Infrared spectrometer optical systems

Because the cast films are relatively thin, the optical density of the light absorbing species can he low and can vary with time of exposure. Additionally, the depth penetration of the absorbed light can be inhomogeneous in some systems. However, thin films can be mounted directly in UV/visible or infrared spectrometers, and so the course of the photopolymerization (and the rate) can be monitored directly in some systems. The most common observation made is the disappearance of monomer (e.g., loss of double bond absorption in the IR) as a function of irradiation time. It must be emphasized that in most thin film compositions important industrially, the monomers used are multifunctional. The polymer which results is then highly cross-linked and simple kinetic arguments are usually not valid. 

Modern NIR equipment is generally robust and precise and can be operated easily by unskilled personnel [51]. Commercial instruments which have been used for bioprocess analyses include the Nicolet 740 Fourier transform infrared spectrometer [52, 53] and NIRSystems, Inc. Biotech System [54, 55]. Off-line bioprocess analysis most often involves manually placing the sample in a cuvette with optical pathlengths of 0.5 mm to 2.0 mm, although automatic sampling and transport to the spectrometer by means of tubing pump has been used (Yano and Harata, 1994). A number of different spectral acquisition methods have been successfully applied, including reflectance [55], absorbance [56], and diffuse transmittance [51]. 

Fig. 7.1. Layout of the infrared spectrometer showing the Michelson Interferometer Optical System. An FTIR spectrometer s optical system requires two mirrors, an infrared light source, an infrared detector and a beamsplitter. The beamsplitter reflects about 50% of an incident light beam and transmits the remaining 50%. One part of this split light beam travels to a moving interferometer mirror, while the other part travels to the interferometer s stationary mirror. Both beams are reflected back to the beamsplitter where they recombine. Half of the recombined light is transmitted to the detector and half is reflected to the infrared source.

A typical IR spectrometer consists of the following components radiation source, sampling area, monochromator (in a dispersive instrument), an interference filter or interferometer (in a non-dispersive instrument), a detector, and a recorder or data-handling system. The instrumentation requirements for the mid-infrared, the far-infrared, and the near-infrared regions are different. Most commercial dispersive infrared spectrometers are designed to operate in the mid-infrared region (4000-400 cm ). An FTIR spectrometer with proper radiation sources and detectors can cover the entire IR region. In this section, the types of radiation sources, optical systems, and detectors used in the IR spectrometer are discussed. 

Modern monochromators consist of a rift system, the optics and the infrared radiation splitting system, which is usually presented by prism or diffraction grid. The following two types of monochromators are most popular in modern infrared spectrometers ... 

In principle, Raman spectroscopy is a microtechnique [161) since, for a given light flux of a laser source, the flux of Raman radiation is inversely proportional to the diameter of the laser-beam focus at the sample, i.e., an optimized Raman sample is a microsample. However, Raman microspectroscopy able to obtain spatially resolved vibrational spectra to ca. 1 pm spatial resolution and using a conventional optical microscope system has only recently been more widely appreciated. For Raman microspectroscopy both conventional [162] and FT-Raman spectrometers [ 163], [ 164] are employed, the latter being coupled by near-infrared fiber optics to the microscope. 

Wiedemann, G., Jennings, D. E., Hand, R. A., Kunde, V. G., Mosley, S. H., Lamb, G., Petroff, M. D., Stapelbroek, M. G. (1989). Postdispersion system for astronomical observations with Fourier transform spectrometers in the thermal infrared. Applied Optics, 28, 139-45. 

In the mid-IR, routine infrared spectroscopy nowadays almost exclusively uses Fourier-transform (FT) spectrometers. This principle is a standard method in modem analytical chemistry45. Although some efforts have been made to design ultra-compact FT-IR spectrometers for use under real-world conditions, standard systems are still too bulky for many applications. A new approach is the use of micro-fabrication techniques. As an example for this technology, a miniature single-pass Fourier transform spectrometer integrated on a 10 x 5 cm optical bench has been demonstrated to be feasible. Based upon a classical Michelson interferometer design, all... 

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