Automated high-speed repetitive GC systems for vapor analysis.

Abstract

Gas chromatography (GC) is one of the most widely used methods for the separation and analysis of lower molecular weight organic compounds. The method can be applied to most organic compounds that have a significant vapor pressure at temperatures below 400$\sp\circ$C. A serious limitation of GC separations, however, is the relatively long analysis time, generally several minutes or more. If analysis times could be reduced, gas chromatography would be a much more attractive technique for applications in which successive, near real-time analyses are required. Two computer-controlled GC systems for the high-speed, repetitive analysis of dilute organic vapors are described. Retention times typically are a few seconds and complete analysis cycle times in the range from 10-20 s. For these apparatuses, a gas-cooled and electrically-heated metal capillary tube is used as a cryo-focussing inlet system and produces injection bandwidths in the 5-15 ms range. A vacuum-pump-operated backflush technique is used to rapidly remove high-boiling-point compounds and to condition the column for subsequent injections. Detection is accomplished by a flame ionization detector in conjunction with a high-speed electrometer. A vapor generator produces test vapors in a variety of dilution gases and over a wide range of sample concentrations. The primary difference between the two high-speed GC systems is in the method of sample introduction. One uses a mechanical valve equipped with a sample loop; while the other uses a completely valveless system in which thermal modulation of the vapor stream is obtained directly by the inlet system. Studies of the two devices show that simple mixtures of volatile organic compounds can often be separated in 10 s or less. During replicate analyses, relative standard deviations for retention time are less than 0.5%; while those for peak height and peak area are typically less than 2.5%. Limits of detection are in the low ppb range. Test mixtures prepared at various concentrations and humidities, 10% to 90%, were measured in order to evaluate the high-speed GC systems for direct-air monitoring applications. Changes in humidity were shown to have no effect on chromatographic performance.