Accuracy of analytical results

Accuracy. In general, the accuracy of analytical results is assured by recovery studies (Wegscheider [1996] Danzer [1995] Burns et al. [2002]). According to the recovery function in the general three-dimensional calibration model (see Fig. 6.3), common studies on systematic deviations (Fig. 4.3), and Eqs. (4.2) and (4.3) the following recovery formulae... 

Alford Stevens et al. [49] carried out a multi-laboratory study of automated gas chromatography-mass spectrometric determinations of polychlorinated biphenyls in soil. The influence of various factors on the accuracy of analytical results were studied. Shaker extraction for 12.5h followed by Florisil chromatography were demonstrated to be the most reliable methods for extraction and clean-up. 

For practical reasons, most analytical laboratories are not in a position to check accuracy internally, without an external source of reference materials. To overcome some of the difficulties in checking the accuracy of analytical results, the IAEA provides the Analytical Quality Control Services (AQCS) Programme to assist laboratories in assessing the quality of their work. AQCS co-ordinates intercomparison studies and supplies reference materials. Participation is on a voluntary basis and at minimum cost. Information supplied by laboratories taking part in the intercomparisons is treated as confidential. 

The data in Tables 34-1 through 34-5 show that we are well advised to adopt a critical attitude regarding the accuracy of analytical results on real samples, even if we perform the analysis ourselves. 

Tschopel P. and Tolg G. (1982) Comments on the accuracy of analytical results in nanogram and picogram trace analysis of the elements, J Trace Microprobe Techn 1 1-77. 

Fine Chemicals.—The many substances we have mentioned under the heading of chemical products, represent only a very small proportion of those in common use, and do not include all that are employed in the laboratory, or those at present of purely scientific interest, which are very numerous. We say at present purposely, for no one can tell how soon they may find practical application. The value of research for its own sake has already been shown in the many examples we have cited of the discovery of elements and compounds, at first merely regarded as scientific curiosities, but sooner or later proved to be of incalculable importance to industry and to the world. So much depends on the accuracy of analytical results, that an adequate supply of chemicals in a sufficiently pure state to be used as reagents is an essential requirement in all laboratories. The statement should be too obvious to mention, but it must be remembered that we buy and sell on analytical data we check and control vast technical operations on such data, and must be able to rely on... 

As a result we propose the technique of Mo, W, and Re determination in production cycle solutions with the use of peak of non-coherent scattered primary radiation in order to take into account the matrix influence on the analytical signal. The accuracy of the results was checked by the input-found technique. 

The accuracy of a determination may be defined as the concordance between it and the true or most probable value. It follows, therefore, that systematic errors cause a constant error (either too high or too low) and thus affect the accuracy of a result. For analytical methods there are two possible ways of determining the accuracy the so-called absolute method and the comparative method. 

A soil sample was taken from a field, transported back to the laboratory by road and stored for three weeks prior to analysis. The analytical procedure consisted of drying the soil in an oven at 100°C for 24 h before the analyte was extracted using 200 cm of dichloromethane. This extract was reduced in volume to 200 til and a 20 p.l aliquot then analysed by HPLC. A calibration was set up by measuring the response from a number of solutions containing known concentrations of the analyte. The resnlt obtained from the unknown , after suitable mathematical manipulation, indicated the original soil sample contained 20 0.05 mgkg of the analyte. Comment on the accuracy of this result. 

In all analyses, there is uncertainty about the accuracy of the results that may be dealt with via sensitivity analyses [1, 2]. In these analyses, one essentially asks the question What if These allow one to vary key values over clinically feasible ranges to determine whether the decision remains the same, that is, if the strategy initially found to be cost-effective remains the dominant strategy. By performing sensitivity analyses, one can increase the level of confidence in the conclusions. Sensitivity analyses also allow one to determine threshold values for these key parameters at which the decision would change. For example, in the previous example of a Bayesian evaluation embedded in a decision-analytic model of pancreatic cancer, a sensitivity analysis (Fig. 24.6) was conducted to evaluate the relationship... 

