Presentation of differential laser-induced fluorimetry as a reference measurement procedure for determination of total uranium content in ores and similar matrices D. P. S. Rathore, Manjeet Kumar & P. K. Tarafder
Accreditation and Quality Assurance Journal for Quality, Comparability and Reliability in Chemical Measurement ISSN 0949-1775 Volume 17 Number 1 Accred Qual Assur (2012) 17:75-84 DOI 10.1007/s00769-011-0838-2
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Author's personal copy Accred Qual Assur (2012) 17:75–84 DOI 10.1007/s00769-011-0838-2
PRACTITIONER’S REPORT
Presentation of differential laser-induced fluorimetry as a reference measurement procedure for determination of total uranium content in ores and similar matrices D. P. S. Rathore • Manjeet Kumar P. K. Tarafder
•
Received: 16 April 2011 / Accepted: 23 September 2011 / Published online: 15 October 2011 Ó Springer-Verlag 2011
Abstract The metrological principle of ‘differential technique in laser-induced fluorimetry’ analysis is discussed and recommended as a reference measurement procedure for determination of total uranium content in ores and similar matrices. The estimated relative expanded uncertainty values obtained for uranium content in standard IAEA samples are, S 1, 0.04 g/kg, S 2, 0.06 g/kg, S 3, 0.04 g/kg, and for S 4, 0.10 g/kg, respectively. These low uncertainty values obtained for uranium show high metrological quality of differential technique. This reference measurement procedure guarantees the quality of an analytical result (accuracy, high precision, reliability, comparability, and traceability). Laser-induced fluorimetry will be useful for the analysis of uranium in ores, certification of reference materials, borehole core assay, and other diverse applications in nuclear fuel cycle. Differential technique in spectrophotometry/laser fluorimetry has inherent high metrological quality. In principle, laser-induced fluorimetry is an ideal technique for D. P. S. Rathore (&) Chemical Laboratory, Atomic Minerals Directorate for Exploration and Research, Department of Atomic Energy, Pratap Nagar, Sector-V Extension, Sanganer, Jaipur 302030, India e-mail: dpsr2002@yahoo.com; dpsrathore.amd@gov.in M. Kumar Chemical Laboratory, Atomic Minerals Directorate for Exploration and Research, Department of Atomic Energy, West Block-VII, R. K. Puram, New Delhi 110066, India e-mail: manjeetsudha2000@yahoo.co.in P. K. Tarafder Chemical Laboratory, Atomic Minerals Directorate for Exploration and Research, Department of Atomic Energy, PO: Tatanagar, Jamshedpur, East Singhbhum District, Jharkhand 831002, India e-mail: ptarafder@sify.com
the very accurate determination of uranium by the use of appropriate fluorescence-enhancing reagents and methodology depending upon the concentration of uranium and sample matrices. Keywords Laser-induced fluorimetry Differential technique Reference measurement procedure Measurement uncertainty Standard materials
Introduction Uranium is a lithophilic element nearly ubiquitous in the nature and environment [1–3]. Importance of uranium is due to its main use as a fuel for nuclear power program [1, 2] as well as to its chemical toxicity [2]. Uranium is present in the environment as a result of leaching from natural deposits, release in mill tailings, and emissions from the nuclear industry, the combustion of coal and other fuels and the use of phosphate fertilizers that contain uranium [3]. There is a continued interest in the development of analytical measurement procedures having high metrological quality (low uncertainty) and traceable for amount of substance in naturally occurring sample matrix in analytical chemistry [4–10]. Efforts have been made for minimization of uncertainty in chemical analysis [11, 12]. Reliable analytical results are often the basis for critical discussions in assessing nuclear operations, environmental pollution, minerals resource potential, production of highquality standard reference materials, and for worldwide inter-comparisons. The challenges in understanding definitions and communication of the metrological concepts in the context of chemical measurements have been methodically discussed in editorials [13, 14]. The metrological
