Optimum sample thickness

Calculations Absorption Factor, Optimum Sample Thickness... 

The optimum sample thickness for PET of a mass density pp r=l-35 g/cm3 in transmission geometry thus is topt,PET = 1 /Ppet 1 mm. If measured in reflection, the PET sample should be at least 3 mm thick. 

Figure 7.3. Effect of absorption in normal-transmission geometry. The total transmitted scattering intensity, It, as a fnnction of the rednced sample thickness. The highest scattering signal is obtained at utopt/cos (26) = 1 with tgpt being the optimum sample thickness...

Sample Thickness If possible choose a sample thickness close to the optimum thickness (Sect. 7.6). Proper background correction will be difficult for extremely thin samples. It is better to shorten the exposure or even to attenuate the primary beam than to decrease the sample thickness far below the optimum thickness. 

Sample thickness and sample uniformity present serious problems. The ideal sample is a uniform foil or a homogeneous solution. For metals the optimum thickness is in the micron range. Most of the samples studied here were powders, ground to pass 300 mesh, dispersed in a ductile microcrystalline wax. This wax dispersion is then spread to a uniform film across a sample holder window. The window has dimensions 3 X 17 mm. Adhesive cellophane tape is frequently spread over the window to assist in the mounting and supporting of the sample. 

Thus, the optimum sample arrangement for Raman spectroscopy of crystal pow ders with a low absorption coefficient is a forward-scattering (0°) arrangement of coarse crystallites with an optimum thickness in a multiple scattering arrangement. These are the conditions for the investigation of colorless samples in the visible range of the spectrum. 

The third dimension in the analysis (depth) is significant because the synchrotron X-rays penetrate deeply in most materials. This has several ramifications. First, buried volumes, such as fluid inclusions, can be analyzed. Second, one needs to know the sample thickness to correct for absorption effects (see quantification discussion below). Third, it is important to minimize the mounting substrate because this material is a source of scattered radiation that contributes to the spectral background. Fourth, the sampling depth depends on the energy of the fluorescence X-ray and therefore is element dependent. This fact can be used to advantage by selecting a sample thickness to achieve optimum sensitivity for the suite of elements of interest. 

A recent analytical study stresses the growing need, prompted partly by l islatory requirements, to differentiate polymorphs and to quantify polymorphic mixtures in pharmaceutical production [126]. The compounds benzil and benzoic add were chosen as a model system for the development of an XRD protocol which could be extended to the quantification of mixtures of drug polymorphs. The study involved the evaluation of sample thickness, the determination of preferred orientation effects, optimum milling conditions and the construction of diffraction intensity-composition calibration curves for mixtures of benzil and benzoic acid. Since the composition of such mixtures can be accurately determined by an independent method, namely HPLC, vaUdation of the quantification of mixtures by the XRD protocol was possible. It was concluded that the protocol is accurate for the model system to within a few percent. It is desirable that the general validity of the approach suggested be tested on a range of real polymorphic systems. 

An IR spectrometer equipped with a prism grating is convenient for most measurements. Liquids between salt plates, or solids in KBr pellets or between salt plates, are used as samples and the sample thicknesses are conveniently adjusted to provide contributive absorption measurements between 0 15 and 0-8 absorption units. This provides optimum resolution and accuracy. Table 11.5 is compiled from various sources and gives the IR assignments useful in urethane analysis. 

It was observed that the values for virgin LDPE polymers are quantitative in the optimised SEE conditions, as opposed to those for recycled polymers. This was attributed to the higher crystallinity of the recycled LDPE and to sample thickness effects. By reducing the sample thickness to 470 /rm and increasing the dynamic extraction time to 30 min ( SEE drastic conditions ) quantitative results were obtained. Optimum conditions for LDPE/(Irganox 1076, Chimassorb 81) were given as pressure, 450 atm dynamic extraction time, 15 min modifier, 10% (methanol) and extraction temperature, 75 C. 

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