Mid-infrared FT-IR spectrometer

The first interferometer incorporating an air-bearing drive was the Block Engineering Model 296. This interferometer was the one used in the first commercial mid-infrared FT-IR spectrometer designed for laboratory use, the Digilab FTS-14 [4]. This instrument was introduced in 1969 and was followed a few years later by the Model 7199 spectrometer made by Nicolet Analytical Instruments which also featured an air-bearing drive. Both of these early FT-IR spectrometers were too large to be placed on a lab bench and the compressor was mounted inside the instrument s cabinetry. Over the next 10 or 15 years, the optics and electronics of FT-IR spectrometers became far more compact and bench-top instruments became commonplace. Since compressors are noisy and occupy too much bench space. 

In many studies, particularly those related to materials and forensic science, it is frequently necessary to measure a mid-infrared spectrum from a trace amount of a sample or a sample of small size. In some circumstances, this may be accomplished by using a beam-condenser accessory within the conventional sample compartment of a Fourier-transform infrared (FT-IR) spectrometer. Perhaps today though, it is more convenient to use infrared microspectrometry (often commonly referred to as infrared microspectroscopy or even infrared microscopy). Based on an optical microscope (or infrared microscope) coupled to an FT-IR spectrometer, it is one of the most useful methods for structural analysis of such samples and can often be undertaken in a non-destructive manner [1, 2]. 

Since the mid-1970s, most measurements of emission spectra of steady flames have used Fourier transform techniques. Figure 5 shows the emission spectrum measured from a premixed, stoichiometric CH4/O2 flame (total pressure equal to 18 torr) to which 3% CF3Br has been added as a flame suppressant. When appropriate, reduced-pressure flames are often studied because at reduced pressure the flame region is expanded, allowing more detailed study. The emission spectrum shown in Figure 5 was measured using a Fourier transform infrared (FT-IR) spectrometer at a resolution of 1 cm"k... 

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... 

IR spectroscopic measurements were performed in the mid and far infrared regions. For mid infrared measurements a Matson Genesis FT-IR spectrometer and the KBr technique (2 mg of sample in 200 mg KBr) were utilized. Far infrared measurements were run on a Bio-Rad-Win-IR FT-IR spectrometer in the 500-100 cm range. In this case HDPE (high density polyethylene) was the matrix material and 128 scans were collected for... 

As with dispersive instmments, FT-IR spectrometers use a Nernst or Globar source, for the mid-infrared region. If the far-infrared region is to be examined, then a high-pressure mercury arc lamp can be used. For the near-infrared region, tungsten lamps are used as sources. 

Infrared spectra (i.r.) Infrared spectra of the materials II and III were obtained with a BROKER IFS48 FT-IR spectrometer purged with nitrogen gas and using a mid band mercury cadmium telluride (MCI) detector. A HARRICK Praying Mantis diffuse reflectance attachment with two ellipsoidal mirrors collected the diffuse reflectance spectra. To avoid residual radiation bands all samples were diluted with KBr powder (Uvasol quality, E. Merck) so that the sample concentration was about 10 %. 

The type of detector used in an FT-IR spectrometer is highly dependent upon the bandwidth (i.e. the spectral frequencies), the modulation rate of the interferometer, and the intensity of the radiant flux. Several types of detectors are used in the infrared regions photoconductive, photovoltaic, bolometers, pyroelectric and Golay cells. A detailed discussion of detectors may be found elsewhere.12 In general, the photovoltaic and photoconductive detectors can be used in the near- and mid-infrared regions as rapid response, high sensitivity detectors. Usually the bandwidths are limited and will not cover the total ran passed by the beamsplitter. Examples of such detectors are given in Table I. As can be seen from the... 

In the analytical laboratory, FT-IR spectrometers are now dominant, if not the almost exclusive mid-infrared tool. Many are also interfaced with FT-IR microscopes that enable high signal-to-noise ratio spectra to be recorded routinely from regions of samples with diameters as low as 20 pm, in some cases 10 pm. There is now a much greater emphasis on sampling techniques appropriate to biological and biomedical samples. 

Figure 2.9 Plot demonstrating the small spot size that can be achieved using synchrotron-sourced mid-infrared radiation. The plot represents the integrated signal intensity from 2000-9000 cm through a 10 pm pinhole scanned on a microscope stage in an FT-IR spectrometer. Reproduced from reference [9] by kind permission of the Advanced Light Source (ALS), Berkeley Laboratory.

The most common mid-infrared source used in FT-IR spectrometers is a resis-tively heated silicon carbide rod, commercially known as a Globar. The typical... 

A few interesting sources for future FT-IR spectrometers have been reported in the past 10 years, including the synchrotron and free electron laser (FEL) [4]. Using the radiation from a synchrotron beam line, spectra of samples as small as 10 pm in diameter (the diffraction Unfit) may be measured with veiy high SNR in times as short as 1 second. Obviously the use of these sources requires the spectroscopist to travel to a synchrotron or FEL facility with a mid-infrared beam line equipped with a FT-IR nficrospectrometer. Such facilities are available in several countries and can be used at minimal cost provided that the potential user can make a good case for the measurement... 

An FT-IR spectrometer is used optimally when detector noise exceeds all other noise sources and is independent of the signal level. This is the usual case for mid-infrared spectrometry but may not be so for shorter wavelengths. The sensitivity of mid-infrared detectors is commonly expressed in terms of the noise equivalent power (NEP) of the detector, which is the ratio of the root mean square (rms) noise voltage, P , in V Hz to the voltage responsivity, R, of the detector, in V W . It is effectively a measure of the optical power that gives a signal equal to the noise level thus, the smaller the NEP, the more sensitive is the detector. The NEP is proportional to the square of the detector area, Ao, with the constant of proportionality being known as the specific detectivity, D that is. 

To achieve a SNR of 10 in the mid-infrared spectrum (Vn x = 4000 cm ), the positional error must be less than 10 cm (1 A) (i.e., about one atomic diameter ). It is a testimony to the power of laser referencing that this specification is easily met by all commercial FT-IR spectrometers. 

To measure such spectra with the same SNR, the dispersive spectrum must be measured M times longer than the FT spectrum. This can be a significant improvement. For example, for a mid-infrared spectrum measured at a resolution of 4 cm , M = 900. Assuming that it takes 15 minutes to measure this spectrum on a grating spectrometer, it would only take 1 second to measure the same spectrum on an FT-IR spectrometer. 

Combining the values of Fellgett s and Jacquinot s advantages, FT-IR spectrometers should be about 2000 times more sensitive than grating spectrometers that operate in the mid-infrared. In practice, however, smaller values are found. To understand why this is the case, we must consider other components in these spectrometers. The same types of source are used in both types of instruments, so we will neglect any discussion of this component. This cannot be said of the detector, however. 

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