Subtraction technique, difference spectra

Mineral standards were hand crushed to -1/4 inch, then ground to a fine powder in a ball mill (alumina elements) or Bleuler Model 526/LFS678 puck mill. The resultant powder was aerodynamically classified in a Bahco Model 6000 micro particle classifier and the finest fraction ( 18 throttle) was collected. A size criterion of 90% or more by weight of particles 5 micron and smaller in diameter was used for the mineral standards. Sizes were verified by Coulter Counter. Duplicate 13 mm KBr pellets were prepared and the spectra were weight-scaled by techniques similar to those reported by Painter (3) and Elliot (4). With one exception, all the mineral standard spectra were averages of spectra from duplicate pellets. The one exception was the iron sulfate spectrum, which was obtained as the difference spectrum by subtracting the spectrum of HCl-washed weathered pyrite from that of the weathered pyrite. A weight correction was applied to the difference spectrum. 

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. 

Kg. 4.1.9. Difference spectrum of oak (Quercus obtusata) obtained by subtracting the spectrum of MWL from the spectrum of the same MWL after NaBH4 reduction. (Instrument Nicolet 20SX D, 4cm 1 resolution, 32 scans, KBr pellet technique)... 

The torsional mode, t(WH2), was located in the INS difference spectrum (Figure 5.8) as a split mode at 385 and 325 cm-1 due to transitions to two split excited librational states (/ = 1, 2), as will be discussed below. INS spectroscopy is a powerful technique to locate such large amplitude vibrations involving hydrogen. Because deuterium does not scatter, as well as protium, modes not due to T 2-H2 can be effectively removed by subtracting the spectrum of the D2 complex or a suitable... 

Little has been published on the use of FT—IR to identify the peaks obtained from an on-line HPLC, but the obvious compatibility of the HPLC with the unstable intermediates of expl decompn suggests that this analytical technique will be extensively explored in the future. The difference spectroscopy (see below) possible with the FT—IR would allow subtraction of the spectrum of the mobile phase and so afford a measure of sensitivity not available with any other technique... 

FIGURE 17 2 IR spectra of water and of an aqueous solution containing water-soluble aspirin. The top spectrum shows the computer-generated difference spectrum obtained by subtracting the water spectrum from that of the sotution. (From Practicsl Sampling Techniques for Infrared Amdysis, P. B. Coteman, ed.. Boca Raton. FL CRC Press. 1993. With permission.)... 

Difference spectroscopy, as the name implies, requires spectral subtraction. Two spectra are acquired. One in which the proton of interest is decoupled and a second reference spectrum in which the irradiation is appHed in an isolated region far from the nearest proton resonance. The two spectra are then subtracted from one another, the resulting difference spectrum highHghting protons that were affected by the decoupling process [34]. Several excellent examples of applications of this technique are found in the monograph by Nakanishi [24]. 

The most straightforward method of analysis for complex spectra is difference spectroscopy. This technique may be carried out by simply subtracting the infrared spectrum of one component of the system from the combined spectrum to leave the spectrum of the other component. If the interaction between components results in a change in the spectral properties of either one or both of the components, the changes will be observed in the difference spectra. Such changes may manifest themselves via the appearance of positive or negative peaks in the spectrum. 

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