Infrared spectroscopy hyphenated techniques

Determination of complex mixtures is one of the most common problems in infrared analysis. Hyphenated techniques such as chromatography provides a useful way of separating mixtures into individual compounds. Gas chromatography (GC) has been by far the best application of combining infrared spectroscopy and a separation technique. One of the principal reasons has been the use of carrier gases which do not absorb in the infrared region. 

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. 

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. 

Gas chromatography is often coupled with the selective techniques of spectroscopy and electrochemistry. We have discussed GC/MS, but gas chromatography can also be combined with several other techniques, such as infrared spectroscopy and nuclear magnetic resonance spectro.scopy, to provide the chemist with powerful tools for identifying the components of complex mixtures. These combined techniques are sometimes called hyphenated methods. ... 

For the purification of compounds, methods including molecular filtration, solid phase extraction (SPE, SPME), solvent extraction, and a variety of basic chromatographic techniques (thin layer, low pressure, ion exchange, size exclusion, etc.), HPLC, and GC (with derivatization of nonvolatile compounds) can be used. Additionally, instrumentation to identify compounds is available, such as the different spectrometric applications, including infrared (IR), mass (MS), ultraviolet and visible (UV-Vis), and NMR spectroscopy. In recent years, the so-called hyphenated techniques (combined chromatographic and spectral methods such as... 

Various modem accessories (ATR crystals, acoustic detectors, infrared microscopes, polarization modulation technique) as well as hyphenation techniques have substantially expanded the field of application of infrared spectroscopy. Applications of IR spectroscopy to surface investigations (characterization of the surface, physi-sorption and chemisorption studies, catalytic properties) are reviewed in [7-9]. Applications of hyphenated techniques, in particular combinations with chromatography, are given in [8]. 

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. 

Improvements in analytical capability for the analysis of complex pyrolysate mixtures have appeared during the last decade high-resolution capillary GC with more polar and selective stationary phases coated on inert fused-silica colmnns coupling of capillary GC with sensitive, selective, and lower-cost mass spectrometric detectors enhanced pyrolysis-MS techniques hyphenated analysis methods, including GC-Fourier-transform infrared spectroscopy (GC/FTIR) and tandem MS and better strategies for handling complex multidimensional pyrolysis data. The present chapter reviews the known chemotaxonomy of miCTOorganisms, summarizes practical considerations for the use of pyrolysis in microbial characterization, and critically discusses selected applications of analytical pyrolysis to microbial characterization. 

The major weakness of classical tga is that it gives information on weight loss but no chemical information. To overcome this problem, some modern tga instruments have been designed to be interfaced to chemical analysis instruments, the most popular being mass spectroscopy and Fourier transform infrared (ftir) spectroscopy. Both can give extremely useful information but care is needed to avoid problems from secondary reactions of volatile products and from loss of volatile products by condensation in transfer lines. The use of these so-called hyphenated techniques has been reviewed (30). 

Under normal circumstances, retention time is a good tool to identify components by GC. Often, however, retention time alone is not definitive because many compounds have similar retention times. Structural information can be obtained independently from several spectroscopic techniques. This has led to hyphenated techniques, such as gas chromatography-mass spectroscopy, gas chromatography-infrared spectroscopy and others. If spectroscopic, reference spectra are available, confirmation of analyte identity is very likely. 

The selective detectors discussed in the previous sections often do not provide enough information to elucidate with 100% probability the nature of the eluting solutes. For this reason, data with selective detectors can be erratic. The future in this respect definitely belongs to the spectroscopic detectors that allow. selective recognition of the separated compounds. Today, the hyphenated techniques CGC-mass spectroscopy (CGC-MS), CGC-Fourier transform infrared spectroscopy (CGC-FTIR), and CGC-atomic emission detection (CGC - AED) are the most powerful analytical techniques available. They provide sensitive and selective quantitation of target compounds and structural elucidation or identification of unknowns. The applicability and ease of use of the hyphenated techniques were greatly increased by the introduction of fused silica wall coaled open tubular columns. The main reason for this is that because of the low flows of capillary columns, no special interfaces are required and columns are connected directly to the different spectrometers. The introduction of relatively inexpensive benchtop hyphenated systems has enabled many laboratories to acquire such instrumentation, which in turn has expanded their applicability ever further. 

The use of instrumental chromatographic methods was described earlier in the role of hyphenated or hybridized techniques when combined with infrared spectroscopy. In this case, the chromatographic front-end acts as a sophisticated sample preparation system for isolation of specific chemical species in a mixture. In cases in which a particular component needs to be removed or separated, it is not necessary to resort to an instrumental method. In such cases, the sample may be passed through a simple column containing the solid-separation phase. A convenient approach for liquids is to prepare small columns of adsorbent or ion-exchange resin in a Pasteur or dropper pipette. This is ideal as a method for sample cleanup or for selectively removing contaminants or specific chemical components. 

Spectroscopic techniques used in essential oil analysis comprise ultraviolet and visible spectrophotometry, infrared spectrophotometry (IR), mass spectrometry (MS), and nuclear magnetic resonance spectroscopy (NMR), including the following H-NMR, C-NMR, and site-specific natural isotope fractionation NMR. Combined techniques (hyphenated techniques) employed in essential oil analysis are GC/MS, liquid chromatography/mass spectrometry, gas chromatography/Fourier transform infrared spectrophotometry (GC/FT-IR), GC/FT-IR/MS, GC/atomic emission detector, GC/isotope ratio mass spectrometry, multidimensional GC/MS. 

Combining separation and analysis techniques (hyphenated techniques) can produce powerful tools for chtiracteriz-ing viscous oils. Thus, liquid chromatography or gas chromatography can be used to separate a sample for subsequent characterization by mass spectrometry (LC/MS or GC/MS). Research into suitable methods for the analysis of viscous oils is underway, but no standard tests have yet been prepared. Extensive research on both proton and carbon-13 nuclear magnetic resonance spectroscopy shows promise as a tool for the analysis of lubricant base oils and other viscous oils. Both near-infrared spectroscopy (NIR) and Fourier-transform IR (FTIR) are the subjects of active research into methods to characterize hydrocarbons and for quality control during production of petroleum products. Standard test methods using these techniques should become available in the future. 

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. 

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