Using a postcolumn-infused internal standard for correcting the matrix effects of urine specimens in liquid chromatography–electrospray ionization mass spectrometry

Matrix effects (MEs) are a major problem affecting the quantitative accuracy of liquid chromatography–electrospray ionization mass spectrometry (LC–ESI-MS) when analyzing complicated samples. While analyzing urine specimens, the wide diversity of endogenous materials and different urine concentrations may result in inaccurate quantification. In this study, we propose a postcolumn-infused internal standard (PCI-IS) strategy for universal correction of MEs in urine specimens. MEs can be effectively corrected by dividing the target analyte signal intensity by the PCI-IS intensity. To evaluate the performance of PCI-IS, we used 6 benzodiazepine (BZD) drugs in 5 different concentrations of urine matrixes as a test model. The divergence of the BZD drug signal responses in 5 different urine matrixes was expressed using their respective coefficients of variation (CV) to evaluate the efficiency of using PCI-IS in correcting matrix effects. The CV of the BZD drug signal intensities in these 5 different concentrations of the urine matrixes were reduced from 10 to 30% to less than 10% when the PCI-IS correction method was employed. When the PCI-IS method was used to correct the 6 BZDs in 25 real human urine samples, over 90% of the test results exhibited quantification errors of less than 20%, and all of the test results had quantification errors of less than 30%. These results demonstrate that the PCI-IS method can resolve the problem of inaccurate quantification that arises from the diversity of urine specimens. The PCI-IS method is particularly useful for clinical analysis or forensic toxicology to improve the quantification accuracy in an economical way.