Adhesive tapes as sampling probes and thermal desorption substrates, in search of direct analysis of particulate solid samples using Soft Ionization by Chemical Reaction In Transfer mass spectrometry

Direct analysis by mass spectrometry (MS) has emerged as a promising strategy for analyzing a broad range of samples. Among the different ionization techniques, plasma-based sources are simple alternatives that can be used to ionize a wide range of analytes, thus providing high analytical versatility. This article describes the design of a simple and cost-effective interface that uses adhesive tapes as both sampling probes and thermal desorption substrates. The particulate solid sample is adhered to the tape in a single and quick step, avoiding particle losses and protecting the interface from clogging. The probe is thermally desorbed in a dedicated chamber, built using commercial elements, in front of the MS inlet. The desorbed analytes are driven by the vacuum system (any supplementary carrier gas is not required), passing through the plasma-based source (Soft Ionization by Chemical Reaction In Transfer, SICRIT®), where they are ionized before being introduced in the spectrometer. The determination of caffeine in coffee has been evaluated as a proof of concept since the complexity of the matrix and the semivolatile character of the analyte can challenge the performance, allowing to check the potential of the new approach. The type of adhesive tape, the parameters related to the desorption and sample dilution have been studied in depth. Working under optimum conditions, LOD was settled down at 0.18 mg g^-1. The intra-day and inter-day precision, expressed as relative standard deviation and calculated at three different concentration levels, were better than 12.5 and 13.7 %, respectively. Finally, the accuracy of this method was calculated by analyzing five different coffee samples and comparing the results with those provided by a reference method. The accuracy was in the range of 82.3-105.5 %. This interface is presented as a promising, simple, solvent-free approach for the direct analysis of solid samples. The interface has been finally applied to the analysis of drug mixtures to demonstrate its versatility.