Composites spectrum subtraction method

Because Eq. (6.26) is very simple, an on-line minicomputer can solve for Nq and Nr very quickly. Providing the peak shapies and positions in the reference spectra are identical to those in the composite spectrum, the method is relatively free of systematic biases in estimating the net intensities. The technique can also be applied to overlap between two line series. For example, the Mn Ky3 lies under the Fe Kcr on energy dispersive spectrometers. The Mn Ka peak area can be used to subtract the Mn interference from the Fe Ka line. However, a word of caution is in order when using Ka/Kj8 or hoc/Lfi ratios for resolving interferences. Occasionally the composite specimen contains an absorption edge between the two lines, which is not present in the pure element reference spectra. Thus, the Ka/Kfi or La/Lfi ratios are different in the composite specimen than in the pure element reference standards. 

A variety of infrared methods have been available for the simple problems of verifying the purity of the initial resin components and mixtures of those components. The simplest possible test of the purity of a reactant is to compare its spectrum with a known standard. Subtractive methods may be employed to emphasize the most significant intensity changes 19). Composition analysis of the blended resins and hardeners becomes more difficult. In some cases, one may simply select characteristic isolated absorption bands for each component and estimate the composition by comparison to standard spectra of known composition 20,21). 

Perhaps the best route to evaluate the composition of a lysoPA mixture is through the methods cited earlier, especially FAB-MS. A subtractive type of analysis can done in which the FAB-MS spectrum of the original sample is obtained then, after each treatment (acid, base, etc.) the sample is purified by thin-layer chromatography, the remaining lysoPA recovered and examined again by FAB-MS. These various profiles will yield information of value in determination of the structural characteristics of the sample under study. Thus, if after an acid reaction followed by a base treatment a chloroform-soluble phosphorus compound remains, it would most likely be the lysoalkyl ether GPA. The FAB-MS spectrum would support such a conclusion. 

For some systems, there is no resonance Raman or SERS effect to be utilized, and the sensitivity becomes the main problem. In this case, a potential difference method will be of great help [11], Here, a spectrum is acquired at potentials where there is no or only a weak surface signal which is subtracted from that at the potential of interest. In addition, a change in the composition of the electrolyte or an isotopic labeling experiment may be considered to identify the surface species and verify its orientation and structure. For temporally resolved studies, electrochemical transient techniques are helpful to understand the surface dynamics and the reconstruction processes of surfaces. For nonuniform surfaces, spatially resolved measurements provide more reliable and complete information on the surface. This is also useful for electrode surfaces that change either chemically or topographically in a microzone upon variation of potential. 

Figure 19. Illustration of the method of extracting the valence-band spectrum of the buried interface between an oxidized carbon fiber and a phenolic resin, with an intermediate titanium alkoxide agent, (a) Valence-band spectrum of oxidized carbon fiber alone, (b) valence-band spectrum of phenolic matrix alone, (c) valoncc-band spectrum of the composite, and (d) difference. spectrum—the result of subtracting spectra (a) and (b) from (c). Spectrum (d) represents the desired valence-band spectrum of the interface. (From Ref. 51.)...

In the case of a crystalline phase in an amorphous matrix, a rough measure of their ratio can be obtained if the compositions are known. This is accomplished by summing the powder diffraction line intensities, subtracting the amorphous (background) intensity, and correcting for Compton-modified scattering. The fact that there is a continuous spectrum of order from well-crystallized to noncrystalline phases is an inherent limitation to the method. 

---