Handling techniques for channel spectra in synchrotron-based Fourier transform spectra

Abstract

Recently, the high radiance of synchrotron sources has been used to enhance FTIR spectrometer performance. However, excessive channel spectra when synchrotron sources are used degrade the quality of retrieved spectral parameters. We have investigated seven different techniques for handling channel spectra. These techniques were used to reduce channel spectra for a test group of seven samples of CO2 mixed with air recorded using the synchrotron source at the Canadian Light Source. The increases in signal to noise ratio (SNR) of spectra handled with each technique were calculated. SNR results showed that transmission spectra, produced using synthetic background spectra with simulated channel spectra, achieved the highest SNR improvement. However, when the spectra groups were fitted using a nonlinear least square fit algorithm, the technique using channel spectra fitting produced the smallest fitting residual. Moreover, the retrieved intensities and air broadening coefficients of 21 spectral lines showed that the spectral fitting technique produced the most accurate values as compared to the HITRAN 2008 database (Rothman et al. J. Quant. Spectrosc. Radiat. Transfer, 110, 533 (2009)). Although the spectral fitting technique was accurate in retrieving spectral line parameters, applying the technique at wider spectral ranges reduced this accuracy.

Fringes in FTIR spectroscopy revisited: understanding and modelling fringes in infrared spectroscopy of thin films

“…Then, the linear parameters in eqn (9) are estimated by regressing the EMSC model spectra onto the spectrum with fringes. Finally, fringes are separated from the pure absorbance spectrum according to eqn (13). All three steps will be illustrated in the following sections by simulated and actual experimental data.…”

“…With the fringe frequency at hand we can now set up the EMSC model according to eqn (9), estimate EMSC parameters and correct the absorbance spectrum with additive fringes (blue spectrum in Fig. 3) according to eqn (13). As a reference spectrum, the spectrum shown in Fig.…”

“…The fact that for the determination of the frequency the region 6000 cm −1 to 3800 cm −1 is required is not a limitation, since spectra can be obtained in this region by most commercially available FTIR spectrometers. When the frequency is determined, the EMSC model can be established according to eqn (9) and the spectrum can be subsequently corrected according to eqn (13).…”

“…[10][11][12] Another non-chemical interference pattern that is frequently encountered in the FTIR spectroscopy of thin films is sinusoidal modulations in FTIR spectra called 'fringes'. 13 The fringes usually result from reflections inside the sample or the sample holder or from reflections between the sample holder and the sample. These reflections lead to additional interferences between the two infrared beams inside an FTIR spectrometer.…”

“…These spikes appear in the interferogram space and are transformed to the sinusoidal waves in the spectrum domain by means of a Fourier transform (FT). 13 They can seriously hinder the interpretation of the chemical bands in FTIR spectroscopy because they change the relative peak heights of chemical spectral bands.…”