DOI: 10.1007/s10967-008-1607-5 Journal of Radioanalytical and Nuclear Chemistry, Vol. 278, No.3 (2008) 761–765
0236–5731/USD 20.00 Akadémiai Kiadó, Budapest
© 2008 Akadémiai Kiadó, Budapest Springer, Dordrecht
INAA with gamma-gamma coincidence for selenium determination in food
A. Di Piero, M. A. Bacchi,* E. A. N. Fernandes
Universidade de São Paulo, Centro de Energia Nuclear na Agricultura, Caixa Postal 96, 13400-970 Piracicaba, Brazil
(Received July 10, 2008)
Selenium detection limits of INAA are normally above its concentration in most biological materials. Gamma-gamma coincidence methodology
can be used to improve the detection limits and uncertainties in the determination of selenium. Here, some edible parts of plants were measured
using a HPGe detector equipped with a NaI(Tl) active shielding, producing spectra both in normal and coincidence modes. The results presented
the reduction of the detection limits of selenium by a factor of 2 to 3 times and improvement in the uncertainty of up to 2 times.
Several studies have addressed the impact of
selenium in human and animal health.
Selenium has a
bimodal nutritional effect, being essential within a
restricted concentration range and toxic above certain
The most known metabolic function of selenium
is as antioxidant. It also has a role in thyroid
and some studies indicate selenium as an
important factor for preventing some kinds of cancer.
The distribution of selenium in the earth is not
regular for being bound to either organic or inorganic
compounds, which favors its lixiviation. The highest
concentrations of selenium were found in arid regions,
with substantial presence of sedimentary rocks, though
the lowest concentrations were found in sandy soils.
The essentiality of selenium for plants is not completely
confirmed, nevertheless the correlation between
selenium concentrations in soils and edible plants is
well-known. Therefore, foodstuff from some parts of the
world can present insufficient concentrations, whereas
products from other regions can present toxic levels.
The determination of selenium of low concentrations
in biological samples is a complex analytical issue.
Several techniques can be used, as ICP-MS, HG-AAS,
GF-ASS, fluorometry and gas chromatography, among
others. However, most methods require sample
digestion, which can cause problems with reagent blank
and selenium losses.
Instrumental neutron activation analysis (INAA) has
several advantages when compared to other techniques
for being non-destructive, which eliminate chemical
treatments, avoiding contaminations by reagents and
losses during sample preparation. Nevertheless, the
concentrations of selenium in food matrices are usually
below the detection limits for normal INAA. Since the
detection limit is largely influenced by the presence of
other radionuclides in the sample, the use of gamma-
gamma coincidence methodologies can considerably
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reduce such effect, making use of the cascade emitted
Several studies have already been conducted
regarding the use of coincidence systems for the
determination of low level concentrations or for
radionuclides emitting low energy photons.
Improvements of up to 6 times were obtained for the
detection limit of some elements.
At least two
reported significant improvements for
selenium determination in biological materials, reaching
detection limits as low as 0.01 mg
. Here, some
selected foodstuffs were evaluated by INAA using
gamma-gamma coincidence to assess the performance
for the determination of selenium.
The measurement system was composed of an
ORTEC HPGe coaxial detector (55% relative efficiency
Co), Model GMX50220, and an active shield of
NaI(Tl) detectors, working in coincidence. The active
shield comprised a 9"×9" annulus crystal surrounding
the main detector and a 3"×3" plug crystal in the top part
of the system. All detectors were arranged inside a lead
shield, with internal layers of cadmium and copper. The
electronic setup allowed the simultaneous acquisition of
spectra in both normal and coincidence mode. In the
coincidence mode only events that were detected
simultaneously in the HPGe and in NaI detectors were
Samples of rice, beans, wheat flour and manioc flour
were taken for being very common in the Brazilian daily
diet. Chickpea, lentil and Brazil-nuts were selected for
being known as nutritional sources of selenium. Three
different varieties or commercial brands of each material
were purchased from the local market of Piracicaba, São
Paulo State. From each sample, portions of 100 g were
taken and processed as necessary, i.e., dried and milled
to fine particle size (<0.5 mm).