Talarna, Vol 39, No 7, pp 715-118, 1992 0039-9140/92 $5 00 + 0 00 F’rrntcd rn Great Bntarn All nghts reserved Copyright 8 1992 krgamon Press Ltcl

DIRECT DETERMINATION OF BERYLLIUM, COPPER AND ZINC IN Al-U MATRICES BY ELECTROTHERMAL ATOMIZATION ATOMIC-ABSORPTION SPECTROMETRY

NEELAM GGYAL, PAW J. PIJROI-IIT, A. G. PAGE and M. D. S,WRY*

Radiochermstry Division, Bhabha Atomic Research Centre, Trombay, Bombay 400 085, Inrha

(Recewed 11 July 1991 Rewed 22 October 1991 Accepted 22 October 1991)

!&unmary-An atomic-absorptton spectrometnc method with electrothermal mode of atormxatron has ken developed for the dtrect determmahon of Be, Cu and Zn m AI-U (3 : 1) matnx samples wtthout pnor chemical separahon of the major matnx. The studres carned out include the effect of the matnx on the analyte absorbance, opt!mmatron of sample ahquot and other expenmental parameters, and analysts of a number of synthettc samples Nanogram amounts of the analytes can be determmed with a solutton ahquot of 5 mtcrohtres contammg 25 mtcrograms of the sample wrth a precrston of 6% or better The analyhcal range obtained for these analytes IS Be: 2-20 ng/l., CW 20-200 rg/l and Zn l-40 pg/ml m the Al-U matnx The analysis of synthetrc samples has shown good agreement wrth then added contents

Trace metal assay forms an essential part of chemical quality assurance programmes for nuclear fuel development. Analytical atomic spectrometric methods are very well suited to the determination of trace metallics. The most sensitive and precise amongst these is the electrothermal atomization-atomic-absorption spectrometric (ETA-AAS) method with or without prior chemical separation of the major matrix. Literature reports on the analysis of Al-U alloys for trace metals by the ETA-AAS method are very fewiv3 and all of them refer to prior chemical separation of the major matrix. A preliminary study on the ETA-AAS method for the determination of silver has been included in a recent publication4 from our laboratory, dealing with the determination of twenty-eight metallics m Al-U alloys by atomic emission spectrometric methods.

In view of the obvious advantages in determination of trace metallics in any matrix without prior chemical separation, ETA-AAS methods have been extended to the direct determination of Be, Cu and Zn in Al-U matrix samples, to assess applicability to Al-U alloys. The present studies include the effect of the matrix on analyte absorbance, opti- mizaton of sample aliquot and other experi-

*Author for correspondence.

mental parameters, and analysis of synthetic samples.

EXPERIMENTAL

Apparatus

A Varian Techtron AAS (model-AA 6) equipped with a carbon rod atomizer (CBA-63) and BC-6 background corrector employing a deuterium lamp as spectral continuum source was used in these studies. The atomizer portion of the spectrometer is enclosed in a glove box as reported earlier.s Varian Techtron hollow cathode lamps for Be, Cu and Zn were operated at slightly higher currents than those prescribed by the supplier. This was necessary as the adaptation of the AAS unit for glove box operation resulted in an increase in the optical path. The 20% increase in the prescribed cur- rent did not affect either the spectral line width or sensitivity for the analyte determination. In case of Zn, the increase in the optical path resulted in significant loss of intensity for the most sensitive line at 213.8 nm due to increase in air absorption. The loss, however, could not be compensated by the increase in lamp current and hence a less sensitive line at 307.6 nm was selected in the analytical procedure. The spectral band width was 0.5 nm for all the analytes. A five microlitre pipette with dispos- able PTFE tips supplied by M/S Eppendorf

TN. 39/7-E 115

116 NEELAMGOYAL et al

Geratebau, Germany, was used for dispensing sample solution.

Preparation of standardr and sample solutions

All the reagents used in the preparation of standards and samples such as nitric acid, hydrochloric acid and water were purified by distilling them twice in a quartz distillation unit. Specpure oxides (Johnson-Matthey, U.K.) of beryllium, copper and zinc were used m prep- aration of stock solutions at 2 mg/ml elemental concentration by dissolving these compounds in 3M mtnc acid. Stock solutions of uranium (12.5 mg/ml) and Al (37.5 mg/ml) were prepared by dissolving high purity uranium m concentrated nitric acid and high purity alummium metal rod m concentrated hydrochloric acid. The alummium chloride solution thus obtained was evaporated to dryness and redissolved in nitric acid. This process was repeated three times to remove any traces of chloride and finally brought to nitrate medium.

