Carbon-based preconcentrators are being developed for use in portable and micro-fabricated GC instruments designed as air analyzers. These preconcentrators are capable of quantitatively collecting organic vapors from large-volume air samples. Thermal desorption then is used to inject the sample into the GC as a vapor plug with plug widths ranging from less than 1 s to several s. The result is a loss in efficiency, a decrease in peak capacity, and a loss in sensitivity. This presents a challenge for fast analyses with relatively short columns where the chromatogram peak width can become dominated by the injection plug width. One solution to this problem is to split the flow coming from the inlet, in a similar manner to that of standard split/splitless inlets. This enables rapid sweeping of the inlet volume without a corresponding increase in column flow rate. This minimizes extra-column band broadening from the inlet dead volume; however, extra- column band broadening from the finite rate of desorption remains an issue. Also, splitting the flow will result in a loss of sample and hence sensitivity for a given sampling time. An increase in sample size may be required for some applications to maintain sufficient detector signal. The use of stop-flow desorption can minimize the effect of extra column band broadening from desorption rate, however may lead to significant decomposition of thermally labile compounds. A second solution to this problem is the efficient use of on-column focusing of mid- and high- boiling point components. Due to their lower vapor pressure, mid- and high-boiling point compounds condense at the mouth of a cold (30°C) column. This condensation results in a much narrower injection plug, which can be released through fast temperature programming of the analytical column. Results and analysis of the benefits and compromises of each solution to the problem will be presented.
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