Reference Materials

RACNT RACNT-2007, BARC, January 44-6, 2007

REFERENCE MATERIALS IN ANALYTICAL CHEMISTRY

January 6, 2007

K.L. Ramakumar Radioanalytical Chemistry Section Bhabha Atomic Research Centre Mumbai 400 085 [email protected]

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Reference Materials in Analytical Chemistry Presentation theme Indispensability of accurate analytical measurements Role of reference materials in the field of chemical metrology.. metrology Definitions, Characteristic features, certification and use, care and storage of reference materials Efforts for preparation and characterisation of reference materials for use in nuclear technology are also enumerated enumerated..

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Statement of Purpose Any analytical measurement should be reliable and invariant in time in space method independent analyst independent Realized by standardisation, harmonisation and assuring the quality of measurements. measurements. Particularly true in nuclear technology where rigorous chemical quality control and assurance measurements are order of the day. day.

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Statement of Purpose The goal of any analytical measurement is to get accurate, reliable and consistent data data.. Prerequisites for achieving accurate results results:: Correct sampling Correct weighing of the sample and standards Use of well maintained and calibrated equipment Qualified operators Validated methods and procedures Use of accurate standards or certified reference materials.. materials Reference materials provide the benefit of comparability— comparability— between results obtained at different times, in different places, by different people, and using contrasting methods.. methods

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When is an analytical result the result acceptable? The result is acceptable when

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it has been obtained by an established method all the uncertainties (systematic and random) have been taken into consideration in the uncertainty value being quoted the method used has been validated by analysing a known secondary (working) reference material the method used has been calibrated with a known certified reference material whose value is traceable to international metrological unit KLR, RACS, BARC

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Primary standard: standard: Material designated or widely acknowledged as having the highest metrological qualities and whose value is accepted without reference to other standards of the same quantity quantity.. e.g. uranium metal or plutonium metal Secondary standard standard:: Material whose value is assigned by comparison with a primary standard of the same quantity quantity.. e.g. uranium dioxide pellets or plutonium oxide oxide.. Their reference value is assigned by comparison with the respective metal standard. standard.

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Working standard standard:: Used routinely to calibrate or check material measures, measuring instruments or reference materials materials.. e.g. Mixed solutions of uranium and plutonium prepared inin-house Certified reference material (CRM) (CRM):: A reference material, accompanied by a certificate, one or more of whose properties are certified by a procedure which establishes traceability to an international unit of metrology..The certified value is accompanied by an metrology uncertainty at a stated level of confidence. confidence.

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Requirements for (certified) reference material •The compounds and concentration should be as similar as possible to the unknown sample sample.. •The material should be 'matrix matched'. matched'. Unfortunately full matrix matching frequently is an unrealistic requirement.. requirement •The reference material should be homogeneous. homogeneous. •If there is a risk for segregation during transport or storage, the material must be re re--homogenized before use use.. •The certified properties of the reference material and the matrix should be stable. stable. The material should be checked for stability as part of the verification process process.. •The uncertainty of the value should be estimated for certified reference materials materials..

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Test area

1

2

3 ….n

Sampling pattern Sampling point

Sampling site

Sampling unit

Increment/Primary sample

Composite/aggregate sample

Sub-sample

Laboratory sample

Drying, sieving, reducing, milling Test sample

Analytical Operations

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RACNT RACNT-2007, BARC, January 44-6, 2007 January 6, 2007

u2CRM = u2bb + u2its + u2sts + u2char A. Van der Veen, Advances in Chemical metrology, November 18, 2003

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General Challenges in the Preparation and Certification Reference materials (RMs) serve as the basis for millions of measurements performed each year year.. The “reliability” of these materials, the associated reference value and the stated uncertainty are of key importance importance.. Critical requirement in NUMAC Total SNM inventory depends on the analytical result and accompanying uncertainty obtained on an aliquot aliquot.. A high degree of confidence needs to be put in the measurements and in the reference materials materials.. Preparation and certification of reference materials is a very demanding task. It requires a thorough understanding of the processes involved.

