Infrared spectrometer transmittance

All infrared spectrometers generate data that are contained in the infrared spectrum (see Fig. 10.1). The spectrum represents the ratio of transmitted intensities with and without sample at each wavelength. This intensity ratio is called transmittance (7 ) can be replaced by percent transmission (%7 ) or by absorbance A = log(l/T). If the experiment is conducted using reflected or diffuse light, pseudo-absorbance units are used (cf. 10.10.2). Finally, it is common to report wavelengths in terms of wave number v (cm-1 or kaysers) knowing that ... 

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]. 

Block diagram of a dispersive infrared spectrometer. The sample beam passes through the sample cell while the reference beam passes through a reference cell that contains only the solvent. A rotating mirror alternately allows light from each of the two beams to enter the monochromator where they are compared. The chart recorder graphs the difference in light transmittance between the two beams. 

The basic instrumentation used for spectrometric measurements has already been described in Chapter 7 (p. 277). The natures of sources, monochromators, detectors, and sample cells required for molecular absorption techniques are summarized in Table 9.1. The principal difference between instrumentation for atomic emission and molecular absorption spectrometry is in the need for a separate source of radiation for the latter. In the infrared, visible and ultraviolet regions, white sources are used, i.e. the energy or frequency range of the source covers most or all of the relevant portion of the spectrum. In contrast, nuclear magnetic resonance spectrometers employ a narrow waveband radio-frequency transmitter, a tuned detector and no monochromator. 

Infrared microscopy combines an optical microscope with an FT-IR spectrometer enabling pico- to femtogram (10 12—10 15 g) quantities of substances to be characterized or very small areas of larger samples to be analysed. Beam-condensing optics focus the radiation onto an area of the sample identified using the optical microscope and either reflectance or transmittance spectra can be recorded. The highly-sensitive MCT detector (p. 283) is normally used as its size can be matched to that of the radiation beam to maximize its response. 

Only in recent years has the use of water as an infrared solvent become fairly routine in the biochemical laboratory. However, Coblentz (1905) had used water as an infrared solvent as early as 1905, and Gore et al. (1949) had studied aqueous solutions of several amino acids in 1949. Blout has published many infrared spectra of biochemical polymers in water and D2O solution, examples of which can be found in Blout and Lenormant (1953) and Blout (1957). Figure 3.9 (Blout, 1957) shows absorption spectra of water and D2O (with and without compensation) of 0.025 mm thickness in the region 4000-600 cm It can be seen that D2O transmits where water absorbs and vice versa, thus making the combination of these solvents useful for examining aqueous solutions. The O—H deformation modes of water are present between 1700 and 1600 cm" and the O—D deformation of D2O lies at 12(X) cm" Except for these regions the spectra show better than 40% transmittance and satisfactory compensation is readily obtained in a double-beam spectrometer. The optimum concentration of a solute is from 5 to 20 %. Two percent solutions have been used (Blout, 1957) and even lower concentrations are possible with a suitable solute, for example, 0.45 % phenol in water (Parker and Kirschenbaum, 1959). 

The method of diffuse transmittance (DT) is based on measurement of the radiation component 7dt (Fig. 1.22) that passes diffusely through an inhomogeneous layer. This method was first applied to the IR spectroscopic analysis of thin films on samples in powder form by Tolstoy in 1985 [116, 117], who obtained DT spectra of water adsorbed onto silica gel. When used in conjunction with a FTIR spectrometer, the method is called diffiise-transmittance infrared Fourier transform spectroscopy (DTIFTS). DTIFTS is the most recently developed IR spectroscopic methods for studying powder surfaces and has already found application in high-performance liquid chromatography (HPLC) and thin-layer chromatography (TLC) [118, 119]. Of increasing popularity are DTIFTS measurements of powders that use an IR microscope to collect radiation [112, 119] (Section 4.3). 

The excellent transmittance of quartz in the near-infrared range has led to a further enhancement of the potential of near-infrared spectroscopy by the introduction of fiber optics. Fiber-optic waveguides are used to transfer light from the spectrometer to the sample and. after transmission or reflection, back to the spectrometer. Most fiber optic cables consist of three concentric compo-... 

A Fourier transform infrared spectroscopy (FTIR) study was obtained by using Perkin-Elmer spectrometer 100, USA. Prior to this analysis, kenaf whiskers were mixed with KBr to prepare homogeneous suspensions and afterwards pressed into transparent pellets and analyzed in transmittance mode within the range of 4000-500 cm. In the case of thin nanocomposite film (Cellulose Acetate Butyrate [CAB] and kenaf whiskers) the analysis was done within the range of 4000-500 cm transmittance mode. 

Most of the errors discussed in this chapter are relatively small. Provided that mid-infrared spectra are measured with an instrument that is equipped with a DTGS or DLATGS detector and care is taken to ensure that the peak absorbance of bands in the spectral region of interest is not excessive, the photometric accuracy of contemporary FT-IR spectrometers is remarkably high. The transmittance scale of these instruments should be accurate to better than 0.001 ( 0.1 %T). 

Analysis for the purpose of accurately determining the quantity of a chemical species existing in a sample is called quantitative analysis. Quantitative infrared spectroscopic analysis mainly deals with the intensity of an infrared absorption band. In this chapter, basic aspects of quantitative spectroscopic infrared analysis for a target substance (the analyte) in solution samples are described. The subjects to be described include the characteristics of a Fourier transform infrared (FT-IR) spectrometer, the relation between percentage transmittance and absorbance, Lambert-Beer s law on the relationship between the intensity of an infrared band and the concentration of a sample, the use of a working curve in quantitative analysis, and the origins of deviations from Lambert-Beer s law. 

The infrared reflection/absorption spectrum of the coatings was obtained using a Perkin-Elmer FT-IR Spectrum 100 spectrometer. The coating spectrum was ratioed against the spectrum of the substrate background. A pure fluoro-organic compound spectrum was obtained on insertion in the KBr pellets in the transmittance mode. 

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