Gas Chromatography Gc

GC is one of the most widely used analyzers in the petrochemical and refining industry. It offers flexibility of applications, high sensitivity analysis, and multicomponent analysis. The widespread use of GC is a result of its versatility. If a sample can be vaporized, an effective separation is often possible.

Process GC sampling is extractive. A small sample of process material is obtained with a sample valve and vaporized in a preheater. The sample is pushed by an inert carrier through a packed solid capillary tube. Components within the sample have varying degrees of affinity for the column packing. The more the component is attracted to the column packing, the slower it moves. As the components come out of the column (that is, are eluted), they are recorded as a function of time by a detector. Depending upon the sensitivity of the detector (see Figure 3.12.3) and separation quality of the instrument, GCs routinely achieve accuracy within 0.25 to 2%. The accuracy of most analyzers is a fixed percentage of their full-scale range.

GC offers continuous results, but the retention time of the sample (i.e., the time taken for the separated components to pass down the column) must be considered. Typical retention times range between 1 and 20 min, depending on the number of components and their vaporization temperatures. A retention longer than 20 min is impractical and generally unacceptable for online analysis.

A big drawback of GC is maintenance. Instruments generally have many electrical and mechanical components. Preventive maintenance is a must. Repairs are not always fast or easy. Repairing a chromatograph can take hours or even days.

FIG. 3.12.3 Basic schematic of a chromatograph.
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