The Background Spectrum

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If preferred, the data system may be used to subtract the background spectrum (Figure 3.14) from that of the analyte (Figure 3.16). This manipulation yields the spectrum shown in Figure 3.17, in which the ions from the background are now totally absent. Care must be taken when adopting this procedure to ensure that any contribution from the analyte is not removed when ions at the same m/z value arise from both the background and the analyte of interest. 

Figure 9. Laser desorption FTMS mass spectra recorded following successive shots of the laser. In each spectrum, (a) shows the results following the first laser pulse, (b) is the second laser pulse at the same spot, (c) is after the third laser pulse, and (d) is the background spectrum (laser off).

Positive going bands indicate the consumption of species, which are present in higher concentration at the potential of the background spectrum ... 

All bands discussed here were also observed with 1 M CH3OH solutions. In order to check that the above discussed bands are due to bulk products accumulated in the thin layer, one series of experiments was performed stepping the potential from 1500 mV down to 450 mV after taking the background spectrum at 0 mV. In this case the bands are observed at all potentials showing that stable bulk products are accumulated in the interface. 

The detection difficulties were then accentuated by the several types of background which were superimposed on the x-ray spectrum of interest. This was overcome by the modelling and subtraction of the background spectrum from the acquired spectrum. 

The sampling station was equipped with an overhead DRIFTS accessory. The sample holder was used for the background spectra without KBr, and 256 coadded scans were taken for each sample from 4000 to 400 cm at a resolution of 16 cm L Single-beam spectra of the samples were obtained, and corrected against the background spectrum of the sample holder, to present the spectra in absorbance units. Spectra were collected in duplicate and used for multivariate analysis. 

Figure 5. Infrared spectrum at room temperature of VSil545 after evacuation at 500 C (a) and after contact at room temperature with 50 torr of propane (b). Reported in the inset is the expansion in the 1300-1550 cm region of spectrum b after subtraction of spectrum a. The background spectrum due to gaseous species has been subtracted.

But bremsstrahlung must also be considered as a part of the background spectrum.17 The main ionization processes of an analyte (M) in argon inductively coupled plasma are ... 

IR spectrum of chemisorbed hexamethyl-disilazane on silica (---) the background spectrum in each case (-) new... 

The most recent results for hydrogen chemisorbed on Raney nickel by Kelly et aj[ (30) are shown in Fig. 8. The inelastic neutron TOF spectra are taken at 77 K in the energy-loss mode with the background spectrum of the reduced Raney Ni sample after desorption of adsorbed species (Fig. 8(a)) subtracted. Figure 8(b) is the pectrum for a hydrogen monolayer. Features at -120 meV (960 cm-i)and -140 meV (1120 cm-1)are in excellent agreement with previous results of Renouprez et al (28,29). In addition, a peak at 78 meV (624 cm"1) is clearly resolved. 

Figure 15.7 Examples of spectral processing. Acquisition parameters listed in Table 15.1 were used to measure a solution of tempol. The background spectrum was collected using water, which shows a feature due to a cavity defect.

Figure 10.3 Infra-red. spectrum obtained after chemisorption ofwBF3 on dehydrated Cab-O-Sil (a) untreated silica, (b) lsO exchanged silica. The dotted line represents the background spectrum of silica before the BF3 chemisorption.

The background spectrum is subtracted (channel by channel) from the reference spectrum. This procedure removes any frequency dependent structure in the background from the reference spectra provided it remains stable for the averaging period. The result of this subtraction is shown in Figure 2c. 

Valves are then reset to admit ambient air and after another stabilization period an ambient air spectrum is acquired for 3 min (Figure 2d) and the background spectrum subtracted (Figure 2e). 

In the design of these experiments a number of considerations are involved In order to minimize interferences from background resonances it is desirable to label sites which give resonances in relatively clear regions of the spectrum From this standpoint, the C-2 carbon represents an attractive labeling site, since the background spectrum of liver is relatively free of interfering resonances at 140 ppm (Table III) However, the chemical shift difference between the oxidized and reduced... 

For the in-situ FTIR analyses, mixed solutions were spread by the same hion onto a house-made minitrough that fitted into the sample chamber of the FTIR instrument. For every in-situ experiment, the close-packed monolayer was initially formed on DI water that was then replaced with ImM CdCl2 solution by careful circulation of the subphase using a peristaltic pump. The background spectrum was taken for a bare water surface when the sample was Ae monolayer on pure water, and was taken for a close-packed monolayer on pure water when the sample was the one on the cadmium-containing subphase. The latter technique enhanced the carboxylate peak relating to ion adsorption. All the experiments were done at room temperature. 

Figure 3 Gamma ray spectra from the same wheat sample (stem) measured in HADES and above ground at IRMM. The background spectrum from HADES is shown as comparison.

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