Screening of cocaine and its metabolites in human urine samples by direct analysis in real-time source coupled to time-of-flight mass spectrometry after online preconcentration utilizing microextraction by packed sorbent

Abstract

Microextraction by packed sorbent (MEPS) has been evaluated for fast screening of drugs of abuse with mass spectrometric detection. In this study, C8 (octyl-silica, useful for nonpolar to moderately polar compounds), ENV ϩ (hydroxylated polystyrene-divinylbenzene copolymer, for extraction of aliphatic and aromatic polar compounds), Oasis MCX (sulfonic-poly(divinylbenzene-co-N-polyvinyl-pyrrolidone) copolymer), and Clean Screen DAU (mixed mode, ion exchanger for acidic and basic compounds) were used as sorbents for the MEPS. The focus was on fast extraction and preconcentration of the drugs with rapid analysis using a time-of-flight (TOF) mass spectrometer as the detector with direct analysis in a real-time (DART) source. The combination of an analysis time of less than 1 min and accurate mass of the first monoisotopic peak of the analyte and the relative abundances of the peaks in the isotopic clusters provided reliable information for identification. Furthermore, the study sought to demonstrate that it is possible to quantify the analyte of interest using a DART source when an internal standard is used. Of all the sorbents used in the study, Clean Screen DAU performed best for extraction of the analytes from urine. Using Clean Screen DAU to extract spiked samples containing the drugs, linearity was demonstrated for ecgonine methyl ester, benzoylecgonine, cocaine, and cocaethylene with average ranges of: 65-910, 75-1100, 95-1200, and 75-1100 ng/mL (n ϭ 5), respectively. The limits of detection (LOD) for ecgonine methyl ester, benzoylecgonine, cocaine, and cocaethylene were 22.9 ng/mL, 23.7 ng/mL, 4.0 ng/mL, and 9.8 ng/mL respectively, using a signal-to-noise ratio of 3:1. B iological samples such as plasma and urine are much more complex than many others due to the presence of proteins, salts, acids, bases, and various organic compounds with similar chemistry to the analytes of interest. As a result, the extraction methods for biological samples have been difficult. If an unsuitable sample preparation method has been employed before the injection, the entire analytical process can be wasted. The purpose of sample preparation is (1) removal of interfering substances to eliminate ion suppression, (2) conversion of the analytes into a more suitable form for injection, separation, and detection, and (3) preconcentration of the analytes to improve sensitivity. The procedure must be highly reproducible, with a high recovery of the target analytes. Microextraction by packed sorbent (MEPS) is a new miniaturized, solid-phase extraction method that can be connected online to a GC or LC system without any modifications. In MEPS approximately 1 to 2 mg of the solid packing material is packed inside a syringe (100 to 250 L) as a plug or between the barrel and the needle (Figure 1a). Sample preparation takes place on the packed bed, which can be coated to provide selective and suitable sampling conditions. This approach to sample preparation is very promising for many reasons: (1) it is easy to use, (2) it can be fully automa...

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