em

, Niistr

Dry ashingWet digestion

ry ans oanrs,

their recoveries. The metal levels evaluated were relatively higher in the dry-ashed samples than thewet-digested samples. However, the results showed non-signicant differences in most data obtained

r contaportanrooquie worlof tech

of the sample. The dry ashing, wet and microwave digestions areused for sample decomposition prior to the determination of traceelements by AAS (Saracoglu, Saygi, Uluozlu, Tuzen, & Soylak, 2007).The dry ashing and wet digestion methods have been applied in

he samples in anestion metps. The reland heavy

2. Materials and methods

2.1. Apparatus

The dry ashing was done using the Uniscope mufe furnace(model SM 9080, Surgifriend Medicals, England), while the wet

Corresponding author. Tel.: +234 803 068 9633.E-mail addresses: olu_akins@yahoo.com (I.O. Akinyele), osshokunbi@gmail.com

(O.S. Shokunbi).

Food Chemistry 173 (2015) 682684

Contents lists availab

Food Che

lsecal laboratories. Many analytical methods including AAS for traceelement determination in food materials require decomposition

in the samples has been checked by spiking some samples withmetal standards and determining their recovery rates.will provide such accurate and reliable data.Flame/graphite furnace atomic absorption spectrometry

(F/GFAAS) is the most utilised instrument for the determinationof trace and heavy metal contents of food samples in most analyti-

digestion methods were applied in processing teffort to recommend the more appropriate digdetermining the eight metals in these food grouof the methods for estimation of trace elementshttp://dx.doi.org/10.1016/j.foodchem.2014.10.0970308-8146/ 2014 Elsevier Ltd. All rights reserved.hod iniabilitymetalsin the developing world, it becomes very necessary to obtainaccurate and reliable data on the concentrations of elements inthe commonly consumed foods. Furthermore, the elemental com-position of food items differs from one group to another due totheir different particulate nature and structural matrix. This makesit quintessential to analyse each food group with the method that

they were applied together with more metals involved, fewer foodgroups were considered. Thus, the aim of this study was tocontribute to the establishment of more reliable, routine methodfor the determination of manganese, iron, copper, zinc, chromium,cadmium, lead and nickel in ve food groups (fruits, leafy and fruityvegetables, tubers, legumes and cereals). Dry ashing and wetTrace elementsHeavy metalsFood groups

1. Introduction

Heavy metals are among the majoand may be considered the most imronment (Zaidi, Asrar, Mansoor, & Fais becoming more serious all over thing countries. Considering the levelafter processing with the two methods. The dry ashing method is recommended for digestion of fooditems in these categories because: it involves lesser amount of chemical reagents and related hazards;it requires simple equipment and achieved better recovery.

2014 Elsevier Ltd. All rights reserved.

minants of food supplyt problem to the envi-, 2005). Such a problemd especially in develop-nological advancement

several analyses of food samples (Akinyele & Osibanjo, 1982;Bahemuka & Mubofu, 1999; Demirel, Tuzen, Saracoglu, & Soylak,2008; Onianwa, Adetola, Iwegbue, Ojo, & Tella, 1999; Onianwa,Adeyemo, Idowu, & Ogabiela, 2001; Onianwa, Lawal, Ogunkeye, &Orejimi, 2000; Saracoglu et al., 2007; Tuzen, 2003). However, inmost cases, these methods were applied separately on the samplesprocessed and fewer metals were evaluated. At other times whenKeywords:

metal concentrations were determined using atomic absorption spectrophotometer according to stan-dard methods. The accuracy of the procedures was conrmed by spiking some samples and evaluatingShort communication

Comparative analysis of dry ashing and wfor the determination of trace and heavy

I.O. Akinyele a, O.S. Shokunbi b,aUniversity of Ibadan, Faculty of Public Health, Department of Human Nutrition, IbadanbBabcock University, Benjamin Carson (Snr.) School of Medicine, Department of Biochem

a r t i c l e i n f o

Article history:Received 7 June 2014Received in revised form 12 October 2014Accepted 17 October 2014Available online 25 October 2014

a b s t r a c t

This study compared the danalysis. The concentratio(chromium, cadmium, leadand fruity vegetables, tube

journal homepage: www.et digestion methodsetals in food samples

geriay, Ogun State, Nigeria

shing and wet digestion methods of processing food samples for elementalf trace elements (manganese, iron, copper and zinc) and heavy metalsd nickel) were determined in varieties of samples classied as fruits, leafylegumes and cereals, obtained from Abeokuta, South-West, Nigeria. Thele at ScienceDirect

mistry

vier .com/locate / foodchem

digestion was done using the Digestion System 40, 1000 heatingunit of JC Tecator. A Buck Scientic Atomic Absorption Spectropho-tometer (210 VGP model, East Norwalk, Connecticut, USA) wasused for determination of the metals. The determination wascarried out in an air/acetylene ame. The operating parametersfor the elements determined were set as recommended by themanufacturer and they are highlighted in Table 1.

2.2. Reagents

concentration of the sample was achieved. The blank digests and

basis.

