GC—IR analysis

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. 

ASTM Standard E 1642-94, Standard Practice for General Techniques of Gas Chromatography Infrared (GC/IR) Analysis, American Society for Testing and Material, Philadelphia, February 1995. 

Measurements of IR absorption of thin films are useful for identification with authentic samples, particularly if in addition pyrolysis and GC/IR analysis of the decomposition products is carried out. The use of specific enzymes is important in aiding the isolation of polysaccharide components from natural sources, and in producing identifiable oligosaccharides that facilitate molecular structural analysis. 

Table 4 lists the specifications set by Du Pont, the largest U.S. producer of DMF (4). Water in DMF is deterrnined either by Kad Fischer titration or by gas chromatography. The chromatographic method is more rehable at lower levels of water (<500 ppm) (4). DMF purity is deterrnined by gc. For specialized laboratory appHcations, conductivity measurements have been used as an indication of purity (27). DMF in water can be measured by refractive index, hydrolysis to DMA followed by titration of the Hberated amine, or, most conveniendy, by infrared analysis. A band at 1087 cm is used for the ir analysis. 

Another analysis handled effectively by use of gc/ir/ms is essential oil characterization which is of interest to the foods, flavors, and fragrances industries (see Oils essential). Even very minor components in these complex mixtures can affect taste and aroma. Figure 4 shows the TRC and TIC for Russian corriander oil which is used extensively in seasonings and perfumes (15). The ir and ms are serially configured. Spectra can be obtained from even the very minor gc peaks representing nanogram quantities in the it flow cell. 

In off-line extraction the extracted analytes are collected and isolated independently from any subsequent analytical technique, which is to be employed next. For example, the extracted analyte can be collected in a solvent or on a solid sorbent. The choice of the collection method affects the possibilities for further analysis. The extracts may be used for final direct measurements (i.e. without further separation), e.g. UV and IR analysis. More usually, however, extraction is a pre-separation technique for chromatography, either off-line (the most common mode of SEE) or on-line (e.g. SFE-GC, SFE-LC-FTTR, etc.). The solvents used in extraction may affect subsequent chromatography. 

The minute sample sizes allowed in SFE-SFC analysis (typically 0.5 mg cf. the approximate weight of 30 mg for a single pellet), which is several orders of magnitude smaller than the sample weights used in GC, HPLC or IR analysis (5-10g), allows us to perform additive dispersion studies on a pellet-to-pellet basis [106]. 

In chromatography-FTIR applications, in most instances, IR spectroscopy alone cannot provide unequivocal mixture-component identification. For this reason, chromatography-FTIR results are often combined with retention indices or mass-spectral analysis to improve structure assignments. In GC-FTIR instrumentation the capillary column terminates directly at the light-pipe entrance, and the flow is returned to the GC oven to allow in-line detection by FID or MS. Recently, a multihyphenated system consisting of a GC, combined with a cryostatic interfaced FT1R spectrometer and FID detector, and a mass spectrometer, has been described [197]. Obviously, GC-FTIR-MS is a versatile complex mixture analysis technique that can provide unequivocal and unambiguous compound identification [198,199]. Actually, on-line GC-IR, with... 

There is a need for increased chromatography-FTIR sensitivity to extend IR analysis to trace mixture components. GC-FTIR-MS was prospected as the method of choice for volatile complex mixture analysis [167]. HPLC-FT1R, SFC-FTIR and TLC-FTIR are not as sensitive as GC-FTIR, but are more appropriate for analyses involving nonvolatile mixture components. Although GC-FTIR is one of the most developed and practised techniques which combine chromatography (GC, SFC, HPLC, SEC, TLC) and FUR, it does not find wide use for polymer/additive analysis, in contrast to HPLC-FTIR. 

A GC-IR-MS system with library search capability has been used to effectively identify the pyrolysis products of polybutadiene and the antioxidant additive 2,6-di-f-butyl-4-methylphenol [199]. Paper for food packaging was analysed by P T (at 100 °C) combined with /i-GC-UV. No specific applications of /rGC-UV to poly-mer/additive analysis have as yet been reported. 

In addition to the above-mentioned restrictions, eluent selection for LC and HPLC is especially important. While the gas used in GC will not interfere with analysis, it is possible for eluent components used in LC or HPLC to interfere with follow-on analysis. This will be true for both MS and IR analysis. Usually, however, all samples can be accommodated if sufficient thought is exercised in selecting both the method of separation and the method of introduction into the follow-on analytical procedure. 

Considering mass alone, why will the gases used in GC typically not interfere with either follow-on MS or IR analysis. 

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... 

Cells with a minimum optical path of a few centimetres are used for gay analysis (Fig. 10.17). If the absorbance under these conditions is too weak, a cell using mirrors to increase the path length can be used. Cells with an optical path of several metres represent a complex and cluttered arrangement. Filliform gas cells with a volume of a few tens of microlitres (1 = 10 cm with diameter < 1 mm) are used for the hyphenated technique of GC-IR. These cells use reflecting side walls. 

IR spectra were recorded on a Perkin-Elmer 577 instrument 1H and 13C NMR spectra were recorded on a Varian XL 200 instrument. GC analysis were performed on a Hewlett-Packard 5880 instrument, FI detector, equipped with a Methyl Silicone fluid capillary column (35 m), by using n-esadecane as internal standard. GC-MS analysis were performed using a Hewlett-Packard 5995 C instrument. 

A 39 g sample of crude 2,3,4-trimethoxy-l-allyloxybenzene in a round-bottomed flask with an immersion thermometer was heated with a soft flame. At 225 °C there was a light effervescence and at 240 °C an exothermic reaction set in that raised the temperature immediately to 265 °C. It was held there for 5 min, and then the reaction was allowed to cool to room temperature. GC and IR analysis showed the starting ether to be gone, and that the product was largely 2,3,4-trimethoxy-6-allylphenol. It weighed 34.4 g. 

An integrated GC/IR/MS instrument is a powerful tool for rapid identification of thermally generated aroma compounds. Fourier transform infrared spectroscopy (GC/IR) provides a complementary technique to mass spectrometry (MS) for the characterization of volatile flavor components in complex mixtures. Recent improvements in GC/IR instruments have made it possible to construct an integrated GC/IR/HS system in which the sensitivity of the two spectroscopic detectors is roughly equal. The combined system offers direct correlation of IR and MS chromatograms, functional group analysis, substantial time savings, and the potential for an expert systems approach to identification of flavor components. Performance of the technique is illustrated with applications to the analysis of volatile flavor components in charbroiled chicken. 

From the point of view of aroma analysis, the ultimate objective of developing so-called "multiply hyphenated" instruments is to produce a device which can automatically determine the identity of all of the constituents of a complex volatile mixture. Integrated GC/IR/MS is a step along that path, but a host of crucial issues remain. 

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