Tatanta2004 by Dr DPS RATHORE

Talanta 62 (2004) 343–349

Analytical applications of a differential technique in laser-induced fluorimetry: accurate and precise determination of uranium in concentrates and for designing microchemielectronic devices for on-line determination in processing industries D.P.S. Rathore∗ , Manjeet Kumar Chemical Laboratory, Department of Atomic Energy, Atomic Minerals Directorate for Exploration and Research, West Block-VII, R.K. Puram, New Delhi 110066, India Received 30 May 2003; received in revised form 23 July 2003; accepted 1 August 2003

Abstract A novel instrumental technique for the direct, fast, accurate, and precise determination of uranium in concentrates and other U-rich materials (as well as to mineralized rocks) is presented. The proposed technique is an absolute methodology, based on the comparison of the fluorescence of the accurately known standard with a sample of similar but unknown concentration in the low operational range of the instrument (on same sample-dilution basis), by the use of H3 PO4 –NH4 H2 PO4 as a fluorescence-enhancing reagent. The relative standard deviation of the proposed technique was 0.5–0.9% (n = 9) at 18.1, 36.2, 61.2, and 99.6% U3 O8 . The proposed technique is suitable for the determination of uranium in samples arising from exploration projects, ores from mining operations, mill process samples, uranium ore concentrates leading to fuel fabrication as well as samples from environmental monitoring containing up to 100% uranium. The results are in good agreement with those obtained by titrimetric, gravimetric, and TBP extraction–H2 O2 spectrophotometric methods. The precision of the technique is within the acceptable ‘pure geochemistry’ type of analysis (R.S.D. ∼ 1.0%) and is comparable even those obtained with titrimetric and gravimetric assay. The proposed differential technique coupled with flow injection may open up new advancement in instrumentation leading to design and development of microchemielectronic devices for direct on-line determination, more compatible with the tools of computer age as also to help in handling of radioactive solutions in chemical laboratories in uranium processing industries. © 2003 Elsevier B.V. All rights reserved. Keywords: Differential technique; Laser fluorimetry; Uranium determination; Concentrates and instrumental advances

1. Introduction Various methods are used for analysis of uranium concentrates and geochemical samples. Selection of the most appropriate method depends on many parameters: on the purpose of analysis, nature of the analyte, the concentration of uranium, the presence and concentration of the other elements in the matrix, the methodology available, the accuracy required, etc. The determination of uranium in concentrates, like mineral beneficiation products such as, yellow cake, impure uranyl nitrate, high-purity uranium compounds, grab sample and other U-rich materials require a precision higher ∗ Corresponding author. Tel.: +91-11-26101913; fax: +91-11-26107358. E-mail address: dpsr2002@yahoo.com (D.P.S. Rathore).

than that obtainable by fluorimetric, spectrophotometric or polarographic methods and in such cases titrimetric methods are almost invariably used [1–4]. In 1964, Davies and Gray [5] published a titrimetric method for uranium, which permits the accurate determination of uranium in the presence of large quantities of impurities. Inspite of the method being simple, precise, reliable, and applicable to a wide range of uranium containing materials, even though several modifications [6–8] have been reported in the Davies and Gray’s method. This is done either by: (i) improving the sensitivity of the visual end point [6–8] or by replacing the end point by electrometric methods [6–8], or (ii) scaled down [8] Davies and Gray’s method by taking small aliquots to reduce the difficulties of handling radioactivity and also to minimize the generated analytical waste. The correction for the reagent blank is