2.5. Statistical analyses

Data were analysed by independent students t-test with SPSSversion 15.0 (Chicago, IL) and differences were considered statisti-cally signicant at P < 0.05.

check (lg/ml) Linear range (lg/ml) Flame type colour

2.50 AA, lean/blue5.00 AA, lean/blue5.00 AA, lean/blue2.50 AA, lean/blue5.00 AA, rich/yellow2.00 AA, lean/blue

20.0 AA, lean/blue

Table 2Comparison of the recovery rates of trace elements and heavy metals after dry ashingand wet digestion of spiked samples.

Metal Method Recovery (%) Metal Method Recovery (%)

Manganese 1 98.2 1.0 Chromium 1 78.0 2.32 97.8 1.3 2 84.3 3.9

Iron 1 99.9 0.8* Cadmium 1 99.5 1.22 90.3 1.4 2 95.1 3.6

Copper 1 97.7 0.7* Lead 1 90.8 2.62 90.4 1.2 2 91.3 2.7

evaluation of the same sample, using the second method.

I.O. Akinyele, O.S. Shokunbi / Food Chemistry 173 (2015) 682684 683Analytical reagent-grade chemicals were employed in theprocessing of all samples. HNO3 and HCl were obtained fromSigmaAldrich (Buchs, Switzerland). Distilled de-ionised waterwas used in all the experiments. All plastics and glasswares wereproperly cleaned by soaking with 2 M nitric acid and rinsedthoroughly with de-ionised water before use. The standard solu-tions of analytes for calibration procedure and spiking, obtainedfrom Buck Scientic, were prepared by diluting a stock solutionof 1000 ppm of the investigated elements.

2.3. Sample collection

Food samples purchased from the local markets in Abeokuta,South-West Nigeria were investigated in this study. The foodgroups include fruits (banana, water melon, orange and apple),leafy and fruity vegetables (garden egg, tomato, onion, Africanspinach (Corchorus olitorius), pumpkin leaves (Telfairia occidentalis)and sokoyokoto (Celosia argentea)), tubers (yam and sweetpotato), legumes (soybean, bean and groundnut) and cereals (rice,maize and wheat). Each food item was purchased by systematicrandom sampling from three to six different stores. A subsampleof each was obtained for analysis. Only wholesome samples wereincluded for the analysis. More so, only edible portions wereincluded, whereas bruised or rotten parts were removed. Sampleswere carefully washed in de-ionised water to remove sand andother impurities. They were then drained and oven-dried at100 C to a constant weight, taking care not to overheat the sam-ples. The dried samples were homogenised using Kenwood blender(model BL335, Watford, UK) and stored in clean dry air-tight poly-ethylene bags until analysis.

2.4. Digestion procedure

2.4.1. Dry ashingA modied method of Crosby (1977) was adopted. One gram of

each dried sample was weighted into a porcelain crucible and dry-ashed in a mufe furnace by stepwise increase of temperature upto 500 C within 1 h and then leaving to ash at this temperaturefor additional 12 h. The residue was dissolved in 1 M nitric acid, l-tered into a 25 mL volumetric ask usingWhatman lter paper andmade up to mark with the nitric acid (1 M). The blank digests wassimilarly processed. The recovery study of this analytical procedure

Table 1Instrumental analytical conditions of investigated elements.

Metal Wavelength (nm) Slit (nm) Sens

Manganese 279.5 0.7 1.25Iron 248.3 0.7 2.50Copper 324.8 0.7 2.00Zinc 213.9 0.7 0.50Chromium 357.9 0.7 2.00Cadmium 228.9 0.7 0.75Lead 283.3 0.7 10.00

Nickel 341.5 0.2 7.00

Buck Scientic (2003).spiked samples were similarly processed. The recovery study ofthis analytical procedure was carried out, as described byOnianwa et al. (2001), by spiking and homogenising severalalready analysed food samples with varied amounts of standardsolutions of the metals. The spiked samples were then processedthe same way as other samples. Average recoveries are as high-lighted in Table 2. Each sample was analysed in triplicate andresults reported as mean standard deviation (SD) on dry weightwas carried out, as described by Onianwa et al. (2001), by spikingand homogenising several already analysed food samples with var-ied amounts of standard solutions of the metals. The spiked sam-ples were then processed the same way as other samples.Average recoveries are as highlighted in Table 2. Each samplewas analysed in triplicate and results reported as mean standarddeviation (SD) on dry weight basis.

2.4.2. Wet digestionWet digestion of samples was performed using mixtures of

acids: HNO3:HCl (3:1), by a modied method of Demirel et al.(2008). Twenty millilitres of the acid mixture was used for each1 g sample digested. Each mixture was heated up to 150 C for2 h on the heating digestion block. Then the acid digest wasallowed to cool and ltered into a 25 mL volumetric ask, usingWhatman lter paper and made up to mark with de-ionised water.This way, organic matter was destroyed in the sample and a highZinc 1 97.9 1.5 Nickel 1 93.5 1.22 97.0 2.8 2 97.2 2.9*

Values are expressed as mean standard deviation (SD).1Dry ashing.2Wet digestion.

