The role of NAA in securing the accuracy of analytical results in the inorganic trace analysis
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The unique value of the neutron activation analysis (NAA) for analytical quality assurance is demonstrated through its contribution to the certification of the reference materials over the time span of 40 years. Ratio primary reference measurement procedures i.e. definitive methods, based on radiochemical NAA, play an important role in confirming the validity of the certified values established in the international intercomparisons. Real analytical situations where the use of NAA was critical to get accurate results are presented. The dangers associated with the possible non-availability of the NAA in the near future are indicated.
KeywordsNeutron activation analysis Ratio primary reference measurement procedure Accuracy Certified reference material Definitive method Quality assurance
Some examples of drastic dispersion of analytical results observed in international interlaboratory comparisons as a function of time
Number of laboratories
Range of results
8 (sel. labs.)
von Lehmden et al. 
Milk Powder IAEA A-11
Dybczyński et al. 
Hay Powder IAEA V-10
Pszonicki et al. 
Milk Powder IAEA A-11
Byrne et al. 
4 (sel. labs.)
4 (sel. labs.)
Whey Powder IAEA-155
Zeiller et al. 
Virginia Tobacco Leaves CTA-VTL-2
0. 383–354 mg/kg
Tea Leaves INCT-TL-1
Soya Bean Flour INCT-SBF-4
Oriental Basma Tobacco Leaves INCT-OBTL-5
Herring Tissue MODAS-3M-3 HerTis
Polkowska-Motrenko et al. (unpublished work, private communication)
Most common sources of errors in inorganic trace analysis 
Inhomogeneity of the test sample with respect to trace analysis
Analyte can be lost from the sample by
Failing to transform the analyte into the desired chemical form
Non-quantitative recovery during preconcentration and/or separation step
Analyte can be added to the sample from
Reagents, acids, solvents etc.
Sampling devices, reaction vessels, Glassware, tubings etc.
Ion exchange resins
Spectral interferences etc.
Trivial errors (e.g. writing erroneous mass or concentration units when reporting the results)
The evolution of the position of NAA in the array of methods used in the inorganic trace analysis
Neutron activation analysis (NAA) has already been known for several decades as a well established technique in inorganic trace analysis. When nuclear reactors with their high fluxes of slow neutrons ranging from 1011 up to 1014 n cm−1 s−1 became available for the civil uses, at the end of forties and beginning of fifties of the last century, NAA became all of sudden the technique providing the best detection limits for several elements in comparison to other methods of analytical chemistry being in use at that time. As the time progressed, new sensitive methods for the determination of elements emerged like: atomic absorption spectrometry (AAS), various variants of emission spectroscopy (ES), X-ray fluorescence (XRF) and finally inductively coupled plasma mass spectrometry (ICP-MS), and NAA was gradually losing its unique position. At the same time, partly due to antinuclear hysteria in some countries, the number of research reactors and laboratories using NAA rather decreased than increased in the world. So, it seems worthwhile to look back and realize what was in the past and what is now the position and value of NAA among other methods of inorganic trace analysis, and what are the prospects for the future.
Contribution of NAA to the certification of the candidate reference materials
From the seventies and up to first half of the nineties of the previous century the domination of NAA is clearly visible. The contribution of NAA in the IAEA intercomparisons starting from the year 1975 was the highest from among all methods used and most often exceeded 50% (cf. Fig. 2). AAS was on the second place, significant contributions were due to ES and XEF methods, with only negligible contribution from mass spectrometry (MS). Similar situation was observed in the INCT intercomparisons (CTA-AC-1, CTA-FFA-1, CTA-OTL-1 and CTA-VTL-2) up to the middle of nineties of the XXth century although the share of NAA was smaller than in the seventies and barely exceeded 40% (cf. Fig. 3). However, at the break of a century NAA lost its leading role and was overtaken by AAS. In the intercomparisons INCT-TL-1, and INCT-MPH-2, considerable share of results was provided by ES and MS (ICP-MS) with minor contribution from XRF method. Ten years later, the four leading methods i.e. ICP-MS, AAS, NAA and XRF provided almost equal share of results in the INCT-OBTL-5 intercomparison. However, after only 5 years more, the leading role of ICP-MS was already beyond discussion and the share of NAA dropped to 12–18% as can be seen from M-2 BotSed and M-3 HerTis intercomparisons (see Fig. 3). The leading role of ICP-MS is not surprising and reflects the current trend in the contemporary trace analysis, but constantly decreasing share of NAA results is regrettable and if this tendency is going to be continued it may become dangerous in the long run for quality assurance, at least in some special cases as will be shown later in this paper.