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necessary [5–8] since the uranium aliquots are small and blank was obtained by extrapolation method [8]. Therefore, even in the modified Davies and Gray’s titrimetric methods, the number of steps is further increased and each step has to be adhered to with a great precaution for a single reliable analysis. An ideal analytical method is desirable to meet the following three basic requirements: reliability, applicability, and practicability. Laser-induced fluorescence (LIF) is a very sensitive, selective, and versatile technique mainly for uranium ultratrace analysis in various fields of the nuclear fuel cycle [9–11]. More recently, we have reported the application of the differential technique in laser-induced fluorimetry (DT-LIF) for the determination of uranium in mineralized rocks [12]. High precision and accuracy of a technique, being an essential criteria, has prompted us to extend the application of DT-LIF method to concentrates and other wide range of diverse U-rich materials. Under the conditions for the use of the differential technique, in which the fluorescence of the unknown concentration in the cuvette is matched with that of the standard of accurately known concentration using H3 PO4 –NH4 H2 PO4 as a fluorescence-enhancing reagent, which amount to a fluorimetric titration. In this way, accuracy and precision of ‘differential technique’ were found to be comparable with differential spectrophotometric technique as well as to classical titrimetric and gravimetric methods. By analogy of differential technique with titrimetry, thus give rise to an absolute methodology [13] based on the use of analytical chemical standards or certified reference materials. The use of certified reference materials as standards ensures calibration, control, and optimization of the quality of analytical data [1,14–17]. In the present work, we report the application of ‘differential technique in laser fluorimetry’ for the direct and fast determination of uranium at a dynamic concentration range from ppm to 100% U3 O8 and its applications in designing microchemielectronic devices for on-line determinations, more compatible with the tools of computer age as also to help in handling of radioactive solutions in chemical laboratories of uranium processing industries.

2. Experimental 2.1. Instrumentation: laser-induced fluorimeter The method developed at this laboratory was studied using the instrument, Scintrex UA-3 Uranium Analyzer (Scintrex Limited, Concord, Ontario, Canada) [10,11]. 2.2. Reagents Fluorescence-enhancing reagent: An acidic buffer mixture of H3 PO4 (1 M) and NH4 H2 PO4 (2.17 M) has been used as a fluorescence-enhancing reagent [12].

Aqueous standard U3 O8 (1 mg ml−1 ) stock solution [1]: Aqueous standard stock solution of uranyl ion of 1 mg ml−1 was prepared from U3 O8 or uranyl nitrate of analytical-reagent grade having 10% HNO3 . The concentration of uranium in this stock solution was verified using the method of Davies and Gray [5]. 2.3. Reference standards IAEA low grade uranium ore, Torbernite (Australia)—S1 (0.471 ± 0.002% U3 O8 ), Torbernite (Spain)—S2 (0.313 ± 0.001% U3 O8 ) and Carnotite (USA)—S3 (0.418 ± 0.002% U3 O8 ) were used as standard reference samples. The solutions of standard reference samples were prepared as per the prescribed procedure. The concentrations of uranium in S1 , S2 , and S3 are 0.00942, 0.00626, and 0.00836 mg ml−1 U3 O8 in 10% HNO3 , respectively. Alternatively, 0.01 mg ml−1 U3 O8 was prepared by diluting 1 ml of the 1 mg ml−1 U3 O8 to 100 ml maintaining 10% HNO3 . Similarly, 0.005 mg ml−1 U3 O8 was prepared by diluting 1 ml of the 1 mg ml−1 U3 O8 to 200 ml maintaining 10% HNO3 . These solutions serve as reference standards. All other chemicals were of analytical-reagent grade. Distilled water was used throughout. 2.4. Differential technique in laser-induced fluorescence (DT-LIF) procedure With an Eppendorf micropipettor, take 100 ␮l aliquots of test samples and at least two reference standard solutions (take at least one higher reference standard containing ∼0.01 mg ml−1 U3 O8 ) separately into 25 ml calibrated flasks (final concentration of U3 O8 up to 40 ng ml−1 ). Add 10 ml buffer mixture of phosphoric acid and ammonium dihydrogen phosphate as a fluorescence-enhancing reagent and dilute to the mark with distilled water. Adjust the sensitivity control of the instrument to give a scale deflection of 6–8 in the panel meter (i.e. more than half of the scale deflection of panel meter) with reference standard solution containing ∼0.01 mg ml−1 U3 O8 in the low operational range of the instrument. Then, measure the fluorescence of the solutions prepared as above. Measure the fluorescence of the reagent blank prepared in the similar manner but containing no uranium. Compute the results using the differential technique as described earlier [12] or as follows: (1) compute a factor for each of the two comparison standards, concentration of U3 O8 (mg ml−1 ) in comparison standard factor = net fluorescence of comparison standard (2) calculate an average from the two factors,

D.P.S. Rathore, M. Kumar / Talanta 62 (2004) 343–349

(3) compute the concentration of U3 O8 (mg ml−1 ) in the original sample, U3 O8 (mg ml−1 ) = average factor × net fluorescence reading of sample × dilution factor (4) compute the percent U3 O8 in the sample, U3 O8 (mg ml−1 ) in the original sample × volume of sample × 100 U3 O8 (%) = weight of sample (mg)

