Improving isotope ratio precision in inductively coupled plasma quadrupolemass spectrometry by common analyte internal standardization

The common analyte internal standardization (CAIS) chemometric technique is extended to improve isotope ratio precision obtained by inductively couple d plasma-quadrupole mass spectrometry (ICP-QMS). A correction function for isotope ratio measurement is derived from a relationship of mass sensitivit y as a function of element mass after considering the degree of ionization and isotopic abundance. An analyte isotope ratio correction factor can be d etermined by simultaneously measuring several internal reference isotopic r atios and the analyte. The CAIS approach corrects for isotopic mass bias an d isotope ratio variation with sample matrix concentration. Isotope ratio p recision is improved from about 0.4 to 0.05%, because long-counting times a llow correcting mass bias and isotope ratio drift during extended data-acqu isition periods. Isotope ratio accuracy is improved from 0.2 to 0.02%.