J.Nat.O.A.M.Institute, Vol.22, No.2,1-8,2005
Some of the investigators [7-12] have reported significantly high levels of cadmium and
arsenic in some species of fish. However, the investigation of DE GOEIJ  and
GUINN  have shown that there is no significant difference of these levels of certain
trace elements, like arsenic, cadmium than those in unpolluted areas.
Whether or not fish will be contaminated will depend on the chemical form of the
element and its concentration in the surrounding medium, microbiological activity in the
marine environment, texture of the sediment, type and age of the fish, etc. However, there
are still insufficient data available in Bangladesh for toxic metals like arsenic, cadmium,
etc. It is strongly believed that this study will consequently be a great help for
Bangladesh’s economy in view of quality assurance for trade as well as the health, safety
and benefit of her people. This paper presents information on the concentration of
arsenic, chromium, selenium and zinc in some varieties of fish and also describes briefly
the chemical procedure followed.
The instrumental neutron activation analysis (INAA) of such elements as As, Cr, Cd, Zn
and Cu in animal organs and fish tissues is difficult because of the 24
Na and 86
activities developed on irradiation. This is because the photopeaks of radioactive products
of these elements are masked, particularly by the Compton continuum of the high 24
matrix activity, thus posing problem not only in handling but also in the computation of
the peak areas from the γ-ray spectra. This problem has necessitated post-irradiation
chemical separation of isotopes of interest. This paper describes such a scheme for
determination of seven trace elements (As, Cr, Se and Zn) in biological materials.
Samples collection and irradiation. Eight varieties of common marine fish, selected in
accordance with their public flavor for Bangladeshi (near coastal belt) in both taste and
cost, namely, Coilia neglecta, Cirrhinna reba,Johnius argantus, Harpondon nehereus,
Setipinna phasa and Lepturacanthus savala were collected from the coastal belt of the
Bay of Bengal and sun-dried after removal of their internal organs, head, skin, and tails.
The dried samples were then chopped into pieces with the aid of a stainless steel knife
(steam cleaned). Only the edible muscle tissue samples were used for analysis. The
sample pieces were dried at 105o
C in an oven until a constant weight was obtained
The dried samples were ground, sieved and thoroughly mixed in a stainless steel rotating
drum for 100 hours to produce a homogeneous powder. The sample powder was finally
preserved in clean and dry polyethylene bottles. Portions of the samples (200-300 mg
each) were heat-sealed in polyethylene bags and irradiated along with a known amount of
MA-A-2 ™, the fish flesh homogenate standard of IAEA in the CIRUS reactor at Bhabha
Atomic Research Center, Trombay, Bombay, India, at a flux of about
(0.5 to 1) .1012
for 20 hours.
Chemical reagents. (1) NH3, HCl, HNO3, HClO4 and acetic acid; (2) Na2SO3 solution:
; (3) NaOH pellets; (4) thioacetamide; (5) H2O2; (6) BaCl2 (=0.2M solution in
water; (7) NH4H2PO4= 1M solution; (8) hydroxylamine hydrochloride (NH2OH.HCl).
Dissolution. Each of the irradiated samples was allowed to “cool” for 4-5 days to enable
the decay of short-lived isotopes and also to reduce the 82
Br and 24
Na activities. About 10
mg carrier for each of the element was added to a 100 ml round-bottomed flask and the
irradiated sample was carefully opened and emptied into it. Then, the sample was
digested in Bethge’s apparatus with a mixture of 6 cm3
concentrated nitric acid and 2 cm3
of concentrated perchloric acid till a liquid remained in the flask.
Chemical separation and determination of metal
After the digestion was completed, the distillates were, evaporated to dryness on a sand
bath to remove nitric acid, and leached with conc. HCl (10 cm3
) and preserved to be
combined with the filtrates from chromium and selenium precipitation.
(1) Determination of chromium: 4 cm3
HClO4 was added to the residue in the flask
and drops of conc. HCl added to the hot solution to distil of CrO2Cl2 . The
distillate was collected in 10 ml sodium hydroxide solution (1N) . To this was 1.5
BaCl2 solution followed by addition of 2-3 drops of H2O2. The pH was set at
6. BaCrO4 was digested in a water bath and was filtered, dried and counted for
Cr. The filtrate was combined with the distillate from decomposition step.
Cr is oxidized to Cr(VI) by HClO4
H2Cr2O7 + 4 HCl → 2CrO2Cl2 + 2H2O(reddish vapor)
CrO2Cl2 + 2NaOH →Na2CrO4 +2HCl (yellow distillate)
Na2CrO4 + BaCl2 → BaCrO4 + 2 NaCl
(2) Determination of selenium: To the residue remaining in the flask 20 ml of conc.
HCl was added and the volume kept at 50 ml. To the warm solution 4-5 cm3
Na2SO3 solution was added and digested in a water bath for 1 hour. The
precipitated Se metal was filtered, dried and counted for 75
Se. This filtrate also
was combined with the distillate from the decomposition step.
