Aspects of Precision and Accuracy in Neutron Activation Analysis Kaj Heydorn

310 Pages·2007·5.99 MB·English

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