SHORT COMMUNICATION

Journal of Radioanalytical and Nuclear Chemistry, Vol. 254, No. 1 (2002) 219–222

Neutron activation analysis of IAEA proposed certified reference materialIAEA-407 (North Sea Fish Homogenate)

M. Wasim, A. Rahman, J. H. Zaidi,* S. Waheed, S. AhmadNuclear Chemistry Division, Pakistan Institute for Nuclear Science and Technology, P. O. Nilore, Islamabad, Pakistan

(Received April 15, 2002)

The proposed reference material from the International Atomic Energy Agency, namely IAEA-407 (Fish Homogenate) was analyzed usinginstrumental neutron activation analysis (INAA). 15 elements, Al, As, Cl, Co, Cr, Fe, Hg, Mg, Mn, K, Na, Sb, Se, V and Zn, were determinedusing two irradiations (5 minutes and 5 hours) and four countings. IAEA-MA-A-1/TM (Copepoda Homogenate), NIST-SRM-1566 (OysterTissue), NIST-SRM-1572 (Citrus Leaves) and NIST-SRM-1577a (Bovine Liver) were utilized as standards in the quantification step.

Introduction

The quality of marine environment has long beenassessed by a number of international organizations likeUnited Nations Joint Group of Experts on the ScientificAspects of Marine Environmental Protection(GESAMP), the Intergovernmental OceanographicCommission (IOC), the Scientific Committee forOceanic Research (SCOR) and the Scientific Committeeon Problems of the Environment Facility (GEF).1

The IAEA Marine Environment Laboratory (IAEA-MEL), Monaco has been conducting intercomparisonexercises through its Analytical Quality Control Services(AQCS) to measure trace elements in marine biota andsediments. The main objectives of these exercises are toallow analysts to check the accuracy of their results, toevaluate data quality on a global and regional level andto satisfy the increasing demand for reference materials(RM) certified for trace levels of contaminants in marinesamples. All the participating laboratories get theopportunity to validate their experimental proceduresand enhance the precision and accuracy of their data.2

COQUERY et al.3 has given a review about the dataquality of the exercises conducted between 1990 and1997 and indicated that a significant improvement in theperformance of laboratories was observed.

The demand of certified reference materials (CRMs)is continuously increasing day by day especially for newmatrices. The formation process of a CRM is verylengthy and requires rigorous data analysis. There are alot of organizations producing reference materials; seeCAROLI’s list.4 A database named “COMAR”5 wasdeveloped to contain the data of reference materialproducers throughout the world. The database alsocovers majority of the matrices from alloys to biologicaland with different chemical and physical properties.

Instrumental neutron activation analysis (INAA) haslong been used for trace element analysis efficientlysince it is a non-destructive, high precision, multielementtechnique and does not involve any blank. For thecertification of most of the SRMs, therefore, INAA isemployed along with the other complementary analyticaltechniques. The National Institute of Standards andTechnology (NIST) uses INAA extensively for thecertification of SRMs. A recent application of thistechnique involves the determination of 23 elements inCoal Fly Ash (SRM-1633b)6 and 21 elements in Spinach(SRM-1570a).7 ZAIDI et al.8 have developed amethodology for the determination of elements indifferent species of fish in Pakistan using INAA andAAS and reported 30 elements. MIYAMOTO et al.9 usedradiochemical neutron activation analysis (RNAA) forthe analysis of food spices and pulses to establishbaseline data.

The Neutron Activation Analysis Laboratory atPINSTECH is a regular participant in the IAEA-MELIntercomparison Exercises and most recentlyparticipated in the analysis of IAEA-405 (EstuarineSediment). All the results submitted by this Laboratorywere found correct with overall z-score of zero.10 Thiskind of quality control exercises has given us sufficientconfidence to publish our data before any finalevaluation of the proposed reference material.

Experimental

Sample preparation

The samples were taken from 15 g bottles withTeflon lined screw caps and sealed in plastic bagscontaining the powder of fish homogenate. Thehomogeneity of this material for trace elements wastested using a standard protocol and found to be satis-factory for the purposes of this intercomparison exercise.The material can easily pick up moisture during storage.

