Page 363 - The Veterinary Laboratory and Field Manual 3rd Edition

332 Susan C. Cork, Willy Schauwers and Roy Halliwell

There are two main types of colorimetry sys- Many substances are either coloured in solution
tem, these can be classified as ‘open’ or ‘closed’. or can be altered to produce a coloured derivative.
Open systems may be adapted for different types In most cases the intensity of the colour will be
of test procedure and are useful in a veterinary related to the amount of substance in the solution.
laboratory specializing in health screening and In colorimetry, the colour intensity of a solution,
survey work. Closed systems are designed to containing an unknown amount of the test sub-
be sold as compact units with specified reagent stance, is compared with a reference or standard
kits provided with each system. The advantage solution which represents or contains a known
of the closed systems is that they are very easy amount of the substance of interest. Colorimetry
to use. The disadvantage is the expense and the is based on the principle that a coloured solution
restriction of having a predetermined set of test can absorb light at a given wavelength in the vis-
profiles offered. ible spectrum (Figure 7.2) and that the extent
to which this occurs is dependent on the colour
intensity. To appreciate the concept of colorim-
7.4 Colorimetry explained etry it is necessary to have an understanding of
light energy. The source of light energy is elec-
The following section has been added to provide tron activity. Light energy is emitted in waves of
some background to the subject of colorimetry varying lengths and these are measured in nano-
with the aim of allowing the laboratory worker metres (nm = 10 m) depending on the distance
–9
to recognize the potential limitations of colori- between peaks and troughs (Figure 7.3).
metric tests and to be able to see the importance
of calibration, quality assurance and monitoring
to ensure that test results are valid. colour and light absorption

Light may be absorbed, reflected or transmitted
the scientific principles when it falls on a coloured solution in a tube.
A coloured liquid seems to appear coloured
Most quantitative or semi-quantitative methods because it transmits a particular wavelength of
used in clinical biochemistry are colorimetric. light from the visible spectrum (Figure 7.4).

Figure 7.2 The principle of wavelength.

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