In their broadest application, CRMs are used as controls to verify in a direct comparison the accuracy of the results of a particular measurement parallel with this verification, traceability may be demonstrated. Under conditions demonstrated to be equal for sample and CRM, agreement of results, e.g. as defined above, is proof. Since such possibilities for a direct comparison between samples and a CRM are rare, the user s claims for accuracy and traceability have to be made by inference. Naturally, the use of several CRMs of similar matrix but different analyte content will strengthen the user s inference. Even so, the user stiU has to assess and account for all uncertainties in this comparison of results. These imcertainty calculations must include beyond the common analytical uncertainty budget (i) a component that reflects material matrix effects, (2) a component that reflects differences in the amount of substance determined, (3) the uncertainty of the certified or reference value(s) used, and 4) the uncertainty of the comparison itself AU this information certainly supports the assertion of accuracy in relation to the CRM. However, the requirement of the imbroken chain of comparisons wiU not be formally fulfilled. 

Mandel, J. Accuracy andPrecision Evaluation and Interpretation of Analytical Results , InTreatise on Analytical Chemistry, ed. by I.M Kolthoff and PJ. Elving, 2nd edn., Vol. 1. New York, Wiley and Sons, Inc., 1978. 

Comparing this value with the order a Zaym result in (3.101) we see that the difference between the exact numerical result and analytic calculation up to order a Zay is about 0.015 kHz for the IS -level in hydrogen, and, taking into account the accuracy of experimental results, one may use analytic results for comparison of the theory and experiment without loss of accuracy. A similar conclusion is valid for other hydrogen levels. 

Analytical techniques have gone through considerable changes in the past 20 years. With the development of more sensitive and selective analytical instrumentation the analyst has been able to detect and identify minute quantities of materials never before seen. This has brought about a keen awareness of the widespread distribution of toxic hazards and also the need to study the long term effects of low level exposures. The development of new methodology is a dynamic process. However, new methods should always be thoroughly tested to demonstrate the precision and accuracy of the results obtained. 

The process of providing an answer to a particular analytical problem is presented in Figure 2. The analytical system—which is a defined method protocol, applicable to a specified type of test material and to a defined concentration rate of the analyte —must be fit for a particular analytical purpose [4]. This analytical purpose reflects the achievement of analytical results with an acceptable standard of accuracy. Without a statement of uncertainty, a result cannot be interpreted and, as such, has no value [8]. A result must be expressed with its expanded uncertainty, which in general represents a 95% confidence interval around the result. The probability that the mean measurement value is included in the expanded uncertainty is 95%, provided that it is an unbiased value which is made traceable to an internationally recognized reference or standard. In this way, the establishment of trace-ability and the calculation of MU are linked to each other. Before MU is estimated, it must be demonstrated that the result is traceable to a reference or standard which is assumed to represent the truth [9,10]. 

FIGURE 8 Composition of error of analytical result related to accuracy of analytical method [4,8]. 

While IQC can be operated in real time to provide a means of deciding whether or not to report a given set of analytical results, EQA is, by its nature, retrospective. While IQC can provide valuable data on imprecision and may indicate its potential causes, EQA compares performance between laboratories and provides information about the accuracy and bias of the results obtained. This additional information can be used to ... 

Distorted Wave Bom Approximation. Quantum scattering calculations are sometimes made using the distorted wave Bom approximation (15). Such calculations have the advantage of almost always being feasible numerically. For simple cases, one can also obtain some results analytically (16). However, the accuracy of the results is generally poor, for most molecular collisions. A... 

Chapter 5 deals with derivation of the basic equations of the fluctuation-controlled kinetics, applied mainly to the particular bimolecular A + B 0 reaction. The transition to the simplified treatment of the density fluctuation spectrum is achieved by means of the Kirkwood superposition approximation. Its accuracy is estimated by means of a comparison of analytical results for some test problems of the chemical kinetics with the relevant computer simulations. Their good agreement permits us to establish in the next Chapters the range of the applicability of the traditional Waite-Leibfried approach. 

Quality Control (QC) QC samples are used to check the performance of the bioanalytical method as well as to assess the precision and accuracy of the results of postdose samples. QC samples are prepared by spiking the analyte of interest and the IS into a blank/control matrix and processing similar to the postdose samples. QC samples cover the low (3 x LLOQ LLOQ = lower limit of quantitation), medium, and high (70-85% of ULOQ ULOQ = upper limit of quantitation) concentration ranges of the standard curve and are spaced across the standard curve and the postdose sample batch. 

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