Page 275 - Vogel's TEXTBOOK OF QUANTITATIVE CHEMICAL ANALYSIS

ELEMENTAL ANALYSIS USINC CAS CHROMATOCRAPHV 9.5

simple calibration with standards is prone to errors (e.g. arising from the use
of a microsyringe with a conventional injection port), and is not generally used;
the commonly used methods are those of area normalisation and internal
standards which allow for variations in the size of sample, etc. In area
normalisation, the composition of the mixture is obtained by expressing the
area of each individual peak as a percentage of the total area of al1 the peaks
in the chromatogram; correction should be made for any significant variation
in sensitivity of the detector for the different components of the mixture (see
Section 9.7).

Quantitative analjwis using the internal standard method. The height and area
of chromatographic peaks are affected not only by the amount of sample but
also by fluctuations of the carrier gas flow rate, the column and detector
temperatures, etc., i.e. by variations of those factors which influence the sensitivity
and response of the detector. The effect of such variations can be eliminated by
use of the internal standard method in which a known amount of a reference
substance is added to the sample to be analysed before injection into the column.
The requirements for an effective internal standard (Section 4.5) may be
summarised as follows:
(a) it should give a completely resolved peak, but should be eluted close to the
components to be measured;
(b) its peak height or peak area should be similar in magnitude to those of the
components to be measured; and
(c) it should be chemically similar to but not present in the original sample.
The procedure comprises the addition of a constant amount of internal
standard to a fixed volume of several synthetic mixtures which contain varying
known amounts of the component to be determined. The resulting mixtures are
chromatographed and a calibration curve is constructed of the percentage of
component in the mixtures against the ratio of component peak arealstandard
peak area. The analysis of the unknown mixture is carried out by addition of
the same amount of internal standard to the specified volume of the mixture;
from the observed ratio of peak areas the solute concentration is read off using
the calibration curve.
Provided a suitable internal standard is available, this is probably the most
reliable method for quantitative GLC. For example, the concentration of ethanol
in blood samples has been determined using propan-2-01 as the internal standard.
Standard addition. The sample is chromatographed before and after the
addition of an accurately known amount of the pure component to be
determined, and its weight in the sample is then derived from the ratio of its
peak areas in the two chromatograms. Standard addition is particularly useful
in the analysis of complex mixtures where it may be difficult to find an internal
standard which meets the necessary requirements.

9.5 ELEMENTAL ANALYSIS USlNG GAS CHROMATOGRAPHY
An important application of gas chromatography is its use for determination
of the elements carbon, hydrogen, nitrogen, oxygen and sulphur in organic and
organometallic samples. A brief account of the procedure used is given here,