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concepts have been elaborated in the ‘International vocabulary of metrology-basic concepts and associated terms’ (VIM) JCGM200:2008 [15] and IUPAC technical report [16]. Meyer [17] reviewed various aspects of the determination of the measurement uncertainty of all validated analytical test procedures as an added value. It is believed that experience is a wonderful thing. It enables us to recognize a mistake when we make it again. The use of primary method or principle of the method in analytical measurement per se does not guarantee a reliable or metrologically traceable result. It has been now well recognized that the quality of measurement result is of highest importance and depends on the realization of the method (strict adherence to the number of steps) into practice [18]. In the VIM, ‘reference measurement procedure’ is described as a measurement procedure accepted as measurement results fit for their intended use in assessing measurement trueness of measured quantity values obtained from other ‘measurement procedures’ for quantities of the same kind, in calibration, or in characterizing reference materials. Metrology is science of measurement, while metrological traceability can be established through traceability chain(s). Accordingly, ‘reference measurement procedure plays an essential practical role in establishing metrological traceability to the base units of SI through metrological traceability chain(s)’. In practice, there are few reference measurement procedures available to chemical metrologists [19]. Among these reference measurement procedures reported so far in analytical chemistry, titrimetric measurement procedures are simplest, rapid, easy to perform, cost-effective and are regarded as absolute method. As of April 2007, the instrumental neutron activation analysis (INNA) [6–10] is now formally recognized by the Consultative Committee for Amount of Substance—Metrology in Chemistry of the Bureau International des Poids et Mesures (BIPM) as a primary ratio method (reference measurement procedure as per VIM). Over the years, the concept of classical titrimetry [20–22] has changed with the advancement in electronics, instrumentation and automation via valves and flowinjection [23]. New approaches of titrimetry have emerged, which give excellent precision and efficiency in comparison with classical titrimetry even at microliter volume level. Various reagents in highly precise microliter aliquots can be delivered in the reaction chamber (equipped with conductometric, potentiometric, fluorimetric and spectrophotometric sensors) from pneumatically pressurized reservoirs fitted with inexpensive high-speed valves in the liquid delivery lines controlled by microcomputers. The use of a ‘differential technique (DT)’ in the recommended ‘reference measurement procedure’ is different to standard procedure recommended by the instrument’s manufacturer
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[1, 24–26]. Differential technique in laser-induced fluorimetry is based on the comparison of the fluorescence of the standard with that of unknown samples on the same sample weight or dilution basis. In this way, the error in analysis is confined to the difference in two concentrations and is so minimized [24, 25]. In conventional fluorimetric method [27], after solvent extractive separation of uranium from accompanying matrix, the liquid sample containing uranium compound is first evaporated carefully to dryness and the residue is fused at a high temperature with a carbonate–fluoride flux to produce a solid disk. The disk is then placed in an optical fluorimeter, where it is illuminated by ultraviolet light to cause uranium present to fluoresce. This conventional method suffers from a lack of sensitivity, limited precision, and complicated time consuming preparative chemistry. Laser-induced fluorescence (LIF) is a very sensitive, selective and versatile technique for uranium analysis in various fields of the nuclear fuel cycle [1, 24–26, 28, 29]. The simple practical approach described in this article decrease the need for exactly matching standard reference materials for a vast diversity of analytical problems and also minimizes uncertainty in analytical measurements. Differential laserinduced fluorimetry is the method of choice, which can be used in the field as well as in a control laboratory. It also fulfills the basic essential requirements of RAP’s: Reliability (accuracy and high precision), Applicability (applicable to diverse sample matrices for wide applications in entire nuclear fuel cycle) and Practicability (inherent high sensitivity, high-performance qualification, simple, rapid and direct, easy calibration and operation, cost-effective). In the continuation of our work on uranium determination using laser-induced fluorimetry [24–26], this paper deals with the presentation of differential laser-induced fluorimetry as a reference measurement procedure for determination of total uranium content in ores and similar matrices. In principle, this methodology can be extended for the determination of other fluorescent compounds/elements.