* Values are signicantly higher (at P < 0.05) than the values obtained from the8.00 AA, lean/blue

Buck Scientic. (2003). Buck scientic 210/211VGP atomic absorption

Table 3nic

valu

684 I.O. Akinyele, O.S. Shokunbi / Food Chemistry 173 (2015) 6826843. Results and discussion

The average recovery rates (%) of the minerals analysed afterthe dry ashing and wet digestion are presented in Table 2. Therecovery values from both dry-ashed and wet-digested sampleswere nearly quantitative (>90%), except in the chromium analysis,where the recoveries were about 80%. This did not vary signi-cantly with the food type spiked. In most of the samples spiked,dry-ashed samples had slightly elevated recovery rates. The recov-ery values of iron and copper from samples processed via dry ash-ing were signicantly higher (P < 0.05) than those of wet digestion.However, the recovery rate of nickel is signicantly higher in wet-digested samples than dry-ashed samples. The recovery ratesobtained for dry-ashed samples in this study were slightly higherthan the 92.6%, 97.3%, 94% and 91% reported respectively for Cu,Zn, Cd and Ni analysed from their spiked samples that weredry-ashed (Onianwa et al., 2000, 2001). Considering our recoverystudies results, dry ashing method is somewhat more sensitivethan this wet digestion method.

The concentrations (lg/g) of the trace elements and heavy met-als in the various food groups analysed are presented in Table 3. Ageneral comparison of the dry ashing and wet digestion methodsshowed no statistically signicant difference (P > 0.05) in all theresults for manganese, zinc, cadmium and lead. This data suggestthat either of the protocols of the dry ashing or wet digestion canbe reliably applied in processing food samples in these food groupsanalysed, while evaluating their manganese, zinc, cadmium andlead contents. In most of the food groups, the concentrations ofthese minerals in the samples processed by dry ashing are slightly

Concentrations (lg/g) of manganese, iron, copper, zinc, chromium, cadmium, lead and

Food group Digestion method Mn Fe

Fruits Dry 6.22 1.25 32.33 11.33Wet 5.97 0.852 33.77 9.36

Leafy and fruity vegetables Dry 15.60 4.60 50.96 30.13Wet 16.27 4.04 44.45 30.00

Tubers Dry 6.58 2.93 7.88 0.73Wet 7.13 2.99 10.63 1.40*

Legumes Dry 22.17 6.96 45.75 15.18Wet 23.19 6.67 41.11 14.76

Cereals Dry 11.27 12.62 17.79 8.74Wet 9.56 10.11 14.45 7.70

Values are expressed as mean standard deviation (SD).* Values are signicantly higher (at P < 0.05) than the values obtained from the ehigher. This is further corroborated by the rates obtained from therecovery studies.

The levels of iron in the wet-digested tubers were signicantlyhigher than those of the dry-ashed samples. As for copper, the con-centrations were signicantly higher (P < 0.05) in leafy and fruityvegetables dry-ashed, whereas all other samples were not signi-cantly different in concentrations. Fruits and tubers wet-digestedhad signicantly higher chromium contents. The nickel contentswere also only signicantly higher in the wet-digested legumes.Previous data published by Saracoglu et al. (2007) showed thatthere were no signicant differences in the levels of manganese,zinc, chromium and nickel of baby food processed via dry ashingand wet digestion methods. Reports of recovery studies using somecertied reference materials (NIST SRM 1537a tomato leaves andNIST SRM 1515 apple leaves) and tomato sauce samples alsoshowed a non signicant difference in the values of copper, zinc,spectrophotometer operators manual. http://www.bucksci.com/catalogs/210-211-Users-Manual.pdf Accessed 30.05.14.

Crosby, N. T. (1977). Determination of metals in foods: A review. The Analyst, 102,223268.

Demirel, S., Tuzen, M., Saracoglu, S., & Soylak, M. (2008). Evaluation of variousmanganese and iron obtained from samples processed by dry ash-ing and wet digestion (Demirel et al., 2008). Thus, these data afrmour ndings.

4. Conclusion

In this study, we have been able to analyse food samples cover-ing wider scope (ve food groups) with the two methods. The dryashing procedure will be recommended because of four reasons: itrequires the use of smaller amount of chemicals hence cost effec-tive; it involves lesser risks associated with chemical usage; itrequires simple equipment (mufe furnace) that is easily handled;and it achieved better recovery in the samples. Moreover, theobtained analytical values make this modied method more suit-able for the analysis of manganese, iron, copper, zinc, chromium,cadmium, lead and nickel in fruits, leafy and fruity vegetables,tubers, legumes and cereals.

References

Akinyele, I. O., & Osibanjo, O. (1982). Levels of some trace elements in hospital diets.Food Chemistry, 8, 247251.

Bahemuka, T. E., & Mubofu, E. B. (1999). Heavy metals in edible green vegetablesgrown along the sites of the Sinza and Msimbazi rivers in Dar es Salaam,Tanzania. Food Chemistry, 66, 6366.

kel in various food groups.

Cu Zn Cr Cd Pb Ni

3.44 0.42 8.23 3.19 0.41 0.32 0.09 0.04