FTIR instrumentation

The primary advantage of multiplexing the spectral resolution elements in the IR is the improvement in the SNR. The improvement arises because the detector irradiance for each measurement is increased. Because the detector noise is constant and independent of the source, the SNR is improved because the multiplexing distributes the detector noise over the intensities of many spectral resolution elements. This distribution process lowers the noise in each spectral resolution element more so than if the spectral resolution element is measured individually by step scanning. 

Mid-IR measurements are typically performed using a Unear scan interferometer and appropriate sampling accessory to guide the light to and from the sample of interest. 

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Vibrational Spectroscopy. Infrared absorption spectra may be obtained using convention IR or FTIR instrumentation the catalyst may be present as a compressed disk, allowing transmission spectroscopy. If the surface area is high, there can be enough chemisorbed species for their spectra to be recorded. This approach is widely used to follow actual catalyzed reactions see, for example. Refs. 26 (metal oxide catalysts) and 27 (zeolitic catalysts). Diffuse reflectance infrared reflection spectroscopy (DRIFT S) may be used on films [e.g.. Ref. 28—Si02 films on Mo(llO)]. Laser Raman spectroscopy (e.g.. Refs. 29, 30) and infrared emission spectroscopy may give greater detail [31]. 

Equipment technology and processing software for FTIR are very robust and provide a high degree of reliability. Concerns arise for only the most demanding applications. For quantitative work on an isolated feature in the spectrum, the rule of thumb is that the spectrometer resolution be one-tenth the width of the band. FTIR instruments routinely meet that requirement for solids. 

In particular, using FTIR instrumentation instead of its dispersive equivalent the user benefits are ... 

FTIR instrumentation is mature. A typical routine mid-IR spectrometer has KBr optics, best resolution of around 1cm-1, and a room temperature DTGS detector. Noise levels below 0.1 % T peak-to-peak can be achieved in a few seconds. The sample compartment will accommodate a variety of sampling accessories such as those for ATR (attenuated total reflection) and diffuse reflection. At present, IR spectra can be obtained with fast and very fast FTIR interferometers with microscopes, in reflection and microreflection, in diffusion, at very low or very high temperatures, in dilute solutions, etc. Hyphenated IR techniques such as PyFTIR, TG-FTIR, GC-FTIR, HPLC-FTIR and SEC-FTIR (Chapter 7) can simplify many problems and streamline the selection process by doing multiple analyses with one sampling. Solvent absorbance limits flow-through IR spectroscopy cells so as to make them impractical for polymer analysis. Advanced FTIR... 

In chromatography-FTIR applications, in most instances, IR spectroscopy alone cannot provide unequivocal mixture-component identification. For this reason, chromatography-FTIR results are often combined with retention indices or mass-spectral analysis to improve structure assignments. In GC-FTIR instrumentation the capillary column terminates directly at the light-pipe entrance, and the flow is returned to the GC oven to allow in-line detection by FID or MS. Recently, a multihyphenated system consisting of a GC, combined with a cryostatic interfaced FT1R spectrometer and FID detector, and a mass spectrometer, has been described [197]. Obviously, GC-FTIR-MS is a versatile complex mixture analysis technique that can provide unequivocal and unambiguous compound identification [198,199]. Actually, on-line GC-IR, with... 

The ATR technique is a commonly used infrared internal reflection sampling technique. It samples only the surface layer in contact with the ATR element the sampling depth probed is typically of the order of 0.3-3 pm [1]. Unless software corrected, compared with a transmission spectrum, the relative intensity of bands within an ATR spectrum increase in intensity with decreasing wavenumber. Several FTIR instrument companies now supply "ATR-correction" software developed to correct for the different relative intensities of bands observed between ATR and transmission spectra, so that ATR spectra can be more easily compared to and searched against transmission spectra. 

If we consider FTIR instrumentation then the situation is trickier, since the equivalent resolution in nm varies across the spectrum. But even keeping the spectrum in its natural wavenumber units, we again find that, except for rotational fine structure of gases, the natural bandwidth of many (most) absorbance bands is greater than 10 wavenumbers. So again, using that figure shows the typical user how he can expect his own measured spectra to behave. 

