Non-Metals
135
dissolved in boiling 6 mol I-I hydrochloric acid. This solution was analysed for selenium on a hanging mercury drop polarographic analyser and copper, lead, and
cadmium were determined in the anodic scanning voltammetry mode using the peaks
appearing at -0.20, -0.5, and -0.7 V vs SeE respectively. Results (mg kg-I) obtained by
this method for crayfish are in good agreement with certified values (reference values
in parenthesis); selenium 0.17 (0.16), copper 3.46 (3.10), lead 0.48 (0.48 ), and cadmium 0.10 (0.05). Relative standard deviations in determinations of selenium, copper,
lead, and cadmium were respectively 12, 5, 15 and 20 %.
Neutron Activation Analysis. This technique has been applied to the determination in
fish of cobalt, chromium, selenium, silver, rubidium, nickel, and zinc [394], and
aluminium, gold, bromine, calcium, chlorine, cobalt, chromium, copper, iron, iodine,
potassium, magnesium, manganese, sodium, rubidium, scandium, vanadium, and
tungsten [395].
Secondary Ion Mass Spectrometry and X-ray Spectrometry. This technique can be
used [396] to provide simultaneous morphological and chemical identifications in
histological sections of fish, molluscs, and crustaceans.
7.2
Non-Metals
Halogens. White [397] developed methods for the detection of trace amounts of
organic halogens in fish. Three fractions (lipid-soluble material, cationic watersoluble molecules, and large macromolecules) were assayed for each sample, the
inorganic halogens being monitored by means of radioactive chloride and radioactive
iodide. Fractions were burned in an oxygen combustion tube, and the resulting
adsorbed halides were assayed on two automatic analytical instruments, chloride and
bromide on one instrument and iodide on the other.
7.2.7
Total Nitrogen
In the Kjeldahl method [398, 399] for the determination of total nitrogen in fish, the
sample is digested with concentrated sulphuric acid, with copper(lI) sulphate added
as a catalyst, in order to convert organic nitrogen into ammonium ion; alkali is added,
and the liberated ammonia is distilled. The ammonia is adsorbed in boric acid and
titrated with hydrochloric acid.
Nitrogen Derived from Total Volatile Bases. This includes the nitrogen content of
those volatile bases which do not react with formaldehyde [399]. Extracts or solutions
are made alkaline with sodium hydroxide in a suitable semi-micro stream distillation
apparatus. The bases, including trimethylamine, so liberated are steam distilled quantitatively into standard acid and the excess of acid is then back titrated with standard
alkali. Formaldehyde is added to the neutralised mixture in order to render amines
other than trimethylamine non-reactive, and the acid released, being equivalent to the
non-reactive amines, is titrated with standard alkali.
135
dissolved in boiling 6 mol I-I hydrochloric acid. This solution was analysed for selenium on a hanging mercury drop polarographic analyser and copper, lead, and
cadmium were determined in the anodic scanning voltammetry mode using the peaks
appearing at -0.20, -0.5, and -0.7 V vs SeE respectively. Results (mg kg-I) obtained by
this method for crayfish are in good agreement with certified values (reference values
in parenthesis); selenium 0.17 (0.16), copper 3.46 (3.10), lead 0.48 (0.48 ), and cadmium 0.10 (0.05). Relative standard deviations in determinations of selenium, copper,
lead, and cadmium were respectively 12, 5, 15 and 20 %.
Neutron Activation Analysis. This technique has been applied to the determination in
fish of cobalt, chromium, selenium, silver, rubidium, nickel, and zinc [394], and
aluminium, gold, bromine, calcium, chlorine, cobalt, chromium, copper, iron, iodine,
potassium, magnesium, manganese, sodium, rubidium, scandium, vanadium, and
tungsten [395].
Secondary Ion Mass Spectrometry and X-ray Spectrometry. This technique can be
used [396] to provide simultaneous morphological and chemical identifications in
histological sections of fish, molluscs, and crustaceans.
7.2
Non-Metals
Halogens. White [397] developed methods for the detection of trace amounts of
organic halogens in fish. Three fractions (lipid-soluble material, cationic watersoluble molecules, and large macromolecules) were assayed for each sample, the
inorganic halogens being monitored by means of radioactive chloride and radioactive
iodide. Fractions were burned in an oxygen combustion tube, and the resulting
adsorbed halides were assayed on two automatic analytical instruments, chloride and
bromide on one instrument and iodide on the other.
7.2.7
Total Nitrogen
In the Kjeldahl method [398, 399] for the determination of total nitrogen in fish, the
sample is digested with concentrated sulphuric acid, with copper(lI) sulphate added
as a catalyst, in order to convert organic nitrogen into ammonium ion; alkali is added,
and the liberated ammonia is distilled. The ammonia is adsorbed in boric acid and
titrated with hydrochloric acid.
Nitrogen Derived from Total Volatile Bases. This includes the nitrogen content of
those volatile bases which do not react with formaldehyde [399]. Extracts or solutions
are made alkaline with sodium hydroxide in a suitable semi-micro stream distillation
apparatus. The bases, including trimethylamine, so liberated are steam distilled quantitatively into standard acid and the excess of acid is then back titrated with standard
alkali. Formaldehyde is added to the neutralised mixture in order to render amines
other than trimethylamine non-reactive, and the acid released, being equivalent to the
non-reactive amines, is titrated with standard alkali.