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tric acid solutions. In the proposed technique, even 10–30% nitric acid concentration in original sample solution has no effect on uranium fluorescence. Since aliquot of sample used is 100 ␮l, which when diluted to 25 ml is in the pH range of 2.19–1.72 and the final pH ∼ 2 of the resulting solution for fluorescence measurements is maintained by fluorescence-enhancing buffer. A 10% nitric acid concentration in original sample solutions is sufficient to keep all metal ions in solution and also for additional dilutions. So, from an analytical point of view, at pH ∼ 2, only UO2 2+ species are present. Thus, a single uranyl fluorescent complex is formed between free UO2 2+ and fluorescence-enhancing reagent.

2.5. Sample solution preparation procedure 3.2. Effect of foreign ions 2.5.1. Rock samples Solution of powdered rock samples (500 mg, 150–200 mesh) is obtained by evaporation with a mixture of hydrofluoric acid (10 ml, 40%) and nitric acid (10 ml, 8 M) to dryness twice, followed by evaporation with 5 ml of concentrated nitric acid thrice (to remove fluoride). The sample is digested with 25 ml of concentrated nitric acid. In titanium rich matrices, after digestion of samples with concentrated HNO3 the sample solution is transferred in a beaker containing a few ml of H2 O2 and then boiled vigorously for further 20–30 min to decompose H2 O2 . If a little unattacked residue remains, it is filtered off, washed, and brought in to solution by sintering and fusing with a minimum amount of a mixture of sodium fluoride and potassium pyrosulphate. The melt after cooling is brought in to solution in nitric acid. Mix the two solutions and make up to 250 ml in a calibrated flask maintaining 10% HNO3 .

The phosphoric acid–ammonium dihydrogen phosphate buffer system is more selective for uranium in the presence of metal ions and found to be most suitable for rock samples of diverse matrices [12]. In the proposed method, the sample solution are suitably one or two step simply diluted with distilled water using Eppendorf single push-button microlitre pipettes [12] to bring the concentration of uranium within the low operational range of the instrument, which also brings the concentration of quencher impurities well below the tolerance limits of the method in the final solution for subsequent fluorescence measurements. The procedure of elimination of interference by simple dilution of sample has the distinct advantage of being quick and very simple to perform because it does not require any chemical preparation or extraction. 3.3. Choice of reference standard

2.5.2. Yellow cake or uranium oxide The sample solution of yellow cake or uranium oxide (500 mg) is obtained by treating the sample with nitric acid (30 ml, 8 M), add a few drops of HF, fume near to dryness followed by evaporation with concentrated nitric acid thrice. The sample is digested with nitric acid (50 ml, 8 M) and boiled for 15–20 min to get a clear solution and make up to 250 ml in a calibrated flask. The final solution is in 10% nitric acid.

3. Results and discussion The fluorescence of the reagent blank prepared as per the prescribed procedure was found to be negligible. 3.1. Effect of high nitric acid concentration High nitric acid concentration as reprocessing medium of samples although has the drawbacks, such as absorption, quenching, and complexation [18]. As the nitrate ion is known to very weakly complex uranyl, and as nitrate concentration is linked to H+ concentration in the case of ni-

The technique used in differential spectrophotometry [19,20] for selecting the appropriate reference standard concentration to obtain the maximum precision possible in any given analyses, can also be applied in fluorimetry [12]. An effort was made to get certified reference materials of yellow cake or an allied material of high grade but failed. It was found that the IAEA low grade uranium ore, such as Torbernite (Australia)—S1 (0.471 ± 0.002% U3 O8 ), Torbernite (Spain)—S2 (0.313 ± 0.001% U3 O8 ) and Carnotite (USA)—S3 (0.418±0.002% U3 O8 ) can be exploited as standard reference samples in the low operational range of the instrument as per the prescribed procedure. Alternatively, 0.01and 0.005 mg ml−1 of U3 O8 solutions in 10% HNO3 suitably diluted from 1 mg ml−1 of U3 O8 stock solution in 10% HNO3 , serve as the most suitable reference standards comparable with and fulfilling all the criteria essential for certified reference materials for such applications [21,22]. 3.4. Reproducibility of the method Under the optimized conditions, the reproducibility of the proposed method was checked by performing nine replicate

