Laser Ablation-Inductively Coupled Plasma-Mass Spectrometry: acquiring basic skills and solid sample meausurements

Transcription

1 Practicum Spectroscopy Fall 2010 LIM Laser Ablation-Inductively Coupled Plasma-Mass Spectrometry: acquiring basic skills and solid sample meausurements Jorge Ferreiro, study degree Chemistry, 5 th semester, fjorge@student.ethz.ch Alex Lauber, study degree Chemistry, 5 th semester, allauber@student.ethz.ch Assistant: Mohamed Tarik Abstract: LA-ICP-MS is a widely used method for trace and ultratrace element detection in solid samples. To get intense signals and low oxidation rates, daily optimization for ICP was necessary. Adjusting parameters were nebulizer gas flow rate, auxiliary gas flow rate, plasma gas flow, RF-power and ion lens voltage; first one being mainly important for optimal signals and low oxide formation. For MS, different dwell times can be set up, showing higher sensitivity for lower dwell time. After daily optimization the LOD and sensitivity for 30 elements of the reference material NIST 610 were calculated, showing higher LODs for unsensitive elements. A qualitative analysis of a 5 centime Swiss coin showed presence of Al, Cu, Na, Ni and Zn. Using a reference material and software Lamtrace a quantitative analysis of a steel sample was performed. In total 17 elements were identified and the results were accurate except for Cu and Cr. Zurich, the J. Ferreiro A. Lauber

2 1 Introduction Laser Ablation-Inductively Coupled Plasma-Mass Spectrometry (LA-ICP-MS) is a widely used method for elemental analysis in solids. The sample is ablated with a pulsed laser beam and introduced as aerosol into the ICP-MS, where it s ionized (ICP) and detected (MS). Figure 1: Standard set-up for LA-ICP-MS equipement. [1] The sample is placed into an airtight, closed ablation-cell which is flushed with a constant He-flow (1 Lmin 1 ). Laser pulses with certain amount of energy are focussed on a solid sample surface. The pulse ablates material from the surface creating a microplasma of ions and neutral particles, which are carried by He/Ar to the plasma of the ICP. Two working modes for ablation are used: scanning mode and drilling mode. The inductively coupled plasma (ICP) atomizes and ionizes the aerosol produced by the laser. Plasma is a state of matter made of ions, electrons and atoms reaching a temperature of about 8000 K. A radio frequency (RF) coil surrounds a part of the torch. The plasma is ignited with a high voltage spark, which ionizes a first Aratom. The generated electron is accelerated and ionizes further Ar-atoms leading to a cascade reaction. The RF-coil supplies the plasma torch with energy to keep it stable. The plasma torch consists of three concentric tubes, which are usually made from quartz. These are shown as the outer tube, middle tube, and sample injector. The gas (usually Ar) used to form the plasma is passed between the outer and middle tubes at a flow rate of Lmin 1. A second gas flow, the auxiliary gas, passes between the middle tube and the sample injector normally at 1 Lmin 1 and is used to change the position of the base of the plasma relative to the tube 2

3 and the injector. A third gas flow, the nebulizer gas, also flowing normally at 1 Lmin 1 carries the sample, in the form of a fine-droplet aerosol, from the sample introduction system. In ICP-MS the ion beam comes through the interface into the mass analyzer. The role of the interface is to transport the ions efficiently from the plasma, which is at atmospheric pressure (760 Torr) to the mass spectrometer analyzer region, which is at approximately 10 6 Torr. The interface consists of two metallic cones (first known as sampler cone and second one known as skimmer cone) with very small orifices, which intermediary space is maintained at a pressure of 2 Torr with a mechanical roughing pump and are normally water-cooled. To focus the ions for the detector an ion lens system (mostly hexapole rods) are used. For the separation by means of a detection of the different elements of the sample, one uses a mass spectrometer (MS). There are many different mass spectrometers with different characteristics and functionalities. For ICP-MS Quadrupole-MS (Q- MS) is widely used because cheap and the sensitivity is sufficient for most analysis. The collimated ion beam enters the mass separator which usually consists of four hyperbolic rods. Each opposite pair of rods are connected with a DC and an AC voltage. According to Mathieu s equations for a given DC and AC amplitude, only ions of a given m ratio will have stable oscillating trajectories and reach the z detector. Widely used detectors are electron- and photomultiplier. For Q-MS a number of instrumental parameter can also be adjusted. The peakhopping mode of operation is generally selected for quantitative analysis. In such a case the dwell time is the period of time that is spend collecting data at a particular mass. Starting with the lowest mass to be analyzed, the instrument collects data (in form of counts-per-second (cps)) for a duration of the selected dwell time and then jumps to the next mass chosen and so on. Normally higher dwell times provoke a smaller error but also fewer mass sweeps in total acquistion period. To get a quantitative analysis of an unknown sample natural internal standards are widely used. (C A ) S = (C A) R (I is ) R (C is ) R (I A ) R (C is) S (I A ) S (I is ) S (1) C indicates concentrations (in ppm), I inentistiy (in cps), A analyte, S sample, is internal standard and R reference material. For all kind of analytical measurements there are always two very important parameters which are given: limit of detection LOD and sensitivity S. The LOD describes the lowest amount of substance which can be distinguished from the background: LOD = 3 σnet S (2) 3

