Page 273 - Vogel's TEXTBOOK OF QUANTITATIVE CHEMICAL ANALYSIS

OUANTITATIVE ANALYSIS BV CLC 9.4

study of mixtures of unknown composition and wide boiling point range;
the solubilities of the higher-boiling substances in the stationary phase are
so large that they are almost completely immobilised at the inlet to the
column, especially where the latter is operated at a relatively low temperature.
The above consequences of isothermal operation rnay be largely avoided by
using the technique of programmed-temperature gas chromatography (PTGC)
in which the temperature of the whole column is raised during the sample
analysis.
A temperature programme consists of a series of changes in column
temperatures which rnay be conveniently selected by a microprocessor controller.
The programme commonly consists of an initial isothermal period, a linear
temperature rise segment, and a final isothermal period at the temperature which
has been reached, but rnay Vary according to the separation to be effected.
The rate of temperature rise, which rnay Vary over a wide range, is a compromise
between the need for a slow rate of change to obtain maximum resolution and
a rapid change to minimise analysis time.
Programmed-temperature gas chromatography permits the separation of
compounds of a very wide boiling range more rapidly than by isothermal
operation of the column. The peaks on the chromatogram are also sharper and
more uniform in shape so that, using PTGC, peak heights rnay be used to obtain
accurate quantitative analy~is.~~

9.4 QUANTITATIVE ANALYSIS BY GLC
The quantitative determination of a component in gas chromatography using
differential-type detectors of the type previously described is based upon
measurement of the recorded peak area or peak height; the latter is more suitable
in the case of small peaks, or peaks with narrow band width. In order that these
quantities rnay be related to the amount of solute in the sample two conditions
must prevail:
(a) the response of the detector-recorder system must be linear with respect
to the concentration of the solute;
(b) factors such as the rate of carrier gas flow, column temperature, etc., must
be kept constant or the effect of variation must be eliminated, e.g. by use
of the interna1 standard method.
Peak area is commonly used as a quantitative measure of a particular
component in the sample and can be measured by one of the following
techniques.
1. Planimetry. The planimeter is a mechanical device which enables the peak
area to be measured by tracing the perimeter of the peak. The method is slow
but can give accurate results with experience in manipulation of the planimeter.
Accuracy and precision, however, decrease as peak area diminishes.
2. Geometrical methods. In the so-called triangulation methods, tangents are
drawn to the inflection points of the elution peak and these two lines together
with the baseline form a triangle (Fig. 9.3); the area of the latter is calculated
as one-half the product of the base length times the peak height, the value
obtained being about 97 per cent of the actual area under the chromatographic
peak when this is Gaussian in shape.