FTIR instruments offer advantages in speed and higher signal-to-noise ratios compared to dispersive IR spectrometers. These advantages combined with the facility for extensive data processing have seen the FTIR technique find increasing applications in polymer studies (11). 

Figure 6. Schematic diagram of an interferometer as used in FTIR instruments.

FIGURE 8.15 An illustration of an FTIR instrument showing the light source, the interferometer, the sample compartment, and detector. 

Dave Segura prepares to use an FTIR instrument for simethicone analysis. Notice the liquid sampling cell in Dave s left hand. 

Obtain a liquid sampling cell from your instructor and fill it with the 20% solution. Place this cell in the FTIR instrument, and obtain the transmittance spectrum according to the instructions specific to your instrument. Record the percent transmittance value at the tip of the 817 cm-1 peak and at the baseline. Repeat with the other three standards, the unknown, and the control sample. 

What is an interferometer and what is its function in an FTIR instrument ... 

NMR spectrum. Fourier transform nuclear magnetic resonance (FTNMR) instruments, which are similar in principle to Fourier transform infrared spectrometry (FTIR) instruments, are popular today. We will briefly describe these instruments later in this section. 

Infrared spectra were taken with a Michelson-Genzel type FTIR instrument (IR/98, IBM Instruments, Inc.) equipped with a liquid nitrogen cooled HgCdTe detector. The optical attachment arrangement is shown in Fig. 2-14. The infrared radiation was p-polarized using ERS-5 wire grid polarizer. 

Practical FTIR solutions have been developed by paying attention to the fundamental design of the instrument. Moving an FTIR instrument out of the benign enviromnent of a laboratory to the more alien environment of either a process line or that of a portable device is not straightforward. A major emphasis on the instrument design in terms of both ruggedness and fundamental reliability of components is critical. Furthermore, issues such as enviromnental contamination, humidity, vibration and temperature are factors... 

An FTIR instrument The three critical components (excluding the sample) are the source, the detector and the interferometer. In terms of enabling technology it is the interferometer that is critical to the measurement. 

As noted, IR instruments fall into several categories that range from simple photometers (single or multiple filter devices) to relatively complex full-spectrum devices, such as FTIR instruments. Today s process engineers often prefer optical methods of measurement rather than the traditional chromatographs because of perceived lower maintenance and ease of implementation. However, the final selection is often based on the overall economics and the practicality of the application. 

Terahertz, or far infrared spectroscopy, covers the frequency range from 0.1 to lOTHz (300 to 3cm ) where torsional modes and lattice vibrations of molecules are detected. It is increasing in use in many application areas, including analysis of crystalline materials. Several dedicated conunercial instruments are available which use pulsed terahertz radiation which results in better signal to noise than those using blackbody sources for radiation (and associated with the terminology far infrared spectroscopy). Work using extended optics of FTIR instrumentation as weU as continuous-wave source THz has also been recently reported. ... 

The basic layout of a simple dispersive IR spectrometer is the same as for an UV spectrometer (Figure 2.1), except that all components must now match the different energy range of electromagnetic radiation. The more sophisticated Fourier Transform Infrared (FTIR) instruments record an infrared interference pattern generated by a moving mirror and this is transformed by a computer into an infrared spectrum. 

Optical and FTIR instrumentation. Absorption spectra were taken on three instruments. A Hewlett Packard 8450 UV/Vis Spectrophometer was used for the visible region of the spectrum. This instrument was computer control led and used to measure standard absorption spectra as well as absorption difference spectra. A Cary 14 Spectrophotometer was used for the near IR region, and an IBM IR/90 Series... 

ATR-IR spectroscopy can be used as a spy inside a reactor for on-line monitoring and control of a reaction. The emphasis in this kind of application of ATR spectroscopy is on the detection of reactants and products in the bulk fluid phase. Such applications benefit from the excellent time resolution of FTIR instruments compared to other analytical tools, such as chromatographs. The method can be used in investigations of kinetics of reactions in batch reactors instrumentation has been developed and even commercialized that allows measurements at elevated temperatures and pressures. 

Figure 10.13—Computer controlled IR spectrometer. Many models of FTIR instruments permit the adaptation of an infrared microscope for the study of microsamples model NEXUS (11>W), reproduced by permission of Nicolet).

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