MIR Spectrometers

There are two types of MIR spectrometers, dispersive and Fourier-transform (FT) spectrometers. Today FT spectrometers are used predominantly. The most significant advantage of FT spectrometers is that radiation from all wavelengths is measured simultaneously, whereas in dispersive spectrometers all wavelengths are measured consecutively. Therefore, a FT spectrometer is much faster and 

Handbook of Spectroscopy, Volume 1. Edited by Gunter Gauglilz and Tuan Vo-Dinh Copyright 2003 WILEY-VCH Verlag GmbH Co. KGaA, Weinheim ISBN 3-527-297S2-0 

Information about the absorption of infrared radiation in the sample is obtained by measuring the intensity ratio of the radiation before and after the sample. In order to obtain this ratio with sufficient accuracy, infrared absorption spectrometers should be double channel instruments. 

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Chromatographic systems have one thing in common most depend on spectro-photometric detection devices, i.e., ultraviolet (UV), visible, fluorescent, and midrange infrared (MIR) spectrometers. High-performance liquid chromatography (HPLC) has been used to, in essence, purify (separate) the constituents from the matrix, then introduce them to a spectrometer for identification or quantification. One reason that spectrometers were not placed in a production setting... 

Developments in MIR spectrometers, particularly Fourier-transform (FT) techniques, have enabled the use of a variety of solid sampling techniques which overcome the disadvantages of classic IR-sampling techniques. Classic sampling techniques [30], such as alkali halide pellet preparation (with KBr or KCl) or mineral oil mull preparations, require a mechanical treatment of the sample and may thus induce solid-solid transformations or desolvations. 

The most common source in MIR spectrometers is a glowing ceramic bar, a so-called glowbar (or globar). More intense emission is provided by the Nernst glower due to its higher operation temperature (black body radiator). A thermocouple or a thermopile is commonly used as detector. The response behaviour of such detectors is slow, which prevents rapid scanning by dispersive MIR spectrometers. 

Computers (not built-in) are part of modern MIR spectrometer. The supplied software manages data acquisition and analysis. Additional software for chemometric evaluation is available. 

Using FT-MIR spectrometers equipped with ATR probes, Chatzi et al. [171], Kammona et al. [172], Hua and Dube [173], and Roberge and Dubd [174] obtained similar results for 2-ethylhexyl acrylate/styrene and VA/butyl acrylate/ MMA emulsion homo- and copolymerizations. Particularly, Hua and Dube [173] present a review about the use of FT-IR-ATR spectroscopy for kinetic studies in polymerization systems. In all cases, MLR or PLS calibration models were used for interpretation of spectral data. 

For our purpose, it is convenient to classify the measurements according to the format of the data produced. Sensors provide scalar valued quantities of the bulk fluid i. e. density p(t), refractive index n(t), viscosity dielectric constant e(t) and speed of sound Vj(t). Spectrometers provide vector valued quantities of the bulk fluid. Good examples include absorption spectra A t) associated with (1) far-, mid- and near-infrared FIR, MIR, NIR, (2) ultraviolet and visible UV-VIS, (3) nuclear magnetic resonance NMR, (4) electron paramagnetic resonance EPR, (5) vibrational circular dichroism VCD and (6) electronic circular dichroism ECD. Vector valued quantities are also obtained from fluorescence I t) and the Raman effect /(t). Some spectrometers produce matrix valued quantities M(t) of the bulk fluid. Here 2D-NMR spectra, 2D-EPR and 2D-flourescence spectra are noteworthy. A schematic representation of a very general experimental configuration is shown in Figure 4.1 where r is the recycle time for the system. 

Serious consideration must be given to the use of more than one vibrational spectrometer. The list of currently available commercial vibrational spectrometers include NIR and NIR-CD, MIR and MIR-CD, FIR, Raman and Raman Optical Activity (RAO). Diasteriomers have distinct non-circularly polarized vibrational spectra. Enantiomers are only distinguishable by CD and RAO [51, 52]. If a synthesis is performed vhere enantiomers are present, then a proper experimental design is needed to solve Eq. (2), and this necessitates the use of CD/ROA. If CD/ROA is not used, then one or more of the pure component spectra ai..asobs are lumped parameters i. e. they represent both enantiomers present. 

To perform chemical imaging of sample surfaces, FT spectrometers can be coupled with a microscope or macrochamber with an FPA detector. CIS are available for Raman, NIR, and MIR spectroscopy. Figure 15 illustrates an optical arrangement for chemical imaging. 

HWGs have successfully been applied to a wide variety of gas-sensing applications [44-52]. Micheels et al. [46] coupled a coiled MIR HWG to a FT-IR spectrometer measuring VOCs in the headspace of water samples. Yang et al. [47,48] partitioned organics from water or the headspace above a soil sample into the coating of a HWG. The waveguide was then inserted into the sample compartment of the FT-IR. 

In the NIR, and, more rarely, in the MIR, interference filters are sometimes employed for spectral dispersion. Sets of individual filters and variable circular filters are used especially in simple spectrometers for production control and environmental monitoring. 

Fig. 46. Optical diagram of the Polytec MIR 160 Fourier spectrometer (No. 5b in Tables 2, 3, 4). M 1, M 2, M 5, M 6, M 7 plane mirrors M 3, M 4 paraboloid mirrors MS spherical mirror MT toroid mirrors G Globar source S high pressure Hg-lamp L He-Ne-laser IS Interferometer scanner BS beampslitter PC photo-cell D pyroelectric detector WL white light source...

Photopolymerization of acrylic monomers and related systems is a challenging area for the application of real-time MIR spectroscopy since the reactions are often completed within seconds, so there needs to be an excellent SNR in the spectra if reliable kinetic data are to be generated. The temporal resolution of an FT-IR spectrometer at a resolution of 16 cm is 11 ms, so 100 spectra can be gathered in the time frame of the fastest reaction, but without multiplexing to improve the SNR. Using a diamond ATR element of area 4 mm that could also be heated to 200 C, uniform irradiation was possible and the cure reactions of films of thicknesses ranging from 1 to 20 pm could be followed (Scherzer and Decker, 1999). 

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