Self absorption effects

Not previously considered in the above reference is a-particle self-absorption phenomena. However, attempts are at present being made to estimate radon and thoron progeny deposited on the surface of materials by y-spectrometry and gross y-count in order to eliminate self-absorption effects. 

The X-ray florescence is proportional to the X-rays absorbed by the sample and therefore can be used to measure the FXAFS oscillations. In general, this technique has a poorer signal-to-noise ratio than transmission FXAFS and there are problems with self-absorption effects, requiring dilute or thin sample sizes. 

In a flame, as the concentration of atoms increases, the possibility increases that photons emitted by excited atoms in the hot region in the centre will collide with atoms in the cooler outer region of the flame, and thus be absorbed. This self-absorption effect contributes to the characteristic curvature of atomic emission calibration curves towards the concentration axis, as illustrated in Fig. 4.4. The inductively coupled plasma (ICP) tends to be hotter in the outer regions compared with the centre—a property known as optical thinness—so very little self-absorption occurs, even at high atom concentrations. For this reason, curvature of the calibration curve does not occur until very high atom concentrations are reached, which results in a much greater linear dynamic range. 

Other sources of error, particularly in quantitative Raman analysis, include laser self-absorption effects leading to attenuation of some spectral bands. Similarly diffuse reflectance of the laser light, which is dependent on the particle size of the formulation components, may increase or decrease the collection volume. However, normalisation techniques can be used to overcome some of these effects [35]. 

Together with these general empirical observations, there have been attempts to quantify the d-occupancy on the basis of XANES data (Mansour et al., 1984). A word of caution is needed for these types of investigations, as the absolute intensity of the absorption edge (white line) can be influenced by self-absorption effects in XAS (Stem and Kim, 1981), and it is essential to demonstrate that the intensities being measured are not influenced by the concentration of element in the sample. In addition, there could be effects associated with adsorbates on the dispersed metal clusters. 

The DeJongh model is similar to the Lachance-Trail model 16), but includes correction for self-absorption effects. The alpha coefficients shown in Table 5 were used as starting values for the straight line regression procedure executed by the PW-1400 software package. 

Table 14.6 gives the characteristics of several self-powered detectors. Rhodium detectors have the best sensitivity but also the largest bumup rate. Their change of sensitivity with time is important and necessitates a correction before the signal is used for the determination of power. The correction is not trivial because of changes in self-absorption effects in the emitter, and may introduce errors unless the detector is calibrated properly. 

Schroeder SLM, Moggridge GD, Lambert RM, Rayment T (1997) Self-absorption effects in grazing-incidence total electron-yield XAS. J Phys IV 7 91-96 Segtnan VH, Sasic S, Isaksson T, Ozaki Y (2001) Studies on the Structure of Water Using Two-Dimensional Near-Infrared Correlation Spectroscopy and Principal Component Analysis. Anal Chem 73 3153-3161... 

Advantages include higher elemental sensitivity and the ability to collect images for multiple elements simultaneously. The disadvantages are that it is more time consuming and self-absorption effects complicate data processing. 

Self-absorption problems were encountered by Paal and Thomson in a study of the retention of [3-H] by a Pt black catalyst. After various pretreatments residual radioactivity was detected by scintillation counting of a suspension of catalyst in a liquid scintillator. The results provided evidence that considerable amounts of tritium were retained by the catalyst. However, the absolute amount present could not be obtained because some of the tritium may have been in the pores and interstices of the Pt, resulting in severe self-absorption effects. 

We started by considering the application of the Winston-Rabl (13) idea to LSC-type systems. But, because the energy of the incident solar light is not preserved in an LSC, we used a detailed balance calculation which gives the intensity of the plate output as a function of size, ignoring self-absorption effects. 

Figure 4.13 Fluorescence self-absorption effects for sodium molybdate NajMoO,.

It is recommended that a metallic arc not be used for calibration purposes, because of the danger of self-absorption. One satisfactory method is to use iron oxide diluted with carbon to minimize the possible self-absorption effect of the metal electrode iron arc. 

The self-absorption effects become negligible if the targets are very thin and of uniform thickness, The elemental concentration mj(gcm ) in various samples can be determined using the expression ... 

Distorted excitation and/or emission spectra—inner filter and self absorption effects. It is important to be aware of the optical geometry of the instrument and the absorption characteristics of the sample in the cell used. The excitation optics are usually arranged to focus the beam into the centre of the cell. In practice this gives a thin beam of excitation light, the width of which... 

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