Experimental Instrumentation Laser-induced fluorimeter [24–26]. The method developed at this laboratory was studied using the instrument, Scintrex UA-3 Uranium Analyzer (Scintrex Limited, Concord, Ontario, Canada). In UA-3 uranium analyzer, a compact sealed molecular nitrogen laser is the light source, emitting very intense but short-lived (3–4 ns) ultraviolet (337.1 nm) pulses of maximum energy of 200 lJ/pulse, at a repetition rate of 15 pulses/s, which selectively excites the fluorescence of the uranyl ions in solution (the instrument was
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originally constructed for the rapid determination of uranium). A nitrogen laser is used in preference to other sources of ultraviolet light because the resultant pulse is intense yet self-terminating, monochromatic and highly directional; the full output power is thus easily directed and focused on the sample cell. The fluorescence of a solution in the cell is detected by a photomultiplier (PMT) isolated by a green-transmitting filter. The amplified and gated (shunt gates that allow transmission only for a 100-ls period starting from 30 ls after the laser pulse) fluorescence signals from the PMT are integrated for 4 s and then displayed on a panel meter, which can be calibrated directly in ng/mL uranium. The samples were also analyzed by digital-readout nitrogen laser fluorimeter fabricated by Laser Applications and Electronics Division, RR Centre for Advanced Technology (CAT), Department of Atomic Energy, Indore, India. Reagents: fluorescence-enhancing reagent [24, 25] An acidic buffer mixture of H3PO4 (1 mol/L) and NH4H2PO4 (2.17 mol/L) has been used as a fluorescenceenhancing reagent. This buffer is sensitive and most selective fluorescence-enhancing reagent for uranium in acidic solution of rock samples. Uranium present as uranyl ion, UO22?, in sample solution strongly complexes with phosphoric acid–hydrogen phosphate buffer at pH & 2 to form UO2(H2PO4)2. H3PO4 as the predominant fluorescent species [24]. Aqueous standard U3O8 (1 mg/mL) stock solution [24, 25] Aqueous standard stock solution of uranyl ion of 1 mg/mL was prepared from U3O8 or uranyl nitrate of analyticalreagent grade having 1.56 mol/L HNO3. The concentration of uranium in this stock solution was verified using the method of Davies and Gray. Reference standards [24, 25, 30] IAEA low-grade uranium ore, Torbernite (Australia)—S 1 (4.71 g/kg U3O8), Torbernite (Spain)—S 2 (3.13 g/kg U3O8), Carnotite (USA)—S 3 (4.18 g/kg U3O8), and Uraninite (Australia)—S 4 (3.75 g/kg U3O8) were used as ‘international standard samples’. The solutions of these samples were prepared as per the procedure and diluted 1 mL of the solution to 100 mL maintaining 1.56 mol/L HNO3. The concentrations of uranium in diluted solution S 1, S 2, S 3 and S 4 are 0.00942, 0.00626, 0.00836 and 0.00750 mg/mL U3O8, respectively. These solutions are used as ‘standard samples’ for calibration of the instrument in the low operational range as per the prescribed procedure.
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Alternatively [25], 0.01 mg/mL U3O8 was prepared by diluting 1 mL of the 1 mg/mL U3O8 to 100 mL maintaining 1.56 mol/L HNO3. Similarly, 0.005 mg/mL U3O8 was prepared by diluting 1 mL of the 1 mg/mL U3O8 to 200 mL maintaining 1.56 mol/L HNO3. These solutions serve as standards for the calibration of instrument. All other chemicals were of analytical-reagent grade. Distilled water was used throughout. Reference measurement procedure With a single push-button micropipettor, take 100-lL aliquots of test samples and three reference standard solutions (take at least one higher reference standard containing &0.01 mg/mL U3O8) separately into 25 mL calibrated flasks (final concentration of U3O8 up to 40 ng/mL). 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 fluorescence indication (Fstd) of 6–8 in the panel meter (i.e., more than half of the fluorescence indication of panel meter) with reference standard solution containing &0.01 mg/mL U3O8 in the low operational range of the instrument. All volumetric operations were carried out using same pipette and glassware followed by fluorimetric measurement operations at controlled room temperature, and then the fluorescence of the solutions prepared was measured as above. Samples containing a higher concentration of uranium are diluted 10-fold and the fluorescence of the reagent blank prepared without uranium was measured in the similar manner. Compute the mass fraction of U3O8 in the original sample, wstd ws ðU3 O8 Þ ¼ Fs Fstd on same sample weight=dilution basis where wstd and ws denote the mass fraction of U3O8 in the comparison standard and samples, respectively. Rock samples solution preparation procedure [24, 25] Among the various dissolution procedures for the determination of total uranium, in various matrices, dissolution with HF and HNO3 mixture in platinum dishes followed by Na2O2 fusion of residue, if any left, is practiced mostly. Solution of powdered rock samples (500 mg; particle size, 74–88 lm) is obtained by repeated evaporation with a mixture of HF–HNO3 to cause complete removal of silicon by vaporization as tetrafluoride and oxidation of tetravalent uranium to more soluble hexavalent uranyl ion and then repeated evaporation with HNO3 (to remove fluoride).
