The Saha ionisation equilibrium shift correction model applied to MPT-OES for analysing complex matrix samples: an example for brine samples

Abstract

The Saha-IESC (Saha Ionisation Equilibrium Shift Correction) is a novel approach designed to correct for ionisation interference in the analysis of various solution samples. It integrates the Saha equation, plasma electroneutrality, the ideal gas equation, and the mass conservation equation to determine electron densities and the densities of various particles. Then, it establishes a correlation between spectral line intensities, particle densities, and concentrations, allowing for accurate quantitative analyses. For plasma sources deviating from LTE (Local Thermodynamic Equilibrium), the model introduces a variety of plasma temperatures. Saha-IESC can accurately quantify both macro and trace elements without strict matrix matching between the standard and samples to be tested. Nor does it require a large number of samples as in machine learning. When combined with the concept of spectral standardisation, it can also correct the spectral intensities. After calibration, the R² values of the calibration function of elements in the certified reference materials (CRM) GWB(E)130459 were improved from 0.7599–0.9971 to 0.9902–0.9999. And the detection accuracies for several elements in the two certified reference samples were in the range of 90–104% (GWB(E)130459) and 89–102% (BWR3030-2016), respectively. The robustness of the model was assessed through spiked recoveries of six elements in four brines. The range of spiked recoveries was 94–150% (external standard), 97–152% (internal standard), and 80–120% (Saha-IESC), respectively. The Saha-IESC can greatly reduce the complexity of experimental design, while effectively suppressing the ionisation interference.