Infrared spectrometer components

Another way to improve the analysis of complex matrices can be the combination of a multidimensional system with information-rich spectral detection (31). The analysis of eucalyptus and cascarilla bark essential oils has been carried out with an MDGC instrument, coupling a fast second chromatograph with a matrix isolation infrared spectrometer. Eluents from the first column were heart-cut and transferred to a cryogenically cooled trap. The trap is then heated to re-inject the components into an analytical column of different selectivity for separation and subsequent detection. The problem of the mismatch between the speed of fast separation and the... 

Effluent gas emerging from a gas chromatograph at atmospheric pressure can be led directly into a heated infrared gas cell via a heated transfer line. Vapour-phase infrared spectra of eluting components can be recorded as they pass through a cell by a Fourier transform (FT) infrared spectrometer enabling a full-range spectrum to be collected and stored in a second or less. 

We discussed the fundamentals of mass spectrometry in Chapter 10 and infrared spectrometry in Chapter 8. The quadrupole mass spectrometer and the Fourier transform infrared spectrometer have been adapted to and used with GC equipment as detectors with great success. Gas chromatography-mass spectrometry (GC-MS) and gas chromatography-infrared spectrometry (GC-IR) are very powerful tools for qualitative analysis in GC because not only do they give retention time information, but, due to their inherent speed, they are also able to measure and record the mass spectrum or infrared (IR) spectrum of the individual sample components as they elute from the GC column. It is like taking a photograph of each component as it elutes. See Figure 12.14. Coupled with the computer banks of mass and IR spectra, a component s identity is an easy chore for such a detector. It seems the only real... 

The data manipulating capability of a computerized infrared spectrometer allows the spectroscopist to delve more deeply into the structural origin of the infrared absorptions by using data processing techniques to purify, manipulate, and correlate the spectra. If one can systematically vary the relative amounts of various structural contributions, absorbance subtraction can be used to isolate the spectral contributions of the structural components. 

Fig. 6.5. Computed structures due to the hydrogen dimer, in the quadrupole-induced (0223,2023) components near the So(0) line center at 120 K (the temperature of Jupiter s upper atmosphere). Superimposed with the smooth free — free continuum (dashes) are structures arising from bound — free (below 354 cm-1) and free - bound (above 354 cm-1) transitions of the hydrogen pair (dotted). The convolution of the spectrum with a 4.3 cm-1 slit function (approximating the instrumental profile of the Voyager infrared spectrometer) is also shown (heavy line) [282].

A mid-infrared absorption instrument generally consists of a Fourier transform design with the same basic components as noted above for the Fourier transform near-infrared spectrometers (broadband light source, Michelson interferometer, and detector optimized for the mid-infrared spectral region.)... 

Principal component analysis is most easily explained by showing its application on a familiar type of data. In this chapter we show the application of PCA to chromatographic-spectroscopic data. These data sets are the kind produced by so-called hyphenated methods such as gas chromatography (GC) or high-performance liquid chromatography (HPLC) coupled to a multivariate detector such as a mass spectrometer (MS), Fourier transform infrared spectrometer (FTIR), or UV/visible spectrometer. Examples of some common hyphenated methods include GC-MS, GC-FTIR, HPLC-UV/Vis, and HLPC-MS. In all these types of data sets, a response in one dimension (e.g., chromatographic separation) modulates the response of a detector (e.g., a spectrum) in a second dimension. 

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. 

An infrared spectrometer essentially consists of a source of continuous infrared radiation, a means for resolving the infrared radiation into its component wavelengths, and a detector. The procedure that is involved in recording the IR spectrum of a sample can be represented mathematically by the following equation ... 

The next component part in the infrared spectrometer is the detector. The most important types of detectors used in infrared spectroscopy are the thermal detectors. In this type of detector, radiation energy is first absorbed and then converted into heat energy. The actual measured value is an electrical voltage, which is produced or changed by the heating. Despite their higher sensitivity, photo electric detectors have a lower popularity due to the limits they have of the ana-lyzable wavelength area. 

The last component part of an infrared spectrometer other than the evaluating computer is the amplifier. The purpose of the amplifier is to amplify the signal coining from the detector to enable the computer to evaluate the signals. 

Most of the component parts used in Raman spectroscopy such as the monochromator and sample chamber have the same functioning principle as in infrared spectrometers. All these were described in detail in section 2.2.1. 

All the component parts used in photometers have the same working principle as those already described in other spectrometers, for example, the infrared spectrometer. The prism and refraction grids are used as monochromators. The detector is usually made of different types of photoresistors depending on the instrument type. 

Other species that may be of interest require more specialized equipment that is more difficult to use and/or interpret results. A Fourier transform infrared spectrometer (FTIR), as can be seem in Figure 33.6, can be used to detect a wide array of components. The FTIR requires that the species of interest be "IR-active." Most polar and polyatomic molecules are IR-active. The FTIR... 

Miscellaneous detectors. TLC, as with other chromatographic methods, is a separation not an identification technique and thus for unambiguous identification of analytes the separated components must be examined by spectroanalytical techniques. Mass [67] and Fourier-transform infrared spectrometry [68] have both been used to good effect and considerable effort is currently being expended to develop robust methodologies and instrumentation in these areas. Instrumentation has recently been developed, for example, which elutes separated components directly onto a measured amount of potassium bromide which is then automatically pressed and introduced into an infrared spectrometer. 

Instruments which measure and record the wavelengths at which samples emit or absorb light are called spectrometers. The basic components of a UV-visible or infrared spectrometer are shown in Fig. 20.5. The components include... 

The basic component of most Fourier Transform Infrared spectrometers is the Michel son interferometer. This is not the only interferometer used in FT-IR, but it is employed more often than other designs. A treatment of many other interferometer designs is available. The Michel son interferometer in a Fourier Transform Infrared spectrometer replaces the monochromator in a dispersive instrument, although the functions cannot be correlated. A monochomator divides a continuous bandwidth into its component frequencies, whereas an interferometer produces interference patterns of the bandwidth in a precise and regulated manner. It should be noted that this type of interferometer is not restricted to the infrared region and its use can be extended to the visible and millimeter regions of the electromagnetic spectrum. 

Thick films (1.0 pm or greater) are made possible today due to improved techniques in cross-linking liquid phases, and also to the more inert fused silica surface. Cross-linking techniques will be discussed later in this chapter. Such thick films show increased retention of sample components—essential for volatile compounds. In addition, their high capacity allows injection of larger samples this can be important when mass spectrometers or Fourier transform-infrared spectrometers are to be used for subsequent analysis. 

Infrared analysis Used to identify the chemical functional groups present in the sample and also the types of bonds in it by detecting the absorption spectra of vibrational levels of atoms or molecules. FTIR In a dispersive IR-Spectrometer, component frequencies are viewed individually with Fourier Transform IR. All frequencies are simultaneously examined in much more details. 

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