Experimental procedure accuracy

Designing an experimental procedure involves selecting an appropriate method of analysis based on established criteria, such as accuracy, precision, sensitivity, and detection limit the urgency with which results are needed the cost of a single analysis the number of samples to be analyzed and the amount of sample available for... 

Numerous examples of standard methods have been presented and discussed in the preceding six chapters. What we have yet to consider, however, is what constitutes a standard method. In this chapter we consider how a standard method is developed, including optimizing the experimental procedure, verifying that the method produces acceptable precision and accuracy in the hands of a single analyst, and validating the method for general use. 

The most common direct methods are the oven, the distillation, and the Fischer methods. They can be made precise by careful standardization of the experimental procedures their accuracy can be assured only by calibration against some accurate reference method. 

In classical mechanics both the position of a particle and its velocity at any given instant can be determined with as much accuracy as the experimental procedure allows. However, in 1927 Heisenberg introduced the idea that the wave nature of matter sets limits to the accuracy with which these properties can be measured simultaneously for a very small particle such as an electron. He showed that Ax, the product of the uncertainty in the measurement of the position x, and Ap, the uncertainty in the measurement of the momentum p, can never be smaller than M2tt ... 

Winge et al. [730] have investigated the determination of twenty or more trace elements in saline waters by the inductively coupled plasma technique. They give details of experimental procedures, detection limits, and precision and accuracy data. The technique when applied directly to the sample is not sufficiently sensitive for the determination of many of the elements at the low concentrations at which they occur in seawater, and for these samples preconcentration techniques are required. However, it has the advantages of being amenable to automation and capable of analyzing several elements simultaneously. 

However, we should emphasize that the NBO/NRT concepts of hybridization, Lewis structure, and resonance differ in important respects from previous empirical usage of these terms. In earlier phases of valence theory it was seldom possible to determine, e.g., the atomic hybridization by independent theoretical or experimental procedures, and instead this term became a loosely coded synonym for the molecular topology. For example, a trigonally coordinated atom might be categorized as sp2-hybridized or an octahedrally coordinated atom as d2sp3-hybridized, with no supporting evidence for the accuracy of these labels as descriptors of actual... 

In Figures 8 and 9 are shown the data for the dependence of the characteristic film buildup time t on Apg and U. In accord with the model, t is found to be independent of U, with only a very weak dependence on Apg indicated. This latter result could in part be a function of experimental inaccuracy. The data reduction for t introduces no assumptions beyond that needed to draw the exponential flux decline curves such as those shown in Figures 2 and 3. However, an error analysis shows that the maximum errors relative to the exponential curve fits occur at the earlier times of the experiment. This is seen in the typical error curve plotted in Figure 10. The error analysis indicates that during the early fouling stage the relatively crude experimental procedure used is not sufficiently accurate or possibly that the assumed flux decline behavior is not exponential at the early times. In any case, it follows that the accuracy of the determination of 6f is greater than that for t. 

It is appropriate at this point to briefly discuss the experimental procedures used to determine polymerization rates for both step and radical chain polymerizations. Rp can be experimentally followed by measuring the change in any property that differs for the monomer(s) and polymer, for example, solubility, density, refractive index, and spectral absorption [Collins et al., 1973 Giz et al., 2001 McCaffery, 1970 Stickler, 1987 Yamazoe et al., 2001]. Some techniques are equally useful for step and chain polymerizations, while others are more appropriate for only one or the other. Techniques useful for radical chain polymerizations are generally applicable to ionic chain polymerizations. The utility of any particular technique also depends on its precision and accuracy at low, medium, and high percentages of conversion. Some of the techniques have the inherent advantage of not needing to stop the polymerization to determine the percent conversion, that is, conversion can be followed versus time on the same reaction sample. 

Measurements of the desired property or properties (usually grouped together under the general title specifications) are expressed as numerical values therefore, the accuracy of these measurements is of the utmost importance. The measurements need to be sufficiently accurate so as to preclude negative scientific or economic consequences. In other words, the data resulting from the test methods used must fall within the recognized limits of error of the experimental procedure so that the numerical data can be taken as fixed absolute values and... 

Standardization Imposition of rules permitting to check or validate the accuracy of a test using live organisms. For example, the use of a well-defined experimental procedure and the use of a reference toxicant are important rules to standardize a test. Test standardization also requires that the test be feasible by many laboratories and yield comparable results with the same test substance. Volume 1(14). 

Whilst modern instruments may provide much more accurate data than those of years ago, new types of instrument are being developed which provide data of somewhat lesser accuracy, but which have other advantages (e.g. speed, throughput, on-line). Advances in computing methods help in the extraction of meaningful information from such data, which in the past would have been impossible, and so bioinformatics has become an essential part of the experimental procedure. 

Reviews integrate, correlate, and evaluate results from published literature on a particular subject. They seldom report new experimental findings. Effective review articles have a well-defined theme, are usually critical, and may present novel theoretical interpretations. Ordinarily, reviews do not give experimental details, but in special cases (as when a technique is of central interest), experimental procedures may be included. An important function of reviews is to serve as a guide to the original literature for this reason, accuracy and completeness of references cited are essential. 

