164
Fish
79-56 % (dimethyllead) and 92-114 % (i. e. 103 ;:;; 11 %) (diethyllead). The relatively
low recovery of dimethyllead indicates that the dialkyllead remained bound to the
tissue, or existed in solution as a complex, unextractable with solvent. It was noticed
that there was a large Pb(II) peak in the fish sample containing spiked dimethyllead,
but such was not found in the standard which was run in parallel but without the
sample. Such low recovery was attributed to the decomposition of dialkyllead in the
fish matrix.
This method was applied to a number of fish samples (Table 7.15). For the first
time, the occurrence of triethyl- and diethyllead compounds was detected in fish.
Birnie and Hodges [495] have given details of a procedure for the determination of
down to om mg kg-I of ionic species of alkyl lead in marine organisms by solvent
extraction and differential pulse anodic stripping voltammetry. The sample is homogenized in the presence of a mixture of salts (lead nitrate, sodium benzoate, potassium iodide,
sodium chloride, EDTA) which effectively releases the di- and trialkyllead species present,
and facilitates their transfer into toluene as a preliminary to back extraction into dilute
nitric acid ready for differential pulse anodic stripping voltammetry. Recoveries were in
the range 70-90 % (Et 3 Pb+, Et 2 Pb 2 +, Melb+) to 10-40 % (Me 2 Pb 2 +).
7.4.3
Mercury
Spectrometric Method. Jones and Nickless [496] have described a dithizone spectrophotometric procedure for the measurement of trace concentrations of methylmercury salts in fish tissue. The application of a simple equation using absorbance measurements taken at two wavelengths cancels out small differences in excess of dithizone
arising between blank and sample, thus ensuring good precision in the range 0.14.0 ~g ml- I . Dithizone reacts with most organomercury salts of the type RHgX where
X is any anion. The extraction procedure used is based on that described by Westoo
[497] in which 40 g of fish is homogenized with water, and then concentrated hydrochloric acid and sodium chloride added. A benzene extract is centrifuged to
provide a clear phase which is then treated with aqueous 1 % cysteine solution. After
acidification, the aqueous phase is then again extracted with benzene, and to the
extract is added a benzene solution of dithizone. This extract is evaluated spectrophotometricallyat 628 and 475 nm. Extraction efficiencies were low (24-32 %) but reproducible. Methyl mercury contents found in tuna fish flesh ranged from 0.15 to
0.69 mg kg-I.
Atomic Absorption Spectrometry. Various workers have applied cold vapour atomic
absorption spectrometry to the determination of organomercury compounds in fish
[498-520]. Various methods have been used to decompose organomercury compounds prior to atomic absorption spectrometry, including digestion with acidic
potassium permanganate [519], sodium hydroxide [499], sulphuric acid-hydrogen
peroxide [505-507, 510], sulphuric acid-nitric acid [498, 511], hydrochloric acid
digestion-benzene extraction [509], hydrochloric acid digestion-toluene extraction
[517], and acidic cupric bromide digestion-toluene extraction [516]. Steam distillation
has also been used to isolate organomercury compounds from fish prior to determination by cold vapour atomic absorption spectrometry [518].
Fish
79-56 % (dimethyllead) and 92-114 % (i. e. 103 ;:;; 11 %) (diethyllead). The relatively
low recovery of dimethyllead indicates that the dialkyllead remained bound to the
tissue, or existed in solution as a complex, unextractable with solvent. It was noticed
that there was a large Pb(II) peak in the fish sample containing spiked dimethyllead,
but such was not found in the standard which was run in parallel but without the
sample. Such low recovery was attributed to the decomposition of dialkyllead in the
fish matrix.
This method was applied to a number of fish samples (Table 7.15). For the first
time, the occurrence of triethyl- and diethyllead compounds was detected in fish.
Birnie and Hodges [495] have given details of a procedure for the determination of
down to om mg kg-I of ionic species of alkyl lead in marine organisms by solvent
extraction and differential pulse anodic stripping voltammetry. The sample is homogenized in the presence of a mixture of salts (lead nitrate, sodium benzoate, potassium iodide,
sodium chloride, EDTA) which effectively releases the di- and trialkyllead species present,
and facilitates their transfer into toluene as a preliminary to back extraction into dilute
nitric acid ready for differential pulse anodic stripping voltammetry. Recoveries were in
the range 70-90 % (Et 3 Pb+, Et 2 Pb 2 +, Melb+) to 10-40 % (Me 2 Pb 2 +).
7.4.3
Mercury
Spectrometric Method. Jones and Nickless [496] have described a dithizone spectrophotometric procedure for the measurement of trace concentrations of methylmercury salts in fish tissue. The application of a simple equation using absorbance measurements taken at two wavelengths cancels out small differences in excess of dithizone
arising between blank and sample, thus ensuring good precision in the range 0.14.0 ~g ml- I . Dithizone reacts with most organomercury salts of the type RHgX where
X is any anion. The extraction procedure used is based on that described by Westoo
[497] in which 40 g of fish is homogenized with water, and then concentrated hydrochloric acid and sodium chloride added. A benzene extract is centrifuged to
provide a clear phase which is then treated with aqueous 1 % cysteine solution. After
acidification, the aqueous phase is then again extracted with benzene, and to the
extract is added a benzene solution of dithizone. This extract is evaluated spectrophotometricallyat 628 and 475 nm. Extraction efficiencies were low (24-32 %) but reproducible. Methyl mercury contents found in tuna fish flesh ranged from 0.15 to
0.69 mg kg-I.
Atomic Absorption Spectrometry. Various workers have applied cold vapour atomic
absorption spectrometry to the determination of organomercury compounds in fish
[498-520]. Various methods have been used to decompose organomercury compounds prior to atomic absorption spectrometry, including digestion with acidic
potassium permanganate [519], sodium hydroxide [499], sulphuric acid-hydrogen
peroxide [505-507, 510], sulphuric acid-nitric acid [498, 511], hydrochloric acid
digestion-benzene extraction [509], hydrochloric acid digestion-toluene extraction
[517], and acidic cupric bromide digestion-toluene extraction [516]. Steam distillation
has also been used to isolate organomercury compounds from fish prior to determination by cold vapour atomic absorption spectrometry [518].