Phase-modulated step-scan

No matter what modulation frequency is used for PA/FT-IR spectrometry, the bands from the upper layers of the sample always dominate the spectrum. In addition, the fact that the thermal wave decay length varies as when the spectra are measured with a rapid-scanning interferometer has always led to suboptimal results. The variation of L with wavenumber may be circumvented through the use of a phase-modulated step-scan interferometer, and most contemporary PA/FT-IR spectra are now measured with this type of instrument. 

Fig. 7.10 Step-scan FTIR spectra of 10 mM ferrocyanide in 1.0 M KCI. (A) Power spectrum (dashed line) and phase spectrum (solid line) recorded by modulating the potential between the limits of 0.02 Vsce and 0.42 Vsce at 1 Hz. The electrode had been pretreated by cycling between -0.3 Vsce and +0.8 Vsce for 1 h to form an adsorbed...

FTIR step-scan photoacoustic spectroscopy was used to study the composition of thermoplastic olefin films, as a function of depth below the surface. Experiments were completed at various modulation frequencies, enabling a stratification model to be developed. The uppermost layer (0-3 im) showed large changes in talc and PP concentration, while the layer below showed a significant decrease in both the phases. In the third layer (6-9 pm), all three phases showed the maximum values. In the fourth layer (9-12 pm), the talc concentration reduced, whilst concentrations of elastomeric copolymer of ethylene and propylene (EPR) and PP were observed, decreasing with depth (44). 

Figure 7.46. In-phase (solid) and quadrature (dashed) potential-modulated ATR-SEIRA spectra of 4-mercaptopyridine (PySH) SAM on 20-nm-thick (80-nm-size particles) Au evaporated electrode in 0.1 M HCIO4. Modulation frequencies are shown. Amplitude of potential modulation was 400 mV, between -0.1 and 0.3 V (SCE). Spectra were recorded using Bio-Rad FTS 60A/896 FTiR spectrometer equipped with dc-coupled MCT detector and bandpass optical filter transmitting between 4000 and 1000 cm. Spectrometer was operated in step-scanning mode using setup shown in Fig. 4.56. Reprinted, by permission, from K. Ataka, Y. Hara, and M. Osawa, J. Electroanal. Chem. 473, 34 (1999), p. 39, Fig. 6. Copyright 1999 Elsevier Science S.A.

Jurdana LE, Ghiggino KP, Leaver IH, and Cole-Clarke P (1995) Application of FT-IR step-scan photoacoustic phase modulation methods to keratin fibres. Applied Spectroscopy 49 361-366. 

The polarization measurements in the mid-infrared (IR) and near-infrared (NIR) were performed on a Bruker IFS 88 FTIR/FTNIR spectrometer utilizing a wire-grid polarizer on KRS5-substrate which could be rotated pneumatically parallel or perpendicular to the selected reference direction. For the experiments in the step-scan mode a mercury-cadmium-telluride (MCT) detector with a DC-coupled preamplifier was used. This allowed an absolute intensity at each mirror position to be recorded and the use of phase-modulation-demodulation techniques, which are often applied for the step-scan mode [28-30] to be avoided. Further specific instrumental details of the individual applications are given in the corresponding sections. 

In Chapter 19 we saw that kinetic processes cannot be studied with a conventional rapid-scanning interferometer when the reaction rate is so fast that the reaction is essentially complete by the time just one or two interferograms have been measured. Instead, very fast reactions must be repeated at each retardation step of a step-scan interferometer. A different but related approach to the measurement of reversible dynamic systems can also be made with step-scan interferometers. In this case, however, very small changes in the state of the sample are introduced by subjecting it to a modulated perturbation of some type. Dynamic information can be obtained when the phase of the induced signal lags behind the phase of the perturbation by several degrees. An example of a reversible modulated process is when a polymer film is subjected to a modulated uniaxial strain, and this measurement is discussed in some detail in the first four sections of this chapter. 

In Chapter 20 we saw how photoacoustic (PA) spectra could be measured with a step-scan interferometer no matter whether the PA signal was demodulated with a lock-in amplifier or by digital signal processing (DSP). For DSP, a Fourier transform (FT) has the same function as the lock-in amplifier. Manning et al. [14] showed that the same approach is feasible in DIRLD spectrometry with a step-scan FT-IR spectrometer but without a PEM. Consider the case where the detector signal contains components caused by simultaneous sinusoidal phase modulation at frequency /pm, and sample modulation at frequency fs. The phase- and sample-modulated components of the signal can be demodulated either with a... 

Step-scanning interferometers can enhance the photoacoustic technique, because the variable modulation frequency of the scanning interferometer is decoupled. Thus, a constant modulation frequency is applied to all wavelengths in the spectral range. One can modulate the IR beam via phase modulation in which an interferometer mirror dithers to oscillate the retardation about the set point of each interferometer step. With this approach, all of the frequencies are modulated synchronously, and lock-in modulation can be used to extract the phase-modulation [53]. When phase-... 

With amplitude modulation the cosine and sine components may be handled in two ways to achieve quadrature detection in fl. They may be acquired in subsequent scans by either incrementing the pulse or receiver phase and the data co-added in the computer memory or they may be acquired sequentially and stored separately. With the first approach direct Fourier transformation yields frequency discrimination in fl but no absorptive lineshapes whilst with the second approach additional processing steps are necessary to achieve both, frequency discrimination and absorptive lineshapes. 

In this magnitude COSY experiment it is possible to obtain sine and cosine modulated data with co-addition using a simple two step phase cycle and consequently the minimum number of scans per increment is 2 or for long acquisitions a multiple of 2. The simplicity of this pulse program is in contrast to the phase sensitive COSY experiment and other Bruker two dimension pulse programs which utilize phase increment commands such as ipl to achieve frequency discrimination in fl. 

The next step is to separate the second harmonic component from the often much larger first harmonic component. This is done with a phase-sensitive AC amplifier (lock-in amplifier) which is synchronized with the scan and tuned to the second harmonic frequency (i.e., twice the modulation frequency). 

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