StationaryPhases
29
7.
Assortment of Sensitive Detecting Systems. Detectors used in gas chromatography are
relatively simple,withhigh sensitivity and fast responses given.
8.
Ease of RecordingData. Output of the gas chromatographdetector can be conveniently
connected to a potentiometer recorder, or integrating systems, computers that can store
a large amount of information.
9.
Automatization. Gas chromatographs can be used to automatically monitor various
chemical processes that allow samples to be taken periodically and injected into the
chromatographic columns tobe separatedanddetected.
An ideal stationary phase is selective and has different adsorptivity for each sample
component in order to ensure separation, aswell as awide array of operating temperatures.
It has to be chemically stable and have a low vapour pressure at high operating
temperatures. Some criteria for selecting anadequate stationaryphase are:
Is the stationary phase selective enough in separating the sample components so as to
separate themonebyone?
Can there be an irreversible chemical reaction and can the mixture components be
separated?
Does the liquidphasehave avapour pressure low enough at operating temperature?
Is it thermally stable?
For instance, we quote a separability criterion of sample components known as “component
sympathy” for the fixedphase.
Thus, a light n-paraffin mixture with “sympathy” for non-polar stationary phase will
separate on grounds of different boiling points; olefins, being polarisable, will show
“sympathy” towards a polar stationary phase. The cis- and trans- components of an olefin
mixture will separate on a stationary phase consisting of a complex of transitional metals,
dissolved in anappropriate solvent, such as polyethyleneglycol.
Identifyingmixture components canbe accomplishedusing chromatographic etalons –pure
substances or known mixtures of components. In these cases, universal detectors may be
used (FID, WCD). For unknown mixtures, mass spectrometry is recommended. For the
universal detectorWCD, only non-fuel componentsmay be used, and for the FID, only fuel
components that donot interact chemicallywith the stationaryphasemaybeused.
The quantitative analysis of a given component is based upon evaluating the
chromatographic peak, which is triangle-shaped when columns with filling are used; its
surface is measured and divided by the total surface, in different percentages for different
types of detectors.
For capillary columns with good resolution, the signal takes the shape of straight lines and
calculating the composition of themixture is done in the order of succession, dividing each
individual line by the total number of lines andusing an adequate calibration curve, drawn
upondeterminations of known compounds.
The results can be expressed as a percentage, g/L component in a known mixture,
dependingon the typeof calibrationor the calibrationunits.
Thermal stability is of great importance, since operating under severe circumstances can
lead to short analysis intervals and incomplete use of the chromatographic column; for
