Optimization of Sorbent Mesh Geometry in Transmission Mode Direct Analysis in Real-Time Mass Spectrometry

Abstract

Targeted trace-level (sub mg/kg) quantitative analyses of volatiles are often performed by gas chromatography–mass spectrometry (GC-MS). Extraction of headspace volatiles onto thin etched sorbent sheets (SPMESH) followed by transmission mode direct analysis in real-time mass spectrometry (TM-DART-MS) can increase throughput by 1–2 orders of magnitude as compared with GC-MS. However, optimization of mesh pattern geometry has not been reported for SPMESH or other TM-DART-MS applications. Herein, a novel cartridge-based SPMESH device was fabricated to allow easier comparison of different mesh designs. SPMESH sheet geometries with varying aperture shape (circular, square), aperture size, and surface areas were fabricated, and their performance on DART-MS was evaluated using two target volatiles (3-isobutyl-2-methoxypyrazine (IBMP) and methyl anthranilate (MA)). For square apertures, an optimum aperture size of 1.5 mm was observed. At lower and higher aperture sizes, a decrease in sensitivity and an increase in detection limits were observed, presumably because of reduced flux through the SPMESH sheet and decreased surface area perpendicular to the DART flow stream, respectively. Further improvements in sensitivity and detection limits were observed for mesh patterns with circular mesh apertures in place of square apertures, possibly because volatiles can be more efficiently desorbed from the interior faces of the SPMESH sheet by the DART gas stream. The best performing design improved detection limits by 24-fold for IBMP (LOD decreased to 4.1 ng/L from 100 ng/L) and by 6-fold for MA (20.9 ng/L from 135 ng/L) as compared with the original SPMESH design. These findings have implications for design of other TM-DART-MS sampling devices.