Determination of Phosphorus and Sulfur with Dynamic Reaction Cell ICP-MS

Transcription

1 FIELD APPLICATION REPORT ICP MASS SPECTROMETRY Determination of Phosphorus and Sulfur with Dynamic Reaction Cell ICP-MS Authors: Kenneth R. Neubauer, Ph.D. Perkin Elmer Life and Analytical Sciences 710 Bridgeport Avenue Shelton, CT Introduction The determination of low levels of phosphorus (P) and sulfur (S) presents a challenge for conventional ICP-MS. First, the ionization potential for both of these element is high ( ev) which results in low-ionization efficiency in the plasma and consequently, low-signal intensities. Therefore, it is desirable to measure the most abundant isotope of each species: 31 P (100% abundant) and 32 S (95% abundant). However, both of these isotopes suffer from major interferences resulting from nitrogen (N), oxygen (O) and hydrogen (H), reacting to form primarily 15 N 16 O + and 14 N 16 O 1 H + at m/z 31 and O + 2 at m/z 32. To overcome this problem, Dynamic Reaction Cell (DRC ) ICP-MS can be used to eliminate the effects of these interferences. Both phosphorus and sulfur react readily with oxygen to form their oxides according to the following reactions: P + + O 2 PO + + O S + + O 2 SO + + O Therefore, phosphorus and sulfur can be measured by monitoring the oxide reaction products PO + at m/z 47 and SO + at m/z 48 as the analytical species. This analytical scheme has been demonstrated previously 1 and this field application report extends the previous work to other matrices. Experimental The instrument used in this work was an ELAN DRC II. General instrumental parameters are shown in Table 1, and more specific parameters are given in the text. All solutions were made from single-element or multielement standards (PE Pure, PerkinElmer Life and Analytical Sciences, Shelton, CT USA; Spex CertiPrep, Metuchen, NJ USA), and all measurements were made against external calibration curves in deionized water or 1% HNO 3. Reaction gases were of % purity.

2 Table 1. Operating Conditions for the ELAN DRC II Parameter/System Setting/Type Analytes PO + (m/z 47), SO + (m/z 48) Reaction Gas O 2 RPq 0.50 RF Power 1500 W Nebulizer Quartz Concentric Spray Chamber Quartz Cyclonic Nebulizer Flow (Standard Mode) 0.96 L/min Nebulizer Flow (DRC Mode) 1.06 L/min Results Figure 1 shows reaction cell gas optimization curves for the formation of PO + (m/z 47) and SO + (m/z 48) in deionized water. These curves clearly demonstrate the formation of PO + and SO + (blue lines) along with the decrease in background (red lines) at the same masses. This results in low background equivalent concentrations (BECs) of 2-3 µg/l for phosphorus and 6-7 µg/l for sulfur. Consequently, low levels of these analytes can be determined. DI + 10 µg/l DI + 10 µg/l DI DI BEC = 6-7 µg/l Figure 1. Oxygen cell gas optimizaton for PO + (left) and SO + (right) in deionized water. Figure 2 shows calibration curves of 5, 10 and 20 µg/l phosphorus and sulfur in 18 MΩ deionized water using tellurium (Te) at 10 µg/l as an internal standard. Tellurium was chosen as an internal standard because its ionization potential (9.01 ev) closely matches those of phosphorus and sulfur (10.5, 10.4 ev, respectively). Three different 18 MΩ deionized water samples from various sources were analyzed against these curves. Additionally, one of the water samples was spiked with 1, 5 and 10 µg/l of phosphorus and sulfur. The results from these analyses are shown in Table 2 and demonstrate that low levels of phosphorus and sulfur can be measured. Figure 2. Calibration curves for PO + (left) and SO + (right) in deionized water. Two potential interferences at m/z 47 and 48 are nitrogen dioxide (NO 2 + ) and titanium (Ti + ). NO 2 + may be expected to result from a nitric-acid-matrix reaction with the oxygen reaction cell gas (NO + + O 2 NO O). However, this reaction does not occur since it is thermodynamically prohibited ( H r = 57.4 kcal/mol) 1. Figure 3 supports this idea by showing oxygen 2

