American Journal of Analytical Chemistry, 2013, 4, 739-743
Published Online December 2013 (
Open Access AJAC
Analysis of Selenium Content in Root and Tuber Plants in
Central Nigeria
Saidu Zarmai, Ishaq S. Eneji*, Rufus Sha’Ato
Department of Chemistry, Centre for Agrochemical Technology, University of Agriculture, Makurdi, Nigeria
Email: *
Received October 30, 2013; revised November 29, 2013; accepted December 3, 2013
Copyright © 2013 Saidu Zarmai et al. This is an open access article distributed under the Creative Commons Attribution License,
which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
Some common staple roots and tubers (cassava, yam, sweet potato and yellow yam) were analyzed for selenium content
level using hydride generation—atomic absorption spectrophotometry (HG-ASS) technique. Results for duplicate
analysis revealed that sweet potato has the highest mean selenium content (19.2 ± 5.20 μg/kg), followed by yellow yam
(18.3 ± 6.97 μg/kg), then yam (13.6 ± 7.12 μg/kg) and cassava the least (13.0 ± 5.84 μg/kg). In comparing our results
with the literature values, most of the results obtained in this work such as <1.00, 1.09, 1.91, 2.35 and 11.0 μg/kg were
lower while a few others like 52.6, 54.2, 72.3 and 81.8 μg/kg were higher than literature values. The variations could be
due to the type of species/variety, geographical location, total selenium concentration in the soil and its bioavailability.
In view of the importance of Se to human health and considering the levels found in the staples investigated, we rec-
ommend that sweet potato be popularized as a staple, much more than cassava as it is now the case in Central Nigeria.
Keywords: Selenium; Content; Roots and Tubers; Central Nigeria; HG-AAS
1. Introduction
Selenium (Se) is a naturally occurring metalloid element
that is found in all natural materials on earth including
rocks, soils, water, air, plant and animal tissues [1,2].
Selenium concentrations in most rock types are generally
low. Sedimentary rocks contain more of the elements
than igneous rocks; however, concentrations in most lime
stones and sandstones rarely exceed 0.05 mg/kg. Se is
often associated with the clay fraction in shales (0.06
mg/kg) than lime stones or sandstones. Some phosphatic
rocks have very high concentrations up to 300 mg/kg [3].
Consequently, coals and other organic-rich deposits can
be enriched in the element relative to other rock types,
typically ranging from 1 to 20 mg/kg. However, values
of over 600 mg/kg had been found in some black shales
and exceptionally high concentrations of 600 mg/kg in
Se rich coals were found in China [4]. Se is often found
as a minor component of sulphide mineral deposits. Dis-
tribution of Se in the geological environment is highly
variable, reflecting the properties of different rock types
In most cases, food forms the main source of selenium
for humans because the concentrations of the element in
water and air are generally low. The concentration of
selenium in food stuffs depends primarily upon the sele-
nium concentration of the soil on which the food was
grown [6]. Brazilian nut contains 554 μg/kg selenium
which is the richest staple food in the world. The forage
crops containing less than 40 μg/kg selenium are gener-
ally associated with deficiency in grazing animals [7].
Selenium concentration of most soils is very low 0.01 to
2 mg/kg but high concentration of up to 1200 mg/kg has
been reported in some seleniferous areas [8].
The nutritional importance of selenium has been docu-
mented. Its improved consumption may decrease cancer
incidence and heart deterioration, arthritis, diabetis and
goiter. It has also been reported that increased selenium
intake enhances the immune systems of HIV/AIDS and
avian flu sufferers. Its improved intake also enhances
reproductive fertility in humans and animals [5].
Selenium toxicity (selenosis) is rare in humans, al-
though cases have been reported. Selenium poisoning
was reported in the United States who consumed a sup-
plement that contained 182 times more selenium than
stated on the label [9]. Its toxicity depends on concentra-
tion and chemical form. Selenate and selenomethionine
are among the most selenium toxic compounds whereas
*Corresponding author.
selenium sulphide is much less toxic due to its insolubil-
ity. Selenite has been shown to be toxic at 5 mg/kg of
dietary supplementation [10]. The most common symp-
toms of selenosis patient include nausea, vomiting, hair
loss, nails changes, irritability, fatigue, tooth decay, skin
lession and peripheral neuropathy.
The approximated safe lower and upper reference nu-
trient intake daily allowances of selenium have been re-
ported [7,11,12]. Root and tuber crops are staple foods
which are the main sources of calories for an estimated
700 million poor people in Africa, Asia and Latin Amer-
ica [10]. In Central Nigeria, there may be little or no in-
formation available for selenium content of some com-
mon roots and tubers. In this work, we therefore obtained
cassava, yam, sweet potato and yellow yam samples
from thirteen (13) major open markets in Central Nigeria
and determined their selenium content. It is believed that
these results would add to the data in the field of analyti-
cal chemistry.
