Mid-infrared spectrometry

P. Melling and M. Thomson, Fiber optic probes for mid-infrared spectrometry, (John WUey Sons Ltd. 2002). 

Grills, D. C. George, M. W. Fast and Ultrafast Time-resolved Mid-infrared Spectrometry using Lasers. In Handbook of Vibrational Spectroscopy, Chalmers, J. M., Griffiths, P. R., Eds. Wiley Chichester, UK, 2002 Vol. 1, pp... 

Quantification of microbial PHA using GC method is rapid, sensitive, reproducible, and requires only small amount of samples (5-10 mg) for the analysis. Other techniques of analysis such as IR spectrometry at 5.75 A (Juttner et al. 1975), two-dimensional fluorescence spectroscopy, flow cytometry (Degelau et al. 1995) HPLC (Karr et al. 1983), ionic chromatography, and enzymatic determination (Hesselmann et al. 1999) were also desalbed. For online determination of PHA content in recombinant E. coli system, Fourier transform mid-infrared spectrometry (FTIR) and microcalorimetric technique (Ruan et al. 2007 Jarute et al. 2004) were also reported. For precise composition determination and structural elucidation of PHA, a variety of nuclear magnetic resonance (NMR) spectroscopy techniques have also been applied and the most commonly used are proton ( H) and carbon-13 ( C) NMR (Doi et al. 1986 Jacob et al. 1986). 

Lendl, B. and Mizaikoff, B. (2006) Optical Fibers for Mid-Infrared Spectrometry. Handbook of Vibrational Spectroscopy,... 

Ultraviolet (UV)-visible spectrometry, near-infrared spectrometry, mid-infrared spectrometry, and Raman spectrometry have all been used for online process analysis (including inline and noninvasive applications). 

Mid-infrared spectrometry Mid-infrared spectrometry (MIR) has disadvantages that have hindered the widespread acceptance of the technique for online analysis. The wavelength of light used in mid-infrared spectrometry is highly attenuated by silica optical fibers. This limits the length of optical fiber that can be used for distancing the instrument from... 

Installing additional accessories in front of the ion source can render analytes amenable to ionization and subsequent mass spectrometric analysis. On-line sample treatment is especially important when analyzing liquid-phase, complex, and/or concentrated samples. For example a thermal vaporizer was used to enable analysis of liquid samples by a process mass spectrometer designed for gas analysis [196], This system has been successfully implemented in the monitoring of an esterification reaction [197]. The obtained data were in a good agreement with those recorded by in-line mid-infrared spectrometry. The setup incorporated a magnetic sector analyzer with two detectors an electron multiplier detector... 

Owen, A.W., McAulay, E.A., Nordon, A., Littlejohn, D., Lynch, T.P., Lancaster, J.S., Wright, R.G. (2014) Monitoring of an Esterification Reaction by On-line Direct Liquid Sampling Mass Spectrometry and In-line Mid Infrared Spectrometry with an Attenuated Total Reflectance Probe. Anal. Chim. Acta 849 12-18. 

Similar situations arise when measurements are made in the infrared region. In mid-infrared spectrometry, the infrared spectrum maps the internal vibrational frequencies... 

Ishiguro, T., T. Ono, K. Nakasato, C. Tsukamoto, and S. Shimada. 2003. Rapid measurement of phytate in raw soymilk by mid-infrared spectrometry. Biosci. Biotechnol. Biochem. 67 752-757. 

For mid-infrared spectrometry, this thickness is about 10 pm. As the center wavelength of mid-infrared spectra is about 5 pm, the sample thickness is approximately equal to the smallest dimension in the x-y plane that can be observed, that is, the diffraction limit when the NA is approximately equal to 0.6. For conventional NIR spectrometry (1200 - 2450 nm), on the other hand, the absorptivities of the stronger bands are in an order of magnitude less than the stronger fundamentals from which they are derived and so the sample thickness should be at least 100 pm. However, the diffraction-limited spatial resolution for NIR measurements is less than 3 pm if optics with an NA of 0.6 are used. Thus, even though the ultimate spatial resolution is in principle determined by the optics of the spectrometer, in practice this resolution is never achievable because the thickness of the sample means that the diameter of the beam waist at the top or bottom surface of the sample is larger than the diffraction-limited spatial resolution. 

Figure 6.1. Spectral energy density of blackbody sources at 1500, 2000, 2500, and 3000°C. The temperature of many sources used for mid-infrared spectrometry is 1500°C, while the temperature of many near-infrared sources is 3000°C.

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. 

Most samples to be investigated by mid-infrared spectrometry, except polymer films and KBr pellets, must be held between two windows. Glass cannot be used as a window material in the mid-infrared, and salt plates are used instead. The two most important properties of the materials that are used for infrared spectrometry are the cutoff and the refractive index these parameters are listed in Table 11.1. 

The chalcogenides are all insoluble in water and other common solvents. ZnSe and CdTe have excellent transmission characteristics. The only problem with these materials is their high refractive index, which leads to high front-surface reflectance (see Section 13.2.2), so that transmission spectra of liquids held in cells fabricated from these materials often give rise to interference fringes (see Section 11.1.3). These materials aU make excellent internal reflection elements. AMTIR (amorphous material that transmits infrared radiation) is a mixture of several chalcogenides. Many optical fibers used for mid-infrared spectrometry are made from this material (see Section 15.4). 

Jackson, R.S. (2002) Continuous scanning interferometer for mid-infrared spectrometry, In Handbook of Vibrational Spectroscopy, Vol. 1 (eds J.M. Chalmers and P.R. Griffiths), John Wiley Sons, Ltd, Chichester, pp. 264-282. 

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