Gas Chromatographic Microsystems for Airborne and Aqueous Volatile Organic Compound Determinations

Abstract

New technologies offering sensitive, selective, and near-real-time identification and quantification of the individual components of complex mixtures of volatile and semi-volatile organic compounds (S/VOCs) are greatly needed in applications such as personal (worker) exposure assessment, air and water pollution monitoring, disease diagnosis, and homeland security. This dissertation describes the characterization of two prototype instruments containing core gas chromatographic microsystems (µGCs); the development and characterization of a microscale vapor extractor (µVE), and its integration with a µGC; and the development of adsorbent materials providing selective preconcentration of polar S/VOCs for use in certain µGC applications. Following a review of the background and significance of the research (Chapter 1), this dissertation then describes the design, modeling, and preliminary characterization of the µVE, which is a passive device containing microchannels and a polymer membrane that transfers dissolved VOCs from aqueous samples passed through the device to the gas phase for analysis by a downstream µGC (Chapter 2). In a proof-of-concept experiment, a hybrid µVE-µGC microsystem extracted four VOCs from a 700 µL sample of synthetic urine in 3.5 min, and then separated, identified, and quantified each VOC in ~80 sec with a projected detection limit as low as 660 parts-per-billion. The hybrid μVE-μGC microsystem may eventually permit rapid field/clinical analyses of water contaminants and urinary biomarkers of exposure and disease. Chapters 3 and 4 describe prototype µGC instruments that are referred to as Personal Exposure Monitoring Microsystems (PEMM-1 and PEMM-2, respectively). PEMM-1 is a laptop-controlled, AC-powered, compact, bench-top unit and PEMM-2 is a battery-powered, belt-mounted unit with embedded controll. Both contain analytical microsystems made from Si-microfabricated components: a dual-adsorbent µpreconcentrator-focuser, a single- or dual-μcolumn separation module, and a μsensor-array detector. The μsensor-array consists of 4-5 chemiresistors (CR) coated with various monolayer-protected Au nanoparticles (MPN), which collectively yield partially selective response patterns that can enhance the recognition/discrimination of VOCs. Other key components include a pre-trap for low-volatility interferences, a split-flow injection valve, and an onboard He carrier-gas canister. In laboratory tests, PEMM-1 demonstrated the determination of 17 VOCs in the presence of 7 background interferences in 8 min. Detection limits were below the corresponding Threshold Limit Values (TLV) of the VOCs. PEMM-2 demonstrated the direct, autonomous determination of 21 VOCs in 6 min, with detection limits ranging from 16−600 ppb, well below TLV levels. A chemometric strategy involving retention time windows was implemented that greatly facilitated vapor recognition and discrimination via the µsensor-array response patterns. Results from a “mock” field test, in which personal exposures to time-varying concentrations of a mixture of five VOCs were measured autonomously, agreed closely with those from a reference GC. Chapter 5 describes the use of a trigonal-tripyramidal room-temperature ionic liquid (RTIL) as a surface modifier for the graphitized carbons, Carbopack B (C-B) and Carbopack X (C-X), used as µpreconcentrator adsorbents. The goal was to impart selectivity for polar compounds, particularly organophosphates and their precursors. Results showed that the capacities for five organophosphorus vapors were consistently enhanced ~2.5-fold with the RTIL-treated adsorbents relative to the untreated adsorbents. Furthermore, the capacities for several non-polar reference vapors were reduced 11 to 26-fold with the modified adsorbents. Implementation in next-generation µpreconcentrator devices is planned.