2. Materials and Methods
Samples of cassava, yam, sweet potato and yellow yam
were collected from six states in Central Nigeria namely:
Niger (Minna and Bida); Nasarrawa (Laffia and Keffi);
Benue (Gboko, Makurdi and Otukpo), Kogi (Idah and
Lokoja); Kwara (Ilorin and Lafiagi) and Plateau (Jos),
including the Federal Capital Territory, Abuja (see Fig-
ure 1) in January, 2012. The samples were packed in
well labeled polyethylene bags appropriately in respect
of food types and area of collection. They were trans-
ported to laboratory and washed thoroughly with distilled
water to remove adhered soil particles and allowed to dry
at ambient laboratory temperature. Each sample was fur-
ther oven dried at 105˚C to constant weight for about 24 -
36 h. The Oven dried samples were crushed in a ceramic
mortar and pestle; sieved with the mesh of aperture 2 mm
and stored in moisture resistance plastic bottles according
to standard method [13].
About 1 g of crushed sample was added to 30 mL cru-
cible and covered with lid. Sample was dissolved in 10
mL of conc. HNO3 (70.5% - 96% AR). Sample was
heated at 60˚C for 30 minutes on a hot plate, then al-
lowed to be cooled. 3 mL of H2O2 (30%) was added to
the sample. Digestion was continued at 120˚C for 1 h.
After cooling the sample, it was diluted with deionized
water and finally filtered. The filtrate was diluted to a
total volume of 100 mL. The filtrates were stored in sam-
ple bottles in refrigerator prior to sample analysis. The
same procedure was repeated for other 51 samples. A
blank sample was prepared to check the possible con-
tamination. Selenium content was determined using hy-
dride generator (GBC-HG-3000)—atomic absorption spec-
trophotometer (GBC-Avanta). Determinations were in du-
plicate. Statistical analysis was carried out using SYS-
TAT 16.0 (SPSS, USA).
3. Results and Discussion
In this research, a total of fifty-two (52) samples of root
and tuber staples were obtained from open markets in
Central Nigeria for selenium content investigation. Table
1 shows the results of roots and tubers statistical analysis
expressed as number of samples (N), mean values with
their standard errors, minimum and maximum values.
Selenium content (μg/kg) obtained from cassava sam-
Source: Longman School Atlas, 2003.
Figure 1. Map of Central Nigeria showing major towns.
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Table 1. Summary statistics of selenium content (μg/kg) of
some common roots and tubers obtained from open mar-
kets—Central Nigeria.
Roots & tubers N Mean ± SD Minimum Maximum
Cassava (Manihot ssp) 13 13.0 ± 5.84 0.01 52.6
Yam (Discorea spp) 13 13.6 ± 7.12 0.01 81.8
Sweet potato
(Ipomea batatas) 13 19.2 ± 5.20 0.01 48.7
Yellow yam
(Discorea cayensis) 13 18.3 ± 6.97 0.01 72.7
ples in Central Nigeria ranged from 0.01 - 52.6 μg/kg
with Abuja sample having the highest (52.6 μg/kg) while
Bida, Gboko, Otukpo, Idah, Ilorin, Lafiagi and Jos sam-
ples having below the detection limit (<1.00 μg/kg) using
hydride generation-atomic absorption spectrophotometer
(HG-AAS) with the mean of 13.0 ± 5.84 μg/kg. Selenium
content (μg/kg) of yam samples varied between 0.01 -
81.8 μg/kg with Makurdi sample containing the highest
(81.8 μg/kg) while Bida, Abuja, Lafia, Gboko, Lokoja,
Idah, Ilorin, Lafiagi and Jos samples having below detec-
tion limit (<1.00 μg/kg) with the mean of 13.6 ± 7.12
μg/kg. Selenium content (μg/kg) obtained from sweet po-
tato samples ranged between 0.01 - 48.7 μg/kg with Bida
sample containing the highest (48.7 μg/kg) while Ma-
kurdi, Lafiagi and Jos samples having below the detec-
tion limit (<1.00 μg/kg) with the mean of 19.2 ± 5.20
μg/kg). Selenium content (μg/kg) obtained from yellow
yam samples varied from 0.01 - 76.3 μg/kg with Lafia
sample having the highest (76.3 μg/kg) while Minna,
Bida, Keffi, Makurdi, Otukpo, Lokoja, and Ilorin sam-
ples containing below the detection limit (<1.00 μg/kg)
with the mean of 18.3 ± 6.97 μg/kg (Figure 2). Most of
the values in this research work fell far bellow the ac-
ceptable range of 40 - 450 μg/kg in staple foods specified
by National Academy of Science [11] as safe lower and
upper reference nutrient intake while few others fell within
the range. Statistical analysis of variance (ANOVA) be-
tween and within the data of roots and tubers (cassava,
yam, sweet potato and yellow yam) showed that there
was no significant difference (P > 0.05) in selenium con-
tent (μg/kg) determined from these different staples in
Central Nigeria. Multiple mean comparisons using Tukey
Honest Significant Difference (HSD) test showed that
there was no significant difference (P > 0.05) in selenium
content (μg/kg) among cassava, yam, sweet potato and
yellow yam. The results of this analysis are direct evi-
dence that root and tuber staples are non-selenium accu-
mulators [11].
