Uncertainty, analytical methods

MCM-41, olefin epoxidation, 418, 421-2 MCM-48, olefin epoxidation, 418 MDA see Malondialdehyde Measurement uncertainties analytical methods, 624 potentiometry, 663 Meat... 

Ultra-trace analysis, hydrogen peroxide determination, 638 Ultraviolet see UV Uncatalyzed sulfoxidations, 472-4 Uncertainty, analytical methods, 624 UN Environment Programme (UNEP) chemicals safety, 745, 747 SIDS database, 622 UnfunctionaUzed olefins... 

If, in the above example, the analytical error was 0.2 per cent then the total error sT would be equal to 3.006 per cent. Hence the contribution of the analytical error to the total error is virtually insignificant. Youden7 has stated that once the analytical uncertainty is reduced to one-third of the sampling uncertainty, further reduction of the former is not necessary. It is most important to realise that if the sampling error is large, then a rapid analytical method with relatively low precision may suffice. 

Other authors used a simple 2 standard deviation criteria or an outlier test (F-test) to check for significant differences between within-bottle and between-bottle results (Martin-Esteban et al. 1997 Quevauviller et al. 1995). The degree of homogeneity of elements and compounds in the materials tested in these studies does not seem to be adequately described and, hence, the asigned uncertainties in the mean values may represent only the bias between the analytical methods used in the certification. 

LGC - VAM Publications (i) The Fitness for Purpose of Analytical Methods, A Laboratory Guide to Method Validation and Related Topics, (2) Practical Statistics for the Analytical Scientist A Bench Guide By TJ Farrant, (3) Trace Analysis A structured Approach to Obtaining Reliable Results By E Pritchard, (4) Quantifying Uncertainty in Analytical Measurement, and (5) Quality in the Analytical Chemistry Laboratory. LGC/RSC Publications, London, England. 

As probabilistic exposure and risk assessment methods are developed and become more frequently used for environmental fate and effects assessment, OPP increasingly needs distributions of environmental fate values rather than single point estimates, and quantitation of error and uncertainty in measurements. Probabilistic models currently being developed by the OPP require distributions of environmental fate and effects parameters either by measurement, extrapolation or a combination of the two. The models predictions will allow regulators to base decisions on the likelihood and magnitude of exposure and effects for a range of conditions which vary both spatially and temporally, rather than in a specific environment under static conditions. This increased need for basic data on environmental fate may increase data collection and drive development of less costly and more precise analytical methods. 

Reports of on-line SFE-FIPLC are rare, perhaps because the majority of analytes that have been extracted using SFE can be separated using either GC or SFC. On-line SFE-HPLC is often used to monitor extraction efficiencies. SFE-HPLC optimised for temperature (120 °C), pressure (384 bar), SCF flow and modifier (methanol) has been used for the quantification of Irganox 1010 and Irgafos 168 extracted from PP. In this case Thilen and Shishoo [12] varied three SFE parameters for optimisation of the extraction efficiency, and five parameters for the collection efficiency, see Figures 7.7 and 7.8. Despite these efforts, low recoveries were observed (Table 7.16). This was attributed to problems associated with the compounding process, and not to uncertainties in the extraction and analytical method. 

AMC (1995) The Analytical Methods Committee Uncertainty of measurement - implications for its use in analytical science. Analyst 120 2303... 

As shown in Sect. 7.1, signal-to-noise ratio S/N can be used to characterize the precision of analytical methods. Noise is a measure of the uncertainty of dynamic blank measurements (of the background ). 

EURACHEM (1995) Quantifying uncertainty in analytical measurement. Teddington EURACHEM (1998) The fitness for purpose of analytical methods. Teddington... 

Any analytical method inherently carries with it limitations in terms of speed, allowable uncertainty (as MDL), and specificity. These characteristics of a method (or analytical technique) determine where and how the method can be used. Table 71-1 shows a method to relate purpose of analytical method to the speed of analysis and error types permitted. 

Table 71-1 Characteristics and allowable uncertainty for different analytical methods...

So the example case results in an uncertainty range from 5.014 to 5.714 with an uncertainty range of 0.7. Therefore if we have a relatively unbiased analytical method, there is a 95% probability that our true analyte value lies between these upper and lower concentration limits. 

If potassium iodide is added first, and then the solution is stirred, acidified and titrated, the loss of residual chlorine is reduced, although still significant. The loss again increases monotonically with stirring for 20 min. However, for a stirring time of 1 minute or less, the loss is not detectable within the uncertainty of the analytical method. There is a loss of chlorine whether the sample is stirred in the titrator or on a stirrer, although the loss seems smaller in the latter case. For a stirring time of 20 min, only 24% of the residual chlorine is lost. Moreover, titrations performed at pH 2 and pH 4 yield the same residual chlorine concentrations. 

A logical approach which serves to minimise such uncertainties is the use of a number of distinctly different analytical methods for the determination of each analyte wherein none of the methods would be expected to suffer identical interferences. In this manner, any correspondence observed between the results of different methods implies that a reliable estimate of the true value for the analyte concentration in the sample has been obtained. To this end Sturgeon et al. [21] carried out the analysis of coastal seawater for the above elements using isotope dilution spark source mass spectrometry. GFA-AS, and ICP-ES following trace metal separation-preconcentration (using ion exchange and chelation-solvent extraction), and direct analysis by GFA-AS. These workers discuss analytical advantages inherent in such an approach. 

