Sampling Method versus Sample Type

One of the most important practical aspects of infrared spectroscopy is that it may be applied to almost any type of sample in any physical state, form, or modification. In this section, sampling will be discussed in terms of physical state and/or physical nature of the sample. One point that will be emphasized throughout is the importance of documentation of all experimental procedures, sampling methods and conditions, and any abnormalities observed during the sample preparation procedure. 

If the sample (solution or mixture) is known to contain water, then not only should this information be recorded but also extreme care must be taken to ensure that the sampling method is compatible with water being present. Comments are provided in Section 5 regarding the removal of water and other volatile materials that can interfere with an analysis. Also, if a sample is seen to be made up of more than one phase, it is helpful to obtain spectra from each separated phase. Techniques such as filtration, decanting, and centrifuging should be used if practical to ensure representative sampling of a particular phase. Record the nature of the phase being 

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In-situ SPV measurements seem possible with minor modifications (1) the tip potential (versus the reference) is set at a value close to the rest potential of the semiconductor in darkness (this must be compatible with the electrochemical response of the tip), and (2) the tip current is quenched by adjusting the sample voltage (versus the reference) with the second feedback system. With p-type materials the method seems more obvious than with n-type specimens, since illumination promotes surface electrons. At n-type materials SPV measurements will induce corrosion since holes are driven to the interface. If absolute measurements of the SPV seem difficult, because they depend on the adjustment of the tip potential, differential measurements appear accessible to experiment. 

Determination of the reaction rate from calorimetric measurements, using DSC technique, is very useful and was applied with success for many template polymerization systems and for blank polymerizations.Two types of calorimetric measurements were described isothermal and scanning experiments. The heat of polymerization can be measured by DSC method, measuring thermal effect of polymerization and ignoring the heat produced from decomposition of the initiator and heat of termination. In isothermal experiments sample is placed at a chosen temperature and thermogram is recorded versus time. Assuming typical relationship... 

One practical approach to1 testing the specificity of an analytical method is to compare the test results of samples containing impurities versus those not containing impurities. The bias of the test is the difference in results between the two types of samples [20]. The assumption to this approach is that all the interferents are known and available to the analyst for the spiking studies. 

If the composition of the crude oil and the equilibrium constants of the components were known p could be calculated using the principles outlined in the previous chapter. The composition and amount of liquid and vapor could be calculated at any temperature and pressure. Using these results, the volume of the liquid could be calculated from the known densities of the various components in the liquid. Similarly, the volume of the residue liquid at stock tank conditions could be computed, p would be the ratio of these two volumes. Again, for most crude oils, calculations of this type are impossible since the composition of the original crude oil is seldom known. Consequently, it is usually necessary to make experimental determinations of p versus P in the laboratoiy using a sample of the reservoir crude oil. If experimental data are not available it is possible to estimate a value of p using one of the following methods for this purpose. 

There are also other BE-type assessments. Interaction studies assess the influence on bioavailability by other individual factors, such as food, alcohol, or other drugs. Such studies are usually single-sequence crossover, but the assessment method remains the same—whether confidence intervals of AUC and Cmax ratios fall within (0.80, 1.25). The same can be said of PK similarity assessments between subject populations, for example, healthy volunteers versus patients. The assessment method is the same as that used for BE, but important differences remain. In typical BE studies, subjects are densely sampled so that individual PK parameters, AUC and Cmax, can be determined with precision. PK similarity assessments are concerned with the differences in different populations, instead of formulations. The assessments are usually based on multiple (parallel) studies, as crossover studies are not possible, and sequence and period effects are not considered. Assessments involve obtaining estimates of average PK parameters in the populations and the 90% confidence intervals for the ratios of the average PK parameters. 

In this presentation, two examples of the use of vibrational spectroscopy to probe water-solid interactions in materials of interest to the food and pharmaceutical sciences are described. First, the interaction of water vapor with hydrophilic amorphous polymers has been investigated. Second, water accessibility in hydrated crystalline versus amorphous sugars has been probed using deuterium exchange. In both of these studies, Raman spectroscopy was used as the method of choice. Raman spectroscopy is especially useful of these types of studies as it is possible to control the environment of the sample more easily than with infrared spectroscopy. 

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