Analytical quality control is needed to ascertain the reliability of results from all types of analysis, including activation analysis. Thea priori precision has to be combined with the statistics of counting in order to reach a state of statistical control. The Analysis of Precision is used to verify the absence of unknown sources of variability and to ascertain the absence of systematic biases or calibration errors by analyzing samples or reference materials with accurately known content or actual samples by an independent method.
Chemical methods are unsuitable for the determination of the specific activity of commerical125I preparations because of the unknown chemical state of the iodine in solutions more than a few weeks old.125I and127I were determined in samples from seven different manufacturers by instrumental neutron activation analysis; the specific
activities found ranged from about 45% to more than 90% of that of the truly carrierfree product. Correlation of the specific
activities with the126I:125I ratios indicates contamination of one of the products with stable iodine during the manufacturing process.
For more than half a century we have been using activation of stable isotopes to determine chemical elements, and significant contributions have been made to a variety of scientific subjects. Nevertheless, activation analysis has not yet become integrated in the field of chemical analysis, and therefore the special features characteristic of activation analysis have not been fully realised in analytical chemistry. At the same time basic chemical knowledge has only to a limited extent been utilised in the development of analytical methods based on activation. This situation has only now become painfully clear, when a world-wide requirement is being made to express the uncertainty of analytical results in accordance with the BIPM philosophy. The identification and estimation of all uncertainty components needed to produce a reliable uncertainty budget requires the combined efforts of all parties. An attempt is here made to extrapolate current trends for the expression of uncertainty in activation and analysis into the future and to show, how the implementation of the BIPM Guidelines with respect to correction for all known or suspected biases, achievement of statistical control, and full traceability, can help bringing analytical chemistry into its rightful position as a scientific discipline in the field of metrology.
The National Physical Laboratory γ-ionization chamber has been used for the measurement of chemical yield by re-irradiation
in a method for the determination of arsenic by neutron activation analysis. Satisfactory accuracy and a precision of 1% were
obtained with a single reading. Discrimination against other radioisotopes is achieved by irradiating for a short time and
measuring at a total decay time of one mean life of the radionuclide to be determined; the resulting discrimination factor
is given as a function of the half-life ratio. Maximum sensitivities for 66 elements with γ-emitting thermal neutron capture
products were calculated for irradiation and decay times both equal to one half-life, and it is shown that the sensitivity
for shorter irradiations at the selected time of measurement is a linear function of the irradiation time. More than a dozen
elements were found suitable for determination at the milligram level by neutron activation followed by γ-ionization chamber
measurement. The error from interfering elements can be directly estimated from their expected concentrations by means of
the calculated sensitivities and discrimination factors presented in the paper.
Results obtained by measurement of radioactivity have traditionally been associated with an expression of their uncertainty,
based on the so-called counting statistics. This is calculated together with the actual result on the assumption that the
number of counts observed has a Poisson distribution with equal mean and variance. Most of the nuclear scientific community
has, therefore, assumed that it already complied with the latest ISO 17025 requirements. Counting statistics, however, express
only the variability observed among repeated measurements of the same sample under the same counting conditions, which is
equivalent to the term repeatability used in quantitative analysis. Many other sources of uncertainty need to be taken into
account before a statement of the uncertainty of the actual result can be made. As the first link in the traceability chain
calibration is always an important uncertainty component in any kind of measurement. For radioactivity measurements in particular
we find that counting geometry assumes the greatest importance, because it is often not possible to measure a standard and
a control sample under exactly the same conditions. In the case of large samples we have additional uncertainty components
associated with sample heterogeneity and its influence on self-absorption and counting efficiency. In this paper we prepared
an uncertainty budget for existing data for 137Cs in Danish soil, which is shown to account adequately for all sources of uncertainty.
The precision of an activation analysis method prescribes the estimation of the precision of a single analytical result. The
adequacy of these estimates to account for the observed variation between duplicate results from the analysis of different
samples and materials, is tested by the statistic T, which is shown to be approximated by a chi-squared distribution. Application
of this test to the results of determinations of manganese in human serum by a method of established precision, led to the
detection of airborne pollution of the serum during the sampling process. The subsequent improvement in sampling conditions
was shown to give not only increased precision, but also improved accuracy of the results.
It has been found that sampling constants vary greatly not only from major elements to trace elements, but also between individual trace elements. A comprehensive investigation of a potential reference material therefore requires the determination of sampling constants for all elements to be certified, and other analytical methods therefore have to be included. For methods in statistical control the described strategy can be applied.