Page 36 - Vogel's TEXTBOOK OF QUANTITATIVE CHEMICAL ANALYSIS

1 INTRODUCTION

so that atoms are raised to excited States causing them to emit energy:
it is the intensity of this emitted energy which is measured. The common
excitation techniques are:
(a) emission spectroscopy, where the sample is subjected to an electric arc or
spark plasma and the light emitted (which may extend into the ultraviolet
region) is examined;
(b) jame photometry, in which a solution of the sample is injected into a flame;
(c) juorimetry, in which a suitable substance in solution (commonly a metal-
fluorescent reagent complex) is excited by irradiation with visible or
ultraviolet radiation.
Chromatography is a separation process employed for the separation of
mixtures of substances. It is widely used for the identification of the components
of mixtures, but as explained in Chapters 8 and 9, it is often possible to use
the procedure to make quantitative determinations, particularly when using
Gas Chromatography (GC) and High Performance Liquid Chromatography
(HPLC).

1.7 INSTRUMENTAL METHODS
The methods dependent upon measurement of an electrical property, and those
based upon determination of the extent to which radiation is absorbed or upon
assessment of the intensity of emitted radiation, al1 require the use of a suitable
instrument, e.g. polarograph, spectrophotometer, etc., and in consequence such
methods are referred to as 'instrumental methods'. Instrumental methods are
usually much faster than purely chemical procedures, they are normally
applicable at concentrations far too small to be amenable to determination by
classical methods, and they find wide application in industry. In most cases a
microcomputer can be interfaced to the instrument so that absorption curves,
polarograms, titration curves, etc., can be plotted automatically, and in fact, by
the incorporation of appropriate servo-mechanisms, the whole analytical process
may, in suitable cases, be completely automated.
Despite the advantages possessed by instrumental methods in many directions,
their widespread adoption has not rendered the purely chemical or 'classical'
methods obsolete; the situation is influenced by three main factors.
1. The apparatus required for classical procedures is cheap and readily available
in al1 laboratories, but many instruments are expensive and their use will
only be justified if numerous samples have to be analysed, or when dealing
with the determination of substances present in minute quantities (trace,
subtrace or ultratrace analysis).
2. With instrumental methods it is necessary to carry out a calibration operation
using a sample of material of known composition as reference substance.
3. Whilst an instrumental method is ideally suited to the performance of a large
number of routine determinations, for an occasional, non-routine, analysis
it is often simpler to use a classical method than to go to the trouble of
preparing requisite standards and carrying out the calibration of an
instrument.
Clearly, instrumental and classical methods must be regarded as supplementing
each other.