Transmission measurements

A thin film for FTIR imaging can be prepared by solvent casting. Solvent casting requires a solvent that dissolves the sample preferably at room temperature, which can be easily removed (high vapor pressure) without formation of bubbles in the sample film. Solvents such as chloroform (boiling point (bp.) 61.2°C), acetone 

Thin samples can be sliced from an appropriately mounted bulk sample. Smaller samples can be imbedded in a wax block. Automated sliding microtomes can be used for slicing. Thin samples are hard to handle and may curl. A microcompression cell can be used to hold the samples. Unfortunately, microtoming can introduce arifacts into the samples by tearing and orientation. 

The range of use of a particular cell depends on the window material. Most common materials for optical windows or fibers are summarized in Tab. 5.2. The refractive index of the window material should be very close to that of the sample in order to avoid reflection or scattering contributions. 

Material Transmission Range/fim Refractive Index ( 20 °C) Use Solubility 

Optical glasses (Si02) Quartz 0.2-2.2 1.6 0.2 pm 1.5 2 pm windows, fibers high resistance to acids (except hydrofluoric acid) 

Sapphire (single crystal AI2O3) 0.2-4.5 1.73 vis, 1.65 4 pm fibers, ATR crystals high resistance to acids and alkali at temperatures up to 1000 °C very hard 

KRS-5 (TiBr/ni) 0.3-40 2.4 windows slightly soluble in water (0.02 g/100 ml HjO), vulnerable to organic solvents, toxic 

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Tye, R. P., ed. (1977). Thermal Transmission Measurements of Insualtion (STP 660). West Conchohocken, PA American Society for Testing and Materials. 

Values of K have been tabulated for particles of various diameters. For extremely small particles K is nearly zero. Its value increases rapidly to between 3 and 5 for particles in the range of approx 0.3 to 0.7 microns. As the size of the particle increases, K drops to a constant value of 2. When values of K are known, and when either the particle diameter or the number of particles is known, the other may be determined from the ratio It/I0. If both n and r are unknown, they may be determined by making transmission measurements at two different wavelengths and setting up simultaneous equations using equation (16)... 

Thru a combination of sedimentation and transmission measurements, a particle size distribution can be found. Tranquil settling of a dispersion of non-uniform particles will result in a separation of particles according to size so that transmission measurements at known distances below the surface at selected time intervals, will, with Stokes law, give the concn of particles of known diameter. Thus, a size frequency distribution can be obtained... 

The preparation and characterization by chemisorption (H/Ft = 0.9 for 1 wt.% catalyst and 1.0 for 0.5 wt.% catalyst) of the catalysts have been described.( ) The data were obtained In the catalyst cell previously described (2 ) either by transmission measurements, measuring... 

The catalyst for the in situ FTIR-transmission measurements was pressed into a self-supporting wafer (diameter 3 cm, weight 10 mg). The wafer was placed at the center of the quartz-made IR cell which was equipped with two NaCl windows. The NaCI window s were cooled with water flow, thus the catalyst could be heated to 1000 K in the cell. A thermocouple was set close to the sample wafer to detect the temperature of the catalyst. The cell was connected to a closed-gas-circulation system which was linked to a vacuum line. The gases used for adsorption and reaction experiments were O, (99.95%), 0, (isotope purity, 97.5%), H2 (99.999%), CH4 (99.99%) and CD4 (isotope purity, 99.9%). For the reaction, the gases were circulated by a circulation pump and the products w ere removed by using an appropriate cold trap (e.g. dry-ice ethanol trap). The IR measurements were carried out with a JASCO FT/IR-7000 sprectrometer. Most of the spectra were recorded w ith 4 cm resolution and 50 scans. 

Imura, K and Okamoto, H. (2006) Redprodty in scanning near-field optical microscopy illumination and collection modes of transmission measurements. Opt. Lett., 31, 1474-1476. 

Conversion electron Mossbauer spectroscopy (CEMS) measurements with back scattering geometry have the merit that spectra can be obtained from a sample with much less isotope content compared with transmission measurements. Another merit is that a sample, deposited on a thick substrate, could be measured, and that because of the limited escape depth of the conversion electrons, depth-selective surface studies are possible. The CEMS technique was found to be best applicable to specimens of 10-100 pg Au cm, i.e., about two orders of magnitudes thinner than required for measurements in transmission mode [443]. This way (1) very thin films of gold alloys, as well as laser- and in beam-modified surfaces in the submicrometers range of depth [443], and (2) metallic gold precipitates in implanted MgO crystals [444] were investigated. 