4 where σ net is the background standard deviation and S is the sensitivity given by S = I c (3) where I is the intensity an c the concetration. 4

5 2 Experimental 2.1 General A quadrupole ICP-MS instrument Perkin Elmer SCIEX ELAN 6100 DRC II, controlled by the software called ELAN was used. For laser ablation a solid state Nd:YAG laser LSX-213, CETAC Technologies (λ = 213 nm) and a solid state Nd: YAG laser LSX-500, CETAC Technologies (λ = 266 nm) were used. A reference material silicate glass NIST 610 was used for the daily optimization. For the data evaluation Lamtrace and the program R were used. 2.2 Daily optimization In order to produce low oxidation rates in the ICP ThO+ should be lower than Th + 0.5%. To monitor complete atomization and ionization U+ 1. U + and Th + are Th + used because both ions have similar ionization energies, masses and concentrations (in NIST 610) and their major isotope has an abundance of > 99%. Adjusting parameters were nebulizer gas flow rate, auxiliary gas flow rate, plasma gas flow rate and ion lens voltage. The nebulizer gas flow rate ( Lmin 1 ) and auxiliary gas flow rate ( Lmin 1 ) were changed in a bigger range than the plasma gas flow rate ( Lmin 1 ). The RF-power was kept at 1350 W. 2.3 Acquisition of transient signals and mass spectra The dwell time influences the homogeneity of transient signals. The effect of three different dwell times (4ms, 10ms, 50 ms) was investigated on 30 elements of NIST 610. For six major elements (Ag, Cu, Li, Ni, Pb, Si) the transient signal was measured. 2.4 Mass spectra acquistion To identify the elements within NIST 610 two mass spectra were acquiered (one blank and one NIST 610). To acquire good mass spectra the multi channel analyzer (MCA) was set to measure 20 points per mass. For the elements Ag, Li, Ni, Pb 5

6 and Si the resolution R (in NIST 610 and blank) was calculated. R = m m (4) 2.5 Analysis of unknown sample The mass spectrum of a 5 centime Swiss coin was acquiered in hole drilling ablation mode to identify all present elements in the sample. The mass spectrum of the background was measured. For the detected elements a transient signal was acquired. 2.6 Analysis of steel sample After daily optimization of the ICP-MS system a steel sample was analyzed in hole drilling mode. In total 10 transient signals were measured: 2 times standard, 6 times sample and finally 2 times standard to assure that the measurement sequence didn t alter the conditions in the plasma. Ni was used as internal standard to perform the quantification of the sample [2]. 6

7 3 Results 3.1 Daily optimization Table 2 in the appendix summarizes the optimal parameter values for each lab day. I cps Nebulizer Gas Flow (L/min) Figure 2: t s (Top) Nebulizer gas flow rate for daily optimization: The best working conditions (low oxide formation and intense signal) are achieved with a flow rate of 0.8 Lmin 1. (Bottom) Auxiliary gas flow rate for daily optimization: With a flow rate of 0.8 Lmin 1 best signals are achieved. Both paramters were changed in a wide range. Colorcodes: Al(red), Na (black), Si (green), Th (blue), ThO (brown), U (yellow). I cps Auxiliary Gas Flow (L/min) t s

