Errors in analysis

Quantitative analysis of multicomponent additive packages in polymers is difficult subject matter, as evidenced by results of round-robins [110,118,119]. Sample inhomogeneity is often greater than the error in analysis. In procedures entailing extraction/chromatography, the main uncertainty lies in the extraction stage. Chromatographic methods have become a ubiquitous part of quantitative chemical analysis. Dissolution procedures (without precipitation) lead to the most reliable quantitative results, provided that total dissolution can be achieved follow-up SEC-GC is molecular mass-limited by the requirements of GC. Of the various solid-state procedures (Table 10.27), only TG, SHS, and eventually Py, lead to easily obtainable accurate quantitation. 

A validation protocol adapted from the experiences during the method development defines the scope of the validation study (goal of the study, regulating guidelines, key method parameters, etc.). To investigate the adequate method performance, these features (e.g., range of analyte concentration), together with a statement of any fitness-for-purpose criteria, have to be specified in the validation protocol. A basic check has to provide that the reasonable assumptions about the principles of the method are not seriously flawed. In this process, sources of error in analysis have to be listed (Table 4) and their effects have to be checked. The validation should, as far as possible, be conducted to provide a realistic survey of the number... 

Systematic errors in analysis can usually be eliminated but true random errors are due to operations in an assay which are not completely controlled. A common type of random error arises from the acceptance of manufacturers tolerances for glassware. Table 1.2 gives the RSD values specified for certain items of grades A and B glassware. 

The upper and lower warning limits (UWL and LWL) are drawn at 2s above and below, respectively, of the mean recovery. The upper and lower control limits (UCL and LCL) are defined at 3s value about the mean. If ary data point falls outside UCL or LCL, an error in analysis is inferred that must be determined and corrected. The recoveries should fall between both the warning limits (UWL... 

Remaining fraction attributed to unknown uronic acids, proteins, and so on, and to recovery errors in analysis techniques. 

The Intermediate Phase Lal2 42 The composition of this phase was further defined with powder pattern data for nine samples (2.36 < I/La < 2.51) which were quenched from the liquid region and annealed just below the melting point for 2-7 days. Certain diffractions of the neighboring Lais and LaL phases were detectable in fairly small amounts (< 0.06 in I/La) so that the interpolated composition is probably uncertain by not much more than 0.01 in I/La, exclusive of systematic errors in analysis. 

One algebraic method involves rates of reactions when the reagent concentration is large compared with [A]o -I- [B]q. Two simultaneous equations are solved at two times of observation, one near the optimum (Section 21-3) and the other when the reaction is nearly complete. This method is less restricted than graphical extrapolation with respect to both [A]q/[B]o and kjk, because Ata[A], need not be negligible compared with A b[B], to make analytically useful observations feasible. With this method the error in analysis increases when the ratio kjk approaches unity and when the second observation is made at a time prior to complete reaction. 

The hydrogen and nitrogen balances support the higher air flow derived from the carbon balance, closing at 102 and 100% respectively by the indirect method. Sulfur and chlorine, present in the fuel in concentrations of 0.03 wt.%, were recovered at less than 60%. Both elements were present in fuel at concentrations close to the detection limits and closures were sensitive to small errors in analysis and material flows. A substantial fraction of chlorine remained in the gas phase past the cyclone at a stack exit temperature of 3S0°C. 

In all cases, the calculation of kc.a should be based on analysis of the gas stream, as a small error in analysis of the liquid stream can lead to large errors in k a. Some suitable chemical systems for determination of kco in the instantaneous regime are given in Table X. 

The results in terms of final sulfur values and pyrite removal are given in Table VIII. Note that pyritic removal computed from either sulfur forms analyses or the diflFerence in total sulfur between processed and untreated coal (Eschka analysis) resulted in essentially identical values of 93-100%. This corresponds to total sulfur removal of 40-70%, depending on the organic sulfur content of the coal. The observation of greater than 100% removal is a result of cumulative error in analysis and the removal of small amounts of sulfate (0.02-0.04%). Presently, these experiments are being duplicated using ferric sulfate, and preliminary analysis indicates the same results. 