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Twenty-five milliliters of concentrate HNO3 and equal amount of distilled water to digest the sample residue were added on hot plate for 1 h to get a clear solution. If solution is not clear, few drops of H2O2 were added and then boiled vigorously for further 15–20 min to decompose H2O2. If a little unattacked residue remains, it is filtered off, washed and brought into solution by sintering and fusing with a minimum amount of a mixture of sodium fluoride and potassium pyrosulfate. The melt after cooling is brought into solution in nitric acid. The two solutions were mixed and made up to 250 mL in a calibrated flask maintaining 1.56 mol/L HNO3.
Results and discussion Theory of differential laser-induced fluorimetry In the case of pulsed excitation, the fluorescence signal (fluorescence indication) at a time ‘t’ can be expressed as follows [29]: l 1 e elc 0 0 FðtÞ ¼ kI0 eCe Dt=s e e c l ð1Þ s0 ec where k is the apparatus factor, I0 is the laser intensity, s0 is the natural fluorescence life time, e and C, respectively, are the absorption coefficient and mass fraction of the fluorescence species, Dt is the time between the laser excitation and the fluorescence measurement, s is the fluorescence lifetime, l is the optical pathway, e0 and c0 , respectively, are the absorption coefficient and mass fraction of species absorbing at the emission wavelength, and e and c respectively, are the absorption coefficient and mass fraction of species absorbing at the excitation wavelength. By taking advantage of high sensitivity of laser fluorimetry [24–26], the interferences are eliminated by simple onestep dilution of the sample aliquots using single push-button microliter pipettes, thereby bringing the concentration of uranium within the operational range of the instrument, followed by measurement with differential laser-induced fluorimetry technique using more suitable acidic buffer mixture of H3PO4–NH4H2PO4, as fluorescence-enhancing reagent for mineralized rock samples. Thus, for very diluted sample solutions, prefilter (species absorbing (e, c) at the laser wavelength, 337 nm) and postfilter (species absorbing (e0 , c0 ) at the maximum fluorescence wavelengths, 480– 560 nm) effects are negligible (verified by spectrophotometry) and therefore, this Eq. 1 simplifies to FðtÞ ¼ kI0
l eCe Dt=s s0
ð2Þ
The integration of this equation to have total fluorescence intensity leads to
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F ¼ kI0
l eCs s0
ð3Þ
In UA-3 uranium analyzer, the amplified and gated (shunt gates that allows transmission only for a 100 ls period starting from 30 ls after the laser pulse) fluorescence signals from the PMT are integrated for 4 s and then displayed on a panel meter, which can be calibrated directly in ng/mL uranium. Equation 3 reduces to, F ¼ KC
ð4Þ
where K is a new constant for the given reagent system and instrument, C is the mass fraction of uranium expressed as w(U3O8) (g/kg), and F is the fluorescence intensity (a fluorescence indication, F). The above equation can be used to determine the direct concentration of uranium (m, kg) by laser fluorimetry in unknown samples by comparing the instrumental response of the accurately known standard with that of unknown samples in a similar manner on same sample-weight basis as per the recommended procedure [24–26, 31]. This can be considered to be methods that operate in two parts each of which is a ‘direct method’. The two parts of the method can be combined to give: Fstd =Fsampl ¼ Cstd =Csampl
ð5Þ
According to the definition recommended in ‘JCGM 200:2008 -International vocabulary of metrology-Basic and general concepts and associated terms (VIM)’ [14, 15], the application of differential technique in laser fluorimetry (DT-LIF) [1, 24–26] can be safely categorized as ‘reference measurement procedure’ having high metrological quality (low uncertainty). Application of ‘differential technique in laser-induced fluorimetry’ (DT-LIF) Differential laser-induced fluorimetry method has been evaluated using standards, SY-2, SY-3, low-grade uranium ore-IAEA reference samples and core samples of diverse matrices. The results are in good agreement with the published data and those obtained by conventional fluorimetry and other methods and of comparable precision to those obtained by titrimetric assay The relative standard deviation of the method was 0.3–0.5% in nine replicates for 0.04–3.4% U3O8 in mineralized silicate rock samples and 0.5–0.9% at 18.1, 36.2, 61.2 and 99.6% U3O8 in concentrates and mineralized grab samples. In this approach, 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 [24–26].
