Page 364 - The Veterinary Laboratory and Field Manual 3rd Edition

Clinical chemistry 333

Figure 7.3 The electromagnetic spectrum.

Figure 7.4 The visible part of the electromagnetic spectrum.

It has been shown that, under appropriate the concentration of an unknown test solution
conditions, when illuminated with light of a suit- can be determined as follows.
able wavelength the amount of light absorbed
by a coloured solution is directly proportional Concentration of an unknown solution =
to the concentration of the coloured solution absorbance of test solution × conc. of ref-
and the length of the light path through the erence divided by the absorbance of the
solution. reference solution
The amount of light absorbed by a substance
in solution equates to absorbance (or optical For example, if the reference solution has a con-
density [OD]) and this depends on (1) the light centration of 20 mg/ml and an absorbance value
path and (2) the wavelength(s) used. The light of 250 (that is, the OD reading), an unknown
path is usually kept constant for a particular test solution with an absorbance value of 240
piece of equipment by choosing good quality has a concentration of 19.2 mg/ml.
optically matched cuvettes (Figure 7.5).
Essentially, the concentration of the reference 20 mg/ml is to 250 what Z mg/ml is to
solution determines the amount of light absorbed 240
by the reference solution (= absorbance). Using
this principle, and a standard reference solution, So Z = 20 × 240/250 = 19.2 mg/ml

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