Page 38 - Vogel's TEXTBOOK OF QUANTITATIVE CHEMICAL ANALYSIS

1 INTRODUCTION

equipment: each kind of ion gives a peak in the mass spectrum. Non-volatile
inorganic materials can be examined by vaporising them by subjecting them
to a high-voltage electric spark.
Mass spectrometry can be used for gas analysis, for the analysis of petroleum
products, and in examining semiconductors for impurities. It is also a very useful
tool for establishing the structure of organic compounds.

Kinetic methods. These methods of quantitative analysis are based upon the
fact that the speed of a given chemical reaction may frequently be increased by
the addition of a small amount of a catalyst, and within limits, the rate of the
catalysed reaction will be governed by the amount of catalyst present. If a
calibration curve is prepared showing variation of reaction rate with amount
of catalyst used, then measurement of reaction rate will make it possible to
determine how much catalyst has been added in a certain instance. This provides
a sensitive method for determining sub-microgram amounts of appropriate
substances.
The method can also be adapted to determine the amount of a substance in
solution by adding a catalyst which will destroy it completely, and measuring
the concomitant change in for example, the absorbance of the solution for visible
or ultraviolet radiation. Such procedures are applied in clinical chemistry.

Optical methods. Those of particular application to organic compounds are:

1. Use of a refractometer to make measurements of the refractive index of liquids.
This will often provide a means of identifying a pure compound, and can
also be used (in conjunction with a calibration curve) to analyse a mixture
of two liquids.
2. Measurement of the optical rotation of optically active compounds.
Polarimetric measurements can likewise be used as a method of identifying
pure substances, and can also be employed for quantitative purposes.

Thermal methods. Changes in weight, or changes in energy, recorded as a
function of temperature (or of time) can provide valuable analytical data. For
example, the conditions can be established under which a precipitate produced
in a gravimetric determination can be safely dried. Common techniques include
the recording as a function of temperature or time of (a) change in weight
(Thermogravimetry, TG); (b) the difference in temperature between a test
substance and an inert reference material (Differential Thermal Analysis, DTA);
(c) the energy necessary to establish a zero temperature difference between a
test substance and a reference material (Differential Scanning Calor~metry,
DSC).

1.9 FACTORS AFFECTING THE CHOICE OF ANALYTICAL
METHODS
An indication has been given in the preceding sections of a number of techniques
available to the analytical chemist. The techniques have differing degrees of
sophistication, of sensitivity, of selectivity, of cost and also of time requirements,
and an important task for the analyst is the selection of the best procedure for