Inhomogeneous sample

Direct sampling of solids may be carried out using laser ablation. In this technique a high-power laser, usually a pulsed Nd-YAG laser, is used to vaporize the solid, which is then swept into the plasma for ionization. Besides not requiring dissolution or other chemistry to be performed on the sample, laser ablation ICPMS (LA-ICPMS) allows spatial resolution of 20-50 pm. Depth resolution is 1-10 pm per pulse. This aspect gives LA-ICPMS unique dit nostic capabilities for geologic samples, surface features, and other inhomogeneous samples. In addition minimal, or no, sample preparation is required. 

Quantification of inhomogeneous samples is a more usual situation encountered in the application of AES. Figure 5.32 (after Seah 1986) show some of the common configurations of elements that may be studied by AES (or XPS). 

It must be considered, though, that limits derived from the SNR characterize mainly instrumental noise and do not, as a rule, include chemical noise, viz such variations of measurement values which come from sample inhomogeneities, sample preparations in the course of the entire analytical... 

The first physical cause considered was the possibility of an inhomogeneous sample. To eliminate this as a possibility, the sample was ground before aliquots were taken. The sample size was still 10 g of sample per 100 ml of water. In this case, however, time constraints permitted only three replicate readings per flask. The results are shown in Table 9-3. 

N and 0, in solid material. The second point is that EXELFS is especially suitable for the study of inhomogeneous samples (structurally and compositionally heterogeneous in the sense discussed in section 2.2 above) because the primary electron beam can be focussed to a diameter of ca 20. Other advantages of EXELFS have been discussed elsewhere (60, 61). The limitations of the technique include (i) the need to select an optimal thickness of sample so as to minimize multiple scattering and (ii) the susceptibility of the samples to suffer radiation damage. 

Matrix effects and inhomogeneous sample charging seriously hinder quantitative analysis of SIMS on technical catalysts. Although full quantitation is almost impossible in this area, the interpretation of SIMS data on a more qualitative base nevertheless offers unique possibilities. Molecular cluster ions may be particularly informative about compounds present in a catalyst. 

From a pharmacological point of view the first two strategies raise several distinct disadvantages. First, the exact structures of these fullerene-based systems in solution are usually unknown and, especially for polymeric materials inhomogeneous samples are frequently obtained. Furthermore, in many cases the amount of incorporated fullerene is not clearly determined. In addition, the presence of other molecules like the hosts or polymeric residues can cause unpredictable side effects and in no case the mode of action or activity can doubtlessly be associated with the fullerenes. However, for systematic investigations on structure-function relationships or extensive testings of toxicological or human availability properties, the use of structurally well-defined and characterized materials is mandatory. 

Real samples. The move to analyze real samples represents a move toward the unknown. Not only are the results of the analysis unknown ahead of time, but other variables relating to sample inhomogeneity, sample preparation variables, additional sources of error, etc. are introduced. A large number (>30) of duplicate samples should be analyzed so that a reliable standard deviation and a reliable control chart can be established. The ultimate purpose of this work is to characterize what is a typical analysis for this kind of sample so that one can know when the method is under statistical control and when... 

Note that energies ED0 and EA0 are both referenced to the conduction band. Significant curvature in Arrhenius plot probably inhomogeneous sample. 

One major problem of all these techniques is the sensitivity in the parameter selected to detect the presence of inhomogeneities. With visible light for example, inhomogeneous samples can appear transparent if the difference in the refractive index between the phases is less than 0.01. Staining (in the case of transmission electron microscopy, TEM), or chemical etching (in the case of scanning electron microscopy, SEM), can be helpful in revealing the structure. 

For an inhomogeneous sample the situation which is most often encountered is that shown in Figure 2, in which a modified surface layer of thickness d overlays an unmodified bulk. This is usually referred to as the substrate/overlayer model. The signal intensity for a core level in the surface layer is given by equation (2). 

It should be noted that the spectral emission is influenced by the. self-absorption of the emitted radiation by the sample. If the temperature distribution is homogeneous, this effect is already included in the determination of the absorptivity. In inhomogenous samples, the self-absorption may be neglected if the absorptivity is below 5%. In this case, the overall emission can be treated as the sum of the emission of all infinitely thin layers into which the sample can be divided (Pepperhoff and Grasz, 1955). Otherwise, the emission of all inner layers must be corrected by transmission factors before summation. For practical calculations, the sample volume can be divided into different layers, each of which is assumed to be in thermal equilibrium. 

Variability in Absorption Estimates In this study, the occurrence of a negative absorption value for one subject and the absence of a significant vitamin C effect raise some questions about the accuracy of the method However, the expected changes in absorption due to dietary treatments may be masked by the analytical variations associated with absorption measurements and biological variabilities of iron absorption Analytical variations can be introduced at several stages of the analytical procedures incomplete fecal collection, inhomogeneous samples, iron contamination, incomplete colorimetric reaction, non-quantitative recovery after chemical ashing, and variations in isotopic measurements due to ion statistics, memory effects, instrument drift, etc Some of these are not as serious as others, for example, contamination with natural iron woiold not affect the estimate of tracer concentrations provided it occurs before the total iron content is measured ... 

Polymers showing a viscoelastic behaviour occupy the intermediate range. Out of all the existing hardness tests, the pyramid indenters are best suited for research on small specimens and microstructurally inhomogeneous samples (Tabor, 1951). Pyramid indenters provide, in addition, a contact pressure which is nearly independent of indent size and are less affected by elastic release than other indenters. 

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