Hyphenated Infrared Techniques

On some FTIRs the pickoff mirror is moved into and out of position via a motor under software control. On older instruments the mirror may need to be placed and removed by hand. Once the IR beam has reflected off the pickoff mirror, it leaves the FTIR via a hole in the side of the instrument s cover which is frequently covered with an infrared transparent window. Once the beam leaves the FTIR it enters an interface box that contains transfer optics and other devices, allowing the FTIR and the second instrument to work together. 

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Standard practices for GC-IR analysis have been described (ASTM E 1642-94). Griffiths [200] has discussed GC-FTIR designs. Sample preparation methods for hyphenated infrared techniques, in particular GC-FTIR, have been reported [201]. The technique has been reviewed repeatedly [167,183,201-204] a monograph [205] has appeared. 

Many hyphenated infrared techniques have structural elements in common. In general, the infrared beam needs to be removed from the FTIR to interface with another instrument. This is accomplished by a pickoff mirror placed in the infrared beam after it has left the interferometer, as shown in Figure 6.1. 

FIGURE 6.1 The IR beam path and interface used to create hyphenated infrared techniques. 

Gurka DF, Pyle SM, Farnham I, et al. 1991. Application of hyphenated Fourier Transform-infrared techniques to environmental analysis. J Chromatog Sci 29(8) 339-344. 

Many techniques can be employed in the analysis of compounds that can be used in the manufacture of explosives and explosive materials. Raman and infrared spectroscopy, nuclear magnetic resonance (NMR), mass spectrometry, and gas chromatography can all be used in the analysis of this type of material. Hyphenated chromatographic techniques, such as GC-MS and LC-MS, can also be used. 

Willis, J. N. and Wheeler, L., Use of a gel permeation chromatography-Fourier transform infrared spectroscopy interface for polymer analysis, in Chromatographic Characterization of Polymers, Hyphenated and Multidimensional Techniques, Provder, T., Barth, H. G., and Urban, M. W., Eds., American Chemical Society, Washington, D.C., 1995, chap. 19. 

VDU screen via suitable electronic amplifying circuitry where the data are presented in the form of an elution profile. Although there are a dozen or more types of detector available for gas chromatography, only those based on thermal conductivity, flame ionization, electron-capture and perhaps flame emission and electrolytic conductivity are widely used. The interfacing of gas chromatographs with infrared and mass spectrometers, so-called hyphenated techniques, is described on p. 114 etseq. Some detector characteristics are summarized in Table 4.11. 

The identification of GC peaks other than through retention data, which are sometimes ambiguous or inconclusive, can be facilitated by the direct interfacing of GC with infrared spectrometry (p. 378 et seq.) or mass spectrometry (p. 426 etseq.), so-called coupled or hyphenated techniques. The general instrumental arrangement is shown in Figure 4.29(a). 

The identification of a compound only by its retention time is somewhat arbitrary. A better method consists of using two different techniques that are complementary. Coupling a chromatograph with a second instrument on-line such as a mass spectrometer or an infrared spectrophotometer realises these objectives. These coupled methods, often called hyphenated techniques, allow us to obtain two different types of information that are independent. Therefore, it is possible to determine with more certainty the nature and concentration of the components in a mixture in nanograms or smaller units of measurement. 

Many other analytical techniques can be coupled to mass spectrometers. These so-called hyphenated techniques, like GC-MS and LC-MS, include but are not limited to ICP-MS (inductively coupled argon plasma), SCF-MS (supercritical fluid), NMR-MS (nuclear magnetic resonance) and IR-MS (infrared). 

The thermal characterisation of elastomers has recently been reviewed by Sircar [28] from which it appears that DSC followed by TG/DTG are the most popular thermal analysis techniques for elastomer applications. The TG/differential thermal gravimetry (DTG) method remains the method of choice for compositional analysis of uncured and cured elastomer compounds. Sircar s comprehensive review [28] was based on single thermal methods (TG, DSC, differential thermal analysis (DTA), thermomechanical analysis (TMA), DMA) and excluded combined (TG-DSC, TG-DTA) and simultaneous (TG-fourier transform infrared (TG-FTIR), TG-mass spectroscopy (TG-MS)) techniques. In this chapter the emphasis is on those multiple and hyphenated thermogravimetric analysis techniques which have had an impact on the characterisation of elastomers. The review is based mainly on Chemical Abstracts records corresponding to the keywords elastomers, thermogravimetry, differential scanning calorimetry, differential thermal analysis, infrared and mass spectrometry over the period 1979-1999. Table 1.1 contains the references to the various combined techniques. 

For identification purposes the classical techniques of infrared spectroscopy and MS are highly suited in hyphenation to thermogravimetry. Table 1.3 shows the main characteristics of TG-FTIR and TG-MS. 

Spectroscopy has become a powerful tool for the determination of polymer structures. The major part of the book is devoted to techniques that are the most frequently used for analysis of rubbery materials, i.e., various methods of nuclear magnetic resonance (NMR) and optical spectroscopy. One chapter is devoted to (multi) hyphenated thermograviometric analysis (TGA) techniques, i.e., TGA combined with Fourier transform infrared spectroscopy (FT-IR), mass spectroscopy, gas chromatography, differential scanning calorimetry and differential thermal analysis. There are already many excellent textbooks on the basic principles of these methods. Therefore, the main objective of the present book is to discuss a wide range of applications of the spectroscopic techniques for the analysis of rubbery materials. The contents of this book are of interest to chemists, physicists, material scientists and technologists who seek a better understanding of rubbery materials. 

In Chapter 6 we saw that, by itself, chromatography is not well suited to qualitative analysis thus it is often combined with other methods. The most successful combination has been GC with mass spectrometry (MS) GC s ability to separate materials and MS s ability to identify them has made the combination one of the most powerful analytical techniques available today. The other forms of chromatography are also being combined with MS and with infrared spectroscopy (IR). The resulting analytical methods are usually designated by their combined abbreviations (e.g., GC/MS or GC-MS) and are known as hyphenated techniques. The current status of these methods will be described briefly. 

Infrared spectroscopy is one of the techniques most frequently coupled to SFE — it accounts for more than 30% of reported SFE hyphenated methods — which is unsurprising as it is one of the most powerful tools available for the elucidation of molecular structures. The specificity of IR spectral information is highly useful for the real-time monitoring of SFE processes with qualitative and quantitative purposes [125]. However, the partial transparency of supercritical CO, in the IR region — it exhibits strong absorption bands at 3800-3500,2500-2150 and below 900 cm — calls for careful background correction. 

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