Recall that the purpose of any analytical instrument is to discriminately measure one variable or component concentration amongst a mix of different chemical components. The fundamental challenge of chemical analysis is how to detect that one “species” of chemical while reliably and accurately being unaffected by the presence of any other species of chemical. Most analyzers rely on a sensor that exploits some unique characteristic of the chemical of interest (its unique ability to pass through a special membrane, its ability to absorb a unique spectrum of light, its tendency to engage in a unique chemical reaction, etc.), but not chromatographs. Chromatographs simply delay the flow rate of different chemical species through a long tube based on those species’ mass and interactions with the stationary phase, using time as the discriminating variable. This allows chromatographs to be nearly universal in the range of chemical species they can measure while using relatively simple and uncomplicated detectors. The detector need only discriminate between carrier gas and anything that isn’t carrier gas to serve its purpose in a gas chromatograph.
In general, chemical compounds having less molecular weight tend to exit the column earlier, while compounds having greater molecular weight exit the column later. The precise sequence of species elution through a column depends greatly on the stationary phase material type, as well as the carrier fluid type. Proper selection of stationary phase and carrier compounds is essential to efficient chromatograph operation and is usually the domain of trained chemists.
Since species identification in a chromatograph is performed with time as the discriminating factor, a chromatograph’s ability to accurately identify chemical compounds depends on its control computer “knowing” when to expect various compounds to exit the column. Chromatographs are calibrated by injecting a sample containing known concentrations of the components of interest (as well as any potentially interfering species), then timing the beginnings and ends of each component peak as each substance exits the column.
Instrumentation Basics 