A number of factors limit the accuracy with which parameters needed for the design of commercial equipment can be determined. The kinetic parameters may be affected by inaccurate accounting for laboratory reactor heat and mass transport, and hydrodynamics correlations for these are typically determined under nonreacting conditions at ambient temperature and pressure and with nonreactive model fluids and may not be applicable or accurate at reaction conditions. Experimental uncertainty including errors in analysis, measurement,... 

The following questions are concerned with making standard solutions. It is essential to become familiar with and competent at calculations such as these. Mistakes in calculating and in making standard solutions are often the source of errors in analysis. Try to plan the steps you would use to make the solutions required. Avoid using pipettes smaller than 2 cm3 or larger than 25 cm3 because in inexperienced hands these can introduce large errors. You will find proposed answers at the end of the chapter. 

Lanthanide Shift Reagents.—The effects of random co-ordinate error in analysis of lanthanide-induced axial pseudocontact shifts have been discussed, " and the contributions of contact and pseudocontact shifts in the n.m.r. spectra of isoquinoline and of endo-norbornenol have been evaluated. An experimental and computational approach to the use of lanthanide-induced shifts as a rigorous method for structure determination has been described. The method was used to predict the lanthanide-induced shift behaviour of a substrate. The recording of experimental data in excellent agreement with the molecular structure was reported. Contact shift contributions to lanthanide isotropic shifts have been found to be important for organic compounds even where the carbon atom is five bonds away from the lanthanide. 

A fuller consideration of the normalisation, and its theoretical justification, can be found in Barrer et al. [10]. As emphasised earlier, to apply thermodynamic analyses to isotherm data it is imperative that the process be demonstrably reversible. In some zeolites the path of the reverse isotherm deviates markedly from that of the forward exchange (Fig.3). This is described as hysteresis and can arise from small errors in analysis, but often it is a consequence of changes in occupancy of heteroenergetic cation sites created by drying the zeolite samples prior to their use in generating the reverse curve. Other causes are the... 

Experiments were confined to alcohol-rich solutions because of the high affinity of the water component for the zeolite phase. By limiting experiments to this composition range, the internal solvent ratio was maintained near unity to assure reasonably reliable results. With one internal component significantly in excess of the other, this ratio is susceptible to sizeable distortion by small errors in analysis of the external phase. 

Noise is the result of random error due to control input/output functions, errors in analysis, digital dither in the electronics, and a potential host of presumably random causes. The noise level may be constant, or may vary over the range of data gathered. In either... 

Figure A4.3 shows the calibration for the low range. Although the error in analysis is relatively high (see Table A4.3), the results for these low values measured in this range are far better than if measured in the normal range between 0 and 10 mg L h Additionally, measurements of these low concentrations would be impossible with a type A analyser.

Finally, Mason et at. (ExxonMobil Research and Engineering) describe the approaches used for assay of fresh and spent reformer catalysts to determine the precious metals (platinum and rhenium) in them. Methods such as WDXRF, ICPAES, and classical wet chemistry methods are used for such analysis. Precise and accurate methods are critical for these analyses, since small errors in analysis can have a large impact in commercial transactions of these catalysts between the catalyst vendors and the oil companies. 

It is important for students to realize that the inability to obtain the correct answer does not necessarily mean that the analyst uses poor laboratory techniques or is a poor chemist. Many causes contribute to poor accuracy and precision, some of which we will discuss in this chapter as well as in later chapters. Careful documentation of analytical procedures, instrument operating conditions, calculations, and final results are crucial in helping the analyst recognize and ehminate errors in analysis. 

There are two principal types of error in analysis determinate or systematic error and indeterminate or random error. 

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