Infrared spectroscopy instrumental features

Today, a number of different instrumental techniques are used to identify organic compounds. These techniques can be performed quickly on small amounts of a compound and can provide much more information about the compound s structure than simple chemical tests can provide. We have already discussed one such technique ultraviolet/visible (UVA/is) spectroscopy, which provides information about organic compounds with conjugated double bonds. In this chapter, we will look at two more instrumental techniques mass spectrometry and infrared (IR) spectroscopy. Mass spectrometry allows us to determine the molecular mass and the molecular formula of a compound, as well as certain structural features of the compound. Infrared spectroscopy allows us to determine the kinds of functional groups a compound has. In the next chapter, we will look at nuclear magnetic resonance (NMR) spectroscopy, which provides information about the carbon-hydrogen framework of a compound. Of these instrumental techniques, mass spectrometry is the only one that does not involve electromagnetic radiation. Thus, it is called spectrometry, whereas the others are called spectroscopy. 

Any technique for gas analysis can be applied to EGA. The most frequently used methods are mass spectroscopy (MS) and Fourier transform infrared spectroscopy (FTIR). Many instrument manufacturers provide the ability to interface their TGAs with MS or FTIR (see Section 3.7, on instrumentation). Temporal resolution between the TGA and the MS or FTIR detector is an important feature, for example, in distinguishing absorbed water from water as a reaction product and in assigning a decomposition product to a specific mass loss. Each method has its experimental requirements, limitations, and advantages. Mass spectroscopy is a very sensitive technique that identifies volatile species by their mass-to-charge ratio, referred to as m/z. The evolution of the sum of all mJz species can be plotted and compared with the derivative TGA plot to ensure temporal resolution between the TGA and the mass spectrometer. The evolution of a specific mJz, associated with species such as water or formaldehyde, can show the distinct evolution of these compounds. The most common ionization is by 70eV electron impact (El), which operates... 

During the first half of the twentieth century many workers extended the spectral database of organic compounds and assigned spectral features to functional groups. While infrared spectroscopy had moved away from being a scientific curiosity it was used very little suitable spectrometers did not exist and few chemists had access to what instruments there were. Over half a century was to pass between Coblentz s original work and the routine use of spectroscopy as a tool indeed, two-thirds of a century would pass before routine NIR measurement made its debut. 

In Chapter 14 you will team about mass spectrometry, infrared spectroscopy, and UVA is spectroscopy, three instrumental techniques that chemists use to analyze compounds. Mass spectrometry is used to find the molecular mass and the molecular formula of an organic compound it is also used to identify certain structural features of the compound by identifying the fragments produced when the molecule breaks apart. Infrared (IR) spectroscopy allows us to identify the kinds of bonds and therefore the kinds of functional groups in an organic compound. Ultraviolet and visible (UVA is) spectroscopy provides information about compounds that have conjugated double bonds. 

The detectors used in infrared spectroscopy are tailored to both the instrumental technique used and the range examined. In general, dispersive instruments are radiation-limited and require sensitive detectors, while interferometric instruments require detectors with a response rate fast enough to detect and transmit rapid energy changes to a recorder. The most common detectors are summarized in Table 4, along with some of the more salient features. Array detectors are still being developed for multiplex analysis in the NIR, but thus far the cost of these detectors has proven to be prohibitive to widespread use. 

Infrared (IR) spectroscopy offers many unique advantages for measurements within an industrial environment, whether they are for environmental or for production-based applications. Historically, the technique has been used for a broad range of applications ranging from the composition of gas and/or liquid mixtures to the analysis of trace components for gas purity or environmental analysis. The instrumentation used ranges in complexity from simple filter-based photometers to optomechanically complicated devices, such as Fourier transform infrared (FTIR) spectrometers. Simple nondispersive infrared (NDIR) insttuments are in common use for measurements that feature well-defined methods of analysis, such as the analysis of combustion gases for carbon oxides and hydrocarbons. For more complex measurements it is normally necessary to obtain a greater amount of spectral information, and so either Ml-spectrum or multiple wavelength analyzers are required. 

This limitation has been largely overcome by linking chromatographic columns directly with ultraviolet, infrared, and mass spectrometers. The resulting hyphenated instruments are powerful tools for identifying the components of complex mixtures (see Section 31A-4). An example of the use of mass spectroscopy combined with gas chromatography for the identification of constituents in blood is given in Feature 31-1. 

Solid-state multi-element detector arrays in the focal planes of simple grating monochromators can simultaneously monitor several absorption features. These devices were first used for uv—vis spectroscopy. Infrared coverage is limited (see Table 3), but research continues to extend the response to longer wavelengths. Less expensive nir array detectors have been applied to on-line process instrumentation (125) (see PHOTODETECTORS). 

In contrast to the well-known difficulties of traditionally applied quantitative IR spectroscopy of mixtures in solid (powdered) samples, the near-infrared reflectance analysis (NIRA) technique [32] has gained importance over the last decade and can now be implemented on a variety of commercially available Instruments In a number of applications to Industrial, agricultural and pharmaceutical analyses. Both the NIRA instruments equipped with grating monochromators and those fitted with filter systems feature built—In microprocessors with software suited to the Intrinsic characteristics of this spectroscopic alternative. Filter Instruments generate raw optical data for only a few wave-... 

Instrumenls for the ultraviolet (Ul% visible, and infrared (IR) regions have enough features in common that they aie often called optical instruments even though the human infrared wavelengths. In this chapter, we consider the function, the requirements, and the behavior of the components of instruments for optical spectroscopy fur all three types of radiation. Instruments for spectroscopic studies in regions more energetic than the ultraviolet cold less energetic than the infrared have characteristics that differ substantially ftvm optica instruments and are considered separately in Chapters 12 and 19. 

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