APPLICATION OF A 33S/36S DOUBLE SPIKE MULTI-COLLECTOR THERMAL IONIZATION MASS SPECTROMETRY (MC-TIMS) TECHNIQUE FOR THE DETERMINATION OF SULFUR ISOTOPIC COMPOSITION (d34S) IN ENVIRONMENTAL SAMPLES

 

Jacqueline L. Mann and W. Robert Kelly (839), Mentors:  W. Robert Kelly, Division: 839-ACD, Lab: CSTL, Rm. B352 Bldg. 227, MS 8391, 301-975-4472, 301-869-0413, jmann@nist.gov, not a member, Category: Chemistry

 


Stable isotope ratios of light elements (H, C, N, O, and S) have been used in a variety of applications in chemistry, physics, geology, ecology, and food and agriculture for determining the fate of inorganic and organic pollutants, the origin of groundwaters, and the origin and method of production of food products.  Stable light isotope ratios are also being used in medicine and in biology for diagnosis of illnesses as well as for improved understanding of metabolisms and biochemical pathways.

 

Variations in stable sulfur isotopic compositions (d34S, R = 34S/32S) are commonly used for tracing sources and chemical transformation of sulfur.  Increasingly studies employing sulfur isotopes are requiring analytical techniques that are capable of high accuracy and precision measurements on very small sample sizes (< 1 mmole).  In the last 4 years NIST has perfected a specialized high-accuracy mass spectrometric method to accurately and precisely measure the isotopic composition of sulfur in samples < 0.5 mmole.  The new technique uses multiple-collector thermal ionization mass spectrometry (MC-TIMS) in combination with a 33S/36S internal standard for the determination of d34S.  The fundamental limitation to accurate and precise isotopic ratio measurements by thermal ionization is that the measured ratio differs from the true ratio in the source as a result of instrumental fractionation during vaporization of the sample from the filament.  To address this changing ratio and to improve precision and accuracy in the 34S/32S, a well-characterized 33S/36S internal standard was added to the samples and was used to calculate a fractionation factor (a) that corrects for this changing ratio (instrumental fractionation) to give the true ratio in the sample. 

 

Three international sulfur standards (IAEA-S-1, IAEA-S-2, and IAEA-S-3) were measured to evaluate the precision and accuracy of the new technique.  The d34S values (reported relative to Vienna Canyon Diablo Troilite (VCDT), d34S (‰) = (34S/32S)sample/(34S/32S)VCDT  –  1) x 1000]), 34S/32SVCDT = 0.0441626) determined were  –0.32‰ ± 0.04‰ (1s n=4) and –0.31‰ ± 0.13‰ (1s, n=8) for IAEA-S-1, 22.65‰ ± 0.04‰ (1s, n=7) and 22.60‰ ± 0.06‰ (1s, n=5) for IAEA-S-2, and –32.47‰ ± 0.07‰ (1s, n=8) for IAEA-S-3. 

 

The first application of this technique was to snowpit samples collected from the Inilchek Glacier, Kyrgyzstan (42.16°N, 80.25°E, 5100 m) and from Summit, Greenland (72.58°N, 38.53°W, 3238 m).  The d34S data from the Summit snowpit are the first continuous high-resolution (≈ 7 samples/1 year) data for this site.  The d34S values for the Inilchek ranged from 2.6‰ ± 0.4‰ (2s) to 7.6‰ ± 0.4‰ (2s) on sample sizes ranging from 0.3 to 1.8 mmol S.  d34S values for Greenland ranged from 3.6‰ ± 0.7‰ (2s) to 13.3‰± 5‰ (2s) for sample sizes ranging from 0.05 to 0.29 mmol S.  For both snowpits the measurement uncertainty is identical, but the total uncertainty is dominated by blank uncertainty for the Greenland samples.  The d34S measurements were used to estimate seasonal sulfate sources contributing to precipitation in these regions. 

 

We also recently applied the technique in a CCQM pilot study (CCQM P-75) to determine the d34S of the essential amino acid methionine.  The d34S determined was 10.33‰ ± 0.1‰ (n=9) with the uncertainty of our results (reported as combined uncertainties) being smaller then that reported by the 5 other laboratories that participated and on half the amount of sample (100 mg).  Two additional measurements were made using a factor of 10 less sample (10 mg) with the results being 10.62‰ ± 0.24‰ (1s) and 10.63‰ ± 0.28‰ (1s).  Both results fall within in our uncertainty reported, again showing the capability of the technique for measuring small sample sizes.  The measurement results for this material are expected to demonstrate measurement capabilities among laboratories and lead to the development of other multi-element stable isotope reference materials.

 

The double spike MC-TIMS method offers several advantages over the more commonly used gas source isotope ratio mass spectrometric (GIRMS) technique or these measurements.  First, the precisions are typically better and the measurements can be made on much smaller samples.  Second, because it uses an internal standard rather than an external standard, it is intrinsically accurate because only isotope ratios need to be measured; therefore complete recovery of the sample is not required for unbiased results.  Also, mass fractionation that may be caused by losses during drying and/or chemical reduction of the sample is accounted for by adding the spike prior to sample processing.  This is a considerable advantage for small sample sizes (< 1 mmol S) where losses can result in potentially large biases without the use of an internal standard. And third, although not presented here concentration data is also obtained by the inherently accurate and precise isotope dilution technique.  GIRMS requires a separated measurement for concentration determinations.