Methods of Measurement and Some Results

Outlying variances, e.g. as reflected by a Cochran test, do reveal that some sets of data suffer insufficient precision compared to the other sets obtained by other laboratories. Such sets affect the final uncertainty of the certified value but not the certified value as such. The technical discussion should address the reason why a set of data lacks precision (day to day bias ) or why in one laboratory the reproducibility figure is much lower than for the rest of the participants (repeatability figure rather than reproducibility, selection of data, not fully independent measurements etc.). Sets of data are rejected if the standard error of the mean (s/n) exceeds the standard deviation of the distribution of all the laboratory mean values. It must be stressed that BCR has accepted and even promoted alternative methods of measurements in some certification exercises, in order to back-up trueness of certified values. Finally, such methods may have shown that their precision was too poor and were not used to calculate the certified value and its uncertainty. In such cases the results are made available to the user of the CRM through the certification report. 

During the decade 1950-60 much work was done on determining the folate content of various foods, and there was sometimes a considerable variation in the results obtained. It is not the purpose here to present an extensive account of this earlier work. More recent methods of measurement and the protection of some labile forms of the vitamin with ascorbate have resulted in higher assay results for many foodstufis. In some instances the... 

In order to use equation (8-15) for determination of T it is necessary to know the value of hGtkr known as the psychrometric ratio. Values of hG and ky can be estimated independently for the particular shape of the wetted surface by correlations like those presented in Chapter 2, using the heat- and mass-transfer analogy if necessary. Alternatively, experimental values of the psychrometric ratio can be used. Henry and Epstein (1970) have examined the data and methods of measurement and have produced some measurements of their own. For turbulent flow of gases past cylinders, such as wet-bulb thermometers, and past single spheres, the results of 18 gas-vapor systems are well correlated by... 

In recent years, the MS has become a useful alternative to the radiation detector for measuring longer-lived radionuclides. It provides a second and completely independent method of measurement to confirm results it also gives the analyst a choice, because some measurements are easier or more reliable by one method than the other. The MS has been used to measure radioactive atoms with half-lives greater than 10 years because the number of these atoms relative to their decay rate is proportional to the half life for half lives greater than 109 y, even the conventional measurements of analytical chemistry are applicable. 

In a search for an accurate method of measuring moisture in foods, one cannot overlook the essential requirements of convenience, speed, and precision. Many currently used methods meet these requirements without necessarily yielding accurate results under the conditions used. Probably most important are the electrical methods (IS, 24, 26, 36), the air- and vacuum-oven methods (/, 2, 6, 18, 25, 28, 36), distillation with organic solvents (1, 3, 7, 12, 13, 26, 35), and the Karl Fischer reagent method (9, 11, 26, 31, 32). Without discussing the relative merits of these methods, it can be assumed that accurate results could be obtained with each method by calibration against some accurate reference method. 

Methods for the instrumental measurement of whiteness are well established but visual comparison remains important, even in well-equipped laboratories. Some degree of quantification is achieved by the method of paired comparisons, in which a panel of observers is presented with pairs of FBA-treated samples and asked to decide, without undue delay, which is the brighter. The total of positive scores can be used as a measure of whiteness and the results presented graphically as shown in Figure 11.7. Although time-... 

Many of the remaining problems, particularly those connected with fine structure and shape and size, cannot be completely solved by purely chemical methods of investigation, and these have to be supplemented by the use of physical methods. Furthermore, in some instances, the results from such physical measurements have to be correlated with those obtained from studying the action of highly purified enzymes on the starch. 

The analyst provides scientific evidence on which important decisions are made. The work of an analyst is devalued if it is merely considered to be making measurements and reporting results. There has to be some added value. This is provided when it can be demonstrated that the results produced have been obtained in an organization that operates a quality management system. It is because of the importance of the work that the quality has to be assured. This means that all of the necessary actions have been taken to make sure that any factor which has an influence on the final result has been considered, understood and reported in a permanent record - that the appropriate measurements have been made and these have been carried out correctly using a validated method. 

Dietary Fibre. An area of some contention is what should be included as dietary fibre. There are several different methods of measuring dietary fibre and all of them will give different results on the same sample. The EU favours the Association of Analytical Chemists (AOAC) method, which includes lignin and resistant starch, while the UK has preferred the Englyst method and has defined fibre as nonstarch polysaccharides from cell walls as the only substances that count as fibre. 

The existence of a correlation between the catalytic activity and the electrical conductivity which follows from the theory was indicated by us back in 1950 (37, 6S), when there were as yet no measurements available that could either corroborate or refute this theoretical prediction. To date we have already a whole series of experimental work in which such a correlation has been observed (e.g., 36, 56, 66-70). A number of authors have measured the electrical conductivity and the catalytic activity of various samples of a semiconductor which differed in the method of preparation and have discovered that these two properties of the semiconductor vary in the same or in opposite directions from one sample to another. The results of some of these experiments are presented in Table II. 

As noted in the last section, the correct answer to an analysis is usually not known in advance. So the key question becomes How can a laboratory be absolutely sure that the result it is reporting is accurate First, the bias, if any, of a method must be determined and the method must be validated as mentioned in the last section (see also Section 5.6). Besides periodically checking to be sure that all instruments and measuring devices are calibrated and functioning properly, and besides assuring that the sample on which the work was performed truly represents the entire bulk system (in other words, besides making certain the work performed is free of avoidable error), the analyst relies on the precision of a series of measurements or analysis results to be the indicator of accuracy. If a series of tests all provide the same or nearly the same result, and that result is free of bias or compensated for bias, it is taken to be an accurate answer. Obviously, what degree of precision is required and how to deal with the data in order to have the confidence that is needed or wanted are important questions. The answer lies in the use of statistics. Statistical methods take a look at the series of measurements that are the data, provide some mathematical indication of the precision, and reject or retain outliers, or suspect data values, based on predetermined limits. 

N03 As discussed earlier, the nitrate radical can be measured using visible spectroscopy and its absorption bands, particularly the one at 662 nm. As a result, visible absorption spectroscopy has been the method of measurement used most extensively for NOv As discussed shortly, a matrix isolation technique has also been applied with success in some studies. 

It remains for workers to develop a method of selecting correlations based on mechanism rather than on adequacy of measurement and to attempt some interpretation of these results in terms of mechanisms of source-term formation. This paper has attempted to show only that the complexity of the phenomenology, while enough to cause trouble, is not beyond analysis. 

A good overview of the different methods of measuring human toxicological effects can be found in Langlais et al. (1991). They summarize results from many human toxicity studies on ozone and ozonation byproducts. Some results on human toxicity are also presented below. 

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