Anal. Chem. 2001, 73, 2547-2553
Determination of Total Sulfur at Microgram per Gram Levels in Geological Materials by Oxidation of Sulfur into Sulfate with in Situ Generation of Bromine Using Isotope Dilution High-Resolution ICPMS Akio Makishima and Eizo Nakamura*
The Pheasant Memorial Laboratory for Geochemistry and Cosmochemistry (PML), Institute for Study of the Earth's Interior, Okayama University at Misasa, Misasa, Tottori-ken 682-0193, Japan
We have developed a new, simple, and accurate method for the determination of total sulfur at microgram per gram levels in milligram-sized silicate materials with isotope dilution high-resolution inductively coupled plasma mass spectrometry equipped with a flow injection system. In this method, sulfur can be quantitatively oxidized by bromine into sulfate with achievement of isotope equilibrium between the sample and spike. Detection limits for 32S+ and 34S+ in the ideal solution and silicate samples were 1 and 6 ng mL-1 and 0.07 and 0.3 µg g-1, respectively. The total blank was 46 ng, so that a 40-mg silicate sample containing 10 µg g-1 sulfur can be measured with a blank correction of 2000. Another difficulty in the determination of total sulfur resides in the plural oxidation states of sulfur. Most wet chemical studies reduce or oxidize total sulfur into H2S or SO2 gases, which are also unstable and easily oxidized, so that great care must be taken (2) Terashima, S. Geostand. Newslett. 1988, 12, 249-252 (3) Hall, G. E.; Vaive, J. A. Geostand. Newslett. 1989, 13, 1-4. (4) Kelly, W. R.; Murphy, K. E. Geostand. Newslett. 1992, 16, 3-8. (5) Hong, Y.-D.; Namgung, S.-W.; Yoshida, M.; Malik, A. Talanta 2000, 51, 291301. (6) Gregoire, D. C.; Naka, H. J. Anal. At. Spectrom. 1995, 10, 823-828. (7) Menegario, A. M.; Gine, M. F. J. Anal. At. Spectrom. 1997, 12, 671-674. (8) Divjak, B.; Goessler, W. J. Chromatogr., A 1999, 844, 161-169. (9) Mason, P. R. D.; Kaspers, K.; van Bergen, M. J. J. Anal. At. Spectrom. 1999, 14, 1067-1074. (10) Feldmann, I.; Tittes, W.; Jakubowski, N.; Stuewer, D.; Giessmann, U. J. Anal. At. Spectrom. 1994, 9, 1007-1014. (11) Wildner, H. J. Anal. At. Spectrom. 1998, 13, 573-578. (12) Makishima, A.; Nakamura, E. J. Anal. At. Spectrom. 2000, 15, 263-267.
Analytical Chemistry, Vol. 73, No. 11, June 1, 2001 2547
Table 1. HR-ICPMS Operating Conditions 1. ICP Operating Conditions 1. 1 kW, frequency 27.12 MHz HF resistant torch with sapphire injection tube 14 L/min 1.2 L/min 1.0 L/min
rf power torch plasma Ar flow rate auxiliary Ar flow rate nebulizer Ar flow rate
2. Nebulizer and Spray Chamber microconcentric nebulizer, MCN-100 (CETAC Technologies Inc., Omaha, NB) Made of PFA Teflon, operated at room temperature 0.12 mL/min (pumped)
nebulizer spray chamber solution uptake rate flow injection valve dispersion (peak intensity of continuousflow injection nebulization) of FI length of the tubing from the FI valve to the nebulizer (m)
3. Flow Injection Parameter A manual Reodyne 3-way Teflon rotary valve (0.8-mm bore) with 0.1-mL sample loop 3
4. Interface made of Ni made of Ni
sampling cone skimmer cone resolution mass range (amu) 32S 34S data acquisition
1
5. Mass Spectrometer Operating Conditions M/∆M ) 3000 at 10% intensity measured by “E-scan mode” at a magnet mass of 29.998 31.961-31.983 33.955-33.980 ion counting mode ime integration method with peak jumping 15 points per one peak with dwell time of 0.005 s per point 500 scans per one run of 180 s
in controlling their yields. The isotope dilution technique (ID) is one of the best solutions to this problem.13,14 To overcome these difficulties, we have developed a new method for the determination of total sulfur at microgram per gram levels in geological materials, applying ID with HR-ICPMS following oxidation of sulfur into extremely stable sulfate. This oxidation is performed by bromine formed by in situ oxidation of HBr rather than by adding bromine directly. EXPERIMENTAL SECTION Geological Reference Materials. We analyzed the following reference materials: PCC-1 (peridotite), BCR-1, BHVO-1 (basalt) from the USGS; JP-1 (peridotite), JB-1, -2, and -3 (basalt), JA-1, -2, and -3 (andesite) from the GSJ; WMS-1 (sulfide) from CANMET; and the Smithsonian reference Allende chondrite powder (USNM3529, split 1, possible 23). As only a very small amount of powder (10-40 mg) was used, the original reference material powders were further pulverized to