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Choice of certified reference standards for differential technique Certified reference materials (CRM’s) provide a benchmark for analytical industry and should be used frequently [32, 33]. They are powered ores, rocks, slags, metals and alloys that have been prepared, analyzed and documented for use in analytical laboratories. They are invaluable in the laboratory for a variety of purposes, e.g., for calibration, quality control and preparation of in-house control materials and methodology comparisons. Micro- and nanotechnology requires standards of physical and chemical measurements with highest accuracy, comparability and traceability to international measurement system. Choice of certified reference standards for differential technique should be made cautiously. The reported recommended/ certified value of analyte in reference standards should be based on the evaluation of analytical values obtained by using primary methods or documented standard methods. The purity of reference material can contribute to uncertainty and has been discussed in detail [11, 12, 32, 33]. The errors inherited (if any) to these certified reference materials will be transmitted to other measurement data by using modern analytical techniques as well as in the preparation/certification of other certified reference materials and so on, if proper checks are not implemented. Alternatively, stock solutions prepared in minimum 1.56 mol/L HNO3 from spec-pure elemental powder or compound serve as the most suitable reference standards comparable with and fulfilling all the criteria essential for certified reference materials (CRM) for such applications similar to wet chemical analysis [25]. In wet chemical procedures, standard elemental/analyte stock solutions are prepared from primary standards; here, arbiter of accuracy is considered to be the chemical purity of standard reagent and the accuracy of the analytical balance. For uranium analysis, selection of appropriate standards concentration to obtain the maximum precision is necessary and is achieved by adjusting the sensitivity control to obtain more than half of the fluorescence indication on the panel meter [24, 25]. IAEA low-grade uranium ores [24, 25, 30], which are characterized by titrimetry and other validated methods, are classified as ‘international standard materials’ and have been used as reference materials. Alternatively [25], suitably diluted solutions from U3O8 stock solution in 1.56 mol/L HNO3 serve as the most suitable standards for the recommended reference measurement procedure. Choice of fluorescence-enhancing reagents and its stability A choice of an appropriate fluorescence-enhancing reagent for different type of sample matrices is very essential. A
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fluorescence-enhancing reagent of pH & 7 is most suitable for water samples. For rock samples, an acidic buffer has distinct advantages over others, as it eliminates the problems of iron precipitation and uranium adsorption by glass. Phosphoric acid–hydrogen phosphate buffer pH & 2 is the sensitive, stable and most selective fluorescence-enhancing reagent for uranium in acidic solution of rock samples [24, 25]. Effect of sample matrix and foreign ions By taking advantage of the high sensitivity and selectivity of the laser-induced fluorimetry, the interferences are eliminated by simple one-step dilution of the sample aliquots using single push-button microliter pipettes, thereby bringing the concentration of uranium within the operational range of the instrument, followed by measurement by differential laser-induced fluorimetry. This procedure is free from the effects of associated foreign ions and sample matrices [24–26]. This procedure has high tolerance to nitric acid [25], but halides interfere seriously and should be absent. Of course, during repeated nitric acid treatment of samples, the presence of halides does not arise. The phosphoric acid–ammonium dihydrogen phosphate buffer system is more selective in the presence of metal ions and found to be most suitable for rock samples of diverse matrices. Uncertainty in differential laser-induced fluorimetry procedure It is the role and responsibility of analyst to make best use of the analytical performance of the instruments to get the results of higher metrological quality. The flow diagram of differential laser-induced fluorimetry analysis is presented in Fig. 1. The Ishikawa diagram with the uncertainty sources of the measurement of the analysis is presented in Fig. 2. The uncertainty in analytical measurements arises mainly due to the following factors, 1. sampling and acidic digestion of samples, 2. instrumental response, 3. uncertainty due to volumetric operations and number of steps involved, 4. uncertainty in the certified concentration in the CRMs/reference materials and 5. uncertainty due to procedure of measurement. Effect of sampling and acidic digestion of samples on uncertainty As this procedure is intended for determination of total uranium in ores and similar matrices, samples need to be transformed into a liquid form before being analyzed. The most frequently encountered uranium ores are the hydroaluminosilicate ores, with over 50% silica (the
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Weighing 0.5 g sample
Weighing 0.5 g standard sample
Solution preparation of samples by acidic digestion and make up to 250 mL in a calibrated volumetric flask maintaining 1.56 mol/L HNO3.
Solution preparation of standard samples by acidic digestion and make up to 250 mL in a calibrated volumetric flask maintaining1.56 mol/L HNO3.