8 I cps Plasma Gas Flow (L/min) Figure 3: (Top) Plasma gas flow rate for daily optimization: Good working conditions are achieved with a flow rate of 17.5 Lmin 1. Although the effects of larger increase or decrease of the plasma flow rate wasn t sudied. (Bottom) Ion lens voltage calibration: The calibration should yield a good linear fit for the meausred points. The unit for y-axis is voltage-level. Colorcodes: Al(red), Na (black), Si (green), Th (blue), ThO (brown), U (yellow). t s 8

9 3.2 Acquistion of transient signals and mass spectra I cps t dwell = 4ms Figure 4: t s (Top) Transient signal acquistion for dwell time = 4ms in hole drilling mode for six elements: The transient signals are very homogeneous. (Bottom) Transient signal acquistion for dwell time = 10 ms in hole drilling mode for six elements: The signals begin to tickle and lose homogeneity. All signals descrease constanteley due to the hole drilling mode. Colorcodes: Ag (blue), Cu(dark green), Li (red), Ni (light green), Pb (dark blue), Si (dark red). I cps t dwell = 10ms t s 9

10 I cps t dwell = 50ms t s Figure 5: Transient signal acquistion for dwell time = 50 ms in hole drilling mode for six elements: Ag (blue), Cu(dark green), Li (red), Ni (light green), Pb (dark blue), Si (dark red). The strong tickeling of transient signals increases more comparing to 4ms and 10 ms. So it s clearly better to settle a lower dwell time. All signals descrease constanteley due to the hole drilling mode. 10

11 3.3 Mass spectra acquistion 40 Ar Na I cps 10 3 Ar ArN ArCl Figure 6: (Top) Mass spectra of gas blank: Very high intensities are shown for masses 40 and 80, which correspond to the plasma gas 40 Ar + respectively the Ar-dimer 80 Ar + 2. The signals in the low mass range show up due to non-metal elements of the matrix. (Bottom) Mass spectra of NIST 610: All desired NIST 610 elements are detected. m z 40 Ar I cps Mn 59 Co Ce Ag 159 Tb 175 Lu 95 Mo Sn Sb 197 Au Th Pb U Bi m z 11

12 3.4 Analysis of unknown sample 65 Cu I cps Na 27 Al 60 Ni 68 Zn Figure 7: (Top) Mass spectra of coin: In order to indentify the elements possibly present in the sample the whole mass spectrum was acquiered skipping m z = 40 (40 Ar + ) and m z = 80 (80 Ar + 2 ) to protect the detector from high concentration of Ar-species. (Bottom) Transient signals for identified elements: After identfication transient signals were meausured showing very homogenous distributioon despite of Na. m z I cps t s 12

13 3.5 Analysis of steel sample Element Corrected bg./cps bg./cps Calc. Concentration/ppm LOD/ppm B 35(13) 108(10) 41(12) 30(2) Ti 6800(1500) 333(15) 1500(300) 13.9(7) V 31000(1300) 664(33) 363(2) 1.04(7) Cr 24700(850) 410(13) 309(3) 0.88(7) Co (10000) 131(10) 3240(15) 0.51(5) Cu 98000(3000) 153(9) 4900(85) 2.0(2) Zr (82000) 120(10) 1200(210) 0.19(2) Nb (150000) 130(7) 4300(960) 0.30(3) Mo 18200(800) 152(13) 641(8) 1.5(1) Ag 725(48) 121(9) 12.5(5) 0.69(6) Sn 11200(331) 113(9) 184(6) 0.60(6) Sb 8100(260) 114(4) 108(3) 0.48(6) Ce 14000(17000) 112(12) 58(59) 0.17(1) Ta (96000) 116(7) 2800(700) 0.30(3) W 76000(4000) 106(6) 2012(35) 0.9(1) Table 1: Calculated concentrations, background(bg.), background correction and LOD for steel sample. Ni was used as internal standard. Ti, Co, Cu, Nb, Ta and W have high concentrations in steel. One can t figure out a trend for LOD because normally the values increase with heavier elements, but in general most elements have a very low LOD so they are easily detected. 4 Discussion 4.1 Daily optimization The nebulizer gas flow rate showed the largest influence on the signal and formation of oxides. Auxiliary gas flow rate and plasma gas flow didn t have such a large influence. Notice this results are for the intervalls in which the paramters were changed. Obviously the size of the particles in the aerosol will determine the qualitity of the signal. Normally the plasma gas flow will also have a strong influence on the sample mainly on the oxide formation but in this case the rate wasn t nearly changed. 13

