Subtractive techniques

As an example of the use of AES to obtain chemical, as well as elemental, information, the depth profiling of a nitrided silicon dioxide layer on a silicon substrate is shown in Figure 6. Using the linearized secondary electron cascade background subtraction technique and peak fitting of chemical line shape standards, the chemistry in the depth profile of the nitrided silicon dioxide layer was determined and is shown in Figure 6. This profile includes information on the percentage of the Si atoms that are bound in each of the chemistries present as a function of the depth in the film. 

Vidrine, D. W., Use of subtractive techniques in interpreting on-line FTIR spectra of HPLC column eluate, ]. Chromatogr. Sci., 17, 477, 1979. 

In the incremental or decremental technique, another designation for the standard addition (or subtraction) technique, one adds increments of standard solution to the sample, or vice versa. (In the decremental technique the standard precipitates or complexes the ion under test.) When the sample itself is incrementally added to the standard, the latter may have received a previous addition of ISA and/or pH adjuster, but in the reverse method this addition may be made to the sample. However, for the specific example of a univalent anion we shall show how the normal incremental method works38 and that in fact the addition of ISA is not necessary. 

Mrad/h). Films were stored at -20° until analysis could be carried out. Oxidized films and derivatized, oxidized films were characterized by iodometry (reflux with Nal in isopropanol/acetic acid) and by transmission Fourier Transform (FT) IR (Perkin Elmer 1500), using the spectral subtraction technique (3, 14). Free radicals were measured by the electron spin resonance technique (e.s.r., Varian E4 spectrometer). 

Selecting an approach Physical separation of the two compounds will not be easy, but mass spectral subtraction techniques may allow you to obtain a spectrum of the peak of interest. 

This paper contributes to the literature by quantifying anionic polymer adsorption onto the clay minerals kaolinite, feldspar, mica and quartz by X-ray photoelectron spectroscopy (XPS). XPS measures the sorbed amount directly rather than by a subtraction technique. This enables an insight into how effective selective flocculation is for obtaining kaolinite from a mineral mixture. Atomic force microscopy (AFM) is also used to image polymer adsorption onto mineral surfaces and the effectiveness of this technique applied to mineral surfaces is discussed here. 

Infrared spectra were recorded on the resist film spun onto a silicon wafer using a JASCO IR-810 spectrometer equipped with a JASCO BC-3 beam condenser or a JASCO A-3 spectrometer. In the measurements on the latter spectrometer an uncoated silicon wafer was placed in the reference beam in order to balance the silicon absorption band. The subtraction between the spectra was carried out on a built-in micro-processor attached to the IR-810 spectrometer, and the resulting difference spectrum was used to detect structural changes in the polymer molecule upon exposure. The subtraction technique was also used to balance the silicon absorption band. 

Sometimes, small structural differences in morphology of polymer samples can be isolated by using a double subtraction technique. For example, with polyethylene terephthalate) PET, differences in the amorphous phase of the melt-quenched polymer and solution-cast polymer can be isolated by first subtracting out the contribution due to the trans isomer and then subtracting the two difference spectra from each other 214). (Fig. 16) shows the resultingdifference spectrum obtained after the second subtraction. Obviously the two amorphous structures are different from each other. 

No discussion has been devoted to the recent use of Fourier transform spectrometers rather than dispersion instruments. The ease with which the spectral data can be manipulated and background subtracted make the FT methods particularly useful for studies of surface species, particularly during catalytic reaction. Recently there has been a surge of interest in the coupling of computer subtraction techniques to conventional grating instruments. For many IR surface studies, where only limited frequency range is required, this... 

This chapter analyzes the subtractive techniques Differential Multipulse Voltammetry (DMPV), Differential Staircase Voltammetry (DSCVC), and Square Wave Voltammetry (SWV). Of these, the most employed SWV will be analyzed in more detail. Interesting alternatives to DSCVC and SWV are Differential Staircase Voltcoulometry (DSCVC) and Square Wave Voltcoulometry (SWVC), which are based on the analysis of the difference of converted faradaic charge signals obtained between two successive potential pulses when a staircase potential or a square wave potential is applied [4, 5], which is very useful for the study of surface-confined redox species. There exists, however, a book in this series devoted entirely to the theory and application of SWV [6], so in some of the reaction mechanisms analyzed, the reader will be directed to this title for a more thorough treatment of the SWV response. 