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Preparation of Reference Materials

1. Definitive method The method must be based on first principles, have very high precision and essentially zero systematic error.. An example is the use of isotope dilution error mass spectrometry for the characteristics of trace level elements in natural matrix elements. elements. The certification is done in a single laboratory

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Methods identified as having the potential to be primary - isotope dilution with mass spectrometry (IDMS) - coulometry - gravimetry a) gas mixtures and b) gravimetric analysis - titrimetry - determination of freezingfreezing-point depression Measurements of amount of substance, to be considered primary, must be made using a method which is specific for a defined substance and for which the values of all parameters, or corrections which depend on other species or the matrix, are known or can be calculated with appropriate uncertainty.. uncertainty

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Preparation of Reference Materials 2. Independent measurement method Two or more reliable independent methods are used.. The method must be proven to give used accurate results results.. The certification is done in a single laboratory laboratory.. 3. Interlaboratory consensus method A number of laboratories analyze in replicate one or more units of the material being characterized characterized.. The participating laboratories may choose their own method or all laboratories may use the same method.. The consensus value is usually taken as method the mean mean..

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Total measurement uncertainty Measurement errors

Systematic errors

Random errors Sampling error

Corrected Systematic errors

uncorrected Systematic errors

Result

Measurement uncertainty u

X

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Total measurement uncertainty 16

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Selection Criteria of CRMs A reference material is selected by comparing the contribution of the reference material (Ir) to the total analytical uncertainty (IT) and by evaluating this as satisfactory, acceptable or incompatible. incompatible. IT / Ir > 10: 10: 10 > IT / Ir > 4: 4 > IT / Ir > 2:

IT / Ir < 2:

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Satisfactory Acceptable Acceptable if no other solution exists;; upward revision of IT exists undoubtedly necessary Unsatisfactory situation situation;; implies reviewing the total uncertainty, the method applied, and the choice of reference material material.. KLR, RACS, BARC

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Use of Reference Materials Method Validation and Measurement Uncertainty Bias estimation One of the most difficult elements Appropriate RMs provide information within the limits of the uncertainty in certified value(s) and the uncertainty of the method Verification of the Correct Use of a Method RMs can be used for training For checking infrequently used methods For trouble shooting when unexpected results are obtained.. obtained Calibration Normally a pure substance RM is used for calibration Other components such as sample digestion, separation and derivatisation are not covered covered.. The uncertainty associated with RM purity will contribute to the total uncertainty of the measurement measurement..

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Traceability of the measurements Consider two laboratories, A and B, carrying out measurements on samples of broadly the same type. Each calibrates their equipment using a reference standard with a known nominal concentration (x1 (x1 and x2 respectively). They calculate their respective results y1 and y2 from a calibration equation including the respective values of x.

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If, however, the two reference standards are both calibrated against some common reference, a comparison becomes meaningful. meaningful. Now, both results are derived from the same value (x0). Both will now have the same units of measurement, (the same scale and units as x0), and direct comparison of the values y1 and y2 is now not only possible but also meaningful. meaningful. By analogy, of course, x0 could also be derived from a yet higher reference to allow global comparisons comparisons..

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Traceabiliy of working standards to secondary and primary standards

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Use of calibration standards (calibrants) Primary calibrator

Reference material producer Manufacturer’s working calibrator

sells to

Manufacturer’s product calibrator

sells to

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Secondary calibrator End user’s laboratory primary calibrator

End user’s Working calibrator laboratory sample KLR, RACS, BARC

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Use of Reference Materials Control charts and reference materials: Demonstration of measurement procedure performing within given limits. Warning limits and Control limits

If the control value is outside the limits, no analytical results are reported and remedial actions have to be taken to identify the sources of error, and to remove such errors.

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Control charting is a powerful and simple tool for the daily quality control of routine analytical work. The basis is that the laboratory runs control samples together with the routine samples in an analytical run. Material of control samples can be standard solutions, real routine samples, blank samples, inhouse control materials and certified reference materials. Warning limit = CL ±2s

Control limit = CL ± 3s

CL + 3s CL + 2s CL CL - 2s CL - 3s

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Periodicity KLR, RACS, BARC

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Milenko Markovic, Bruce O. Fowler and Ming S. Tung, Preparation and Comprehensive Characterization of a Calcium Hydroxyapatite Reference Material J. Res. Natl. Inst. Stand. Technol. 109 109,, 553 553--568 (2004) Detailed physicochemical characterization of HA, prepared from an aqueous solution, was carried out employing different methods and techniques techniques:: chemical and thermal analyses, x-ray diffraction, infrared and Raman spectroscopies, scanning and transmission microscopies, and Brunauer, Emmett, and Teller (BET) surface surface--area method method.. The contents of calcium (Ca2+), phosphate (PO43–), hydroxide (OH–), hydrogenphosphate (HPO42– ), water (H2O), carbonate (CO32–), and trace constituents, the Ca/P molar ratio, crystal size and morphology, surface area, unitunit-cell parameters, crystallinity, and solubility of this HA were determined.. This highly pure, homogeneous, and highly determined crystalline HA is certified as a National Institute of Standards and Technology (NIST) standard reference material, SRM 2910 2910..

