Postcolumn Introduction of an Internal Standard for Quantitative LC−MS Analysis

A method for introducing internal standards by direct infusion into the LC effluent for the quantitative LC−MS analysis of environmental samples is described. This postcolumn introduction method was found to be effective in correcting quantitative errors associated with matrix signal suppression. However, unlike surrogate or volumetric internal standards, the performance of the postcolumn method does not depend on the selection of an internal standard that shares identical elution time with the target analyte. As a result, either structural analogues (target analyte derivatives) or isotopically labeled compounds may be applied as effective internal standards. Furthermore, the postcolumn introduction method allows the application of one internal standard to address signal suppression effects for several analytes in a single LC−MS run. In contrast, volumetric and surrogate introduction methods require an isotopically labeled internal standard for each analyte to be quantified.