3 Table 2. Determination of Phosphorus and Sulfur in Deionized Water Sample PO + SO + Source Source Source Source Source Source reaction gas optimization curves for phosphorus and sulfur in 10% HNO 3. Comparing these curves to those in deionized water (Figure 1) shows that both are similar in shape and yield equivalent BECs. These similarities confirm that NO 2 + does not interfere with PO + or SO +. 10% HNO µg/l 10% HNO µg/l 10% HNO 3 BEC = 4-6 µg/l 10% HNO 3 Figure 3. Oxygen cell gas optimization for PO + (left) and SO + (right) in 10% nitric acid. The second potential interference with PO + and SO + is titanium (Ti) which has a major isotope at m/z 48 (73.8%) and a minor isotope at m/z 47 (7.3%). Therefore, phosphorus and sulfur determination in Ti-containing samples might be expected to be difficult. However, Ti + reacts readily with O 2 to form TiO + (Ti + + O 2 TiO + + O) at the same rate as PO + formation and faster than SO + formation. As a result, Ti + is displaced to a higher mass (TiO + at m/z 63 and 64) as PO + and SO + form. Proof of this concept is evident in Figure 4 which shows O 2 optimization plots for PO + and SO + in a 10 mg/l Ti matrix. Again, the BECs are similar in the Ti matrix to those obtained in deionized water alone, thus showing that the presence of Ti does not affect phosphorus and sulfur determination. 10 mg/l Ti + 10 µg/l 10 mg/l Ti + 10 µg/l BEC = 7-8 µg/l 10 mg/l Ti 10 mg/l Ti Figure 4. Oxygen cell gas optimizations for PO + (left) and SO + (right) in 10 mg/l titanium Further evidence of Ti removal is seen in Table 3 which shows PO + and SO + measured in the presence of 1 and 10 mg/l titanium. The similar results obtained in the differing titanium levels indicates that Ti + is removed and does not affect PO + and SO + determination. 3

4 Table 3. Determination of Phosphorus and Sulfur in Titanium Sample PO + SO + 1 mg/l Ti mg/l Ti mg/l Ti + 5 µg/l mg/l Ti + 5 µg/l mg/l Ti + 10 µg/l mg/l Ti + 10 µg/l In samples containing large amounts of calcium (Ca), low-level SO + at m/z 48 may be hampered by the minor 48 Ca isotope (0.18% abundant). Unfortunately, Ca does not react with oxygen, so it cannot be displaced to higher mass. However, the 50 SO + isotope at m/z 50 can be monitored if such a situation is encountered. The ability to measure PO + and SO + as part of a multielement, multimode method is shown in the analysis of a silver matrix. The silver concentration entering the instrument is 50 mg/l, and the analytical levels or phosphorus and sulfur are 10 µg/l. Other analytes were measured at 0.1 µg/l. The matrix was made by dilution of a 10,000 mg/l silver standard, and the analytes were spiked at the desired levels. Table 4 shows the analytes of interest, the reaction cell conditions and the internal standards, which were spiked at 10 µg/l. Quantitative measurements were made against an external calibration curve in 1% HNO 3. Table 4. Reaction Cell Conditions for Analysis of a Silver Matrix Analyte m/z Int. Std. Reaction Gas Flow (ml/min) RPq Be 9 6 Li V 51 Ga Mn 55 Ga Co 59 Ga Ni 60 Ga As 75 Ir Mo 98 Ga Cd 114 Ga Cr 52 Ga NH PO 47 Te O SO 48 Te O Se 80 Te O Table 5 shows three consecutive analyses of the silver matrix spiked at the desired analytical levels: 20 µg/l for phosphorus and sulfur and 0.1 µg/l for all other elements. The results indicate that low levels of several elements (including phosphorus and sulfur) can be determined in a single multielement, multimode method. 4

5 Table 5. Analysis of 50 mg/l Silver + Spike Analyte Spike Level Be PO SO V Cr Mn Co Ni Mo As Se Cd Conclusion This work demonstrates the ability of DRC ICP-MS to measure low levels of phosphorus and sulfur, which are difficult to determine by conventional ICP-MS. These measurements are accomplished effectively by moving the analytes away from the interfering species through reaction with oxygen to form oxides (creating PO + and SO + at m/z 47 and 48, respectively). Using this process, phosphorus and sulfur determinations can be made in a variety of matrices, as well as part of a multielement, multimode analysis. This latter point is made possible by using oxygen as a reaction gas since oxygen can be used for a variety of analytes and is not specific for phosphorus and sulfur. References 1. D.R. Bandura, V.I. Baranov, S.D. Tanner Anal. Chem. 74, 7, (2002), PerkinElmer Life and Analytical Sciences 710 Bridgeport Avenue Shelton, CT USA Phone: (800) or (+1) For a complete listing of our global offices, visit /lasoffices 2004 PerkinElmer, Inc. All rights reserved. The PerkinElmer logo and design are registered trademarks of PerkinElmer, Inc. Dynamic Reaction Cell and DRC are trademarks and PerkinElmer is a registered trademark of PerkinElmer, Inc. or its subsidiaries, in the United States and other countries. ELAN is a registered trademark of MDS Sciex, a division of MDS, Inc. All other trademarks not owned by PerkinElmer, Inc. or its subsidiaries that are depicted herein are the property of their respective owners. PerkinElmer reserves the right to change this document at any time without notice and disclaims liability for editorial, pictorial or typographical errors. The data presented in this Field Application Report are not guaranteed. Actual performance and results are dependent upon the exact methodology used and laboratory conditions. This data should only be used to demonstrate the applicability of an instrument for a particular analysis and is not intended to serve as a guarantee of performance _01