Accuracy of quantitative measurements

Multiple reflections within the specimen can also reduce the accuracy of quantitative measurements (51,52). The intensity of light transmitted through a plane parallel plate, after the interference fringes have been eliminated, is given by (53) ... 

Certain raw materials, such as free fatty acids, are too polar to analyse directly by GC or LC. Traditionally they are converted into methyl esters to improve their volatility for GC. This sample preparation was time-consuming, and sample was quite often lost, thereby reducing the accuracy of quantitative measurements. Capillary SFC can handle these polar fatty acids without derivatization [38]. Figure 9.10 shows an SFC chromatogram of Cs-C g... 

The above mentioned aberrations can be measured (Zemlin et al. 1977) and, in principle, corrected for in a TEM. However, in practice, small residual aberrations are left due to the limited accuracy of the measurement and the stability of the microscope. Nonetheless, these residual aberrations have to be taken into account for a quantitative image analysis. 

There are a variety of other factors that influence the accuracy of quantitative analysis. Noise, in the form of baseline disturbances and baseline drift, affects area more than it does height, as it can cause area to be lost at the tailing edges of the peaks where they are widest. Peak asymmetry and detector saturation or nonlinearity, however, have a more detrimental effect on peak height. Figure 7.6 shows a calibration curve comparing peak height measurements with peak area measurements.13... 

The chemical composition of the surface can be evaluated from the integrated intensities of the core-level emissions [31]. Unfortunately, the accuracy of quantitative analysis with photoelectron spectroscopy is generally limited to a few percent even when good standards are available [31]. Therefore, even from a large number of measured samples it was not possible to observe a... 

A quantitative analysis is not always required. In particular, when searching for the source of poisoning of a catalyst, a qualitative analysis, followed by a semi-quantitative estimate is sometimes sufficient. If the data base from which the semi-quantitative program takes its standards includes many standards of a composition similar to that of the sample, the accuracy of the measurement may be of the order of a percent. On the other hand, if the library does not contain standards of a similar composition, the error may be around 50% or even more. As the problem of catalyst poisoning is frequently encountered, the accuracy of the measurements is generally fairly good. The table below is a comparison of the results of semi-quantitative and quantitative analyses conducted on an used post-combustion catalyst. 

Equation 4.14 makes use of the fact that the scattered intensity is proportional to the amount of a particular phase, e.g. see Eqs. 2.17 to 2.19 in Chapter 2, with a correction to account for different absorption of x-rays by two components in the mixture. Since the ratio of intensities fi om a pure phase and a mixture is employed, diffraction patterns from both the pure material and from the analyzed mixture must be measured at identical instrumental settings, in addition to identical sample characteristics such as preparation, shape, amount, packing density, surface roughness, etc. King s equation becomes a simple intensity ratio when two phases have identical absorption coefficients, i.e. when p/pa = p/pb. We note that the composition of the second phase (or a mixture of all other phases) should be known in order to determine its mass absorption coefficient. Otherwise, mass absorption should be determined experimentally. When absorption effects are ignored, the accuracy of quantitative analysis may be lowered drastically. 

More direct approaches for monitoring skin absorption have been proposed - for example, measuring the rate of disappearance of the chemical at the application site. However, the generally low permeability of the skin means that the rate of disappearance is often very slow, and the accuracy of the measurement will depend on analytical techniques that are capable of accurately quantifying minute differences. Reliable results can only be obtained with chemicals that are rapidly absorbed and/or easily quantitated analytically. The main use of this technique is monitoring the loss of radioactivity from the skin surface, but it should be appreciated that... 

Apart from the deficiencies of the measuring devices the available plate material had not been suitable for a long time. With the improvement of the thin-layer plates and, above all, by introducing the high performance thin-layer plates (HPTLC-plates) presented in 1975 by Ripphahn and Halpaap 4), the accuracy of quantitative thin-layer chromatography improved considerably. 

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