Determination of lidocaine in urine at low ppm levels using dispersive microextraction and attenuated total reflectance–Fourier transform infrared measurements of dry films

Abstract

IR spectra provide valuable information about biological systems and can be obtained with compactable and affordable instruments, but the lack of sensitivity of this technique hampers its use in the determination of drugs in clinical fluids. Taking lidocaine as a target molecule, in this paper we introduced a methodology for determining drugs in urine samples using infrared spectroscopy. The lack of sensitivity of the IR was compensated with the combination of an effective and straightforward dispersive liquid–liquid microextraction and the measurement of the dry film of the organic extracts through attenuated total reflectance (ATR). The method developed improves the sensitivity by eliminating the solvent and preconcentrating the analyte in the surface of the ATR crystal. Urine samples were taken from 15 volunteers, and 9 samples were spiked at 6 concentration levels ranging from 0 to 33mgL−1. Multivariate models based on partial least squares regression and science-based calibration were performed and validated with a separated set of spiked urine samples from other patients, obtaining ratio prediction to deviation (RPD) values higher than 3.5. The limit of detection values obtained were 0.5mgL−1 for the univariate calibration and 1.7mgL−1 for a reliable multivariate calibration. Therefore, the procedure is limited to only levels of lidocaine higher than 2mgL−1 but serves as an untargeted, fast, and versatile screening tool, which maintains all the advantages of the widespread application of the IR spectroscopy to the clinical analysis, such as simplicity, compactability, and minimum use of reagents and solvents.