Take 100 µL sample solution, add 10 mL of fluorescence enhancing reagent and make up to 25 mL with distilled water in a volumetric flask.
Take 100 µL standard sample solution, add 10 mL of fluorescence enhancing reagent and make up to 25 mL with distilled water in a volumetric flask.
Measure fluorescence of solutions (after adjusting the standard to full scale deflection).
Adjust the standard having higher concentration to full scale deflection for fluorescence measurement.
Compute the result by comparison with standards (on same sample weight or dilution basis).
Fig. 2 The Ishikawa diagram with the uncertainty sources of the measurement of the analysis shown in Fig. 1
sample weighing
sample preparation
repeatability
sample dilution & preparation of solution for measurement dilution
clear sample solution
effects cancel out
(complete recovery) buoyancy effects cancel out
temperature mass fraction of uranium in sample temperature
uncertainty of standard buoyancy cancel out
effects cancel out clear sample
solution repeatability (complete recovery)
Standard sample weighing
standard sample preparation
uranium-mineralized silicate rock samples of diverse matrices from various uranium projects have the composition ranges of mass fraction listed in Table 1) and the carbonate–hematite–magnetite ores, containing large mass
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dilution
standard samples dilution & preparation of samples for measurement
fraction of iron (up to 700 mg/g).The uranium in ores is present chiefly as pitchblend, uraninite, torbernite, uranophane, autunite, carnotite, coffinite, thucholite, etc. In the decomposition of minerals, HF reacts with both silica and
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Effect of instrumental response on uncertainty
Table 1 Presence of other elements Components
Mass fraction range (mg/g)
Element
Mass fraction range (lg/g)
SiO2 TiO2
499.5–532.1
Zr
125–143
14.3–28.0
Sr
62–141
Al2O3
97.5–134.3
Rb
41–102
Fe2O3
9.2–44.5
Ba
245–1001
FeO
4.3–107.3
Y
20–25
MnO
0.1–2.4
Ni
41–60
MgO CaO
0.7–35.7 1.0–61.7
Co Cr
37–56 21–48 205–285
Na2O
1.3–29.1
V
K2O
9.6–17.6
Cu
P2O5
0.5–3.5
Pb
ThO2
53–3000 102–160
\0.100
silicates and converts them to the volatile and very easily hydrolyzed silicon tetrafluoride. The latter can be removed by volatilization to give a silicon-free solution. The addition of HNO3, being a strong oxidizing agent, converts tetravalent uranium to hexavalent uranium. Practically all oxygen-containing primary uranium minerals dissolve easily in concentrated nitric acid. Secondary uranium minerals containing carbonates, phosphates, sulfates and vanadates decompose equally well. The estimation of measurement uncertainty using the results of replicate analyses of these IAEA standard samples was performed according to the ‘top-down’ model as per the prescribed procedure. The results were found to be in very close agreement with the recommended values. The contribution of the effect of acidic digestion on total uranium determination was found to be negligible (Table 2).The recovery of uranium contents in these samples was found to be in the range 98.4–100.6%. A 0.5 g of powdered rock sample (particle size 74–88 lm) is recommended to minimize sampling error.