Taken as a whole, the evidence indicates that all four major phases can be satisfactorily determined by QXDA if an adequate experimental procedure is employed (e.g. Ref. G30) or by calculation. Alite, belite and the total content of interstitial material can also be determined by microscopy. With any of these methods, assuming the best available procedures to be used, the absolute accuracy is probably 2-5% for alite and belite, and l-2 /o for aluminate and ferrite. 

Integral cross sections for selected electron-impact excitation and ionisation processes have been largely obtained by measuring optical excitation functions. These need to be corrected to a varying degree of accuracy for effects such as cascade contributions and photon polarisation. The details of the experimental procedures, sources of errors and data evaluation have been discussed by Heddle and Keesing (1968). 

Kinetic parameters from selected publication on CO2 reactivity are shown in Table 1. A wide span of apparent activation energies is published for biomass chars, 80.3 kJ/mol -318 kJ/mol and for coal chars, 79 kJ/mol -359.5 kJ/mol Some of the discrepancy can be explained as due to different experimental procedures (such as sample load, particle size and sample preparation) and the application of different analysing equipment. Concerning the latter one, the potential role of systematic errors in temperature measurements among various thermobalances is evident. Variations might also be explained by different extraction procedures, lack of accuracy caused by the approximations used in the different computational methods and the kinetic compensation effect. 

The high pressure-high temperature adsorption isotherms have been measured using a RUBOTHERM magnetic suspension balance The adsorption isotherms and the detail of the experimental procedure are presented elsewhere [11,12] Considering the measurement accuracy of each sensor (mass, temperature and pressure), the relative errors on the adsorbed mass and on the pressure are estimated respectively to 0 3 and 0 5 % the temperature is measured with an accuracy of 0 1 K The main cause of experimental error is not the lack of accuracy of the sensors but rather the buoyancy effect on the adsorbent sample on the one hand and on the adsorbed phase on the other hand The first contibution is taken into account by... 

As an additional remark, it should be noted that traceability should not be confused with accuracy which covers the terms trueness (the closeness of agreement of the measured value with the true value ) and precision (the closeness of agreement between results obtained by applying the same experimental procedure several times under prescribed conditions). In other words, a method which is traceable to a stated reference is not necessarily accurate (i.e. the stated reference does not necessarily correspond to the true value ), whereas an accurate method is always traceable to what is considered to be the best approximation of the true value (defined as a value which would be obtained by measurement if the quantity could be completely defined if all measurement imperfections could be eliminated ). 

Use a Web search engine such as Google to find sites dealing with potentiometric titrators. This search should turn up such companies as Spectralab, Analyticon, Fox Scientific, Brinkmann, Metrohm, Mettler-Toledo, and Thermo Electron. Set your browser to one or two of these and explore the types of titrators that are commercially available. At the sites of two different manufacturers, find application notes or bulletins for determining two analytes by potentiometric titration. For each, list the analyte, the instruments and the reagents that are necessary for the determination, and the expected accuracy and precision of the results. Describe the detailed chemistry behind each determination and the experimental procedure. 

The experimental procedures for obtaining ECD and NIMS data have been described. Examples of the calculations are given for the various classes of molecules. For each group specific test molecules are provided. The aromatic hydrocarbons and aldehydes are Eql(l/lor 1/2) molecules, CS2 is a Eql(2/2) molecule, haloalkanes are DEC(l) molecules, and the halobenzenes and nitromethane are DEC(2) molecules that dissociate via a molecular ion. A graphical procedure for obtaining parameters from ECD data and the calibration of NIMS data using SF6 and nitrobenzene is presented. The use of multiple electron affinities of O2 to define negative-ion states from ECD data is illustrated. A method for the analysis of published NIMS spectra measured at two temperatures reveals the electron affinities of molecules when combined with substitution effects. We then explored the use of precision and accuracy plots and timelines for the evaluation of electron affinities. 

With a very simple apparatus and experimental procedure it is possible to elaborate a stable, reproducible standard specimen for kinetic studies. With this apparatus, the hydration ratio of incompletely hydrated plasters or plaster rocks can be determined with accuracy. Finally this apparatus demonstrates experi-... 

In the interpretation of the numerical results that can be extracted from Mdssbauer spectroscopic data, it is necessary to recognize three sources of errors that can affect the accuracy of the data. These three contributions to the experimental error, which may not always be distinguishable from each other, can be identified as (a) statistical, (b) systematic, and (c) model-dependent errors. The statistical error, which arises from the fact that a finite number of observations are made in order to evaluate a given parameter, is the most readily estimated from the conditions of the experiment, provided that a Gaussian error distribution is assumed. Systematic errors are those that arise from factors influencing the absolute value of an experimental parameter but not necessarily the internal consistency of the data. Hence, such errors are the most difficult to diagnose and their evaluation commonly involves measurements by entirely independent experimental procedures. Finally, the model errors arise from the application of a theoretical model that may have only limited applicability in the interpretation of the experimental data. The errors introduced in this manner can often be estimated by a careful analysis of the fundamental assumptions incorporated in the theoretical treatment. 

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