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Table 1 Determination of uranium in the stock solution of U3 O8 and IAEA reference solutions U3 O8 in standard sample solution (mg ml−1 ), calculated

U3 O8 a in standard sample solution (mg ml−1 ), found

0.500b 1.000b 1.000b 1.361c 1.848c 1.893c

0.502 1.007 1.002 1.341 1.849 1.913

± ± ± ± ± ±

0.004 0.005 0.004 0.011 0.016 0.011

Relative standard deviation (%) 0.84 0.50 0.42 0.80 0.89 0.58

Mean ± S.D. (n = 8). Different aliquots of standard stock solution of 1 mg ml−1 U3 O8 were taken and diluted to different volumes. c In case of IAEA samples, different weights of samples were taken and made up to constant volume; all these sample solutions were analyzed as per the prescribed procedure. a

obtained by using titrimetric and gravimetric methods. In titanium rich grab sample matrices containing up to 32% TiO2 in 10% nitric acid, the data between methods differ by less than 0.5% relative to Davies and Gray’s method. However, higher contents of TiO2 in 10% nitric acid cause hydrolysis of sample solution. Even on hydrolysis due to higher titanium contents, the data between methods differ by less than 2% relative, which may be attributed to variations in the uranium contents. The proposed work is a unique application of the differential technique to the real samples, tested, evaluated and is applied to a large number of samples of diverse matrices from different on-going uranium projects over a period of more than 5 years in different regional chemical laboratories. 3.6. Comparison with other analytical methods

determinations of 18.1, 36.2, 61.2, and 99.6% U3 O8 over a period of nine consecutive days. The relative standard deviation of the method was 0.5–0.9% irrespective of different analysts, reagents, instruments, times, and laboratories, which is within the acceptable limits of ‘pure geochemistry’ type of analysis [23] and is comparable even those obtained with titrimetric and gravimetric assay. Hence, the method appears to be reliable.

A comparative performance of the proposed differential technique with other analytical methods for the determination of uranium is presented in Table 4. The proposed technique is most suitable for the determination of uranium in samples containing up to 100% U3 O8 .

3.5. Determination of uranium

3.7.1. Designing of direct concentration reading laser fluorimeter In the determination of uranium by DT-LIF method, there is only one-step sample-dilution for fluorescence measurement without any separation step from accompanying matrix elements. By keeping all the parameters such as, fluorescence-enhancing reagent system, sensitivity control, dilutions/sample weights as constant, the relationship between fluorescence (F) and concentration of uranium can be expressed in a simplified way as follows,

In order to check the validity of the proposed method, the method was applied to the analysis of different aliquots of standard uranyl nitrate solution, grab samples, yellow cake, and purity of uranyl nitrate analaR grade (BDH Chemicals Ltd., Poole, England). The samples have been analyzed as per the prescribed procedure. The results obtained are given in Tables 1–3. As is evident from Table 3, the data between proposed method and Davies and Gray’s method in yellow cake samples differ by 0.3–0.6%, which is within the precision figure of the differential technique as well as with those Table 2 Determination of purity of uranyl nitrate analaR grade (BDH) No. of replicate values

Uranyl nitratea (%)

5 2 5 3

99.14 100.70 99.38 100.00

values values values values

n = 15

99.60 ± 0.55b (R.S.D., 0.55%)

a Sensitivity control of the instrument is adjusted to give scale deflection of 6, 7, and 8.6 in the low range, respectively with reference standard solution of ∼0.01 mg ml−1 U3 O8 . Measure the fluorescence of five replicate of a suitably diluted (100-fold) sample solution of uranyl nitrate. Compute the percent concentration of U3 O8 in the sample. The original solution of uranyl nitrate was prepared by dissolving 0.5081 g in 250 ml as unknown sample in 10% HNO3 . b Mean ± S.D.

3.7. Application of differential technique in instrumental advances

F = KC

(1)

where K is a constant of proportionality. This Eq. (1) is used to determine the direct concentration of uranium in unknown samples by just comparing the fluorescence response of the CRM. In UA-3 uranium analyzer, there are low and high range of the instrument each differing by a factor of 10. In UA-3 uranium analyzer, there is always a decrease in laser intensity on prolonged use of the instrument. But, we performed a large number of experiments on UA-3 uranium analyzer, in which laser tube was reconditioned [24] (sealed-off) by Instrumentation and Control Division, Centre for Advanced Technology, Department of Atomic Energy, Indore, India, and no change in the laser intensity with prolonged use of the instrument was observed over a period of 3 years. More recently [25], Laser Instrumentation Section, has developed world’s smallest sealed-off nitrogen laser module measuring only 145 mm × 75 mm × 50 mm with integrated H.V. power supply.

