Method for internal standard introduction for quantitative analysis using on-line solid-phase extraction LC-MS/MS.
A novel approach for on-line introduction of internal standard (IS) for quantitative analysis using LC-MS/MS has been developed. In this approach, analyte and IS are introduced into the sample injection loop in different steps. Analyte is introduced into the injection loop using a conventional autosampler (injector) needle pickup from a sample vial. IS is introduced into the sample injection loop on-line from a microreservoir containing the IS solution using the autosampler. As a result, both analyte and IS are contained in the sample loop prior to the injection into the column. Methodology allowed to reliably introduce IS and demonstrated injection accuracy and precision comparable to those obtained using off-line IS introduction (i.e., IS and analyte are premixed before injection) while maintaining chromatographic parameters (i.e., analyte and IS elution time and peak width). This new technique was applied for direct analysis of model compounds in rat plasma using on-line solid-phase extraction (SPE) LC-MS/MS quantification. In combination with on-line SPE, IS serves as a surrogate IS and compensates for signal variations attributed to sample preparation and instrumentation factors including signal suppression. The assays yielded accuracy (85-119%), precision (2-16%), and analyte recovery comparable to those obtained using off-line IS introduction. Furthermore, on-line IS introduction allows for nonvolumetric sample (plasma) collection and direct analysis without the need of measuring and aliquoting a fixed sample volume prior to the on-line SPE LC-MS/MS analysis. Therefore, this methodology enables direct sample (plasma) analysis without any sample manipulation and preparation.