Metals
131
Nitric Acid-Perchloric Acid Digestions [386]. To carry out this digestion, 0.5-3 g of
freeze dried fish ball mill ground sample is digested in a silica flask with 10-20 ml
concentrated nitric acid, then 5-10 ml of 1: 1 nitric: perchloric acid to dryness. The
residue is dissolved in dilute hydrochloric acid-nitric acid and adjusted to pH 8 with
ammonia. This solution is extracted with a 0.02 % solution of dithizone in chloroform. Metals are then back extracted from the organic phase with 2 moll-I hydrochloric acid prior to atomic absorption spectrometry. Using this method the following
values (mg kg-I) were obtained for a NBS reference kale sample (nominal values in
parentheses); cadmium 0.9 (O.84), zinc 29.9 (31.8), lead 2.6 (3.2), copper 4.2 (4.9), and
cobalt 0.05 (O.056). Concentrations (mg kg-I) metals found in whale tissues were; zinc
26-103, lead 0.45-1.37, copper 1.2-7.6, nickel 0.17-0.60, cobalt 0.07-0.38, silver 0.020.04, and cadmium, not detected. Kale brought from Iceland contained the following
concentrations, zinc 39, lead 0.89, copper 2.6, nickel 0.34, cobalt 0.14, silver 0.04 mg
kg-I, and cadmium, not detected.
Nitric Acid-Hydrogen Peroxide Digestions. Van Hoos and Van San [387] worked on
fish samples that had been calcined at 450°C prior to digesting the ash in 2.5 : 1 v / v
14N nitric acid: 30 % hydrogen peroxide. Elements determined included copper,
zinc, cadmium, and chromium. Low recoveries of at least some of these elements
would be expected under these conditions.
Borg et al. [389] digested 10 mg freeze dried fish livers with concentrated nitric acid
at 50°C for 2 h in quartz tubes, then slowly raised the temperature to 110°C over 18 h.
Hydrogen peroxide (30 %) is added to the cooled samples which are again heated to
110°C for 6 h to digest fats completely. When made up to a standard volume, this
digest was used for the determination of copper, lead, cadmium, and zinc by graphite
furnace atomic absorption spectrometry. Table 7.6 compares results on fish livers
obtained by this procedure, and by neutron activation analysis. The high metal
concentrations found in the livers reflect that the fish were taken in an area subject to
heavy contamination originating from ore smelting activities.
Nitric Acid Bomb Digestion. Ramelow et al. [385] determined cadmium, lead, copper,
manganese, zinc, and chromium in wet fish by digestion of 0.5-1.0 g sample with
2-3 ml concentrated nitric acid in a Teflon lined bomb at 150°C for 1.5 h. Elements
were determined in the digest by flame atomization or graphite furnace atomization
atomic absorption spectrometry. Concentrations found in whole fish in an unpolluted
area are shown in Table 7.7, as opposed to concentrations found in fish livers in a
polluted area (Table 7.6).
Intercomparison Studies. The International Council for the Exploration of the Sea
has arranged series of intercomparison studies of the determination of trace elements
in fish by techniques based on atomic absorption spectrometry. A summary of the
more recent test results is given in Table 7.8.
Despite the large number of participants in the fourth exercise, the results for the
analysis of copper, zinc, and mercury demonstrated that most analysts were continuing to produce reasonably comparable and accurate data for these metals at levels
typical of those found in fish muscle and shellfish tissue. The results for mercury were
particularly good in view of the relatively low concentrations present.
131
Nitric Acid-Perchloric Acid Digestions [386]. To carry out this digestion, 0.5-3 g of
freeze dried fish ball mill ground sample is digested in a silica flask with 10-20 ml
concentrated nitric acid, then 5-10 ml of 1: 1 nitric: perchloric acid to dryness. The
residue is dissolved in dilute hydrochloric acid-nitric acid and adjusted to pH 8 with
ammonia. This solution is extracted with a 0.02 % solution of dithizone in chloroform. Metals are then back extracted from the organic phase with 2 moll-I hydrochloric acid prior to atomic absorption spectrometry. Using this method the following
values (mg kg-I) were obtained for a NBS reference kale sample (nominal values in
parentheses); cadmium 0.9 (O.84), zinc 29.9 (31.8), lead 2.6 (3.2), copper 4.2 (4.9), and
cobalt 0.05 (O.056). Concentrations (mg kg-I) metals found in whale tissues were; zinc
26-103, lead 0.45-1.37, copper 1.2-7.6, nickel 0.17-0.60, cobalt 0.07-0.38, silver 0.020.04, and cadmium, not detected. Kale brought from Iceland contained the following
concentrations, zinc 39, lead 0.89, copper 2.6, nickel 0.34, cobalt 0.14, silver 0.04 mg
kg-I, and cadmium, not detected.
Nitric Acid-Hydrogen Peroxide Digestions. Van Hoos and Van San [387] worked on
fish samples that had been calcined at 450°C prior to digesting the ash in 2.5 : 1 v / v
14N nitric acid: 30 % hydrogen peroxide. Elements determined included copper,
zinc, cadmium, and chromium. Low recoveries of at least some of these elements
would be expected under these conditions.
Borg et al. [389] digested 10 mg freeze dried fish livers with concentrated nitric acid
at 50°C for 2 h in quartz tubes, then slowly raised the temperature to 110°C over 18 h.
Hydrogen peroxide (30 %) is added to the cooled samples which are again heated to
110°C for 6 h to digest fats completely. When made up to a standard volume, this
digest was used for the determination of copper, lead, cadmium, and zinc by graphite
furnace atomic absorption spectrometry. Table 7.6 compares results on fish livers
obtained by this procedure, and by neutron activation analysis. The high metal
concentrations found in the livers reflect that the fish were taken in an area subject to
heavy contamination originating from ore smelting activities.
Nitric Acid Bomb Digestion. Ramelow et al. [385] determined cadmium, lead, copper,
manganese, zinc, and chromium in wet fish by digestion of 0.5-1.0 g sample with
2-3 ml concentrated nitric acid in a Teflon lined bomb at 150°C for 1.5 h. Elements
were determined in the digest by flame atomization or graphite furnace atomization
atomic absorption spectrometry. Concentrations found in whole fish in an unpolluted
area are shown in Table 7.7, as opposed to concentrations found in fish livers in a
polluted area (Table 7.6).
Intercomparison Studies. The International Council for the Exploration of the Sea
has arranged series of intercomparison studies of the determination of trace elements
in fish by techniques based on atomic absorption spectrometry. A summary of the
more recent test results is given in Table 7.8.
Despite the large number of participants in the fourth exercise, the results for the
analysis of copper, zinc, and mercury demonstrated that most analysts were continuing to produce reasonably comparable and accurate data for these metals at levels
typical of those found in fish muscle and shellfish tissue. The results for mercury were
particularly good in view of the relatively low concentrations present.