and to construct a baseline (Figure 9.25) for all standards and samples. The baseline absorbance is
measured at the point where a perpendicular line from the peak maximum intersects the baseline and
this absorbance is subtracted from the peak absorbance in all cases.
Figure 9.25
Construction of a baseline for
quantitative analysis.Most cells used in infrared spectrometry have sodium chloride windows and the path length is likely to
vary with use because of corrosion. For quantitative work, therefore, the same cell should be used for
samples and standards. In general, quantitative analysis in the infrared region of the spectrum is not
practised as widely as in the ultraviolet and visible regions, partly because of the additional care
necessary to obtain reliable results and partly because the technique is generally considered to be less
sensitive and less precise; a precision of 3–8% can be expected.
Sampling Procedures
Solid samples can be handled in a number of ways. A mull or paste is made by grinding a few
milligrams of sample in an agate mortar with a drop of a liquid paraffin (e.g. Nujol) or
hexachlorobutadiene. The paste is formed into a thin film by compressing it between sodium chloride or
potassium bromide plates which are mounted in a suitable sample holder. A complete spectrum of the
sample can be obtained from separate mulls in Nujol and hexachlorobutadiene. The C—H bands in the
former obscure the sample bands in that region but these can be observed in the spectrum of the
chlorinated butadiene mull. An alternative procedure is to grind a few milligrams of sample with about
200 mg of dried potassium bromide and to compress the mixture into a thin, homogeneous disk using a
hydraulic press. Liquid samples can be examined neat as a thin film between sodium chloride or
potassium bromide plates or as a solution in a cell of path-length 0.1–10 mm. Three solvents in
widespread use, and which are largely transparent in the infrared region, are carbon tetrachloride,
chloroform and carbon disulphide. Spectra of the surface of solids and liquid samples can also be
obtained either by attenuated total reflectance (ATR) or by diffuse reflectance. The former involves
placing the sample in contact with the