Gas Chromatography (GC) is a technique which separates a gas mixture to determine the presence and concentration of gases and impurities in a sample. Applied correctly, GC can measure down to ppb levels, making it well suited for use in high purity processes.
In GC, gas mixture components are separated by circulating a gas sample, using an inert carrier gas, into a flow-through circular tube known as a column. The different gas constituents are separated due to their interaction with the column material, which cause the different molecules in the sample to elute at different times. These specific retention times are detected by a sensor at the column exit, as the individual molecular properties of each gas cause it to travel through and exit at a different time.
The comparison of retention times allows users to qualitatively identify gas types by the order in which they elute from the column. If conditions are constant, a particular gas will elute with the same retention time, allowing specific gas types to be deduced from the area of the peak. In addition, the relative volume of each gas concentration can also be measured by the detector as each gas elutes from the column.
The conditions by which GC operates for a given application are invariably different and require individual optimization. The majority of GC analyzers are therefore preset at the factory, with application specific valve timings, flow and temperature settings and peak detection parameters.