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30 May / June 2021


High-Throughput Thermal Desorption Analysis of Volatile Compounds Using Selected Ion Flow Tube Mass Spectrometry


Vaughan S. Langford*, Diandree Padayachee, Kalib J.M. Bell, Jing Ma Syft Technologies Ltd, 68 Saint Asaph Street, Christchurch 8011, New Zealand


Abstract


Direct mass spectrometry combined with automation has potential to significantly increase sample throughputs in routine laboratory analysis. This article describes the performance of direct-analysis selected ion flow tube mass spectrometry (SIFT-MS) integrated with automated thermal desorption. The performance of thermal desorption-SIFT-MS in several air quality applications of conventional thermal desorption-gas chromatography mass spectrometry is described. Compared to gas chromatography mass spectrometry analysis, one of the most significant advantages of thermal desorption tube analysis using SIFT-MS is that volatiles are analysed as they are desorbed, providing at least four-fold greater throughputs.


Introduction


Thermal desorption tubes (TDTs) are widely used air sampling media because of the convenience of in-built sample preconcentration, robustness, economical shipping, and their re-usability (including low cost of cleaning). Applications that use TDT sampling include environmental monitoring [1,2], indoor air quality [3,4], and breath research [5].


However, sample throughput has been a critical bottleneck for conventional TDT analysis, because of the relatively slow gas chromatography-mass spectrometry (GC/ MS) analysis with which thermal desorption has been coupled. Direct analysis techniques eliminate the throughput-limiting step: the chromatographic separation, and in doing so yield signifi cant throughput improvements for routine analysis. Selected ion fl ow tube mass spectrometry (SIFT-MS) is a technique that directly analyses volatile compounds in air in real time with high sensitivity and selectivity. High selectivity in real-time is provided by application of multiple rapidly switchable, soft chemical ionisation agents (so-called reagent ions) [6].


Figure 1. Desorption profi le for BTX spiked onto a TDT packed with Carbopack™ X sorbent (Method 325). Instantaneous concentrations in the SIFT-MS fl ow tube are shown on the left- hand axis, while the TD temperature ramp is shown in red on the right-hand axis.


Therefore, by coupling thermal desorption with SIFT-MS, there is opportunity to signifi cantly improve analysis throughput for TDTs.


This article describes the initial application of a fully integrated thermal desorption- SIFT-MS (TD-SIFT-MS) system to several important environmental analyses. First, suitability for outdoor monitoring is


considered in terms of ambient and fenceline methods (United States Environmental Protection Agency (US EPA methods TO-17R [1] and 325 [2], respectively). Second, the application to indoor air quality measurement is demonstrated by vehicle interior air quality (VIAQ) analysis. In these case studies, the analytical performance of TD-SIFT-MS is presented. In practical terms, throughputs


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