MCl2 + SO2 +2H2O → M (metal) + 2HCl + H2SO4
(3) Determination of arsenic: The acidity of the first distillate, with the filtrates from
chromium precipitation and selenium precipitation mixed together, was adjusted
to 1M HCl and 1 ml of a 1% solution of thioacetamide was added to the boiling
solution. This was digested in a water bath. Precipitates of sulfides of As were
filtered off, washed, dried and counted 76
CH3CSNH2 + H2O → CH3CONH4 + H2S
CH3CONH2 + H2O→ CH3COONH4
H2S + M+++
→ MS + 2H+
[M = As]
(4) Determination of zinc. The filtrate from (3) was evaporated to dryness in a boiling
water bath followed by addition of drops of conc.HNO3 until the ammonium salts
were completely destroyed. To this was added 1 cm3
1M NH4H4PO4 solution. The
pH of the solution was then adjusted to 6 and it was heated for a few minutes. The
mixture was then kept in a water bath for 1 hour. The precipitates obtained were
filtered off, washed, dried and counted for 65
ZnCl2 + NH4H2PO4 → ZnNH4PO4 + 2HCl [M= Zn]
The samples and the standards were counted on a 45 cm3
HPGe detector connected to
a 4096 channel pulse-height analyzer (Ortec PCA-MCA card). The energies (in keV)
chosen for the evaluation of the peak areas were: 76
As(26.4 h), Eγ=657 keV (since,
the photopeaks of 122
Sb (561keV) interfered with photopeak 76
As at 559 keV, the 657
keV photopeak was chosen to measure 76
As photopeak area) ; 51
Se (119.77 d), Eγ = 264.66 keV; 65
Zn (243.9 d), Eγ= 1115.5 keV.
Accuracy and precision
Experiments were initially carried out using radioactive tracers and the corresponding
carriers to evaluate the recoveries. The yields were in the range of 93% to 98%. The
accuracy of the method was evaluated by analyzing homogenate fish flesh (IAEA)
Standard Reference Material, MA-A-2™. Our results (values in μg/g) are in good
agreement with the IAEA certified values.
RESULTS AND DISCUSSION
The As, Se, Cr and Zn concentrations determined in the fish species are presented in
Table 1. The values are expressed in μg.g-1
dry weight. The range of concentrations
found in the fish samples are As (2.843-3.920), Se (2.961-6.274), Cr (0.498-1.8430),
and Zn (37.59-101.19). These variations are likely to be due to the migratory nature
and feeding habits of the different species of fish.
Normally, the levels of As (3 ng/ml), Se (0.09 ng/ml) and Zn (0.01 ng/ml) have been
found to be quite low in sea-water , but the levels of these metals as found in fish
samples under investigation are higher. This is due to the tendency of various species
of fish to concentrate certain elements in the tissue more than surrounding medium.
The mean concentrations of arsenic, selenium, chromium and zinc in fish are 3.234
μg/g, 4.385 μg/g, 1.007 μg/g and 59 μg/g, respectively.
Using neutron activation, HAMILTON AND MINSKI  reported the mean values
for As in fish as 2.0±0.08 μg fresh weight. Up to 174 ppm has been found in prawns
from the coastal waters of Britain  and 42 ppm in shrimp from the southeastern
coastal waters of the United States . Fish flour for human consumption has been
reported  to contain 1.8 ppm Se and tuna meal median levels as high as 5.1 and
6.2 ppm (dry basis). High natural Se levels in tuna  and marine mammals 
have been reported. Open ocean water contains as little as 10 μg/l of Zn at the surface
 although coastal seawater usually contains 0.5-2 μg/l of Zn as a result of river
inputs and sewage outfalls [24-26].
Arsenic, selenium, chromium and zinc in different varieties of fish
dry weight basis)
Arsenic Selenium Chromium Zinc
0.36220 3.025 4.751 1.843 80.48 79.75
0.31093 3.076 2.961 1.287 49.55 84.85
0.32212 3.920 3.856 0.550 37.59 83.45
0.27691 3.070 6.274 0.871 101.19 81.82
0.31377 3.469 3.839 0.995 50.73 82.68
0.36220 2.843 4.627 0.498 38.02 81.73
- 2.52 - - 51.0 -
Rita rita - 3.78 3.15 - - -
Mean - 3.234 4.385 1.007 59.59 82.36
Range - 2.843-
Taking 6 g of fish as the maximum consumption  per person per day for
Chittagong and coastal areas of Bangladesh, it is estimated that the average intake of
arsenic, selenium, chromium and zinc through fish is 3.881 μg, 5.262 μg 1.208 μg and
71.51 μg, respectively. The daily intake of arsenic is quite low. Table 2. gives the
comparison of the levels of arsenic, selenium, chromium and zinc in fish by various
researchers in different countries.
The present study indicates that the radiochemical separation scheme should be used
for the isolation nuclides with mutually interfering γ-rays (e.g., 75
Se, with 203
265 keV ) . In the case of 76
As, due to the interference of 122
Sb, at 559 keV, we have
chosen the less sensitive 657 keV peak for computation of arsenic.
Eight tropical marine fish species collected from the Bay of Bengal were analyzed in
order to assess the level of trace toxic elements in this food item consumed by the
Trace element concentrations (mg. Kg-1
wet weight ) in muscle tissue of various
species of fish from different areas of the world
Source Area Cr As Se Zn Species
Et al (1977)
Israel 2.8-4.9 - - 14.9-
- 0.4-3.5 - 3.0-4.5 8 species
- - 6 species
6 species (coilia
*The present data were calculated on dry weight basis
population of Bangladesh. The results indicate that the concentrations of these
elements are much below the toxic levels.
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