Analysis of sulfur in soil and plant digests using methane as a reaction gas for ICP-MS

Quantitation of sulfur (S) is vitally important for analysis of agricultural soil and plant samples due to the requirement of S in living organisms. Although inductively coupled plasma mass spectrometry (ICP-MS) is a commonly used and robust instrument for multi-elemental detection, S is usually analysed by ICP-optical emission spectroscopy (OES) since S quantitation poses a particular challenge for ICP-MS due to interferences on all S isotopes. The requirement for analysis by two instruments increases time and cost for sample analysis, hence analysis by one instrument is desirable. The use of reaction gases in ICP-MS can improve the performance by shifting S to a mass for detection where no interference is present. This work explored the potential of methane as a reaction gas for analysis of S in soil and plant samples to give users an alternative option to oxygen. The product ion clusters CH₂SH⁺ were monitored (m/z 47 and 49 on ICP-MS and with mass shift of +15 from Q1 → Q2 using 32 → 47 and 34 → 49 on triple quadrupole ICP-MS). As expected, triple quadrupole ICP-MS performed better than single quadrupole ICP-MS containing a reaction cell due to the ability to preselect the m/z of choice and remove ions that may react with methane in the reaction cell. The method detection limit (MDL) was 150 mg kg^-1 S for plants and 53 mg kg^-1 S for soils which is fit for requirements. This is the first-time methane has been reported as a reaction gas for analysis of S and shows promising results for agricultural samples when using a triple quadrupole ICP-MS. Results compared well to those obtained via the more commonly used ICP-optical emission spectroscopy (OES) method with results <20 % for all samples. Interlaboratory comparison samples were within 2 Z-scores of the consensus mean. In the absence of ICP-MS/MS, Q-ICP-MS with detection of cluster m/z 47 was deemed to be suitable for detection of S in plant samples, with acceptable MDL (250 mg kg^-1 S), acceptable precision (<20 % RSD) and <20 % variation to the reported ICP-OES result.