* E-mail: jamshed@pinstech.org.pk

0236–5731/2002/USD 17.00 Akadémiai Kiadó, Budapest© 2002 Akadémiai Kiadó, Budapest Kluwer Academic Publishers, Dordrecht

M. WASIM et al.: NEUTRON ACTIVATION ANALYSIS OF IAEA PROPOSED CERTIFIED REFERENCE MATERIAL

It is, therefore, necessary to determine the water contentof the material at the time of analysis from a sub-sample(not taken for analysis) by drying to a constant weight at105 °C usually for 24 hours.

Preparation of multi-element standard

The standards were prepared from the stock solutionsof the respective elements under investigation containing1 mg.cm–3 ultra pure spectrographically standardizedsubstances (Johnson, Matthey & Co., Ltd., London).These solutions were diluted accordingly to give a widerange of standards for each element. The solutions weredried on ash less filter papers and sealed inpolyethylene/quartz capsules for irradiations. Blank filterpapers were also irradiated to determine theircontributions and necessary corrections were made.Standards of appropriate concentrations (checked withthe CRMs) for all the elements under investigation wereused, i.e., according to the level of respective elements inthe samples.

Sample irradiation and counting

Approximately 150–200 mg of sample (in triplicate)and comparator standards were taken and each waswrapped in a clean piece of paper. All of these wrappedsamples were then packed in a polyethylene “rabbit” andsent for irradiation in Pakistan Research Reactor-II(PARR-II), which is a 27 kW, Miniaturized NeutronSource Reactor (MNSR) with a thermal neutron flux of1.1012 n.cm–2.s–1. After irradiation each sample was

transferred to a pre-cleaned and pre-weighedpolyethylene capsule for counting. The capsules were re-weighed to get the net weight of the counted sample. Theoptimization of the activation scheme was carried out forthe analysis of ultra-short, short, medium and long-livedindicator radionuclides given in Table 1.

The experimental setup consisted of a HPGe detector(Canberra Model AL-30) connected to PC-basedIntertechnique Multichannel Analyzer (MCA).“Intergamma, version 5.03” was used for dataacquisition. The system has a resolution of 1.9 keV at1332.5 keV peak of 60Co and peak to Compton ratio of40:1. The sample to detector distance was 15 cm forshort irradiation and 10 cm for long irradiation. The datafiles, containing γ-spectra information (peak energy,peak area, etc), were then used by our indigenouslydeveloped computer programme for activity andconcentration calculations. In the calculation stepbackground subtraction was applied for each irradiation.

Quality assurance

For the reliability of the results, we follow qualityassurance techniques, which include three types ofcalibrations on instrument that is energy, resolution andefficiency calibration. At each irradiation thermal fluxmonitoring is performed. Thermal neutron flux monitors,e.g., Au, Ni, Co and Al foils, are inserted between thesamples and standards to monitor the fluctuations in thethermal neutron flux gradient. In the present study thefluctuation of flux was insignificant.

Table 1. Optimum activation scheme for biological samples

Irr/dec/meas Isotope Half-life Gamma-raytimes used used, keV

5m/5m/5m 28Al 2.24 m 1779.027Mg 9.46 m 843.852V 3.75 m 1434.1

5m/10m/30m 38Cl 37.24 m 2167.742K 12.36 h 1524.656Mn 2.58 h 846.824Na 14.96 h 1368.6

5h/2d/30m 76As 26.32 h 559.1

5h/2-3d/16h 60Co 5.27 y 1173.251Cr 27.7 d 320.159Fe 44.5 d 1099.2203Hg 46.61 d 279.2124Sb 60.2 d 1691.075Se 119.77 d 264.785Sr 64.84 d 514.065Zn 243.9 d 1115.6

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Table 2. Analysis of reference materials for quality control (in µg.g–1)

Element Our value Certified value

NIST-SRM-1572 (Citrus Leaves)Al 95.0 ± 16.0 92 ± 14.7As 3.1 ± 0.3 3.1 ± 0.3Cr 0.76 ± 0.10 0.8 ± 0.2Fe 89.0 ± 3.0 90 ± 9.9Hg 0.08 ± 0.01 0.08 ± 0.02K 18534 ± 752 18200 ± 612Mn 22.8 ± 0.9 23.0 ± 2.0Na 170 ± 18 160 ± 22Zn 30.0 ± 2.0 29 ± 2

