A Gas Sample Pre‐concentration Device Based on Solid Phase Microextraction (SPME) and Temperature Programmed Desorption (TPD)

Abstract

A sample pre‐concentration device, based on solid phase microextraction (SPME) fiber and temperature programmed desorption (TPD), was developed and tested in the analysis of volatile organic compounds. The pre‐concentrator device consists of a low dead volume quartz tube that houses the SPME fiber during the extraction/ desorption steps, a Ni:Cr heating element, and a programmable power supply. Compounds are extracted from air into the SPME fiber according to their affinity and solubility to the fiber material. Following this extraction step, samples are desorbed by a pre‐programmed temperature ramp for their subsequent detection. The SPME fiber was exposed and sample extracted for a pre‐determined amount of time (1 min, in this work). Because of the limited specificity of the stationary phase in SPME fibers towards the analyte, background interfering compounds are unavoidably co‐extracted during this process. A TPD step was then applied to sequentially desorb and detect analytes and interferences. The temperature ramp was designed so as to maximize temporal resolution between target analytes and background chemical interferences. The SPME/TPD pre‐concentrator was tested with a binary chemical system consisting of dimethyl‐methylphosphate (DMMP, a chemical warfare simulant) and toluene (a common commercial solvent). Analyte desorption was monitored on‐line with a quadrupole ion trap mass spectrometer. Five SPME fibers with different stationary phases were tested: polyacrylate (PA), polydimethylsiloxane (PDMS), carboxen–polydimethylsiloxane (CAR/PDMS), polydimethylsiloxane–divinylbenzene (PDMS/DVB), and carbowax/divinylbenzene (CW/DVB). Best results were obtained with the PDMS (100 µm) fiber. Two other SPME fibers, CW/DVB and CAR/PDMS, along with the TPD step, were also successful in the temporal resolution the of DMMP and toluene. The approach is rapid in that the extraction and desorption steps take less than 3 min. Implications and applications of the SPME/TPD system as a sample pre‐concentration step in handheld devices are discussed. The simplicity of the device makes it amenable to miniaturization by available microfabrication techniques.