The advances in laser tube, optics, electronics and instrumentation design that have taken place in recent years have produced digital instruments with extraordinary stability in signal responses. These advances have minimized the uncertainty to a great extent in instrumental signal responses. The instrumental error is minimized by selecting a suitable concentration of reference standard for adjusting the sensitivity control of the instrument, to obtain the maximum precision possible in fluorescence intensity measurement [24]. The instrumental response was found to be stable and reproducible. The time needed for a uranium analysis is only 1–2 min. However, it was observed that at low laser intensity, the instrumental response fluctuates. If fluctuation persists, then the instrument should not be used for analysis. It requires replacement of laser tube as recommended in the instruction manual of the instrument. Uncertainty due to volumetric operations and number of steps involved Proper use of volumetric glass wares and selection of pipettes for dilution of samples are required for minimization of uncertainty. The uncertainty due to volumetric operations, which include preparation of solution in a volumetric flask and making up to volume, transferring a required volume of liquid with a pipette and making up to volume, preparation of working standards and their stability problems associated with aging, etc., is often underestimated [24, 25, 34–36]. Dilution of sample solutions can be performed using single push-button microliter pipettes [24, 25, 34–36]. These pipettes are direct displacement pipettor without air cushion. Their specifications are defined gravimetrically on a five place balance using 15 pipettings of distilled water in a stable environment at approximately 20 °C. These pipettes combine superior accuracy and precision with ease of use for applications where one cannot afford to make a mistake, inaccuracies of
Table 2 Measurement uncertainty in standard samples using differential laser-induced fluorimetry procedure a
w(U3O8), g/kg Found
w(U3O8), g/kg b Recommended value
urel[w(U3O8)] Relative combined standard uncertainty
S 1 IAEA sample
(4.70 ± 0.02)
4.71
0.02
S 2 IAEA sample S 3 IAEA sample
(3.12 ± 0.01) (4.16 ± 0.02)
3.13 4.18
0.03 0.02
S 4 IAEA sample
(3.72 ± 0.03)
3.75
0.05
Sample
a
The results are an average of eight determinations ± standard deviation (reproducibility) on eight replicates
b
Results by the following methods were used to evaluate the uranium content: S 1 av. Photo: thio, 0.473; dibenzoyl methane, 0.470; vol. cerium sulfate titration, 0.469. S 2 av. Photo: thio, 0.314; arsenazo, 0.313; vol. cerium (IV) sulfate quant., 0.313. S 3 av. Photo: thio, 0.420; dibenzoyl methane, 0.419; arsenazo, 0.0.415; vol. cerium sulfate titration, 0.417. S 4 av. photo.: thio, 0.377; DBM, 0.376; arsenazo, 0.372; vol. cerium(IV)sulfate quant.%:0.377
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±0.6% and imprecision \0.2% over the volume range from 100 to 1000 lL. These micropipettes are most suitable for large dilution in a single step. As reported in laserinduced spectroscopic studies [28], increasing temperature results in an increase in dynamic quenching due to increase in molecular collision rates and consequently to a decrease in uranium fluorescence. Thus, all volumetric operations were carried out using same pipette and glassware followed by fluorimetric measurement operations at controlled room temperature for more accurate and precise results.
Uncertainty due to differential laser-induced fluorimetry procedure of measurement
fluorescence response of one standard sample on same sample weight/or dilution basis. The use of different reference standards of accurately known concentration in this recommended ‘reference measurement procedure’ ensures calibration, control and optimization of the quality of analytical data and also serves as a sound quality assurance/quality control program in an analytical laboratory. In this way, it is a self-standardized methodology of measurement and guarantees the quality of an analytical result [1, 24–26] (accuracy, high precision, reliability, comparability and traceability). Differential technique fulfills the essential requirements of both equipment and method calibration as well as is traceable to SI (mass fraction, g/kg) via international standard comparisons. It is practically true because it cancels the uncertainties to a large extent [1, 24–26, 37, 38] associated with dilution steps, measurement of signal, overall uncertainties associated with the method of measurement. As per Eq. 5, the uncertainty in the measurement result is due to the uncertainty associated with the ratio of fluorescence indication (scale deflections) due to standard and sample solutions plus the uncertainty in standard material. The uranium mass fractions and their uncertainties are presented in Table 3. As per the definition of differential laser-induced fluorimetry using single standard for adjusting the fluorescence indication to full scale of laser fluorimeter, the combined relative uncertainty urel[w(U3O8)] in the measurement results of samples can be written down as, 1=2 urel ðwðU3 O8 ÞÞ ¼ u2std þ fuðFstd Þ uðFs Þg2 ð6Þ
It is a noteworthy to emphasize upon here that in differential laser-induced fluorimetry procedure of measurement, the whole methodology is checked by the use of reference materials, as appropriate having varied concentration as well as matrix compositions, which are subjected to exactly the same measurement steps as the sample. In differential technique, first, the concentration of uranium in other standard samples is calculated with respect to the
The uncertainty in reading the deflection of the analog scale is triangular pdf (probability distribution function), and the standard uncertainty is equal to 1/2 [small division of analog scale]/H6 [39]. If S 3 standard sample solution is used as per the prescribed procedure to set the fluorescence indication (scale deflection) of 9 (45 small divisions of analog scale of the UA-3 instrument) for subsequent fluorescence measurement, the value in terms of uranium
Uncertainty in the certified concentration in the CRMs/ standard materials The standard uncertainty in the standard material, such as IAEA low-grade uranium ore samples (in which uranium values are evaluated based on the results of average values using different methods (Table 2), is triangular distribution (because the values close to the recommended value is more likely than near the bounds). An estimate [30, 38] is made in the form of a maximum range described by a symmetric function using the mean value of average results by different methods (Mean ± standard deviation) and for S 3, it will be (4.18 ± 0.02) g/kg. The standard uncertainty in standard sample is equal to the standard deviation divided by H6. In case of S3, it will be (0.02 g) /H6 = 0.00816 g.