14 4.2 Acquisition of transient signals and mass spectra A higher dwell time leads to a more homogenous background and slightly better intensity. In general higher dwell times lead to a longer sweep which can result in a loss of signal. So in this case the suggestion is to take a dwell time between 0.1 ms up to 10 ms because the mass scan is performed hopping in steps of 1 m z although the scan range is big. 4.3 Mass spectra acquistion With one point per mass (MCA = 1) one can t calculate a resolution for two adjacent signals since there is only a maximal value but it isn t possible to draw a line shape. Although the scan speed is higher than for multiple points per peak. One observes for heavier elements better mass resolution for two adjacent peaks. Because of isobaric and polyatomic interferences normally one has a loss of sensitivity. 4.4 Analysis of unknown samples Al, Cu, Na, Ni and Zn were identified through the mass spectrum of the coin. For all signals the transient signal was very homogenous despite for Na, which is one of the major contamination sources for analysis. Na is present in salt which is present on human skin. So the coin was always in contact with Na. The signal appaered to be inhomogenous because the analysis was performed in hole drilling mode and the Na is probably only present on the surface. 4.5 Analysis of steel sample Despite of Co and Cr for all detected elements the concentration was very accurate compared to the certified sample. 14

15 5 Literature [1] Thomas R., A beginner s guide to ICP-MS, Spectroscopy Tutorial [2] Longerich H.P., Jackson S.E., Günther D., J. of Analytical Spectrometry, 1996,11, [3] Uriano G.A.;Certifacte of Analysis - Standard Reference Material 1261a - AISI 4340 Steel, National Bureau of Standards, 1981, 56. [4] Uriano G.A.;Certifacte of Analysis - Standard Reference Material 1262a - AISI 4340 Steel, National Bureau of Standards, 1981, Appendix 6.1 Tables Parameter 1 st day 2 nd day 3 rd day Nebulizer gas flow rate [l/min] Auxiliary gas flow rate [l/min] Plasma gas flow [l/min] Table 2: Optimal parameter values summarized for each experimental day. The parameters remained quite constant for each working day. The nebulizer gas flow rate and auxiliary gas flow rate were changed in bigger ranges than the nebulizer gas flow. Although the last parameter will have a strong influence over the measurements. For the first day the oxide formation rate and complete ionization/atomization were calculated. ThO+ Th + = 0.2% and U+ Th + = 91% 15

16 Element bg./cps bg. corr./cps LOD/ppm S/cps ppm 1 Li (4) 28(7) Na (3) 100(21) Al (27) 28(8) Si (2665) 12(3) Ca (571) 100(26) Cr (14) 173(51) Mn (1) 206(54) Fe (70) 205(81) Co (5) 182(46) Ni (3) 150(39) Cu (3) 138(35) Zn (114) 51(25) Mo (2) 400(97) Pd Ag (8) 260(56) Cd (7) 147(48) Sn (5) 535(128) Sb (5) 291(66) Te Ba (2) 582(150) Ce (2) 555(135) Tb (2) 566(145) Lu (2) 501(134) Pt Au (8) 128(39) Pb (6) 289(63) Bi (4) 241(54) Th (3) 334(85) U (3) 321(76) Table 3: Background (bg.), corrected background, LOD and sensitivity for 30 analytes of NIST 610: Elements with high LODs have a very low sensitivity. 16

17 Isotope Resolution 7 Li 7 29 Si Ni Pb W 204 Table 4: Calculated Resolutions for 5 elements of NIST 610: For heavier elements the resolution is higher. Resolution was calculated at FWMH. Element Calc. concentration/ppm Sample 1262 a/ppm Ref a/ppm Rel. Error /% 11 B 41(12) Ti 1500(300) V 363(2) Cr 309(3) Co 32.40(15) Cu 4900(85) Zr 1200(210) Nb 4300(960) Mo 641(8) Ag 12.5(5) Sn 184(6) Sb 108(3) Ce 59(58) Ta 2800(700) W 2012(35) Table 5: Calculated concentrations, Sample 1262 a and reference (ref.) material 1261a and relative errors. The relative errors are high since the reference material was only used for calibration. Although the calculated concentrations are very accurate for some elements. [2],[3] 17

18 6.2 Program codes 6.3 Lamtrace files 6.4 Labjournal 18