Quantitative modulated excitation Fourier transform infrared spectroscopy

Abstract

The detection of weak absorption changes induced by an external excitation is often hindered by intense background absorptions as well as by noise. Modulation spectroscopy is an adequate tool to be applied in such a case, provided the system may be periodically stimulated, leading to a periodic reversible or pseudoreversible response. In modulated excitation ͑ME͒ Fourier transform infrared spectroscopy the phase sensitive detection ͑PSD͒ used for the demodulation of the periodic system response is generally performed during data acquisition, i.e., applied to the intensity of the interferogram. This leads to a number of problems in quantitative analysis and the requirement of optional equipment. In this article, a method is presented to perform an off-line vector PSD of conventional time-resolved spectra after data acquisition. A detailed mathematical analysis of PSD applied to the spectral intensity, the interferogram intensity, and to time-resolved spectra is presented. It is shown, that vector PSD applied to a set of time-resolved spectra is straightforward and avoids any additional mathematical corrections. Furthermore, it will be shown how ME spectroscopy can be used for experimental separation of overlapping bands and a detailed description for the determination of absolute modulation amplitudes and phase lags is given.

ATR-FTIR Spectroscopy of Membrane Bound Ras Protein

“…It was a cyclic stimulation and the initial spectrum was restored at the end of the measurement. This was further supported by the characteristic similarity of the difference spectra (Figure 45a) to the modulated excitation response [242]. The interpretation of our data was additionally leading to the conclusion that; NO-aided loosening of the membrane networking was occurring along with the stabilizing interactions of the lipid C=O region (Figure 45b).…”

“…Such a modulated response was not observed during the GDP nucleotide circulation (Figure 40), but was observed after the NO-mediated GTP exchange of the Ras protein ( Figure 45a). Lipid methylene stretching vibrations were not shifting ( Figure 37a and 50); so, the observation (Figure 45a) was not due to the effect of temperature [242,243].…”

“…Modulated excitation (ME) is applicable to the ATR-FTIR, provided that the sample of interest admits (periodic) excitation by the modulation of an external parameter and the weak absorption changes which are hindered due to a huge background noise are revealed by this means [242,243]. Such a modulated response was not observed during the GDP nucleotide circulation (Figure 40), but was observed after the NO-mediated GTP exchange of the Ras protein ( Figure 45a).…”

Reduction and re-oxidation of Cu/Al2O3catalysts investigated with quick-scanning XANES and EXAFS

“…For this purpose, the Cu/Al 2 O 3 catalysts were reduced at 350°C and the gas atmosphere was periodically switched between oxidizing and reducing conditions using 21%O 2 /He and 5%H 2 /He, respectively. The periodic change of gas atmosphere provided the possibility to average over several cycles corresponding to modulation excitation spectroscopy (MES), a useful technique in time-resolved spectroscopy to improve signal-to-noise ratios without loosing resolution in time or energy [14].…”

Time Resolved Operando X-ray Techniques in Catalysis, a Case Study: CO Oxidation by O2 over Pt Surfaces and Alumina Supported Pt Catalysts

“…In the second category, methods of advanced analysis, then we might point to three significant areas: the recent and increasing awareness within the catalytic community of modulation excitation spectroscopy analysis (MES) [184], and more advanced approaches toward the modelling of XANES and EXAFS data that can provide deeper insight or resolve problems that more conventional approaches struggle with. Within the time resolved, structural-kinetic, remit of this review by far the most important of these, if only as it is a method that is independent of the applied probe method, is MES, though we also note some other advanced methods in relation to getting the most from X-ray spectroscopy.…”