NIST-SRM-1566 (Oyster Tissue)As 12.94 ± 1.75 13.40 ± 1.8Fe 195.1 ± 22.5 195 ± 33Mg 1434 ± 208 1280 ± 100Mn 17.72 ± 1.21 18 ± 1Na 4991 ± 184 5100 ± 294V 2.18 ± 0.08 2.3 ± 0.1

IAEA-MA-A-1/TM (Copepoda)As 7.1 ± 1.4 6.7 ± 0.6Co 0.13 ± 0.01 0.12 ± 0.01Hg 0.264 ± 0.016 0.28 ± 0.01Sb 0.08 ± 0.02 0.07 ± 0.03Se 3.0 ± 0.3 3.0 ± 0.2Zn 161 ± 9 158 ± 2

NIST-SRM-1566 (Bovine Liver)As 0.052 ± 0.005 0.047 ± 0.006Cl 2780 ± 87 2800 ± 98Fe 195 ± 22 194 ± 19K 9840 ± 75 9960 ± 70Mg 614 ± 19 600 ± 15Mn 9.8 ± 0.7 9.9 ± 0.8Na 2500 ± 144 2430 ± 129Zn 124 ± 8 123 ± 8

For quality control, normally two or three certifiedreference materials are analyzed with the samples. Theresult of three such comparisons are given in Table 2.Our results are in good agreement with the certifiedvalues.

Results and discussion

The irradiation and counting conditions are shown inTable 1 and the results for IAEA-407 are summarized inTable 3. Concentration values in Table 3 are mean of atleast three determinations and standard deviations valuesare at 1σ. The limit of detection is based on 3 times thestandard deviation of the spectrum continuum countvalues. All the results are corrected for moisture content.The moisture content was BL = CL = OT = 407 = ca.5%; while CP = 9%.

56Mn was measured in the second counting(5m/10m/30m) because its peak was well-resolved form28Mg at 843 keV. Interference correction was applied

for 203Hg at 279.2 keV due to the presence of 75Se at279.5 keV. The peak of 65Zn at 1115.6 keV wascorrected for 152Eu peak at 1112.1 keV, this interferencewas not present in all samples. 65Zn was also correctedfor 160Tb (1115.1 keV) in Citrus Leaves. The peak areaof 60Co in all the spectra was corrected for backgroundbecause this radionuclide is present in our detectorbackground. The rest of the elements were found freefrom any interference. Nuclear interferences were foundnegligible because the fast flux in MNSR type of reactoris very low.

In a sample four elements were measured using asingle standard including Al (quantified with CitrusLeaves) V (determined using Oyster Tissue) and Co andSb (measured with Copepoda standard). Confidence inthe result was gained because a large number of valuesproduced by different irradiations were in goodprecision. Cl, K, Mg, Mn and Na were determined usingBovine Liver, Citrus Leaves and Oyster Tissue.

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Table 3. Results submitted to the IAEA-407 certification project

Element Unit Content St. dev. RSD, %

As mg/kg 12.5 0.8 6Cl mg/kg 23.6 1.0 4Co mg/kg 69.0 2.0 3K mg/kg 12.2 0.6 5Mn mg/kg 3.30 0.10 3Na mg/kg 13.6 0.6 4Se mg/kg 2.3 0.1 4V mg/kg 1.30 0.10 8Zn mg/kg 61.0 4.0 7

Indicative values

Element Unit Content

Al mg/kg 70Cr ng/g 900Fe mg/kg 150Hg ng/kg 40Mg mg/kg 2.5Sb ng/g 10

Except for Mn, Copepoda does not provide certifiedvalues for these elements. The remaining elements weremeasured either with two or a combination of threestandards. Ni was also demanded by IAEA but we couldnot detect this element, however, the detection limit for65Ni was found 21 ppm in the sample.

The majority of the elements (Co, Mn, Cl, Na, Se, K,As, Zn, and V) have %RSD (relative standard deviation)below 10%, four elements (Al, Hg, Mg, Fe) below 5%,Sb and Cr below 20%.

Conclusions

INAA has been used to certify IAEA-407 (FishHomogenate), which will be useful as CertifiedReference Material in future work and will fulfilldemand for new biological reference material. It is freefrom spectral or nuclear interferences except for Hg andZn, which are resolvable. This standard will also providebase line elemental data of North Sea fish.

References

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