Relative usage of analytical methods used in exploration geochemistry in North America in 1971,
(numerical values taken from )
Percent of total number of samples
Number of elements by each method (max.)
Atomic absorption (AAS)
Emission spectrography (ES)
X-ray fluorescence (XRF)
Selective ion electrodes
A case illustrating the problems with the accurate determination of scandium
Elements which were certified on the basis or results by NAA only
Elements certified on the basis of interlaboratory comparison with the use of results by NAA only
Peruvian soil Soil-5
Cs, Dy, Eu, Hf, Lu, Nd, Sc, Sm, Ta, Tb, Yb
Lake Sediment (SL-1)
Br, Eu, Hf, Na, Sb, Sc, Sm, Yb
Apatite Concentrate (CTA-AC-1)
Hf, Sc, Ta
Fine Fly Ash (CTA-FFA-1)
Hf, Sc, Ta, Tb
Oriental Tobacco Leaves (CTA-OTL-1)
Ce, Eu, Sm, Tb, Th
Virginia Tobacco Leaves (CTA-VTL-2)
Cs, Hf, La, Sb, Sm, Tb, Th, U, W
Tea Leaves (INCT-TL-1)
Mixed Polish Herbs (INCT-MPH-2)
Hf, Hg, Sc, Ta
Corn Flour (INCT-CF-3)
Soya Bean Flour (INCT-SBF-4)
Oriental Basma Tobacco Leaves (INCT-OBTL-5)
Br, Hf, Sc, Ta
Polish Virginia Tobacco Leaves (INCT-PVTL-6)
Br, Hf, Sc, Ta
MODAS-3 Herring Tissue (M-3 HerTis)
When ICP-MS method with its excellent detection limits became available to many laboratories, one could presume that the problems with the determination of low concentrations of all elements also in biological matrices will suddenly disappear. In reality, at least in the case of scandium this does not seem to be the case. In the recent M-3 HerTis intercomparison (cf. Table 1) the eleven results for Sc (in ng g−1) provided by the participating laboratories were as follows: (1) 2.74(NAA); (2) 2.93(NAA); (3) 3.09(NAA); (4) 3.30(NAA); (5) 3.50(NAA); (6) 3.58(NAA); (7) 5.92(ICP-MS); (8) 6.39(NAA); (9) 10.4(ICP-MS); (10) 42.5(ICP-MS); (11) 154 (ICP-MS).
The results (7)–(11) were rejected as outliers in the process of evaluation of results used in our Laboratory [23, 24]. Scandium was certified on the basis of results (1)–(6) as: 3.2 ± 0.4 ng g−1 (H. Polkowska-Motrenko et al. private communication).
