As stated earlier, the survey of the Irish Sea required analysis
of 8,000 fish samples over a period of 15 months, so it was
highly desirable to use a relatively simple digestion procedure
involving one reagent, and the simplest possible glassware.
Extensive analysis of biological samples at Liverpool
University indicated that simple digestion by nitric acid was
adequate, without the prolonged refluxing and volume reduction
practised by other workers using just nitric acid, (Ministry
of Agriculture, Food and Fisheries – MAFF internal method).
This method, described in Table 1 and now in routine use, has
been tested by spiking samples with mercury and by inter-laboratory
cross-calibration studies. A long sequence of quality
control, reference tissue analyses was used to confirm its
reliability (see Table 4).
Recoveries of mercury from spiked fish samples were
92–102%. The inter-laboratory calibration study, with the
Ministry of Agriculture, Food and Fisheries laboratory at
Burnham on Crouch, Essex, centred on 21 samples from three
marine and one freshwater fish species. The 21 samples contained
varying amounts of fat or oil and muscle mercury concentrations
in the range 0.13–1.27 mg kg-1 fresh weight. The
results showed recoveries in the range of 95–104% of those
obtained by MAFF, the mean recovery being 96.2%. These
recoveries are consistent with the standards of interlaboratory
calibration for mercury analysis – Munns and Holland [6],
and are acceptable when compared with other studies – Uthe
et al. [8]. The lowest recoveries were associated with fish
containing more oil/fat than average.
Figure 3.