Rationale: The concentration-gradient-method (CGM) was previously introduced as a precise and accurate method for isotope ratio determination by quadrupole-based inductively coupled plasma mass spectrometry (ICP-QMS). The investigation of its potential and advantages in the analysis of analytes with a poor signal-noise ratio (S/N) is important to establish routine isotope ratio analysis industrial applications on these widely used instruments.
Methods: The CGM was applied on isotope measurements of Sr near its limit of quantification (LOQ) and of sulphur where there was a massively interfered 32 S isotope signal in gypsum samples of different origin, in order to demonstrate the advantages of the CGM over the commonly used measurement and evaluation approach. The comparison between the CGM and the classical measurement and evaluation approach was performed with high and low concentration Sr standard solutions, to prove the robustness of the isotope ratio determination.
Results: In both cases the CGM reached a recovery rate of approximately 103 %, whereas the classical approach became increasingly inaccurate at lower S/N (recovery of 123 %). In the case of sulphur isotope ratio determination only the CGM enabled a differentiation between geogenic and flue gas desulphurisation plant-originated gypsum samples.
Conclusions: The robustness of the CGM approach was illustrated for gypsum samples with trace strontium concentration and its general applicability to the determination of sulphur isotope ratios by means of quadrupole-based ICP-MS was demonstrated using the example of sulphur in gypsum.
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