Vapor-phase spectra

Aldehydes —CHO (See also Table 7.49 for C—H.) All values given below are lowered in liquid-film or solid-state spectra by about 10-20 cm Vapor-phase spectra have values raised about 20 cm Saturated 1740-1720 ... 

Several manufacturers offer GC-FT IR instruments with which a vapor-phase spectrum can be obtained on nanogram amounts of a compound eluting from a capillary GC column. Vapor-phase spectra resemble those obtained at high dilution in a nonpolar solvent Concentration-dependent peaks are shifted to higher frequency compared with those obtained from concentrated solutions, thin films, or the solid state (see Aldrich, 1985). 

In the various GC-FTIR systems that are commercially available, three essentially different types of GC-FTIR interfaces can be identified (137). With the most commonly used interface, the GC column effluent flows through a heated light-pipe, and vapor-phase spectra are collected in real time at 1 s intervals. This... 

Molecular specificity is afforded by control of the potential energy difference across the electrode-solution interface. The selectivity or resolution of electrochemistry is not nearly as great as we would like, and the inability to distinguish between two or more species can be a serious weakness. Solution electrochemistry suffers by comparison with mass spectrometry in the same sense that optical spectra in solution suffer by comparison with vapor-phase spectra. 

FTIR spectroscopy to a particular pesticide, the methods have general applications to numerous compounds. Most of these utilize the high sensitivity of FTIR, and the data manipulation capability of the system. In several of the gas evolution studies, spectra were acquired at less than one-minute intervals. While this is not really "rapid scanning," the high resolution required for vapor phase spectra would not have been possible with a normal dispersive instrument. Several other techniques using FTIR show promise in the area of pesticide analysis. 

A literature search indicated that in most cases the available vapor phase spectra are insufficiently detailed for this purpose, even at near-room-temperature conditions. 

Vapor phase spectra of trifluoro-acetic acid were reported by Christian and Stevens 70) and of oxalic acid by Pava and Stafford 71). 

In vapor-phase spectra at low pressures (<10 torr) no problems of environmental broadening, collisions and intermolecular interaction are encountered. For the majority of aromatics and aza-aromatics, solids or liquids at room temperature, only very low pressures are achieved at 20 °C and in the experimentally practical temperature range 273 to 373 K. 

Thermostated sample cells for all physical states are commercially available. To obtain vapor phase spectra of materials with low vapor pressure, heatable gas cells with a standard length or multiple reflection cells are needed. 

The vibrational frequencies 972, 996, 453, and 437 have been observed in the infrared vapor phase spectra of MoOgCl by Barraclough and Stals (IJ.). These frequencies were assigned to the symmetric and as3nnmetric Mo-0 and Mo-Cl stretches, respectively. Similar Infrared data for these four frequencies have been reported by lorns and Stafford (IJ,) and Ward and Stafford (1 ). The rest of the frequencies are from the Infrared and Raman studies of crystalline MoOgCl by Adams and Churchill (M). Assignments are made by comparison with data reported for CrO Clgd) by Miller et al. (15). 

HRGC-FTIR analysis was carried out with a Nicolet 20 SXB system interfaced to a Dani 6500 gas chromatograph. A J W fused silica DB-5 colujnn, 30 m x 0.32 mm id, df = 0.25 pm, was used. PTV injection (4(T -200°C) was performed. The temperature program was 60° to 250°C at 5°/min. Light pipe and transfer line were held at 200°C He (2 ml/min) was employed as carrier gas. Vapor phase spectra were recorded from 4000 - 700 cm- with a resolution of 8 cm-. ... 

Figure 10. FTIR vapor phase spectra from pyranoid (Z)- and (E)-linalool oxides(left, top to bottom) as well as from corresponding epoxy derivatives (right, top to bottom).

Chromatographic interfaces are based on three common approaches the flow-through cell (light pipe) and solvent elimination with either matrix isolation or cold trapping [2,198,201]. Flow-through cells provide a simple and convenient interface for GC-FTIR, since typical mobile phases are transparent in the mid-infrared region. Mobile phase elimination interfaces are used primarily to increase sensitivity, and to obtain high-resolution or condensed phase spectra, for improved confidence of identification by library search techniques. Vapor phase spectra have characteristic broad absorption... 

Dirinck et al., " successfully apphed GC/FTIR to the analysis of amphetamine-like compounds in judiciary exhibits. With fight-pipe GC/FTIR, unique vapor-phase infrared spectra were generated, allowing the unambiguous differentiation between closely related amphetamines. The obtained vapor-phase spectra were submitted to a spectral search on a laboratory-made vapor-phase FTIR library. Several amphetamine analogues have been identified in confiscated powders and tablets using this approach. 

Spectra in the 3500-1900 A. region were taken with a Cary 14 spectrophotometer purged with nitrogen. The vapor phase spectra were obtained using quartz ga.s celts, with path lengths from 2 to 10 cm. The vapor phase spectral measurements were extended to 1700 A.usinganitrogen-purged Perkin-Elmer Model 350 spectrophotometer. 

In the latter publication, for example, the vapor-phase IR spectra of all the four isomers of pulegol and dihydrocarveol are shown, which have been extracted from a GC/FTIR run. These examples convincingly demonstrate the capability of distinguishing geometrical isomers with the aid of vapor-phase IR spectra, which cannot be achieved by their mass spectra. A broad application of GC-FTIR in the analysis of essential oils, however, is limited by the lack of suf dent vapor-phase spectra of uncommon compounds, which are needed for reference use, since the spectra of isolated molecules in the vapor phase can be signi cantly different from the corresponding condensed-phase spectra. 

After infrared spectra at low resolution (usually 4 cm ) have been recorded as a function of time and temperature, the further procedure is similar to GC/FT-IR (see Section 17.5.1.7) and, in fact, GC/FT-IR. software, such as Gram-Schmidt thermogram reconstruction and library search with a vapor-phase spectra database is commonly used. Helpful for the identification in the vapor phase is the virtual absence of interaction between molecules, and therefore the nondestructive IR detection enables in many cases an easy and fast interpretation of single and overlapping absorption bands. 

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