Transmission sampling

These three main classes of process sample streams are in increasing order of difficulty for near-infrared process analysis. In general, liquid streams are best measured in a transmission sampling mode, solids (powders) in diffuse reflectance mode, and slurries in either diffuse reflectance or diffuse transmission according to whether the liquid phase or the suspended phase is of greater analytical signihcance. If the... 

Cylindrical samples, which are common in the Debye-Scherrer cameras Figure 3.2), are also used in powder diffractometry. Similar to flat transmission samples, small amounts of powder are required in the cylindrical specimen geometry. This form of the sample is least susceptible to the non-random distribution of particle orientations, i.e. to preferred orientation effects. 

Both flat and cylindrical transmission samples are commonly used in combination with position sensitive or image plate detectors. The major disadvantage of the transmission geometry arises from the fact that self-focusing of the diffracted beam is not as precise as in the Bragg-Brentano... 

Conversely, transmission geometry requires that the sample is minimally absorbing. This is usually not a problem if the studied specimen is a molecular substance. However, when the material is a dense alloy or intermetallic compound containing heavy elements, the preparation of a high quality specimen for transmission powder diffraction may be problematic. With flat transmission samples the best approach is to try to arrange no more than a single layer of particles mounted on the film. When cylindrical specimens are employed, the radius of the capillary should be reduced to a practical minimum. Unfortunately, these measures usually reduce the... 

In the transmission geometry the requirements are different. When a flat transmission sample is used, the aperture of the incident beam is defined by the largest Bragg angle of interest, since at 0 = 0 the sample is perpendicular to the incident beam (and not parallel, as in the Bragg Brentano geometry). Equation 3.1 then becomes as follows (where the notation are the same as in Eq. 3.1)... 

Investigations of the acidity of specific surface sites may be accomplished by studies coordinated with spectroscopic methods, such as infrared (JR) spectroscopy, nuclear magnetic resonance (NMR) spectroscopy, or mass spectrometry (MS). Surface characterization with Fourier transform infrared (FTIR) spectroscopy can provide quantitative results with experimental methods that are easily performed. However, the transmission sampling techniques traditionally employed for infrared studies may introduce experimental artifacts on the analyzed surface (10,... 

Analyses were performed on a Siemens Elmiskop 102 TEM. Transmission samples were prepared by impregnation of the xerogel with an epoxy resin to which an amine was added to serve as a hardener. Hardening went on for 48 h after which a 60 nm slice was cut up with a Reichert Supernova ultramicrotome. 

Figure 6.10 Effect of streptokinase (SK) on fibrinogen concentration and platelet aggregation induced by ADP in platelet-rich plasma. Platelets were incubated with streptokinase for 1 minute prior to the addition of ADP, lfimo L and aggregation monitored by light transmission. Samples for fibrinogen were collected in aprotinin to prevent continued lysis. The fibrinogenolytic and proaggregatory effects of streptokinase were concentration dependent and were inhibited by aprotinin and e-aminocaproic acid (e-ACA)...

The alkali-halide disk method The alkali-halide disk technique is another traditional transmission sample presentation method, in which an intimate mixture of the finely ground solid sample and dry powdered alkali-halide, most commonly KBr, is pressed into a self-supporting disk. Finely powdered dry potassium bromide will coalesce to form a clear disk with high transmission when it is pressed under high pressure in an evacuated die. 

Infrared Spectroscopy (I.R.). I.R. spectra were obtained using a Perkin Elmer I.R. DS with either KBr disc or film transmission samples as appropriate. 

Low transmission sample (microsample or optically opaque material) 2- to 8-min scan, with a spectral resolution of 4-8 cm , providing a S/N of 500 1 or better. The use of a MCT detector is suggested for samples with an overall transmission of 5%T or less, especially in the absence of an optimizing accessory (such as a beam condensor for microsamples). When using a MCT detector, it is important that the potential for nonlinearity is appreciated. Also, it is important to be aware of the occurrence of detector saturation, which will lead to extreme nonlinearity, and spectral distortion. 

Figure 9.2. A transmission sample that does not completely cover the illuminated area of the infrared radiation will lead to stray Ught.

