Determination of low-level (sub-microgram) sulfur concentrations by isotope dilution multi-collector inductively...

The certification of sulfur (S) in Standard Reference Materials™ by the National Institute of Standards and Technology (NIST) has been exclusively performed using isotope dilution thermal ionization mass spectrometry (ID-TIMS). The ID-TIMS measurement method is limited in its capability for low concentration measurements (<1 µg/g) due to the blank associated with the chemical reduction procedure (≈0.2 S µg). Newly developed materials and applications, such as biofuels made from soy and nanomedicine, pose a challenge to the ID-TIMS technique because of their very low concentrations (<<1 µg/g) of S. As described here, a measurement technique with low S blanks is essential for low-level S measurements.

An isotope dilution (ID) multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS) method combined with an internal normalization approach for mass bias correction has been used to determine low-level S concentrations in synthetically prepared mixtures using a (33)S-enriched spike material. Dilute sulfur solutions (1 µg S/g) were prepared from NIST SRM 3154 (Sulfate Spectrometric Solution) to test the capability of this technique for measuring very low-level S concentrations.

The concentration results for each solution were 0.983 ± 0.004 µg/g (95% CI, n = 2, k = 2), 1.006 ± 0.005 µg/g (95% CI, n = 2, k = 2), and 0.999 ± 0.003 µg/g (95% CI, n = 2, k = 2), in excellent agreement with the gravimetric determination, deviating less than 0.35% and suggesting the technique can yield unbiased and accurate results. The blanks averaged 13 ± 0.0017 ng S (1s).

The data results provide a clear indication that the ID-MC-ICP-MS method for the determination of low-level S concentrations is feasible. The more than one order of magnitude reduction of the blanks suggests that it is a better alternative to the ID-TIMS method for very low S materials such as are encountered in biofuels and some biochemical species. Published 2012. This article is a US Government work and is in the public domain in the USA.