Comparing the values obtained with literatures cited, it
was observed that most of our results were lower while
few others were higher than values reported for foods in
the USA (white rice 12 μg/kg, brown rice 10 μg/kg, wal-
nuts 5 μg/kg, cod 32 μg/kg, egg 14 μg/kg, bread 4 μg/kg,
cheedar cheese 4 μg/kg, Tuna canned 63 μg/kg, Maca-
roni 15 μg/kg) [6]. National Academy of Science [11]
reported the range of staple foods 40 - 450 μg/kg. Abu-
lude et al. [14] also reported high mean of selenium con-
tent (cassava flour 10 ± 3.8 mg/kg, yam <2 ± 3.2 mg/kg,
millet 5 ± 3.4 mg/kg, maize 5 ± 3.4 mg/kg, soybeans 5 ±
3.0 mg/kg) using Atomic Absorption Spectrophotome-
try(AAS). The mean results (Figure 3) of root and tuber
foods analysed in this work, revealed that sweet potato
has the highest selenium content (19.2 ± 5.20 μg/kg),
followed by yellow yam (18.3 ± 6.97 μg/kg), then yam
(13.6 ± 7.12 μg/kg) and cassava the least (13.0 ± 5.84
The variations in the selenium contents of the root and
tuber staples we selected and those in literatures may be
due to the type of species, geographical location, total
selenium concentration in the soil, bioavailability of the
selenium in the soil and analytical methods used [6].
Differences in the results of this analysis could also be
attributed to the presence or absence of anthropogenic
activities on the soils such as application of fertilizers,
pigment in glass and ceramic manufacture, antifungal
Figure 2. Selenium content distribution in µg/kg of some
common roots and tubers staples from different sampling
areas—Central Nigeria.
Figure 3. Mean selenium content (µg/kg) in root and tuber
staples using HG-AAS.
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agent in pharmaceuticals, fossil fuel combustion and light-
sensitive photoconductor layer in photocopiers [4]. Con-
sequently, as we consumed a varied diet obtained from
many geographical locations, it is not likely that sele-
nium deficiency in the soil in a few areas will cause sele-
nium deficient in Central Nigerians’ diet. Individual in-
takes will differ about the determined mean, depending
on the amount of protein in the diet and in particular on
the amount of selenium rich foods which are consumed
There have been many documented maximum intakes
of selenium. The National Academy of Science [11] re-
ported 450 μgSe/day while selenium intake of 400 μgSe/
day has been recommended by WHO [12]. According to
Food Standard Australia New Zealand (FSANZ) [16]
quoted Australian RDI to be 85 μgSe/day and Nordic
project [17] considered an intake of 280 - 350 μgSe/day.
Comparison of our analytical results with the litera-
tures, it was observed that good food sources of selenium
were sweet potato and yellow yam. Yam and cassava
contained some significant content of selenium and it is
unlikely that selenium deficiency occurs in Central Nige-
ria. The values obtained were far below those intakes
referred to as the unsafe dose.
4. Conclusion
Analysis performed by using HG-AAS technique for Se
content indicated that few samples contained below the
detection limit while most samples contained relatively
large content of the element. The different levels of Se in
these roots and tubers investigated could be due to spe-
cies or variety differences, total Se concentration in the
soil where they were grown, bioavailability of Se in the
soil, and local anthropogenic activities; this requires fur-
ther investigation to be more conclusive. However, since
the mean selenium content in the four roots and tuber
staples revealed that sweet potato contained the highest
Se, followed by yellow yam, then yam and cassava the
least, and considering the importance of Se nutrition to
the health conditions of certain populations, it is recom-
mended that sweet potato should be made to be a more
popular staple rather than cassava, which is now the most
popular one.
5. Acknowledgements
The authors are highly grateful to the immediate family
members for their support and encouragement. Our ap-
preciations also go to the members of staff Fugro Nig.
Ltd. Port-Harcourt for given us opportunity to use their
laboratory for this research work. We are also grateful to
the anonymous reviewers for their suggestions which
brought our report to this level.
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