In transmission sampling the infrared beam passes through a thin film of sample and then impinges on the detector as shown in Figure 4.1. 

The letter L in the figure denotes the pathlength of the sample, which is the thickness of sample encountered by the infrared beam. Since the beam passes through the entire sample, bulk contributions are emphasized and surface contributions are minimized, as shown in Figure 4.1. Sample thicknesses of 1 to 20 microns are typical. Transmission sampling should not be confused with transmittance, which is a y-axis unit used to plot spectra, which was discussed in Chapter 1. 

In general, transmission sampling of liquids is faster and easier than that of solids because it is easier to turn liquids into a thin him. There are two widely used methods for obtaining the hansmission spectra of liquids the capillary thin him method and... 

A discussion of the y-axis units used to plot DRIFTS spectra is needed. When percent transmittance or absorbance units are used to plot a spectrum, it is assumed that transmission sampling was used, as illustrated in Figure 4.1. DRIFTS is not a transmission technique but a reflectance technique, and the proper y-axis units for DRIFTS spectra should be reflectance or some unit related to it. It is technically... 

Beer s Law relates concentration, a sample property, to absorbance, a spectral property, and forms the basis for many quantitative spectroscopic analyses. It assumes that the spectrum is being measured using a transmission sampling method as illustrated in Figure 5.1. 

FIGURE 5.1 An illustration of transmission sampling where the infrared beam passes through the sample before impinging on the detector. The letter L denotes the pathlength, the thickness of sample through which the IR beam passes. Beer s Law is derived assuming this experimental setup. 

Microscope transmission samples suffer from the same opacity problem as macroscopic samples. The solution to this problem is to prepare samples that are 1 to 20 microns thick. Fortunately, many microscopic samples can be turned into thin films by applying pressure and flattening them. This works because it does not take a lot of force to put a lot of pressure on a sample of small area, as shown in Equation 4.3. There are many ways to apply pressure to a microscopic sample to flatten it. In the author s opinion the best tool for this purpose is a roller knife, as shown in Figure 6.6, so called because it has a roller at one end and a scalpel at the other. 

Capillary Thin Film A transmission sampling technique used to obtain spectra of liquids. Typically, a drop of liquid is placed between two infrared transparent windows, which are then placed directly into the infrared beam. The capillary action of the liquid holds the two windows together. 

Cast Films A transmission sampling technique used to analyze polymer films. The polymer is dissolved in a solvent, and the solution is evaporated onto an infrared transparent window giving a polymer film. The window/film combination is then placed directly in the infrared beam. 

Diamond Anvil Cell A device used to prepare samples for transmission sampling by infrared microscopy. The cell consists of two diamonds with flat faces. The sample is placed on one diamond face, and the second diamond face is brought into contact with the sample to flatten it. The entire assembly is then placed in the infrared beam of an infrared microscope. 

KBr Pellet A transmission sampling technique most commonly used on powders and solids. The technique involves grinding the sample and KBr, diluting the sample in the KBr, then pressing the mixture to produce a transparent pellet. The pellet is then placed directly in the infrared beam. 

Mull A transmission sampling technique where the sample is ground then dispersed in an oil or mulling agent. The oil/sample mixture is then sandwiched between two infrared transparent windows and placed in the infrared beam. 

Opacity Problem An issue with transmission sampling where the sample has to have the right product of thickness times concentration to obtain a usable spectrum. 

Sealed Liquid Cells A transmission sampling technique used to obtain the spectra of liquids. The cell consists of two infrared transparent windows held a fixed distance apart by a gasket. The cell is filled with liquid then placed in the infrared beam. 

Transmission Sampling A sampling method where the infrared beam is passed through a thin film of sample before impinging on the detector. Samples frequently need to be diluted or flattened to render them into thin film form. 

All IR transmission samples require a substrate on or a cell in which they can be placed. Common cell materials are summarized in Table 2. The ideal cell material would be 100% transparent in the spectral region of interest, non-interactive with the sample, hard but not brittle, inexpensive, non-hygroscopic, and insoluble in water. Needless to say, no materials fulfill all these requirements, so cell materials are chosen to have the properties necessitated by the sample or experiment. 

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