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Availability of Nuclear Reference materials Institute for Reference Materials and Measurements (EC, IRMM), 2440 Geel, Belgium; http://irmm.jrc.cec.eu.int/ New Brunswick Laboratory, (NBL), 9800 S. Cass Ave., Argonne IL 60439, USA; http://www.nbl.doe.gov/ CETAMA, CEA/ValrhoCEA/Valrho-Marcoule, DCC/DRRV, BP 171, 30207 Bagnols sur Ceze, France V.G. Khlopin Radium Institute, 22-nd Murinsky Av. 28, 194021 St. Petersburg, Russia Ural Electrochemical Integrated Plant (UEIP), Materials are available through TENEX http://www.tenex.ru/izotop_en.html#uran

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Laboratory ware used for storage and sample treatment Borosilicate Glass Used extensively. extensively. Resistant to most acids Should not be used with HF or boiling H3PO4 Alkaline solutions should not be heated or stored. stored. Borosilicate glass can contribute a variety of contaminants. contaminants. Should not be heated using a hot plate (500 °C) C).. do not use borosilicate glass for ashing in muffle furnace Porcelain Popular material used for ashing purposes. purposes. Can be heated up to 1370 K Coated with a glaze which is about 70 % SiO2, with roughly equal amounts of the oxides of Al and Ca, and lesser amounts of Na and K. If alkalis are present, then the sample is typically treated with conc conc.. H2SO4 prior to ashing. ashing. Do not use porcelain: porcelain: HF; HF; boiling H3PO4; and the oxides, hydroxides, or carbonates of the alkali or alkaline earth elements

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Quartz •Opaque quartz has the highest trace element concentration and should not be used for trace analysis. •Transparent quartz comes in four different varieties Types I & II are made from naturally occurring quartz crystals or sands. Type I is created by electric melting and type II by flame melting. Type II has slightly less impurities than type I (some impurities are volatilized by the flame). Type III quartz is made synthetically by vapor phase hydrolysis of pure silicon compounds such as SiCl4. This type of quartz is more pure than the natural quartz, with the exception of Cl which is ~ 50 ppm. Type IV quartz is synthetically made from SiCl4 using a process involving electrical fusion of the oxidized staring material. It is as pure as type III, with respect to trace metal content, and contains far more Cl . Use the synthetic type III quartz whenever possible. Quartz is typically 99.8+% SiO2. It is attacked by HF, boiling H3PO4, and the alkali and alkaline earth oxides, hydroxides, and carbonates. It can be heated to 1100 °C . Its main advantage over that of porcelain is that major contamination occurs from only Si -- however, this contamination can be significant

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Platinum Expensive but a popular container material. Heats up and cools down rapidly, making it excellent for % ash determinations where the % ash is at low levels. Resistant to attack by most acids and reagents. Avoid concentrated H3PO4 at high temperatures, HCl + HNO3 mixtures and fusions using Li2CO3, Na2O2, or the alkali hydroxides. Avoid heating at prolonged temperatures in excess of 1370K. 0

+2

Avoid using Pt for a sample containing high levels of Cu or Cu especially if present in an organic matrix. A sample containing trace levels of Cu in an organic matrix will not ruin the platinum, but it is likely to be lost to the platinum during the ashing process. Since platinum has alloying tendency, it is best to avoid its use with samples containing elements other than those that have no tendency to form the metal (i.e. - alkali, alkaline and rare earth elements). Platinum is known to contain trace amounts of the other precious metals and should not be used for their preparation. Avoid samples containing Hg in any form. Hg metal is easily formed and alloys very readily at room temperature with platinum. Also avoid ashing samples containing P in any form, including the phosphates.

