In the present context, the term ‘automated gas chromatographs’ (autoGCs) refers to systems that are beginning to be used in air monitoring network stations to provide frequent, usually hourly, analysis of ambient air for VOCs, over monitoring periods of days to months. Typically, the hardware for an autoGC consists of a benchtop gas chromatograph equipped with: (1) a sample enrichment unit (preconcentrator) for VOCs; (2) non-specific detectors such as the flame ionization detector (FID), electron capture detector (ECD), or photoionization detector (PID); and (3) a data acquisition system for formatting, storage, retrieval and transmission of monitoring data. The preconcentrator can be single- or multi-stage. The most common configuration is the two-stage preconcentrator, as shown in Figure 6.1(a). This consists of: (i) a primary trap (usually a multi-adsorbent trap, sometimes cooled to assist in trapping) for VOCs, through which a known volume of sample air is passed; and (ii) a secondary trap on which VOCs are ‘focused’ before release into the GC. The primary trap is backflushed with an inert carrier gas to clear oxygen and other non-adsorbed gases and then heated to cause thermal desorption of VOCs to the secondary trap. The secondary trap may consist of a cooled inert surface such as silanized glass wool in a short section of deactivated fused silica capillary column, or a small volume of solid adsorbent in a narrow diameter tube. The objective for using the secondary trap is to deliver VOCs to the column in a small volume so that chromatographic resolution is limited only by the inherent resolving power of the GC column.