To minimize experiment time a very strong Co/Rh source was used, with an initial source strength of about 350 mCi at launch. Instrument internal calibration is accomplished by a second, less intense radioactive source mounted on the end of the velocity transducer opposite to the main source and in transmission measurement geometry with a reference sample. For further details, see the technical description in Sect. 3.3. 

In conclusion, IR analysis of polymer/additive extracts before chromatographic separation takes advantage mainly of straightforward transmission measurements. Without separation it is often possible to make class assignments (e.g. in the reported examples on plasticisers and carbodiimide hydrolysis stabilisers) it may eventually be necessary to use multivariate techniques. Infrared detection of chromatographic effluents is dealt with in Chapter 7. 

In solvent-elimination LC-FTIR, basically three types of substrates and corresponding IR modes can be discerned, namely, powder substrates for diffuse reflectance (DRIFT) detection, metallic mirrors for reflection-absorption (R-A) spectrometry, and IR-transparent windows for transmission measurements [500]. The most favourable solvent-elimination LC-FTIR results have been obtained with IR-transparent deposition substrates that allow straightforward transmission measurements. Analyte morphology and/or transformation should always be taken into consideration during the interpretation of spectra obtained by solvent-elimination LC-FTIR. Dependent on the type of substrate and/or size of the deposited spots, often special optics such as a (diffuse) reflectance unit, a beam condenser or an FITR microscope are used to scan the deposited substances (typical diameter of the FITR beam, 20 pm). 

ATR is one of the most useful and versatile sampling modes in IR spectroscopy. When radiation is internally reflected at the interface between a high-refractive index ATR crystal (usually Ge, ZnSe, Si, or diamond) and the sample, an evanescent wave penetrates inside the sample to a depth that depends on the wavelength, the refractive indices, and the incidence angle. Because the penetration depth is typically less than 2 pm, ATR provides surface specific information, which can be seen as an advantage or not if surface orientation differs from that of the bulk. It also allows one to study thick samples without preparation and can be used to characterize highly absorbing bands that are saturated in transmission measurements. 

The basic situation is illustrated in Figure 27-1. What we have here is a simulation of an ideal case a transmission measurement using a perfectly noise-free spectrometer through a clear, non-absorbing solvent, with a single, completely soluble analyte dissolved in it. The X-axis represents the wavelength index, the T-axis represents the measured absorbance. In our simulation there are six evenly spaced concentrations of analyte, with simulated concentrations ranging from 1 to 6 units, and a maximum simulated... 

In Chapter 41, based on reference [2] we derived the following expression for the noise of a transmission measurement, for the case of constant detector noise, as is commonly found in IR and NIR spectrometers ... 

Fig. 14. Infrared absorption spectrum of anode films prepared at Ts = 25°C with boron fractions xg = 0 (top), 0.25, 0.5 0.75, and 1 (bottom), respectively, in the gas. The film thickness and the transmission measured at v = 4000 cm-1 are given for each curve. From C.C. Tsai (1979).

Fig. 2.16. G-mode frequency of SWNTs as a function of pump-probe time delay obtained from transient transmission measurement using a sub-10 fe pulse at 2.1 eV. From [55]...

In the normal-incident transmission measurements of LB films deposited on transparent substrates, the electric vector of the infrared beam is parallel to the film surface (Figure 5A). Therefore, only absorption bands which have the transition moments parallel to the film surface can be detected by this method. On the other hand, in the above-mentioned RA measurements, in which the p-polarized infrared beam is incident upon the LB film prepared on Ag-evaporated substrates at a large angle of incidence, we have a strong electric field perpendicular to the film surface as shown in Figure 5B. Therefore, in this case, only absorption bands which have the transition moments perpendicular to the film surface can be detected with a large intensity enhancement. Thus, if the molecules are highly oriented in the LB films, the peak intensities of particular bands should be different between the transmission and RA spectra. 

This expression is interpreted as the relative uncertainty in the concentration, as related to the relative uncertainty of the transmittance measurements, ST/T. The graph below illustrates the effect of a 1% uncertainty in transmission measurements on the percent relative uncertainty in the concentration (Fig. 5.8). 

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