Despite a long-time studying of superoxide production by mitochondria, an important question is still debated does mitochondria produce superoxide under physiological conditions or superoxide release is always a characteristic of some pathophysiological disorders resulting in the damage of normal mitochondrial functions Uncertainties in this question arise due to the different results obtained with the use of respiratory inhibitors and different analytical methods. 

The implications of the analysis have to be considered before taking the sample or devising a sampling scheme. It is the responsibility of the analytical chemist, through discussion with the customer, to establish the real nature of the problem. How much cadmium is there in this sample is not sufficiently specific. You must always ask why the information is required. The answer affects both the sampling plan and the choice of analytical method. These will depend on the acceptable level of uncertainty in the final result. 

Selection of a suitable analytical method can be made once the reason for carrying out the analysis is well understood. Analytical methods may be (a) qualitative or (b) quantitative or semi-quantitative. The former usually pose few problems if only an indication is required as to whether a particular analyte is present or not - certainly not how much with a value having a small uncertainty. If a negative result is required (i.e. confirmation of absence from the product), then one has only to worry about the limit of detection of the test used. Many tests to confirm the absence of impurities in pharmaceutical products fall into this category. Equally, rapid tests for positive confirmation are often made on unknown substances. These may subsequently be confirmed by other, quantitative tests. Quantitative methods are used in a variety of situations and a variety of different methods can be employed. What you must always remember is that the method used must be fit for the purpose. 

This chapter deals with handling the data generated by analytical methods. The first section describes the key statistical parameters used to summarize and describe data sets. These parameters are important, as they are essential for many of the quality assurance activities described in this book. It is impossible to carry out effective method validation, evaluate measurement uncertainty, construct and interpret control charts or evaluate the data from proficiency testing schemes without some knowledge of basic statistics. This chapter also describes the use of control charts in monitoring the performance of measurements over a period of time. Finally, the concept of measurement uncertainty is introduced. The importance of evaluating uncertainty is explained and a systematic approach to evaluating uncertainty is described. 

Note that some organizations may not use the terminology used in this book and may not distinguish between SOPs and WIs. Standard Operating Procedures provide details of how a series of operations are carried out. An example of a SOP would be the detailed instruction for carrying out a particular analytical method. Work Instructions give details of how a specific operation is carried out. What might be classed as a WI is how to operate a particular instrument, how to estimate measurement uncertainty or how to calibrate a piece of equipment. 

Subsampling can be done by hand, although several machines for subsampling are available. Another approach is to have an analytical method or methodologies that allow analysis of many different analytes in one sample. In this way, there is no question of variability between subsamples. This is not always possible, but minimizing steps in any procedure will decrease variability and uncertainty in the analytical results. 

Although quantification of the elements present in the y spectrum can in theory be achieved from first principles using the equation given above, in practice uncertainties in the neutron capture cross-section and variations in the neutron flux within the reactor mean that it is better to use standards. These standards must be included in each batch of samples irradiated in order to account for variations in neutron flux inside the reactor. For analysis of minor and trace elements calibration is easier than with other analytical methods provided that the major element composition remains reasonably constant, as the y ray intensity is proportional to concentration over a very wide range of concentrations. However, for analysis of major elements, e.g., silver in silver coins, the relationship between intensity and concentration is more complex, due to progressive absorption of neutrons as they pass through the specimen. In such cases y ray intensity will also depend on the thickness of the sample and therefore specialized calibration methods are required (Tite 1972 277). 

Although the finite element method can provide the most accurate means for analyzing structures for blast loads, the uncertainty associated with determination of loads generally does not justify its use. Also, the effort associated with finite element model development and interpretation of results is often greater that what is required by the simplified methods outlined above. The simpler SDOF based analytical methods are recommended for use except in those cases, as described above, where the inaccuracies associated with SDOF approximations may be unacceptable. 

Accuracy, uncertainty, and traceability. A certified value is the best approximation of the true concentration of the analyte. During the certification process, a variety of analytical methods may be used to determine this true value. Uncertainty estimates ultimately based on this process, together with information about the material s homogeneity can give a certified reference material traceability, needed for true international comparability. 

Although ocean water is well mixed with respect to Li, and many laboratories measure Li isotopes in seawater, the range in values reported in the literature (8 Li = +29.3 to +33) cast some uncertainty on this reservoir. As precision of analytical methods improves, a check of the viability of the seawater standard should be carried out. 

Real geological samples rarely exhibit a specific number of mineral components. Such samples are composed of a multitude of minerals that are qualitatively referred to as major, minor, and trace components. The sensitivity of the particular analytical method limits our ability to resolve these minerals. Here we determine the number of mineral components (NC) from the elgenanalysls of the raw data matrix. The raw data matrix Is approximated by a successively Increasing number of eigenvectors. When this approximated data matrix Is within the expected uncertainty of the data, we... 

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