Reliability of Analytical Measurements

For the characterization of the reliability of analytical measurements the terms precision, accuracy, and trueness have a definite meaning. 

Precision is defined as the closeness of agreement between independent test results obtained under stipulated conditions (Fleming et al. [1996b] Prichard et al. [2001]). Precision characterizes the random component of the measurement error and, therefore, it does not relate to the true value. 

According to the conditions under which the measurements are carried out, it is to distinguish between 

Repeatability is the closeness of the agreement between the results of successive measurements of the same measurand carried out under the same conditions of measurement . Repeatability conditions include the same measurement procedure, the same observer, the same measuring instrument, used under the same conditions, the same location, and repetition over a short period of time (ISO 3534-1 [1993]). 

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This Handbook aims to explain terminology widely used, and sometimes misused, in analytical chemistry. It provides much more information than the definition of each term but it does not explain how to make measurements. Additionally, it does not attempt to provide comprehensive coverage of all terms concerned with chemistry, instrumentation or analytical science. The authors have addressed primarily those terms associated with the quality assurance, validation and reliability of analytical measurements. The Handbook attempts to place each term in context and put over concepts in a way which is useful to analysts in the laboratory, to students and their teachers, and to authors of scientific papers or books. This approach is particularly important because official definitions produced by many international committees and organisations responsible for developing standards are frequently confusing. In a few cases the wording of these definitions completely obscures their meaning from anyone not already familiar with the terms. 

The variety of complex terms used in the Quality Assurance aspect of analytical measurement can be the cause of considerable confusion.This unique handbook explains the most widely-used terminology in language that is readily understood, and attempts to place each term in context. Concepts are described in a way that is useful to all practitioners, particularly those concerned with quality assurance, validation and reliability of analytical measurements. Explanations of terms are always in line with the "official definition", often developed by international committees. 

The UK Government has, for more than six years, funded the Valid Analytical Measurement (VAM) Programme, which is aimed at improving the quality and comparability of analytical measurements. The work undertaken within VAM is key to the underpinning of a modern physico-chemical and biochemical National Measurement System, By disseminating the activities of VAM across international boundaries and linking with other national measurement system VAM also aims to ensure the comparability of data worldwide. Thus VAM provides an infrastructure under which reliable measurements can be made for trade, regulation and health and safety provision. 

Both qualitative observations and quantitative measurements cannot be reproduced with absolute reliability. By reason of inevitable deviations, measured results vary within certain intervals and observations, mostly in form of decision tests, may fail. The reliability of analytical tests depends on the sample or the process to be controlled and the amount of the analyte, as well as on the analytical method applied and on the economical expenditure available. 

Analytical laboratories need to check their performance with regard to the production of accurate results with satisfactory precision. The most desirable way to ensure the reliability of analytical results is the participation of laboratories into regular interlaboratory tests. An interlaboratory study has to be understood as a study in which several laboratories measure a quantity in one or more identical portions of homogeneous, stable materials under documented conditions, the result of which are compiled into a single document (IUPAC [1994] Prichard et al. [2001]). 

The purpose of an analytical method is the deliverance of a qualitative and/or quantitative result with an acceptable uncertainty level. Therefore, theoretically, validation boils down to measuring uncertainty . In practice, method validation is done by evaluating a series of method performance characteristics, such as precision, trueness, selectivity/specificity, linearity, operating range, recovery, LOD, limit of quantification (LOQ), sensitivity, ruggedness/robustness, and applicability. Calibration and traceability have been mentioned also as performance characteristics of a method [2, 4]. To these performance parameters, MU can be added, although MU is a key indicator for both fitness for purpose of a method and constant reliability of analytical results achieved in a laboratory (IQC). MU is a comprehensive parameter covering all sources of error and thus more than method validation alone. 

Results of analytical measurements are a kind of a product of the chemical analyst s work. Both manufactured products (object of analysis) and analytical results must be of an appropriate quality. In addition, the quality of analytical measurements appears to have its own accumulative requirement the quality of every product is a result of comparison of the obtained value (anal3dical result) with the reference value (expected, standard, norm, required). In order for the obtained result to be comparable (authoritative, reliable) to the reference value, its (high) quality must be documented and maintained. The quality of analytical results must be assured in the first place before drawing conclusions about the quality of the examined products. 

The aim of the VAM programme is to encourage the use of the above procedures to improve the quality of the analytical measurements in the United Kingdom. The work centres around three main areas of activity defining and disseminating best analytical practice which will enable laboratories to deliver reliable results every time developing the tools which enable laboratories to implement best analytical practice working with analysts in other countries to ensure the comparability of analytical measurements across international boundaries. 

Some authors have stated that human hair can be employed as an index for an excess, or a deficiency, of specific nutrients in the diet or as an index of absorption of contaminants from the environment [39]. Analysis of hair is considered to give an indication of the integrated dose of elements ingested by a person over some months. In addition, it is easier to collect hair than blood or urine. Therefore, analysis of human hair is performed by a number of organisations to monitor the level of exposure to heavy metals of a population or an individual [40]. However, this monitoring is often hampered by an insufficient reliability of the measurements. The BCR has thus decided to provide laboratories with a mean of checking their analytical performance by... 

To obtain reliable analytical data it is essential to examine the reliability of all the steps involved in an analytical process — sampling, black box, data processing — as well as the reliability of the instruments used. The complexity of the sample is key to reliable analytical information as the complexity of the sample influences the selection of the analytical process and the instrument used for analysis. In addition to its complexity, the history of the sample must also be considered. Usually, the sampling process is a most critical aspect, as its reliability affects the results considerably. The quality and reliability of analytical information cannot be guaranteed unless standards are used for measurements. Conversely, only reliable methods can be considered for standardization. 

The importance of estimation of uncertainty in chemical analysis is due to its direct relationship with the quality of the analytical information, and with the reliability of the measurement. Taking into account the definitions given in a previous chapter of this book for the reliability of the analytical information — as a function of the reliabilities or uncertainties of different steps of the analytical process — it follows that the reliability of the analytical information (RAI) is... 

Once the methodology for trace element determination in petroleum products by ICP-MS was validated, it was applied to several crude oils (supplied by Petrobris) and their fractions originating from a Brazilian petroleum basin. In order to prove the reliability of analytical results obtained after oil fractionation, total mass balance (TMB) was calculated for each element measured. Table 3 shows an example of crude oil and its fractions analyzed by ICP-MS. TMB results were in the range of 90-110 % for most of the elements analyzed. In order to confirm the repeatability of oil fractionation and analysis, each sample (oil + fractions) was analyzed three... 

The measurement of absorption of ultraviolet-visible radiation is of a relative nature. One must continually compare the absorption of the sample with that of an analytical reference or blank to insure the reliability of the measurement. The rate at which the sample and reference are compared depends on the design of the instrument. In single-beam instruments there is only one light beam or optical path from the source through to the detector. This usually means that one must remove the sample from the light beam and replace it with the reference after each reading. Thus, there is usually an interval of several seconds between measurements. 

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