IR dispersive spectrometer

Experimental limitations initially limited the types of molecular systems that could be studied by TRIR spectroscopy. The main obstacles were the lack of readily tunable intense IR sources and sensitive fast IR detectors. Early TRIR work focused on gas phase studies because long pathlengths and/or multipass cells could be used without interference from solvent IR bands. Pimentel and co-workers first developed a rapid scan dispersive IR spectrometer (using a carbon arc broadband IR source) with time and spectral resolution on the order of 10 ps and 1 cm , respectively, and reported the gas phase IR spectra of a number of fundamental organic intermediates (e.g., CH3, CD3, and Cp2). Subsequent gas phase approaches with improved time and spectral resolution took advantage of pulsed IR sources. 

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). 

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. 

A group in Latvia and Russia [78] analysed microbial cell mass by a fairly simple classical infrared spectroscopic method, recording spectra of dried biomass embedded in potassium bromide pellets with a dispersive IR spectrometer. They could calibrate the evaluation for four major groups of cellular... 

In the years to follow the key to the measurement of vibrational circular dichroism was the development of photoelastic modulators suitable for work in the infrared spectral region. The first successful measurements of circular dichroism originating from vibrational transitions in the infrared were done by Hsu and Holzwarth (1973) on thin slices of monocrystalline a-NiS04 6 H2O and a-ZnSe04 6 H2O. For this measurements the authors used a normal dispersive IR spectrometer supplemented by a linear polarizer and a photoelastic modulator made from Germanium. 

Dispersive IR spectrometers can be either single-beam or double-beam usually they are double-beam instruments. The use of a double-beam instrument significantly reduces the problem of possible interference from carbon dioxide and water vapor in the atmosphere during analysis. The absorption due to a solvent may also be nearly cancelled by placing an equivalent pathlength solvent cell in the reference beam. 

Figure 1 Data path of a ratio-recording, dispersive IR spectrometer (Reproduced by permission from ref. 1)...

What is/are the advantage(s) of using an FTIR instead of dispersive IR spectrometer ... 

Dispersive Infrared Spectroscopy The dispersive IR spectrometer generally incorporates an IR broadband source, sample cell, a diffraction grating and one or more IR detectors. Dispersive IR instruments may provide simultaneous or sequential measurements. Respectively, the instrument may have a fixed grating and many detectors, or a movable grating and a single detector. In some cases, the grating may be replaced by one or more optical filters to resolve the desired wavelengths. A reference cell and associated optics to perform simultaneous differential analysis are also incorporated to improve sensitivity or reliability of measurement. 

Real-time, autonomous, fluid monitoring eliminates the need for sampling and remote laboratory analysis, thus providing timely data at reduced costs. IR sensors are already in service by the US military [23]. The sensors shown in Fig. 16.5 are a ferrous/non-ferrous metal sensor, a miniaturized dispersive IR spectrometer... 

The development of Fourier transform infrared (FTIR) spectroscopy during the past two decades has revitalised the field of IR spectroscopy. This is due not only to the superior performance of FTIR spectrometers by comparison to that of the dispersive IR spectrometers that they have virtually replaced, but also to a number of other factors. First, the development of FTIR... 

Figure 3 Schematic diagram of a double-beam dispersive IR spectrometer.

FTIR spectrometers have largely replaced dispersive IR spectrometers. 

The technique developed by Bewick for use with a dispersive IR spectrometer is termed electrically modulated infrared spectroscopy (EMIRS) and is essentially a direct development of the UV-VIS specular reflectance technique [50], modulated specular reflectance spectroscopy (MSRS). As in MSRS, radiation is specularly reflected form a polished electrode surface while the electrode potential is modulated with a square wave between a base potential and the working potential at which the process of interest occurs the wavelength range of interest is then slowly scanned. Only that... 

In the recirculation system concentrations were monitored by means of non-dispersive IR spectrometers (CO Ultramat, Siemens CO2 Uras 2T, Hartmann Braun) and a magnetic device (O2 Magnos, Hartmami Braun). 

Why are nondisperstve IR instruments often used for the delerntination of gases rather than dispersive IR spectrometers ... 

The major problem with the use of NaCl, KBr, and similar ionic salts is that they are very soluble in water. Any moisture, even atmospheric moisture, can dissolve the surface of a polished salt crystal, causing the material to become opaque and scatter light. Optics and sample containers made of salts must be kept desiccated. This hmitation is one of the reasons salt prisms are no longer used in dispersive IR spectrometers. 

An important variant is the FTIR spectrometer. The main advantages of FTIR are that it is much quicker because it measures all the frequencies simultaneously and it is more sensitive than dispersive IR spectrometers. 

---