Ratio primary reference measurement procedures (RPRMP): definitive methods
The use of certified CRMs with the type of matrix and concentration level of the analyte possibly closely matching those in the test samples
Application of the methods with “guaranteed accuracy”
The possession of methods that would give truly accurate results has always been a dream of people working in the field of trace analysis, where, as was indicated earlier, the chances of making gross errors are both real and multiple. Various names such as e.g. “definitive” and “primary” etc. have been associated in the past with such methods of “guaranteed accuracy”. In the Institute of Nuclear Chemistry and Technology (INCT) the work on “definitive methods by radiochemical NAA” started more than 30 years ago. The principal idea is the combination of neutron activation with selective and quantitative isolation of the desired radionuclide by column chromatography followed by γ-ray spectrometric measurement. Several rules which should be obeyed when devising such methods were formulated [26, 27, 28]. The definition of “definitive methods” formulated by Uriano and Gravatt  sounded: “Definitive methods of chemical analysis are those that have a valid and well described theoretical foundation, have been experimentally evaluated so that reported results have negligible systematic errors, and have high levels of precision.”. In the real sense this definition is very close to the more exact definition formulated by CCQM (5th Meeting, 1999)  namely: “A primary method of measurement in the SI is a method having the highest metrological qualities whose model (mathematical equation)and realization are completely described and understood in terms of SI units.
The use of a primary direct method results in a value of an unknown quantity without reference to a standard of the same quantity.
The use of a primary ratio method results in a value of the ratio of two values of the same quantity without reference to a standard of the same quantity.
In both cases, the results must be accompanied by a complete uncertainty statement.”
The above definement was included into the definition of the “Primary reference measurement procedure” as given in VIM 3 [14, 31]. Our definitive methods can now be termed: Ratio primary reference measurement procedures (RPRMP) [28, 31]. In the high accuracy RNAA methods devised in the INCT i.e. definitive (RPRMP) methods, the mass fraction of an element being determined is described by a mathematical equation . The methods provide the results with very low levels of uncertainty, traceable to SI units. The expanded uncertainty (k = 2) is usually of the order of 2.3–2.8% (e.g. for Cd, Co, Cr and Fe determination). Only in the case of RPRMP for molybdenum, the expanded uncertainty is greater (3.4–5.2%) . While the other RPRMPs were devised as single element methods, the separation scheme and the whole method for Mo had to include the determination of uranium, because the same radionuclide which is used for the determination of Mo by NAA i.e. 99Mo–99mTc, may be also formed in U(n,f)99Mo reaction [32, 33].
Our definitive (RPRMP) methods can be an option or alternative to ID-MS methods, which however cannot be used for monoisotopic elements. Generally, the availability of two primary methods based on various physicochemical principles could be an ideal solution for the certification of certain elements in the candidate reference materials. It should be remembered that our RPRMPs were elaborated, so far, only for biological materials, because in this case it can be safely assumed that the decomposition and dissolution of the test samples will be complete. During a quarter of a century the definitive methods were elaborated for As, Cd, Co, Cu, Fe, Mo, Ni, Se and U. Recently, new RPRMP for chromium was published by Zuba and Polkowska-Motrenko . It should be remembered that even the primary method has only potential to give the results of the highest metrological quality and the execution of the method is as important as the method itself. Therefore in our definitive methods we use a series of criteria [27, 28, 31] which must be simultaneously fulfilled before the result can be acknowledged as being obtained by RPRMP. The specialists in the ID-MS also issued a warning to potential users of their method writing: “ID-MS is not a “magic bullet”. Like any other technique, failure to understand all sources of error and how to deal with them may lead to a significant bias in the results” .
The hypothetical imminent danger of non-availability of NAA and its consequences
For several decades NAA played an important role in quality assurance providing dominant share or results in the process of certification of the candidate reference materials. As a nuclear method, the NAA, if performed properly, is generally an accurate method, less dependent than other methods of trace analysis on the matrix effects. NAA has the advantage that in many cases the analysis may be performed purely instrumentally and is essentially free from the blank problems. On the other hand, elaborated in the INCT Ratio primary reference measurement procedures (definitive methods) by RNAA, are together with ID MS methods an important instruments for verification of accuracy of other methods used in inorganic trace analysis. Diminishing number of good NAA laboratories around the world raises the concern that some scientific competences may be lost in the near future with the obvious harm to the problems of quality assurance in inorganic trace analysis. It should be in the interest of the whole scientific community, not only radioanalytical chemists, to keep this method alive still for many years to come.
Thanks are due to dr Z. Samczyński for fruitful discussions.
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