Table 3 Uranium mass fractions and their uncertainties Description
Uranium mass Standard fraction, g/kg uncertainty, g/kg
Relative standard uncertainty
Uranium content in S 3 standard sample
4.18
0.00816/4.18 = 0.00195
(0.02 g/kg) /H6 = 0.00816 g/kg
*The fluorescence indication of S 3 standard sample solution 4.18 adjusted to 9 (45 small divisions) using sensitivity control of the instrument
(0.0928 g/kg) /(2H6) = 0.0189 g/kg 0.0189/4.18 = 0.00452
Fluorescence indication of sample solution of S 1
(0.0928 g/kg) /(2H6) = 0.0189 g/kg 0.0189/4.71 = 0.00401
4.71
* Sample solution of standard S 3 was used to set the fluorescence indication of 9 on panel meter using the sensitivity control of the instrument
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concentration corresponding to one small division of scale is 0.0928 g/kg U3O8. The standard uncertainty in reading the scale deflection will be (0.0928 g/kg) /(2H6) = 0.0189 g/kg. The combined relative uncertainty urel[w(U3O8)] in the result of sample having mass fraction of uranium say, 4.71 g/kg U3O8, can be calculated using the Eq. 6 as follows: urel ðwðU3 O8 ÞÞ ¼ ð0:00816=4:18Þ2 þ f0:0189=4:18Þ 1=2 ð0:0189=4:71Þg2 and is found to be 0.002. The estimation of measurement uncertainty using the results of replicate analyses in differential laser fluorimetry The IAEA standard samples for uranium have been analyzed by the application of differential technique using UA-3 Laser Fluorimeter (Laser intensity, 5.5 on panel meter) as per the recommended procedure. The estimation of measurement uncertainty using the results of replicate analyses of these IAEA standard samples was performed according to the ‘top-down’ model for individual laboratories as per EURACHEM/CITAC Guide [37, 38]. In this case, the combined standard uncertainty is simply the combination of the standard deviation associated with these determinations and the uncertainty in the IAEA standard samples value. Using the recommended coverage factor k = 2 (this corresponds approximately to the 95% confidence interval), the relative expanded uncertainties for uranium in standard IAEA samples are the following: S 1, 0.04 g/kg; S 2, 0.06 g/kg; S 3, 0.04 g/kg; and S 4, 0.10 g/kg, respectively. The samples were also analyzed by digital-readout nitrogen laser fluorimeter fabricated by Laser Applications and Electronics Division, RR Centre for Advanced Technology (CAT), Department of Atomic Energy, Indore, India. The results obtained were found to be comparable. The uncertainty values obtained by both approaches show high metrological quality (low uncertainty) of measurement result using DT-LIF method. This differential technique has been developed, tested, evaluated and applied to a large number of samples in our laboratories [24, 25]. The uncertainty values are irrespective of different analysts, nitric acid concentrations, reagents, instruments, times and laboratories. In principle, laser-induced fluorimetry is an ideal technique for the very accurate determination of uranium by the use of appropriate fluorescence-enhancing reagents and methodology depending upon the concentration of uranium and sample matrices [1, 24–26].
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Conclusions Differential laser-induced fluorimetry has high metrological quality and warrants its introduction as a ‘reference measurement procedure’ for determination of total uranium in ores, certification of reference materials, core assay, beneficiation products and other diverse applications in nuclear fuel cycle. In principle, laser-induced fluorimetry is an ideal technique for the very accurate determination of uranium by the use of appropriate fluorescence-enhancing reagents and methodology depending upon concentration of uranium and sample matrices. This methodology may give direction in the area of microchemistry in developing new reliable methods for substance measurement by measuring at micro/nanogram masses using microvolumes (and lower) with the use of more sophisticated and compact instrumentation for clinical pathology, biochemistry, medical research, pharmaceutical industry, inorganic analysis and other related diverse applications. Acknowledgments Authors thank Director, AMD, for his kind permission to publish this work. Authors thank to the reviewers for their invaluable comments and suggestions to improve the manuscript in the present form.
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