A stock solution of Al-U matrix at 25 mg/ml concentration was prepared from the above U and Al solutions m 3 :l proportion. A high concentration “master” standard for the analytes was prepared m 5 ml by mixing appro- priate ahquots of mdividual elemental stock solutions and their intermediate stage dilutions to obtain Be at 0.1 ,ugg/ml, Cu at 1.0 fig/ml and Zn at 200 pgg/ml concentration. A set of six 5-ml working standards was prepared by adding graded proportions of the master standard and a fixed proportion of the Al-U matrix stock solution. The concentrations of the analytes in these standards were: Be, 0.001-0.05 pgg/ml; Cu, 0.01-0.5 pg/ml and Zn, 2.0-100 pg/ml. An additional low concentration standard was prepared specially for use in standardization for Zn at 1 pg/ml concentration by appropriate dilution of a higher concentration solution. Al-U samples were dissolved in concentrated hydrochloric acid, treated with concentrated nitric acid and finally made up to volume in O.lM nitric acid. Synthetic samples in the Al-U matrix were prepared at five intermediate range concentrations by mixing known aliquots of the master standard with the matrix solution. Another set of single element synthetic samples was prepared by mixing known aliquots of the elemental solutions with uranium (5 mg/ml) and Al-U matnx solutions as well as the aqueous solution. The latter set was prepared for studying the effect of the matrix on analyte absorbances.

Procedure

The effect of the Al-U matrix on the analyte absorbance was studied for all the analytes by varying the concentration of the matrix in the range 5-20 mg/ml in an intermediate concen- tration of analyte standard. The experimental parameters such as temperatures and the time durations for “dry”, “ash” and “atomize” stages were subsequently optimized for each analyte at 5 mg/ml concentration of the Al-U matrix. A series of standards was then studied to obtain analytical ranges for the analytes. Five spiked samples were analysed by the optimized procedure, providing a check on the accuracy of analyte determinations. Repetitive analyses of these samples were carried out to evaluate the precision of determinations. In order to study the effect of the Al-U matrix at 5 mg/ml on the analyte absorbance in comparison with aqueous and uranium (5 mg/ml) matrices, single element synthetic samples were prepared in these three matrices and the absorbances were measured with the optimized procedure.

RJBULTS AND DISCUSSION

The influence of the Al-U matrix on the atomization behaviour of the analytes was examined. It was observed that the non-specific absorbance signal, as evaluated by the back- ground corrector, increased significantly with an increase in the matrix concentration. This can be seen from Fig. 1 where variation of the matrix absorbance at 324.8 nm (Cu) as a func- tion of matrix concentration is shown. Such correction to the analyte signal, required because of the matrix, is very significant, especially at low analyte concentrations, mean- ing that the background correction system could not adequately compensate for the background.

os-

04-

z 03-

6

0 5 IO 15 20

(Al-U) Concentratton. mg/m(

Fig 1 Variation of blank absorbance with matrix con- centration for 324.8 nm Cu lme.

Direct determmatton of Be, Cu and Zn m Al-U matnccs by ETA-AAS 711

Table 1 Expenmental parameters and analytrcal results

Carbon rod atomtzer settmgs (“C) Linear Llmlt of

Element and Dry Ash Atomize analytical quantrtatrve wavelength, Temp Temp Temp. range, determmation, nm (60 set) (60 =c) (3 set) lIglml !I

Be 100 950 2700 0 002-O 02 10 x 10-u 234 9 CU 100 950 2400 0.02-O 2 10 x IO-” 324 8

Zn 100 950 2700 IO-400 5 x 10-g 307 6

Senslhnty~

0.052 A/ng

0 006 A/ng

0 02 Alpg

*Based on 25 pg Al-U matrix m 5 ~1 soluhon tAs per IUPAC conventton, tt 1s expressed as slope of the analyttcal curve, A bemg the absorbance.

Instead, a blank correction method was adopted in the procedure for standardization in which the absorbance signal due to the matrix alone was subtracted from the gross analyte signals obtained for the standards. In the ETA-AAS studies carried out earlier6 on the determination of trace metallics in uranium, the matrix concentration was optimized at 20 mg/ml. However, in the present case, a matrix concen- tration above 5 mg/ml was found to substan- tially affect the reproducibility of the analytical signal due to the high value of non-specific absorbance. The high non-specific absorbance appears to be due to the presence of a large amount of alummium. It is known’ that aluminium, when heated under carbon rod atomizer (CRA) conditions, forms gas phase A&C* species in addition to commonly known Al,C3, contributmg to the high non-specific absorbance observed. The detection limits achieved in the present work for all the analytes are very low and hence the relative limits evalu- ated on the sample basis, viz. 5 mg/ml, are able to meet adequately the purity requirement of the fuel matenal.

