A compact comprehensive two-dimensional gas chromatography (GC[times]GC) approach for the analysis of biogenic VOCs
We describe the development of a compact comprehensive two-dimensional gas chromatograph suitable for the measurement of biogenic VOCs in the atmosphere at part per billion mixing ratios. The design seeks to minimise instrument size and power consumption and maximise portability and autonomy. The instrument concept is to achieve high analyte selectivity for complex VOC mixture analysis using comprehensive two-dimensional GC (GC[times]GC), rather than hyphenation with larger more expensive detectors such as MS. Key features of the analytical approach are a custom-built miniature thermal desorption trap to collect and concentrate VOCs from the sample gas stream, a copper conducting direct column heating system and a valve-modulated interface to enable GC[times]GC. The high power and large form-factor turbulent GC oven is replaced by direct column heating (and cooling below ambient) by thermal transfer from copper bobbin holders with heating and cooling input from Peltier devices. The combination of two independent copper bobbins allows for independent control of the two columns needed for comprehensive GC. A heated two position 1/16[prime or minute][prime or minute] diaphragm valve is used to enable flow modulation between two columns, with analyte detection at the outlet of the second column using a miniaturised low cost photo-ionisation detector. The instrument sub-components are controlled by a Compact RIO computer (National Instruments) and purpose designed software written in LabVIEW allowing autonomous measurements. The complete system weighs 15 kg, is around the size of a desktop computer and has a mean power demand of 112 W when battery powered. Results on the sensitivity and linearity for isoprene collection and analysis of standard gas mixtures are presented along with a discussion of limiting factors that hinder field device performance.