Risø-R-419 Aspects of Precision and Accuracy in Neutron Activation Analysis Kaj Heydorn Risø National Laboratory, DK-4000 Roskilde, Denmark March 1980 Aspects of Precision and Accuracy in Neutron Activation Analysis Aspects of Precision and Accuracy in Neutron Activation Analysis Kaj Hey dorn Isotope Division Risø National Laboratory Roskilde, Denmark July 1978 Denne afhandling er af Den polytekniske Læreanstalt, Danmarks tekniske Højskole, antaget til forsvar for den tekniske doktorgrad. Lyngby, den 14. december 1979. Peter Lawaetz Paul Carpentier Rektor Administrationschef This Thesis has been accepted by the Technical University of Denmark for public defense in fulfilment of the requirements for the Degree of Doctor Technices. Lyngby, December 14, 1979. Peter Lawaetz Paul Carpentier President Secretary Abstract Analytical results without systematic errors and with accurately known random errors are normally distributed around their true values. Such results may be produced by means of Neutron Activation Analysis both with and without radiochemical separation. When all sources of random variation are known a priori, their effect may be combined with the Poisson statistics characteristic of the counting process, a.id the standard deviation of a single analytical result may be estimated. The various steps of a complete neutron activation analytical procedure are therefore studied in detail with respect to determining their contribution to the overall variability of the final result. Verification of the estimated standard deviation is carried out by demonstrating the absence of significant unknown random errors through analyzing, in replicate, samples covering the range of concentrations and matrices anticipated in actual use. Agreement between the estimated and the observed variability of replicate results is then tested by a simple statistic T based on the chi-square distribution. It is found that results from neutron activation analysis of biological samples can be brought into statistical control- In routine application of methods in statistical control the same statistical test may be used for quality control when some of the actuai samples are analyzed in duplicate. This Analysis of Precision servjs to detect unknown or unexpected sources of variation of the analytical results, and both random and systematic errors have been discovered in practical trace element investigations in different areas of research. Particularly, at the ultratrace level of concentration where there are few or no Standard Reference Materials for ascertaining the accuracy of results, the proposed quality control based on the Analysis of Precision combined with neutron activation analysis with radiochemical separation, with an a priori precision independent of the level of concentration, becomes a powerful tool for controlling accuracy too. UDC 543.53 : 519.25 I False facts are highly injurious to the progress of science for they often endure long; but false views if supported by some evidence., do little harm for everyone takes a salutary pleasure in proving theU falseness. Darwin Preface False data are probably no less common than they were 100 years ago, and methods for nullifying them are therefore still worth pursuing. Analytical results for trace element concentrations are sometimes pejoratively referred to as random numbers [To'g 1976); but nothing is wrong with random numbers when they belong to a normal distribution with an accurately known standard deviation. In fact, information concerning the precision and accuracy of an analytical result marks the distinction between qualitative and quantitative analysis. However, this information is not always available, and if available it may not be reliable. My interest in this subject was initiated while working in 1965-66 with Professor Vincent P. Guinn at General Atomic in San Diego, California, where I was engaged in neutron activation analysis, a field in which so many important contributions have been made by this laboratory. After my return to Denmark I was fortunate enough to be able to continue work in activation analysis for medical research under the sponsorship of Professor Mogens Faber. With the support of Dr. C. F. Jacobsen, a small group was later established as part of the Risø Radioisotope Laboratory; recently, co-operation with several other scientific disciplines has been sponsored by the National Research Councils. The present work is therefore largely based on results previously published as parts of other projects. However, a considerable number of additional analytical investigations are presented here, and the emphasis and treatment of the data differ in many respects, particularly from that of the earlier publications. Reference is made to the most frequently quoted publications by Roman numerals |l - XI], but these papers do not in themselves form part of the dissertation. The scientific literature is taken into account up to and including 1977. but no attempt has been made to make a complete bibliography of neutron activation analysis. Only papers with special relevance to the subject under discussion are included in the references, in particular representative or important papers on precision and accuracy that are often difficult to locate in a computer search. II In the preparation of the text the guidelines given by Vanderborght [Unisist 1976] have been combined with the formal instructions for a Risø Report [Steenbuch 1974]. The type-written text was processed by the Univac 1110 computer. K. Heydorn July 15, 1978 The printed version is identical to the original type-written version, except for purely editorial changes, including up-dating of literature references and the correction of misprints. K. Heydorn March 15, 1980 IIMIS-descriptors: Accuracy Errors Neutron Activation Analysis Quality Assurance ISBN 87-550-0648-5 ISSN 0106-2840 Contents 1. Introduction 1 Four decades of neutron activation analysis 1.1. Exposition of the Subject 2 Selection of elements Outline of work 1.2. Definition of Terms 12 Precision and accuracy Analytical terminology Symbols and units 2. Factors Affecting Precision and Accuracy 17 Types of error Inherent errors Organization Methodology 2.1. Sampling 20 Quality Quantity Validity Stability Sampling procedure 2.2. Choice of Comparator 27 Principles Composition Form Quality 2.3. Conditioning 37 Treatment Blank problems 2.4. Irradiation 41 Nuclear reactor activation Irradiation and decay times Irradiation conditions IV 2.5. Separation 56 Radiochemical separation Multiple carrier addition Performance characteristics Interference Reproducibility 2.6. Measurement 70 Sensitivity and selectivity Counting times Count-rate Counting geometry Live time control 2.7. Yield Correction 87 Assumed yields Tracer addition Carrier addition Precision and accuracy 2.8. Calculation 96 Total count methods Peak area methods Actual methods Peak boundary selection Discussion Precision of the Analytical Method 113 Classical methods Contemporary methods Neutron activation analysis 3.1. Estimation 115 Distribution of results A priori precision Counting statistics Overall precision 3.2. Analysis of Precision 123 Precision of irradiation Verification of precision Test of precision Statistical control 3.3. Control of Precision 138 Modifications Replicate analysis Continuous quality control Sampling conditions 4. Accuracy of Analytical Results 148 Types of systematic errors Tests for accuracy Referee methods 4.1. Verification of Analytical Results 151 Internal control Specific activity 4.2. Detection of Systematic Errors by the Analysis of Precision 159 Detection characteristics Experimental performance 4.3. Blanks 167 Known blank Estimated blank Unknown blank Insignificant blank 4.4. Reference Materials 183 Biological reference materials Arsenic in SRM 1571 Consensus values Improved certifications 5. Practical Applications 201 Types of problems 5.1. Medical Samples 203 Biological variation Short-term effects Long-term arsenic Essential elements 5.2. Other Problems 227 Radioiodine isotope ratios Lithium in reference materials Vanadium in reference materials 5.3. Ultratrace Analysis 243 Arsenic in serum Vanadium in serum Manganese in serum 6. Conclusion 256 Appendix VII Acknowledgements XI Sammendrag XIII References XVII