Internal reflection, sample-handling

This layer is then analysed directly by internal reflectance infra-red spectroscopy. Since there is no handling of the sample, contamination is reduced to a minimum. However, only infra-red spectral analysis is possible with this system since the material absorbed on the germanium prism is always a mixture of compounds, and since the spectrophotometer used for the production of the spectra is not a high-precision unit, the information coming from this technique is limited. While identification of specific compounds is not usually possible, changes in spectra, which can be related to the time of day, season, or to singular events, can be observed. 

Internal-reflection spectroscopy is used to obtain IR spectra of hard-to-handle or hard-to-prepare samples such as solids with limited solubility, films, pastes, adhesives, and powders. Reflection occurs when a beam of radiation passes from a denser to a less dense medium. The fraction of incident beam which is reflected increases as the angle of incidence becomes larger. Beyond a certain critical angle, reflection is complete. During the reflection process the beam penetrates a small distance into... 

The classical book on techniques by W. J. Potts (108) is still the book of choice for sample preparation and quantitative methods. Practical aspects of theory, sample handling, and application are well-described by A. L. Smith (109). ASTM E-13 is expected to release a new practice on qualitative analysis in 1985, and its "Manual on Practices in Molecular Spectroscopy" (27) covers nomenclature, instrument testing, microanalysis, internal reflection, and quantitative analysis. 

Modem IR instmments often have reflectance of other sampling capabilities for obtaining IR spectra that eliminate the necessity of salt plate cells and simplify sample handling. The most useful is an internal reflectance method called attenuated total reflectance (ATR). The sample is pressed on a diamond substrate and the infrared radiation penetrates the sample, being reflected internally, and then exits for detection. 

Generalized requirements for validation of the stability of stock and sub-stock solutions were described in Section 10.2.7. The FDA guidance document (FDA 2001) does not, however, recommend any exact procedures about how this should be done Conditions used in stability experiments should reflect situations likely to be encountered during actual sample handling and analysis. The procedure should also include an evaluation of analyte stability in stock solution . Also the following The stability of stock solutions of drug and the internal standard should be evaluated at room temperature for at least six hours. If the stock solutions... 

Coarse or hard powders are not well served by either the compressed pellet or mull technique, mainly because of difficulties associated with grinding. In such situations, the best approaches require the use of an accessory, such as a diffuse reflectance or photoacoustic detector. Both diffuse reflectance and photoacoustic methods [99,100] may be applied to most forms of powdered solids. As a rule, photoacoustic measurements, which are the only form of true absorption measurement, are not significantly influenced by sample morphology. An alternative procedure for powders is ATR, especially a horizontal accessory, preferably equipped with a pressure applicator. Note that the use of pressure is recommended to ensure intimate contact between the sample and the IRE (internal reflectance element) surface. Normally, the sample must conform to the surface of the IRE, and because the strength of the IRE is typically limited, the procedure is recommended only for soft powders. However, with the introduction of diamond-based ATR accessories [101-103], it is possible to handle most types of powdered material. 

Figure 13.4a-c shows schematically, apparatus using IREs of trapezoidal shape, in which internal reflection occurs multiple times these have been widely used for many years in combination with mid-infrared spectrometers. The advantage of using an IRE with multiple internal reflection lies in the wide diversity and variability of its use as shown in Figure 13.4b, the sample can be in close contact with both the upper and lower surfaces of the IRE, and as shown in Figure 13.4c, a small-area sample can also be handled. In other... 

The C—H stretching bands in the internal reflection spectrum would seem to indicate only sp C—H stretching modes. This band, however, is due to an impurity on the internal reflection crystal possibly occurring as a result of handling the crystal. It is very difficult to eliminate such impurity films on internal reflection crystals, and a spectrum of the crystal should be obtained without sample before obtaining the sample spectrum. 

The sample of desorbed tritide is placed inside a quartz tube that is connected to a gas-handling manifold by a TorrSeal . A quartz sleeve with Silicon Carbide (SiC) in the annular space is placed around the end of the quartz tube, surrounding the sample with microwave susceptor. The quartz tube and susceptor sleeve are thermally insulated from the rest of the microwave cavity. An internal thermocouple measures the temperature of the sample and provides the temperature signal for process control of the desired temperature. A shine block (alumina foam), attached to the thermocouple, blocks radiant heating of the TorrSeal and the upper area of the quartz tube and manifold. An IR pyrometer is used as a secondary measure of the temperature of the susceptor, and therefore of the sample. A stainless steel shield reflects microwaves from the quartz tube not in the susceptor sleeve, eliminating the production of a plasma at low pressure in the quartz tube. 

Diffuse-reflectance MIRS has found a number of applications for dealing with hard-to-handle solid samples, such as polymer films, fibers, or solid dosage forms. Reflectance MIR spectra are not identical to the corresponding absorption spectra, but sufficiently close in general appearance to provide the same level of information. Reflectance spectra can be used for both qualitative and quantitative analysis. Basically, reflection of radiation may be of four types specular, diffuse, internal, and attenuated total. 

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