Validating Results

Recognising that aU analytical results represent estimates of the concentration of an analyte within a food/biological matrix, the most important characteristic of any result is a statement of its uncertainty interval. Typically, the repeatability of an analysis performed on a particular 

Having persisted through this introduction to atomic spectroscopy, you might want to consult the following references for a more detailed discussion of the appropriate topic. 

Slavin, Graphite Furnace AAS - a Source Book , Perkin-Elmer Corporation, Ridgefield, CN (1984). 

Slavin, Atomic absorption spectrometry , in Metallobiochemistry, Part A, J. F. Roirdian and B. T. Vallee (eds.). Methods in Enzymology, Vol. 158, Academic Press Inc., New York, NY, p. 145 (1988). 

Atomic absorption spectrometry , VCH Publishers, Weinheim, Germany (1985). 

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But, with the use of digitization, 2D quantitative measurements are allowed for industrial radiography. These can be done by powerful tools, like estimation of defect extension, automatic segmentation, recognition of individual defects and image analysis (figure 7). For validation, results can be compared with destractive examination of metallic objects. 

Various ion-optical tricks have to be used to compensate for the spread of energies of the extracted ions, which limit mass resolution unless corrected for. In the latest version of the atom probe (Cerezo et at. 1988), spatial as well as compositional information is gathered. The hole in the imaging screen is dispensed with and it is replaced by a position-sensitive screen that measures at each point on the screen the time of flight, and thus a compositional map with extremely high (virtually atomic) resolution is attained. Extremely sophisticated computer control is needed to obtain valid results. 

The standard requires validation results to be recorded and design failures to be documented in the validation records. 

Figure 55. Logarithmic plots of the cross-validation results as a function of the number of factors (rank) used to construct the calibration.

Control of Monitoring and Measuring Devices (Calibrate measuring equipment for valid results)... 

Validation results obtained from factor analysis of Table 31.2, containing the retention times of 23 chalcones in 8 chromatographic methods, after log double-centering and global normalization. The results are used in the Malinowski s f-test and in cross-validation by PRESS. 

In the second phase, analysts in participating laboratories prepare and analyze a minimum of two conttol samples and two samples fortified at the proposed tolerance concenttation. This phase allows analysts to become familiar with the method before the analysis of samples that will be part of the method validation. Results from the second phase should demonsuate that the control samples are without interference and that the analysts in the participating laboratories can achieve acceptable recovery of analyte from the samples. It is not uncommon for an analyst to have to repeat the second phase several times before adequate results are obtained. Failure at this phase of the trial can cause a method to fail the Uial. Often the problems are related to a poorly written SOP that does not adequately describe the procedure. 

To avoid any subjectivity in the judgement of performance characteristics presented by applicants, the permitted ranges of several parameters are fixed (e.g., recovery, repeatability, highest intensity of blank signals compared with the LOQ). However, in other cases professional judgement of the referee is required to assess validation results. Some of these aspects are discussed below. 

The sensitivity achieved (LOD) is not normally presented. It is recognized that different laboratories determine dissimilar values for this parameter and even within a laboratory the repeatability of the LOD is low. Most often, the lowest validated concentration gives an impression about the lowest levels that can be analyzed generally with acceptable results. A measure of selectivity is the intensity of blank results. This intensity is discussed by the participants of inter-laboratory validation studies. However, results are not reported and limits are not defined by CEN TC 275. The results of method validations of the several multi-residue/multi-matrix methods are not reported in the same way, but newer methods with limited scope generate analogous tables with validation results (as an example, see Table 7). 

Table 7 Presentation of the validation results in EN 13191 2000 Nonfatty foods - Determination of dithiocarbamate and thiuram disulfide residues - Part 1 ...

In contrast to many other validation protocols, the description of the NMKL validation process starts with the protocol of planned validation. This protocol should include, e.g., the needs of the client, available equipment, the chemical form in which the analyte occurs (i.e., in pesticide analysis the residue definition), matrix types, the availability of reference materials and the working range. Consequently, an extra paragraph is dedicated to the requirements for the documentation of validation results, which refers to the rules in Section 5.4.4 of EN 45001 (amended by ISO 17025). 

Selectivity and sensitivity of available instruments are tested in all laboratories in the initial step of validation. The crops used for fortification experiments and the concentration levels are identical in all laboratories. Recoveries are determined with all available detection techniques, but after discussion of the results each laboratory selects individually one valid result for each analyte-matrix-level combination. Only this result is used for the calculation of the final mean recovery and standard deviation. Typical criteria for the acceptance of methods are given in Table 11. 

If analytical methods are validated in inter-laboratory validation studies, documentation should follow the requirements of the harmonized protocol of lUPAC. " However, multi-matrix/multi-residue methods are applicable to hundreds of pesticides in dozens of commodities and have to be validated at several concentration levels. Any complete documentation of validation results is impossible in that case. Some performance characteristics, e.g., the specificity of analyte detection, an appropriate calibration range and sufficient detection sensitivity, are prerequisites for the determination of acceptable trueness and precision and their publication is less important. The LOD and LOQ depend on special instmmentation, analysts involved, time, batches of chemicals, etc., and cannot easily be reproduced. Therefore, these characteristics are less important. A practical, frequently applied alternative is the publication only of trueness (most often in terms of recovery) and precision for each analyte at each level. No consensus seems to exist as to whether these analyte-parameter sets should be documented, e.g., separately for each commodity or accumulated for all experiments done with the same analyte. In the latter case, the applicability of methods with regard to commodities can be documented in separate tables without performance characteristics. 

Predicted skeletal americium concentrations were compared to values observed in a USTUR case and the predicted and observed values agreed reasonably well. However, the same USTUR case was used to derive values for model parameters and, therefore, agreement with the observations would be expected. No other validation results are described. 

Transferability of subsystems of large systems is an assumption often invoked in the study of physical objects where a direct analysis of the complete system is cumbersome. The study of subsystems, either in isolation or as parts of a smaller object is often simpler than the study of the original large system yet in many instances, some of the results obtained for the subsystem can be safely extrapolated to the large system. Whereas transferability has proved to be a very useful concept that leads to important and valid results when used with appropriate caution, it is also a concept that is sometimes poorly justified and may lead to erroneous conclusions. 

Consent Yield Externally Valid Results - an Empirical-Test , Psychopharmacology no, no. 4 (1993) 437-42... 

Validated result, n - a result produced by the spectroscopic (or instmmental) method that is equivalent, within control limits, to the result expected from the reference method so that the result can be used in lieu of the direct measurement of the sample by the reference method. 

In a sense, therefore, there have been two conflicting views with respect to the suitability of formalin as a fixative, in the face of demands that biopsy tissues may be examined not only by traditional morphologic methods, but also by IHC, in situ hybridization (ISFI) and, following extraction procedures, by other molecular methods. Both views recognize that these newer methods do not perform well, or at all, on routinely processed FFPE tissues. One view advocates the development of new fixatives that are molecular friendly, the other view holds that AR-based methods may be employed to achieve accurate valid results of IHC, ISH, and other molecular methods using FFPE tissues. 

Equation 13.5-2 is the segregated-flow model (SFM) with a continuous RTD, E(t). To what extent does it give valid results for the performance of a reactor To answer this question, we apply it first to ideal-reactor models (Chapters 14 to 16), for which we have derived the exact form of E(t), and for which exact performance results can be compared with those obtained independently by material balances. The utility of the SFM lies eventually in its potential use in situations involving nonideal flow, wheic results cannot be predicted a priori, in conjunction with an experimentally measured RTD (Chapters 19 and 20) in this case, confirmation must be done by comparison with experimental results. 

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