Abstract
Abstract. A reliable method of sample introduction is presented for online gas chromatography with a special application to in situ field portable atmospheric sampling instruments. A traditional multi-port valve is replaced with a valveless sample introduction interface that offers the advantage of long-term reliability and stable sample transfer efficiency. An engineering design model is presented and tested that allows customizing this pressure-switching-based device for other applications. Flow model accuracy is within measurement accuracy (1%) when parameters are tuned for an ambient-pressure detector and 15% accurate when applied to a vacuum-based detector. Laboratory comparisons made between the two methods of sample introduction using a thermal desorption aerosol gas chromatograph (TAG) show that the new interface has approximately 3 times greater reproducibility maintained over the equivalent of a week of continuous sampling. Field performance results for two versions of the valveless interface used in the in situ instrument demonstrate typically less than 2% week−1 response trending and a zero failure rate during field deployments ranging up to 4 weeks of continuous sampling. Extension of the valveless interface to dual collection cells is presented with less than 3% cell-to-cell carryover.
Highlights
In the atmosphere, organic compounds play a central role in the processes that generate tropospheric ozone and secondary organic aerosol (SOA)
Many thousands of these organic compounds have been identified in both the gas and particle phases (Hamilton et al, 2004; Lewis et al, 2000) using various chromatographic techniques, including gas chromatography (GC), and these most likely represent only a small fraction of what is present (Goldstein and Galbally, 2007)
Two flow states are used during operation of the thermal desorption aerosol gas chromatograph (TAG) instrument: (1) concurrent collection and analysis (“load”), wherein excess helium carrier gas flow over that required for the column is bled into the collection and thermal desorption (CTD) cell, and (2) sample transfer from the cell to the column (“inject”) is conveyed by a second helium source
Summary
Organic compounds play a central role in the processes that generate tropospheric ozone and secondary organic aerosol (SOA). When a mass spectrometer is employed, these leaks can cause oxidation of the ion source, shortened electron impact filament lifespan, and reduced sensitivity To significantly reduce such problems and to improve the reliability and longterm instrument stability, we sought a suitable substitute for the classical sample injection valve. Inc) on our thermal desorption aerosol gas chromatograph (TAG) instrument with a novel valveless injection (VLI) system It was arrived at independently, the fundamental design of the VLI system described here is a natural extension of the early pressure switching work of Deans (1972, 1973, 1984a, b), who argued that with such a design one could maximize transfer efficiency and increase reproducibility and reliability at the same time. Extension to the transfer of highly polar compounds using online derivatization and the adaptation of the integrated-manifold VLI to a dual-cell system for simultaneous paired sample collection is demonstrated
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