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Table 3 Determination of uranium in the yellow cake, high grade grab samples, and mineralized rock samples Sample

Grab Grab Grab Grab

U3 O8 found by proposed methoda (%)

sample sample sample sample

Yellow Yellow Yellow Yellow Yellow Yellow Yellow

a b c d

cake cake cake cake cake cake cake

Total U3 O8 Leachable U3 O8

U3 O8 found by Davies and Gray’s titrimetric method (%)

U3 O8 found by gravimetric homogeneous precipitation method (%)

18.20 36.52 36.60 40.02

± ± ± ±

0.18; 0.32; 0.34; 0.35;

0.99 0.88 0.93 0.87

18.10 36.52 37.19 40.70

– – – –

61.24 62.50 62.50 74.15 75.52 76.90 74.60

± ± ± ± ± ± ±

0.51; 0.30; 0.31; 0.37; 0.32; 0.35; 0.39;

0.83 0.48 0.50 0.50 0.42 0.46 0.52

61.05 62.10 62.21 74.17 75.37 76.62 74.88

61.2 62.4 62.4 – – – –

2.79 ± 0.00 2.65 ± 0.00

– –

U3 O8 found by TBP-extraction–H2 O2 spectrophotometric method (%) 17.73 36.77 – – – – –

– –

Other elements present in grab samples a (%)

b (%)

c (%)

d (%)

SiO2 TiO2 Al2 O3 Fe2 O3 (T) CaO MgO P2 O 5 MnO Na2 O K2 O LOI

5.11 32.00 2.44 3.06 5.66 0.07 0.08 0.24 0.78 0.24 5.59

3.48 38.67 0.49 2.31 2.84 0.12 0.37 0.04 1.21 0.14 3.75

2.67 39.33 0.42 1.65 2.86 0.16 0.66 0.05 1.01 0.14 5.38

26.91 15.10 10.33 4.31 6.72 3.59 0.07 0.14 1.00 2.27 8.51

Mean ± S.D. (n = 8); %R.S.D.

Table 4 Comparison of the analytical performance of the proposed technique with other methods for the determination of uranium Conventional fluorimetry [1]

Solvent-extractive spectrophotometry [1]

Classical titrimetric and gravimetric methods [1–5]

DT-LIF method [12] and present work

(1) Time consuming, multiple steps

Time consuming, multiple steps

(2) Separation step is mandatory

Separation step is mandatory

Titrimetric method

(3) Less precision (4) Moderate accuracy (5) Applicable up to 500 ppm U3 O8

Less precision Moderate accuracy Applicable to more than 500 ppm to 1% U3 O8

High precision Most accurate Applicable to more than 5% U3 O8

(6) Quantity of sample needed is moderate but analytical waste generated is maximum (7) Cost of chemicals for analysis per sample after solution preparation is six times of DT-LIF methoda ,b (8) Uses calibration method which is based on assumptions

Quantity of sample needed is moderate but analytical waste generated is maximum Cost of chemicals for analysis per sample after solution preparation is 5–10 times of DT-LIF methoda ,b Uses Calibration method which is based on assumptions

Quantity of sample needed as well as analytical waste generated are maximum Cost of chemicals for analysis per sample after solution preparation is 20 times of DT-LIF methoda ,b Uses weighing/or volume by difference method. Hence, more reliable

Analysis time is 1–2 min per sample No separation step is needed. Simple dilution of sample High precision Most accurate Applicable to dynamic range of concentration (from 100 ppm to 100% U3 O8 ; R.S.D. < 1%) Quantity of sample needed as well as analytical waste generated are minimum Cost of chemicals for analysis per sample after solution preparation is lowest (Rs. 2 per sample)a ,b Differential technique using reference standards guarantees the quality of an analytical result. It is a self-standardized method

Besides this, foreign exchange on a proprietary fluorescence-enhancing reagent FLURAN III is also saved by substituting it with indigenously developed H3 PO4 –NH4 H2 PO4 as a fluorescence-enhancing reagent used in DT-LIF. a Difference in cost of analysis per sample between conventional fluorimetry and DT-LIF method after sample solution preparation is Rs. 10. For 10,000 samples, the difference in cost is ∼Rs. 1 lakh (significant saving of national exchequer). b Difference in cost of analysis per sample between Davies and Gray titrimetric [5] and DT-LIF method after sample solution preparation is Rs. 37. For 10,000 samples, the difference in cost is ∼ Rs. 3.7 lakh (significant saving of national exchequer).

