Figure 4.27
Graph of log adjusted retention times of n-alkanes
versus carbon number. The open circles show log
retention times of n-alkane standards. The retention
of an unknown (full circle) can be determined from
the graph as shown.Quantitative Analysis
Gas chromatography has become one of the most useful separation techniques because quantitative
information can so readily be obtained from it. Standardization of operating conditions is of prime
importance and detector response factors must be known for each compound to be determined. The
integrated area of a peak is directly proportional to the amount of solute eluted. Peak height can be used
as a measure of peak area, but compared to an area measurement the linear range is less and it is more
sensitive to changes in operating conditions. A more reliable method is to multiply the peak height by
the retention time as these parameters are inversely related. Heights are measured by drawing
perpendiculars from the peak maxima to the baseline or projected baseline if there is drift or the peaks
to be measured overlap. Measurement of area is accomplished by one of the following methods, which
vary considerably in precision (Table 4.12).
Geometric Methods
As normal peaks have a Gaussian profile, which approximates to an isosceles triangle, their area can be
estimated by multiplying the height by
Table 4.12 Precision of methods of peak area measurement
Method of measurement Relative precision, %
computing integrator 0.4
cutting out and weighing peaks 1.7
height × weight at 1/2 height 2.6
1/2 base × height 4.0