The analysis of the influence of the non-faradaic component corresponding to the converted charge-potential (Q-E) and current-potential (I-E) curves, is very different. A short discussion of this influence in some of the subtractive techniques analyzed follows. 

The effect of the subtraction technique on the appearance of the difference spectra can be investigated by plotting the frequency of significant bands in the difference spectra (such as the v, CH2 band mentioned above and the vs S-O band discussed frirther below) as a function of the subtraction scalar, X. Table m shows... 

Figure 3.26 Subtraction technique for elimination of effect of sample heat capacity change. The endotherm from the DTA trace represents both the latent heat of transformation as well as a shift in heat capacity of the sample during the transformation. The baseline (which is the sample temperature lag relative to the reference) shifts most rapidly near the center of the endotherm, where the conversion of reactant to product is most fervent. The right-hand trace represents a DTA endotherm with the effects of sample heat capacity changes subtracted out. Note that in this case, where the total heat capacity of the product is less than the reactant, this subtraction has resulted in an endotherm of larger area.

One of the most common uses of subtraction techniques is to remove a solvent spectrum from the spectrum of a solution. In most every case there is a solute-solvent interaction, and the spectra of all components in the solution have changed from those of the pure compounds. The changes can occur in band frequencies, intensities, and shapes. The user may gain some insight by performing the subtraction, but the spectra will be distorted. Examples of typical distortions are shown in Fig. 5-7 (6). The top spectrum is a mixture of EDTA and water, and the second spectrum is that of pure water. Two substractions are shown. The first was based on removal of the water band... 

Very useful adjuncts in the group classification and in verifying the identity of organic compounds are the so-called subtractive techniques [42]. With these techniques, a loop containing a chemical that either entraps or substantially retards a certain group of substances is included in the GC pathway. Such a procedure may be considered as an extreme... 

The cysteine residues were derivatized to introduce the R1 side chain (Fig. 5A), and the EPR spectra analyzed in terms of R1 mobility (Langen et al., 1999). The EPR spectra each revealed two components reflecting R1 populations of different mobility. Using simulation and subtraction techniques, the two spectral components (a and /3) were resolved and analyzed separately (see Fig. 7B for examples). In each case, the most... 

Typical features of the spectra of polymers are the changes introduced when the polymer chains are oriented by strain. When observed with polarized radiation the changing orientation of the molecular chains is visible by pleochromism of the infrared bands or the changes of the polarizability in the Raman spectra. Stress relaxation and the effects of fatigue and fracture may be observed, especially when the spectral subtraction technique is applied. These methods are well described by Siesler and Holland-Moritz (1980). In the Atlas of polymer and plastics analysis by Hummel and Scholl (1991) the methods of polymer analysis are described exhaustively and the spectra of plastic material and its constituents are collected. 

Petcavich et al. (1978) employed IR subtraction techniques to elucidate the mechanism of the oxidative degradation of polychloroprenes at 60 °C. The spectra were taken at 60 2°C. The results lead to the conclusion that 1,2- and 3,4-structural irregularities are involved in the initial stage of the thermal oxidation of these compounds at 60 °C. In addition, a simple free radical mechanism seems to be consistent with the experimental results. The observed results suggest that polychloroprenes may be stabilized towards oxidative degradation by eliminating the 1,2- and 3,4-structures by chemical modification of the polymer after synthesis. 

Methods that monitor ions at nominal mass [23, 43, 55, 63, 64] have interferences from higher PCBs, toxaphene, and chlordane-related compounds, all of which have similar GC-retention times to PCAs and similar molecular masses to PCAs (i.e., 350-500 Daltons) [14]. Reiger and Ballschmiter [43] have applied a simple subtraction technique to compensate for the effects of these interferences, but it is unclear how this procedure compromises the accuracy of their quantitation method. 

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