The formula assumes that the detector sensitivity is the same for each component. If this is not the case,
the response of each must first be determined using a set of standards. Areas are then multiplied by
correction factors obtained by setting the response of one component equal to unity.
Internal Standardization
An accurately known amount of a standard is added to the sample before it is chromatographed. The
ratio of peak area of standard to that of the component of the sample to be determined is calculated.
This ratio is converted to weight of component using a previously prepared calibration curve (p. 9). The
internal standard should have a retention time close to those of the components being determined but
well resolved from them. Preferably it should be present at a similar concentration level.
Standard Addition
If a pure sample of the component to be determined is available, the sample can be chromatographed
before and after the addition of an accurately known amount of the pure component. Its weight in the
sample is then derived from the ratio of its peak areas in the two chromatograms.
The advantages of internal standardization are that the quantities of sample injected need not be
measured accurately and the detector response need not be known, as neither affect the area ratios.
Standard addition is particularly useful in the analysis of complex mixtures where it may be difficult to
find a suitable internal standard which can be adequately resolved from the sample components.
Combination of Gas Chromatography with Other Analytical Techniques
The identification of GC peaks other than through retention data, which are sometimes ambiguous or
inconclusive, can be facilitated by the direct interfacing of GC with infrared spectrometry (p. 378 et
seq.) or mass spectrometry (p. 426 et seq.), so-called 'coupled' or 'hyphenated' techniques. The general
instrumental arrangement is shown in Figure 4.29(a).
GC-Mass Spectrometry
Identification of separated components can be achieved by feeding the effluent gases from a GC column
directly to a mass spectrometer. The interfacing of the two instruments presents difficulties in that the
mass spectrometer operates at very low pressures (10–^7 to 10–^9 N m–^2 ) whereas gas emerging from the
column is at atmospheric pressure. For packed columns, a jet-orifice separator (Figure 4.29(b)) enables
the carrier gas (usually helium) to be pumped away whilst allowing the sample vapours to pass on into
the mass spectrometer. It is constructed of glass and consists of two fine-bore jets separated by a 0.5– 1
mm gap and surrounded by an evacuated tube. As the GC effluent is passed through the separator, the
momentum of the relatively large and heavy sample molecules carries them