Page 371 - The Veterinary Laboratory and Field Manual 3rd Edition

340 Susan C. Cork, Willy Schauwers and Roy Halliwell

Chapter 3). Plasma levels of pepsinogen in rumi- Principles of chemical reactions
nants can also be used to assess the severity of
abomasal damage. To understand the principles behind biochemical
tests it is necessary to have a basic knowledge
and understanding of chemical reactions. There
7.6 General principles and are a wide range of text books on the subject
methodology of basic of biochemistry and it is a good idea to become
biochemical tests familiar with the periodic table of the elements
and the atomic weight of commonly used ele-
Most automated biochemistry systems use ments and compounds. Some useful texts are
quantitative methodology to determine the exact listed in the bibliography at end of this chapter.
amount of a given substance in a sample. Manual Basically, compounds and their constituents
tests may also give quantitative results but to (elements) are made up of atoms. Each atom is
ensure reliability it is very important to follow composed of neutral (neutrons), positive (pro-
the protocols exactly. In both systems, clean tons) and negative (electrons) subunits that
glassware and good quality water are required. determine the structure and reactivity of an
For accurate chemical analytical work or when element. An element is defined by its atomic
using new glassware it is advisable to soak it number, mass and position in the periodic
in 2–5% (20 ml/l) hydrochloric acid (HCl) fol- table (see Table 7.4), for example, carbon has
lowed by washing and rinsing in two changes of a nucleus containing six neutrons and six pro-
tap water and three changes of de-ionized water tons (Figure 7.10), an atomic number of six and
(see Chapter 2) before use. an atomic mass of (6 + 6) = 12. The number
The following section outlines the princi- of electrons depends on the ‘charged state’ of
ples behind common biochemical tests and the the element, carbon atoms may gain or lose
practical aspects which need to be considered. electrons depending on the compound that is
Quantitative and semi-quantitative analysis is formed. These characteristics determine how
commonly done using either of the following carbon atoms will behave in chemical reactions.
methods: The way in which elements combine to form
compounds, by the transfer or sharing of elec-
1 volumetric analysis, in which a substance in trons, is known as valency, for example:
solution is measured by titration
.
2 colorimetric techniques, by which a sub- Na + C + H + 3 O = NaHCO (sodium
2–
+
+
3
stance reacts with reagents to give a coloured bicarbonate)
product which is measured by an absorption + 2–
filter colorimeter/ spectrophotometer. 2H + O = H O (water)
2

Na + Cl = NaCl (salt)
+
–
The majority of modern routine biochemical
assays are based on chemical reactions that In these examples carbon (C) has a valence
produce coloured substances. Other methods, of +4, hydrogen (H) and sodium (Na) have a
including flame emission and liquid chromatog- valence of +1, chlorine (Cl) has a valence of –1
raphy will not be considered further here. and oxygen (O) has a valance of –2.
Some elements have more than one valance,

for example iron (Fe and Fe ). Elements are
3+
2+
composed of one type of atom but may combine

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