Quantitative surface scanning by Direct Analysis in Real Time mass spectrometry

Abstract

Only a few ambient ionization sources have been demonstrated to work quantitatively for surface scanning. A modification of the Direct Analysis in Real Time mass spectrometry (DART-MS) interface is needed to improve the precision during the scanning of a high-performance thin-layer chromatography (HPTLC) plate or any other surface or planar substrate, especially for quantitation without an internal standard correction. The substrate movement relative to the ion source outlet and the mass spectrometer inlet was optimized to improve the desorption, ionization, and capture of analytes. The substrate carrier was mounted at an angled position, thus reducing collisions between the deflected gas stream and the inner transfer tube wall. A special transfer tube, whose edge was angled towards the substrate and allowed a narrow set-up of the ambient air gap, captured the deflected DART gas stream. For the repeated DART-MS scanning along five identical deposited bands of butyl-4-hydroxybenzoate a mean precision of 2.7% was obtained. A signal decay of 62% was observed after five scans. After HPTLC of methyl-4-hydroxybenzoate and butyl-4-hydroxybenzoate, mean determination coefficients of 0.9937 and 0.9906 were obtained for five calibrations on five plates, respectively. The mean recovery of two control standards was 94% with a mean repeatability of 9% (%RSD, n = 5) obtained on five different plates. The DART SVPA-3DS system remained compact and the access to the substrate was kept wide open despite the optimized scan lane (spatial resolution at full width at half maximum 0.8 mm, height 3 mm). The performance data showed that the quantitative surface scanning was improved as well as the desorption efficacy and detectability using this modified DART-MS interface.