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Graphite Inexpensive and relatively clean, but very messy to work with. It is an inexpensive way to perform Li2CO3 fusions where the crucible slowly oxidizes away over the course of 77-10 fusions. It does not wet by some melts which can be poured out quantitatively. Losses due to the porosity of graphite should exclude its use for ashing samples containing trace metals. Graphite's main advantage to the trace analyst is being a material that can withstand fusions that might destroy platinum. Plastics Plastics are very important to the trace analyst. Whenever possible, the analyst should attempt to use plastics for sample collection, storage, preparation, and measurement. Their major disadvantage is the inability to be used for high temperature operations, such as ashing or fusion.

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Indigenous Efforts

The necessity of reference materials for quantifying analytical measurements for QC / QA and other requirements on nuclear materials had been recognized long ago in BARC Limited availability of reference nuclear materials Not all of them accessible to us

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Indigenous Efforts Uranium 1950-60’s 195060’s:: Pure uranium metal available from Analytical Chemistry Division was being used as reference material From late 1960 1960’s ’s:: High purity stoichiometric U3O8 was prepared and used as reference material material.. 1990’s:: Rubidium uranium sulphate (Rb2U(SO4)3) was 1990’s prepared and characterised for use as reference material for uranium in RLG G.M. Nair et al. al., BARC BARC--560( 560(1971 1971)) K.D. Singh et al. al., J. Radioanal Radioanal.. Nucl Nucl.. Chem Chem.., 240, 240, 183( 183(1999) 1999)

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Indigenous Efforts Plutonium Anhydrous plutonium potentiometry

sulphate

for

use

in

The material is stoichiometric with impurity content less than 0.07 07% %. In use for more than two decades potassium plutonium sulphate (K4Pu(SO4)4) was prepared and characterised for use as reference material for plutonium in RLG G.M. Nair et al. al., BARC/I BARC/I--517 517((1979 1979)) K.D. Singh et al. al., BARC BARC--1496( 1496(1990 1990))

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Indigenous Efforts Trace Elements Elements:: NonNon-Metallics Matrix matched reference materials are not available F and Cl Cl:: NaF and BiOCl are homogeneously blended with high purity U3O8 Carbon:: WC or stainless steel standards are used Carbon Generally employed for validation of the analytical technique.. Hence a matrix matched reference technique material may not be strictly necessary G.M. Nair et al. al., BARC/I BARC/I--517 517((1979 1979))

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Indigenous Efforts Trace metallics 1970’s:: First concerted effort for U3O8 with known boron 1970’s content.. content 1980-90’s 198090’s:: Sets of U3O8, ThO2 and graphite materials containing known amounts of trace constituents including rare earth elements were prepared for use in spark source mass spectrometry spectrometry..

K.L. Ramakumar et al al.., Fresenius’ Z. Anal. Anal. Chem. Chem., 318 318,, 12 12((1984) 1984) K.L. Ramakumar, Ph. Ph.D. Thesis, University of Bombay(1985 Bombay(1985)) B.P. Datta, Spectrochimica Acta, B52 52,, 471( 471(1997) 1997)

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Indigenous Efforts 2000’s: Interlaboratory experiments by NFC, and BARC to characterize U3O8 and ThO2 with respect to trace elements. Prepared following a wellwell-defined analytical protocol. Different analytical techniques employed in the participating laboratories Rigorous statistical treatment of the analytical results Consensus values have been assigned to the individual elemental concentrations. DAE-Interlaboratory comparison experiment (ILCE) for trace metal DAEassay of uraniumuranium-Phase II: Experiments: development of certified reference materials, compiled by radiochemistry division, BARC and NFC, Hyderabad (2002) Preliminary report on the DAE interlaboratory comparison experiments for determination of trace metals in thorium oxide, Radiochemistry Division, BARC (2004)

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Conclusions Reference materials for any analytical measurement, more so in nuclear technology whether it is for chemical quality control or for accountability purpose are indispensable.. indispensable Availability of reference materials enables any analyst to come as close to absolute as analytically feasible feasible.. At the same time these are not panacea to all the analytical requirements and one has to be judicious enough in their proper selection selection.. It is necessary to prepare and characterise indigenous reference materials for use in nuclear technology, as many of these are not accessible to the users in India India..

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Epilogue 1. Beware of contamination 2. Understand matrix effects (*) •

Use standard addition where ever necessary(*)

•

Calibrate the method (*)

•

Determine precision and accuracy(*)

•

Ensure proper sampling plan

•

Know your Statistics

Need for RMs. Use appropriate reference materials

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