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Automated Chromatographic Analysis of Atmospheric Gases in Environmental Samples

Environmental Chemistry Laboratory, USDA-ARS, Beltsville, MD 20705

G. Blicher-Mathiesen

Department of Freshwater Ecology, National Environmental Research Institute, Silkeborg, Denmark

*Corresponding author (gmccarty{at}asrr.arsusda.gov).

ABSTRACT

The need for a single-injection gas chromatographic procedure for detection of the principal atmospheric gases found in environmental samples has led to development of an automated system that permits measurement of CO₂, N₂O, O₂-Ar, and N₂ in a single detector without interference from water vapor in samples. The injection of gas samples on the system was automated by interfacing a commercial headspace sampler with a gas chromatograph (GC) that was fitted with an ultrasonic detector and two automated multiport switching valves. The first switching valve was configured with short precolumns of Porapak Q (PQ) to retain and vent the water vapor in samples. The other valve was configured with two analytical columns connected in series that permit separation of the gas components. After water is removed by the precolumn, the gas sample flows onto a PQ column where CO₂ and N₂O are retained and separated while the composite peak of N₂ and O₂-Ar elutes onto a molecular sieve (MS) column. The valve is then switched, causing reversal of the order of columns with respect to the gas flow without reversing the direction of flow through the columns. This allows the CO₂ and N₂O peaks to elute from the PQ column to the detector while the O₂-Ar and N₂ components are separated on the MS column and later flow to the detector. The system has proven to be robust in that it permits routine, unattended analyses of moist gas samples with high sensitivity and without interference from water peaks. This, in conjunction with the highly controlled equilibrium conditions permitted by use of the headspace sampler, makes the system suitable for gas analysis of environmental samples including dissolved gas analysis of natural water samples.

Received for publication November 10, 1995.

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