Repeatability analytical method

The earliest studies on the automation of chemical analysis in oenology by FIA were carried out by Sarris et al. (1970). However, it was only from 1974 onwards that this technique started to become widely used in routine oenological determinations with the development of reliable and repeatable analytical methods. 

The oxidation of teUurium(IV) by permanganate as an analytical method has been studied in some detail (26). The sample is dissolved in 1 1 nitric-sulfuric acid mixture addition of potassium bisulfate and repeated fuming with sulfuric acid volatilises the selenium. The tellurite is dissolved in 10 vol % sulfuric acid, followed by threefold dilution with water and titration with potassium permanganate ... 

Precision The repeatability characterizes the degree of short-term control exerted over the analytical method. Reproducibility is similar, but includes all the factors that influence the degree of control under routine and long-term conditions. A well-designed standard operating procedure permits one to repeat the sampling, sample work-up, and measurement process and repeatedly obtain very similar results. As discussed in Sections 1.1.3 and 1.1.4, the... 

The precision of an analytical method is a measure of the variability of repetitive measurements. Contributions from numerous sources affect precision, but the major components are within-laboratory (repeatability) and between-laboratory (reproducibility) variations. Precision is expressed as the relative standard deviation (or CV)... 

To demonstrate the validity of an analytical method, data regarding working range/ calibration, recovery, repeatability, specificity and LOQ have to be provided for each relevant sample matrix. Most often these data have to be collected from several studies, e.g., from several validation reports of the developer of the method, the independent laboratory validation or the confirmatory method trials. If the intended use of a pesticide is not restricted to one matrix type and if residues are transferred via feedstuffs to animals and finally to foodstuffs of animal origin, up to 30 sets of the quality parameters described above are necessary for each analyte of the residue definition. Table 2 can be used as a checklist to monitor the completeness of required data. 

Analytical measurement Pros - Results obtained reflect well reality - Repeatability and reproducibility of results (at least between good qualified labs) - Measurements are independent of information/data sources - Multipurpose analytical methods can cover many compounds on a single run - Even the best model will ultimately need to be experimentally checked - Discovery of new emerging contaminants is possible... 

Analysis precision, n - a statistical measure of the expected repeatability of results for an unchanging sample, produced by an analytical method or instrument for samples whose spectra represent an interpolation of a multivariate calibration. The reader is cautioned to refer to specific definitions for precision and repeatability based on the context of use. 

An analytical method based on matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) was applied to provide information on the structure of copolymer 29, for example, repeat units and end groups <2002ANC6252>. 

If the analytical method used by participants in the proficiency testing round has been validated by means of a formal collaborative trial, then the repeatability and reproducibility data from the trial can be used. The repeatability standard deviation gives an estimate of the expected variation in replicate results obtained in a single laboratory over a short period of time (with each result produced by the same analyst). The reproducibility standard deviation gives an estimate of the expected variation in replicate results obtained in different laboratories (see Chapter 4, Section 4.3.3 for further explanation of these terms). 

The precision of a test method is the variability between test results obtained on the same material using a specific test method (ASTM, 2004 Patnaik, 2004). The precision of a test is usually unrelated to its accuracy. The results may be precise, but not necessarily accurate. In fact, the precision of an analytical method is the amount of scatter in the results obtained from multiple analyses of a homogeneous sample. To be meaningful, the precision study must be performed using the exact sample and standard preparation procedures that will be used in the final method. Precision is expressed as repeatability and reproducibility. 

The first two points are best dealt with as part of the process for developing vahdated analytical methods. Vafidation should include testing the robustness of a method in repeated use over a period of time determining the precision and accuracy and study of potential interferences. As an example, it would be expected that in the capillary GC—TEA method for organic explosives, a peak should be at least three times the basefine noise to be counted as a real signal, and that the relative retention time should be within 1.0% of the standard for volatile compounds and within 0.5% for the rest. The relative retention time is simply the ratio of the analyte s retention time compared with that of an internal standard. Use of relative retention times significantly improves the repeatabdity of GC analysis... 

The study of the precision of a method is often the most time and resource consuming part of a method validation program, particularly for methods that are developed for multiple users. The precision is a measure of the random bias of the method. It has contributions fi om the repeatability of various steps in the analytical method, such as sample preparation and sample injection for HPLC [5-9], and from reproducibility of the whole analytical method fiom analyst to analyst, fiom instrument to instrument and fiom laboratory to laboratory. As a reproducibility study requires a large commitment of time and resources it is reasonable to ensure the overall ruggedness of the method before it is embarked upon. 

In order to set up an analytical method based on factor scores or analytical deviation in a Q.A. environment, several precautions would need to be followed. For instance, changes in raw materials would need to be monitored and either new reference materials used or factor analyses repeated. Changes in chromatographic conditions would require a system which allowed constant updating to avoid shift in factor scores. While the peak ratio technique is the simpler of the two, factor scores provide a more sophisticated method which can be used where overlap of components does not lend itself to the simpler methods. These two techniques for deriving the concentrations of the multicomponents in a mixture were shown to predict sensory response when one multicomponent was altered. However it also could be used in a multidimensional formula to predict response when more than one multicomponent has been altered. 

The precision of an analytical method is usually expressed as the standard deviation or relative standard deviation (coefficient of variation) of a series of measurements. Precision represents repeatability or reproducibility of the analytical method under normal operating conditions. Precision determinations permit an estimate of the reliability of single determinations and are commonly in the range of 0.3 to 3% for dosage form assays. 

The precision of an analytical method can be defined as the pattern of variation of single assays on a uniform sample. The precision serves to identify random errors and is described by the repeatability (variability within a laboratory) and reproducibility (variation between different laboratories). 

Repeatability Repeatability is a measure of the precision under the same operating conditions over a short interval of time, that is, under normal operating conditions of the analytical method with the same equipment. It is sometimes referred to as intra-assay precision. 

To construct a cause-and-effect diagram of uncertainty sources from the information contained in the procedures and equations of an analytical method, follow these steps. First, draw a horizontal right-facing arrow in the middle of a sheet of paper. Label the arrow end with the symbol for the measurand. Starting from the sources identified by the equation for the value of the measurand, draw arrows to this line at about 45°, one for each of the quantities in your equation plus any other sources identified that are not already counted, plus one for repeatability. Label the start of each arrow with a symbol for the quantity. Figure 6.3 shows a draft cause-and-effect diagram for the purity of the acid. 

Table 26.3 Numerical Requirements for Accuracy and Repeatability of Analytical Methods in the European Union...

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