Single-beam background

The procedure used to acquire the data was as follows. First a dc interferogram is measured with the 200-MHz transient recorder in the step-scan mode without sample excitation. The measured interferogram is used to calculate the phase spectrum, 0v, and the single-beam background spectrum, Sr v). The time-resolved measurement at a given OPD is then started by a laser flash at each step. The 200-MHz transient recorder measures the time-domain signal from 100 ns to 20 ps and the 200-kHz recorder monitors from 5 ps until the end of the reaction. At every step, several photocycles are accumulated to improve the SNR. Because of the fact that... 

Note that the PA spectrum has significant intensity only at those wavelengths where the sample absorbs and the single-beam background is completely suppressed. The same concept was applied commercially by Briihl and Kjaer in Denmark, which developed a portable instrument for monitoring industrial atmospheres, the sensitivity of which was in the low parts-per-milfion range. However, this instrument no longer appears to be marketed. 

An infrared spectrum may be defined as a sample-dependent change induced on the intensity distribution of infrared radiation emitted by a source over the entire infrared region. The intensity distribution (spectrum) of the mid-infrared radiation emitted by a source is shown in Figure 3.2a. This spectrum does not have absorptions by any sample (except for absorptions of atmospheric water vapor and carbon dioxide), and it is denoted as the reference spectrum (sometimes called the single-beam background spectrum) B v). ( ) is deter-... 

Only the single-beam type of photometric system of an FT-IR spectrometer is commercially available at present. This requires that two measurements must be made the first is for a reference single-beam background spectrum recorded without the sample, and the second for the sample placed in the beam. As these two measurements are made under the same optical conditions, accuracy of spectral intensity can be expected. Care must be taken, however, not to change the conditions of measurements during the time interval between the two measurements. 

For the reference spectrum, a single-beam background spectrum of the aperture in place, but without the sample, is measured. 

Figure 5 shows the single beam (background and the sample) and the ratioed spectra of polystrene film recorded in 30 seconds at 4cm-l spectral resolutions. The ATR technique can be used effectively for obtaining the FT-IR spectra of difficult samples such as carbon-filled polymers. Figure... 

Figure 45.2 ATR single beam spectmm of a catalyst thin-film on ZnSe crystal under argon upon UV-irradiation (a) P Ti02 catalyst, (b) Pd/AbOs catalyst, and (c) Au/T102 catalyst difference spectra reveals the background shift on (d) Pd/Ti02 catalyst, (e) Pd/Al203 catalyst, and (f) Au/Ti02 catalyst.

In Fig. 2.12 characteristic IR reflectance spectra of concentrated (3 M) methanol on platinum at potentials between 0.45 V and 1.3 V are shown. The single beam spectrum at 0 mV was taken as background. The following characteristic bands allow the identification of bulk products ... 

As for all absorbance-based spectral measurements, the intensity data represented in a raw (single-beam) chemical image are a combination of the spectral response of both the instrument and the sample. In order to remove the instrument response component, it is necessary to ratio the data to a background reference. For reflectance measurements, the background is a separate data cube typically acquired from a uniform. 

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. 

Background for a single-beam ultraviolet instrument, subtract the 550 from the reaction mixture from the 550 for the blank control ... 

The initial single beam dispersive spectrometers that did not, at the time, produce digitised spectra (this would have allowed for baseline correction) were soon replaced by double beam spectrometers. This more complex arrangement can directly yield the spectrum corrected for background absorption. The use of two distinct but similar optical paths, one as a reference and the other for measurement, allows the alternate measurement of the transmitted intensity ratios at each wavelength. 

Application of this principle is used in two types of background absorption correction set-up. Single beam atomic absorption instruments have an electromagnet at the level of the graphite furnace (or flame) and a polariser in the optical path (Fig. 14.14). However, this accessory is quite expensive. 

Single-beam spectra for neat (without solvent) frans-free fat (background) and trans fat (test samples). Reprinted from Adam et al. (2000) with permission from the American Oil Chemists Society. 

In FTIR instrumentation, single-beam spectra (Fig. D 1.7.IB and DI.7.2B) are measured separately for both a test sample and an appropriate reference background material, and then ratioed to obtain an absorption spectrum... 

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