. The electrical leads to the ceramic socket are threaded through the mount and sealed with epoxy. The height of the mount, and thus the position of the coil relative to the view port, is adjusted with a machine screw that pushes against the top of the housing. When the height is properly adjusted, the lowest edge of the W-coil is just above the level of the view port. The purge gas (10% H 2 in Ar) flows at a rate 77

88 Figure 1. Picture of portable WCAES. 78

89 Figure 2. Picture of Aluminum Atomization Cell. 79