The accumulation of the Al-U matrix has been found to affect the reproducibility of absorbance signals during the first forty atom- ization cycles. For the subsequent one hundred cycles, however, the absorbance remained constant. Such an effect was also observed earlier’ for some of the analytes in a uranium- plutonium matrix. A prior conditioning of the atormzer tube with the Al-U matrix was thus required for the determination of beryllium, copper and zinc. During forty atomization cycles, a total of 1 mg of the Al-U matrix would get deposited in the graphite tube. The con- ditioning of the tube was therefore carried out by firing the same total amount of the matrix but spread over four atomization cycles, thereby preserving the tube for further use. This treat-

ment was found to be adequate for obtaining reproducible results during subsequent atomiz- ation cycles. It appears that initially, carbon available from the tube surface helps the atom- ization of the analytes oxides thus showing high analyte signal which reduces on successive atomization cycles due to the formation of stable uranium carbide. The analyte signal stabilizes to a low value on formation of a uniform layer of uranium carbide at the loading surface of the graphite tube furnace. This stage is reached on atomization of about 1 mg of the AI-U matrix. Thus on conditioning of the tube, reproducible analyte signals are observed, the effect being primarily due to the uranium-graphite reaction.

The optimized experimental parameters, the linear analytical range and the lowest absolute amount determined as well as the sensitivity for each of these analytes in the Al-U matrix are given in Table 1. The ash/atomization tempera- ture optimized here for all analytes refer to the values obtained in the presence of the Al-U matrix and thus differ significantly from those

Table 2. Analysis of synthetrc samples*

Element

Sample concentratron, &ml

Expected Obtained

Be 0004 0.0042 0.008 0.082 0012 0.012 0.016 0.017 0.020 0019

CU :z 0.042

0’120 0.065 0 130 0 150 0.145 0.200 0.190

Zn 4.0 4.05 8.0 I 20

120 12.7 16.0 152 24.0 23.5

*Analyses based on 10 replicate analyses.

NDXAM GOYAL et al

Element

Table 3. Matnx effect on analyte absorbance

Absorbance Concentratton, AlUrmmUm-

figglml Aqueous Uramum* uraniumt

Be 001 0503 0.375 0434 CU 0 05 0 257 0231 0.314 Zn 100 0.302 0 131 0 339

l Uramum matnx concentration IS 5 mg/ml tAfter correctmg for matnx-reagent blank as well as non-specrfic ab-

sorbance due to matnx

observed without it. The matrix appears to play a significant role in the dissociation/atomization process of the analytes. The ash temperature for all three analytes is minimum (200”) m aqueous medium as against 950” in the presence of Al-U matrix. The poor sensitivity attained for zinc is the result of using a less sensitive spectral line m the determination of zinc. The performance of the method was evaluated by analysing a number of synthetic samples. Typical analysis results for five samples are shown in Table 2. The results obtained for these analytes indicate close agreement with the added contents using standards prepared in the Al-U matrix. The precision of determinations calculated from repetitive analyses of these samples was found to be better than 6% RSD for all the three analytes.

The data shown in Table 3 give absorbances observed for the specific concentrations of the analytes in aqueous, uranium and alu- minium-uranium matrices. As seen from the table, the analyte absorbance appears to be reduced when aqueous medium is replaced by the uranium matrix, but the absorbance values get significantly restored to their original values when studied in the presence of the Al-U matnx. The studies reported* earlier by the authors have revealed that the appearance temperatures determined for these analytes are close to the temperature at which the partial pressure of oxygen increases due to the reaction U308+U02, viz. 1300”, resulting in suppression of the absorbance in the presence of uranium matrix. The aluminium-uranium (3 : 1) matrix contains only 1.25 mg/ml of uranium as com- pared to 5 mg/ml of uranium in uranium matrix. The reduced uranium content in the alu- mimum-uranium matrix reduces the availability of oxygen by the aforesaid reaction and results

in partial restoration of absorbance signals for the three analytes to their aqueous values. As a result, the method is developed by using a matching composition of the Al-U matrix for both the samples and standards.

The ETA-AAS methods described here are the first report on the direct determination of beryllium, copper and zinc in Al-U matrix samples and should be applicable to the analysis of Al-U alloys.

While some Al-U samples were dissolved, as described in the experimental section, and analysed by the reported method, their purity exceeded the limits detectable here.

Acknowledgement-The authors are grateful to Dr P. R. NataraJan, Head, Radmchemistry Division, B.A.R.C. for his keen interest and encouragement m the course of this work

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