Integrating in-vial thin film microextraction using polysiloxane-based adhesive tapes with low-temperature plasma ionization mass spectrometry: A solvent-free approach for determining cocaine and methamphetamine in saliva samples

Abstract

The direct combination of sample preparation and mass spectrometry (MS) arose as an alternative to classical chromatographic methods, reducing solvent consumption while increasing the analysis throughput. In this combination, selectivity relies on the efficient isolation of the analytes from the sample matrix and the discrimination power of the instrumental technique. Low-temperature plasma-based ionization has demonstrated potential for these couplings, typically including a thermal desorption (TD) step to transfer the analytes to the gaseous phase. Thermal-resistant adhesive tapes (TRT) are promising materials in microextraction-TD couplings because they are thermally stable and can interact with target analytes thanks to their polysiloxane-based adhesive. In this paper, a novel in-vial microextraction technique based on TRT is described, and its direct coupling to Soft Ionization by Chemical Reaction In Transfer (SICRIT) MS is explored. The TRT is attached to a commercial headspace metallic cap from which the septum has been previously removed. The cap is finally screwed in a 5 mL vial containing the sample where the extraction takes place. After the extraction, the cap containing the TRT is analyzed by a TD-SICRIT-MS/MS in a dedicated interface. This first approach has been evaluated using the determination of cocaine and methamphetamine in saliva samples as proof of concept. The limits of detection (LODs) were fixed at 1.5 microg·L-1, so the sensitivity fits well with the concentration level of these drugs in biofluids. The intra-day and inter-day precision, both expressed as relative standard deviation (RSD), were better than 15.9 and 16.7 %, respectively. The accuracy, expressed as relative recovery, was in the range between 85-119 % for both analytes. Once extracted, the analytes were stable in the tapes for at least two weeks opening the door to on-site extraction workflows. The low price of the tapes (1000 TRT segments cost ca. 7.5 $) is a positive aspect for popularizing this microextraction technique. Furthermore, the method is free of organic solvents and does not require additional gases (such as N2 or Ar) to carry out the desorption and ionization of the analytes, which makes the process environmentally friendly, simple and safe.