PA-IR spectroscopy

A spectral image in absorbance units can be calculated pixel by pixel as  

An alternative approach recently proposed by Sommer et al. is the use of a single or double-pass prism monochromator rather than a grating system [8], This provided a large bandwidth, up to 3000 cm, at the expense of a reduced spectral resolution, especially in the C—H stretching region. Another recently described approach to extend the spectral coverage was to stack two gratings with a low and 

It should be pointed out that, in good part because of the use of large off-axis mirrors, optical aberrations can lead to significant curvature of the images under certain instrumental configurations. This has a detrimental impact on spectral resolution when multiple rows are birmed. Pelletier et al. have reported a data processing procedure to minimize this effect for situations where experimental limitations prevent improving the optical set-up [9]. In future commercial 

Figu re 13.4 Evolution of the peak-to-peak noise level In 100% noise spectra as a function of acquisition time using an MCT PA-IR spectrograph (with an without pixel binning) and an FT-IR spectrometer. Reproduced with permission from Ref [7]. 

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PA-IR spectroscopy allows very rapid measurements, with millisecond or submillisecond time resolution, with good to excellent SNRs. By measuring 100% lines spectra, Rabolt et al. reported that a peak-to-peak noise level of 2.7 x 10 absorbance units can be obtained in 17 ms using an InSb FPA system [6]. A similar value was obtained in 8.7ms with an MCT FPA system [7]. As expected, longer acquisition times lead to increased SNRs. The data in Figure 13.4 show that peak-to-peak noise level in PA-IR spectra decreases with acquisition time, with a slope close to the expected square root improvement [1, 7]. Noise levels are similar to those obtained with an FT-IR spectrometer for the same measurement time. 

Snively et al. used a PA-IR spectrograph equipped with a 64 x 64 MCT rollingmode FPA to push the time resolution of PA-IR spectroscopy to less than 100 ts per spectrum [27]. The small size and the acquisition mode of this FPA, both contributed to enable such time resolution. First, a smaller FPA allows faster frame rates than a larger one (3.2 ms per image in this specific case), at the expense of bandwidth and/or resolution. Second, the 100% duty cycle of a rolling-mode FPA was put to profit. In a snapshot-mode FPA, all pixels measure the signal intensity simultaneously for -100 ts, but the electronic read-out time can be as... 

As noted above, step-scan FT-IR can provide a better time resolution than PA-IR spectroscopy for time-resolved studies, as well as full spectra at the desired resolution. On the other hand, its major limitation is that the phenomenon under study must be perfectly repeatable-information which often is not available before an experiment is carried out. Another problematic aspect to consider is that sufficient relaxation time must be allocated for the sample to return to its initial state between consecutive perturbations. Unfortunately, this parameter is also often not known a priori before the experiment is performed, and may risk artifacts appearing in the data. In contrast, a single perturbation is required in a PA-IR experiment to record the time-resolved data, eliminating the requirements of repeatability and an a priori knowledge of the relaxation time. PA-IR spectroscopy was used to assess directly the repeatability of the orientation/reorientation cycles for 5CB [27]. Table 13.1 shows the switch-on and switch-off time constants determined individually for a series of 300 consecutive reorientation cycles. As expected for this well-studied LC, the time constants did not evolve systematically as a function of the number of cycles. In this case, however, the repeatability was demonstrated experimentally and not only assumed, as is often necessary in step-scan studies. 

In addition to high-speed measurements, PA-IR spectroscopy distinguishes itself from FT-IR by the possibility of performing 1-D imaging. Figure 13.12a... 

Although still in its early stage of development, PA-IR spectroscopy represents a promising approach to mid-lR spectroscopy. The combination of focal plane array detectors with a spectrograph can ... 

There have been a few other experimental set-ups developed for the IR characterization of surfaces. Photoacoustic (PAS), or, more generally, photothemial IR spectroscopy relies on temperature fluctuations caused by irradiating the sample with a modulated monocliromatic beam the acoustic pressure wave created in the gas layer adjacent to the solid by the adsorption of light is measured as a fiinction of photon wavelength... 

Using IR spectroscopy and NMR, one can analyze the chemical structure of PA. The molecular weight and molecular weight distribution can be analyzed by endgroup analysis, viscometry, and high-pressure liquid chromatography (HPLC). The crystalline order can be analyzed by WAXS, small-angle X-ray spectroscopy... 

Spectroscopy, 490. See also 13C NMR spectroscopy FT Raman spectroscopy Fourier transform infrared (FTIR) spectrometry H NMR spectroscopy Infrared (IR) spectroscopy Nuclear magnetic resonance (NMR) spectroscopy Positron annihilation lifetime spectroscopy (PALS) Positron annihilation spectroscopy (PAS) Raman spectroscopy Small-angle x-ray spectroscopy (SAXS) Ultraviolet spectroscopy Wide-angle x-ray spectroscopy (WAXS)... 

ATR FT-IR spectroscopy has also been employed to monitor the solid-phase synthesis of substituted benzopyranoisoxazoles [180]. Finally, Huber et al. [181] have also reported that this technique is particularly suitable for the characterization of supported molecules in combinatorial chemistry, as well as for the identification of side products and for Photoacoustic (PA) FT-IR. 

Photo-acoustic spectroscopy (PAS) is a kind of infrared (IR) spectroscopy which is a popular choice for real-time monitoring of VOCs at ppbv levels. Recently there has been a great revival of interest in PAS because it offers much greater sensitivity than conventional spectroscopic techniques. All spectroscopic methods yield quantitative and qualitative information by measuring the amount of light a substance absorbs PAS simply measures this in a more sensitive way. 

IR Infra-Red Spectroscopy, including FTIR Fourier Transform IR PAS-IR IR Photo Acoustic Spectroscopy RS Raman Spectroscopy... 

The strength of solid acids and bases is characterized by proton affinity (PA). For a base B, PA is equal to the enthalpy of reaction B + H " -> BH in gas phase, where B is electrically neutral base and BH" is its protonated form. The methods have been developed to determine PA for various compounds by the combined application of different indicators, sorbents and IR spectroscopy methods [22]. 

IR spectroscopy was applied for the measurements of adsorption and desorption of pyridine. Prior to the pyridine adsorption, all samples were activated in vacuum (lO Pa) at 675K except when they were previously evacuated at 975K. Pyridine adsorbed on Bronsted acid sites, true Lewis acid sites and sodium cations was characterised by IR bands at -1550, -1455 and -1442 cm", respectively [12]. Table 2 shows the ratio of the absorbance at -1550 cm 1 (PyB) to that at -1455 cm 1 (] L) for various samples. These results indicate that all samples, even when evacuated at 975K possessed Bronsted acid sites. However, the Lewis acid sites dominated except with the samples treated at lower temperatures. One should stress that evacuation at 975K led to a higher reduction of Bronsted acid sites than heating at the same temperamre in air. The lower PyB/PyL ratio observed on niobium-modified NH4NaY... 

IR spectroscopy was used to characterise the anions [TiF4(S04)2]2, [TiF2(S04)2]2 and [Ti302F2(S04)4]4. 69 vTiF was seen in the IR spectrum of [Ba8Ti6F3012(Cp )6(hm pa)6]2+ the single peak shows the high symmetry of the core of this complex.70... 

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