Abstract

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