Single-beam background spectrum

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... 

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 9.20 (a) Single beam FTIR spectrum of background a plot of raw detector response versus... 

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. 

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 ... 

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. 

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. 

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... 

A further advantage of spectrophotometers is the ready availability of a number of low-cost instruments with sufficient accuracy and reproductivity for dyebath analysis. Much of the work in the current study was carried out on a single-beam grating spectrophotometer costing approximately 2,000. The computations necessary in the analysis can be conveniently carried out on low-cost desk calculators or microprocessors. The calculations necessary for a four-dye mixture (or three dyes plus background) can be handled on a system costing less than 1,000. The least-squares fit of 16 points of the absorption spectrum can be carried out on a 3,000 minicomputer. Development of these low-cost instrument-minicomputer systems is largely responsible for consideration of dyebath reuse as a practical reality for the textile industry. 

It is interesting to compare the single-beam spectra [35] (Figure 5) recorded on an nm-Pt/GC electrode at 0.0 and 0.70 V, respectively. In normal cases, such as CO adsorption on a massive Pt electrode, IR bands of CO adsorbates often cannot be observed in single-beam spectrum since the IR absorption of COad is usually too small and is buried in the strong background, only the CO2 band that is derived from COad oxidation and is in the thin layer may appear due to the strong IR absorption... 

I is the intensity measured in a single beam spectrum of sample and I0 is the intensity measured in the background spectrum. Transmittance is often expressed as %T, which forms the scale of the vertical axis in Figure 9.20c. The spectrum can also be presented as absorbance (A) versus wavenumber as shown in Figure 9.21. The absorbance is calculated from the transmittance. 

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