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D.P.S. Rathore, M. Kumar / Talanta 62 (2004) 343–349 AIR LASER FLUORIMETER

TO WASTE AUTO SAMPLER

PVC MIXING COIL

CELL

BUFFER REAGENT PROPORTIONATING PUMP

RECORDER FOR DIGITAL READ-OUT or SPECTRUM RECORDER

Fig. 1. Auto analyzing flow-injection laser fluorimeter.

We found that at a fixed sensitivity control of the instrument, the value of common factor as per the prescribed procedure followed remains constant up to three places of decimal. This is an interesting observation and can be exploited in practically designing pre-calibrated laser-induced fluorimeter. In this way, with the advancement in laser tube, digitization and computer controlled electronic signal processing, the instrument can be practically pre-calibrated to read directly uranium in percentage or mg ml−1 in test samples at different ranges of concentrations (i.e. without any separation step as well as further dilution of sample aliquots). 3.7.2. Automation of laser fluorimetric method for uranium determination The application of proposed DT-LIF coupled with unique features of flow-injection analysis (FIA) [26–28], can be exploited to enable us to design and develop microchemielectronic devices. Such microchemielectronic devices will be more compatible with the tools of computer age as also to help in handling of radioactive solutions in chemical laboratories of uranium processing industries at various stages of nuclear fuel cycle. The recommended analytical procedure for fluorescence measurement can be accomplished in a similar fashion as in the automation of spectrophotometric methods using flow-injection system for introducing sample and reagent transportation. The flow-injection is a simplest

100uL SAMPLE PERISTALTIC PUMP SAMPLING LOOP

BUFFER REAGENT

LASER FLUORIMETER

MIXING COIL

PVC TUBING

TO WASTE

BY PASS

way of carrying out automatic wet chemical analysis rapidly and efficiently. The heart of a continuous flow instrument, be it segmented or non-segmented, is the peristaltic proportionating pump system and are capable of delivering remarkably reproducible volumes of samples and reagents The volume is controlled by the inside diameter of the tubing and by the pumping time (Figs. 1 and 2). Such other microchemielectronic devices may prove valuable for the determination of other fluorescent actinides and lanthanides in solution in different complexing media by taking advantages of triple selectivity of time resolved laser-induced spectrofluorimetry. Among the actinides and lanthanides [29], the ones that are fluorescent in solution are uranium, curium, americium, europium, terbium, dysprosium, samarium, gadolinium, cerium, and thulium.

4. Conclusion The simplicity, rapidity, free from matrix effects, no separation steps, minimum generation of radioactive analytical waste, maximum throughput, cost-effectiveness, accuracy, and high precision are the significant features of the proposed differential technique. Differential technique is unique and warrants its use for routine determination of uranium in samples of diverse matrices over a dynamic range of concentrations from ppm to 100% U3 O8 . The ‘flow-injection’ coupled with ‘differential technique in laser-induced fluorimetry will be most promising microchemielectronic device for uranium determination in future advancement and may also prove valuable for the determination of other fluorescent actinides and lanthanides in solution in different complexing media by taking advantages of triple selectivity of Time Resolved Laser-Induced Spectrofluorimetry.

Acknowledgements RECORDER

Fig. 2. Segmented flow-injection laser fluorimeter.

We thank Dr H.C. Arora, Associate Director, Shri D.B. Sen, Regional Director, Shri A.K. Bagchi, Additional

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Director, and Shri R.K. Gupta, Director, AMD, for their kind permission to publish this work. The authors wish to thank Shri P.K. Srivastava and Dr. D.S.R. Murthy for their constant inspirations and suggestions on the theme of this work. Thanks are also due to my all colleagues for their help, critical comments, and valuable discussions during the progress of this work.

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