Direct powder sampling

Transparent solid samples can be analyzed directly by placing them in the IR beam. Most solid samples, however, are opaque and must be dispersed in a more transparent medium before recording a traditional transmission spectrum. If a suitable solvent is available, then the solid can be analyzed by preparing a solution and analyzing as described earlier. When a suitable solvent is not available, solid samples may be analyzed by preparing a mull of the finely powdered sample with a suitable oil. Alternatively, the powdered sample can be mixed with KBr and pressed into an optically transparent pellet. 

W. E. Petit and G. Horlick. Spect. Acta. 41B, 699, 1986. Describes an automated system for direct sample-insertion introduction of 10-pL liquid samples or small amounts (10 mg) of powder samples. 

In the powder diffraction technique, a monochromatic (single-frequency) beam of x-rays is directed at a powdered sample spread on a support, and the diffraction intensity is measured as the detector is moved to different angles (Fig. 1). The pattern obtained is characteristic of the material in the sample, and it can be identified by comparison with a database of patterns. In effect, powder x-ray diffraction takes a fingerprint of the sample. It can also be used to identify the size and shape of the unit cell by measuring the spacing of the lines in the diffraction pattern. The central equation for analyzing the results of a powder diffraction experiment is the Bragg equation... 

Atsuya I, Itoh K, Akatsuka K (1987) Development of direct analysis of powder samples by atomic absorption spectrometry using the inner miniature cup technique. Fresenius Z Anal Chem 328 338-341. 

From (4.56) and Table 4.3, we derive the relative intensity ratios 3 2 1 1 2 3 for the hyperfine components of a Zeeman pattern of a powder sample. The transition probability for the case of the polar angle 6 = Oq can readly be calculated by integrating (4.56) only over the azimuthal angle (j). One obtains a factor (1 + cos 0o)/2 and sin 0o for m = 1 and m = 0, respectively, which are multiplied by the square of the Clebsch-Gordan coefficients. As a consequence of the angular correlation of the transition probabilities the second and fifth hyperfine components (Fig. 4.17) disappear if the direction k of the y-rays and the magnetic field H are parallel (0q = 0). 

This relation is only valid for a crystal with isotropic /-factor. The effect of crystal anisotropy will be treated in Sect. 4.6.2. The function h(6) describes the probability of finding an angle 6 between the direction of the z-axis and the y-ray propagation. In a powder sample, there is a random distribution of the principal axes system of the EFG, and with h 6) = 1, we expect the intensity ratio to be I2J li = I, that is, an asymmetric Mossbauer spectrum. In this case, it is not possible to determine the sign of the quadmpole coupling constant eQV. For a single crystal, where h ) = — 6o) 5 delta-function), the intensity ratio takes the form... 

Different analytical procedures have been developed for direct atomic spectrometry of solids applicable to inorganic and organic materials in the form of powders, granulate, fibres, foils or sheets. For sample introduction without prior dissolution, a sample can also be suspended in a suitable solvent. Slurry techniques have not been used in relation to polymer/additive analysis. The required amount of sample taken for analysis typically ranges from 0.1 to 10 mg for analyte concentrations in the ppm and ppb range. In direct solid sampling method development, the mass of sample to be used is determined by the sensitivity of the available analytical lines. Physical methods are direct and relative instrumental methods, subjected to matrix-dependent physical and nonspectral interferences. Standard reference samples may be used to compensate for systematic errors. The minimum difficulties cause INAA, SNMS, XRF (for thin samples), TXRF and PIXE. 

Table 8.54 indicates the main characteristics of X-ray powder diffraction. Powder samples of lmg or greater can be analysed. The principal advantage is that substances (e.g. BaS04) can be identified directly, and not indirectly via their elements (e.g. Ba and S). This is particularly advantageous if both silica and silicates (kaolin, talc) are present. Detection limits are matrix dependent, but ca. 3% in a two-phase mixture, with synchrotron radiation ca. 0.1 %. 

Direct solid sampling (bulk metal, thin film, compacted powder, solution residues)... 

Figure 5.7 Magnetization (a) and susceptibility (b) of the Dy3 complex obtained on oriented single crystals and powdered samples. The Z direction is orthogonal to the Dy3 plane. The inset in the left panel shows the difference between the X and...

Figure 5.8 Static magnetic data for the Er(trensal) complex in the case of a powdered sample and an oriented single crystal measured in parallel or perpendicular direction to the C3 rotation axis, (a) Temperature dependence of the magnetic susceptibility...

Powdered samples that cannot be cast as films can be measured by this technique. Quite simply, the IR light beam shines on a quantity of the powdered sample held in a cup. In this case, the light scatters, or reflects in all directions. Some of this light is captured by the detector. A disadvantage is that the light intensity is decreased substantially by the scattering, hence the term diffuse. See Figure 8.27. 

Reflectivity spectra can be registered in two different modes (i) direct reflectivity or (ii) dijfuse reflectivity. Direct reflectivity measurements are made with well-polished samples at normal incidence. Diffuse reflectivity is generally used for unpolished or powdered samples. Figure 1.7 shows the experimental arrangements for measuring both types of spectra. 

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