Purification of air for use in gas chromatography as a carrier gas and in a flame ionization detector

Abstract

An economical and convenient source of pure air is needed in gas chromatography to support combustion for the flame ionization detector (FID) and as a carrier gas ( I ) . The use of purified laboratory air and electrolytic hydrogen would eliminate the use of bottled gases if air were used as a carrier gas. In addition, a previous publication (2) showed that using purified air and electrolytic hydrogen improved the sensitivity of the FID by an order of magnitude.King ( 3 ) has developed a batchwise process for air purification using a modified version of the heatless dryer (4). This technique, using carbon as a purification medium, does not eliminate methane from the air which can occur naturally at a concentration up to 10 ppm by volume. Some "laboratory" compressed air supplies have over 200 ppm of methane present ( I ) . A few ppm of methane in the oxidant supply for the FID would cause some background noise and limit the sensitivity. More significantly, the same quantity of methane in the carrier gas would cause a much higher background noise. This is due to burner design and diffusion flame considerations-that is, the methane in the oxidant supply does not reach the "ionization" sensitive portion of the combustion region. For the specific analysis of methane, quantities below the background would result in negative peaks. Carrier gas supplies must also be well regulated and constant if reliable retention times are to be obtained. Thus, unless a large ballast were used, a batch processor for air would not have a constant delivery.A gas chromatograph has been developed, the NRL Total Hydrocarbon Analyzer ( I ) , which uses air as a carrier gas and incorporates the principles of backflush chromatography ( 5 ) . The chromatograph, developed for use on nuclear submarines, uses ships' air for the carrier gas and oxidant for the FID. Since this air source contains significant concentrations of methane ( I ) and halogenated hydrocarbons ( 6 ) , treatment to supply pure and well-regulated air is mandatory for this application.Studies conducted on air purification using catalytic oxidation show that chlorinated hydrocarbons as a group of compounds are difficult to oxidize (7, 8). This publication describes an air purification system employing the t.echnique of catalytic oxidation which delivers a constant flow of air essentially free of organic contaminants. EXPERIMENTALThe catalysts studied were 0.5% palladium on Ys-and %e-inch alumina pellets (Engelhard Industries, Inc.), 0.3% platinum on %-inch alumina pellets (Baker and Company, Inc.) and hopcal-

Catalytic Decomposition of Halogenated Hydrocarbons over Hopcalite Catalyst

“…The gas for the burner was prepared from compressed air taken from the laboratory supply line. This air was purified (Williams and Eaton, 1974), humidified to about 50% r.h. at room temperature and pressure, and mixed with the halogenated hydrocarbons (Musick, et ah, 1972) before being admitted to the burner. All the bubblers, tubes, valves, and other components to which the contaminated burner air was exposed prior to analysis were constructed of either glass or stainless steel.…”