Condensed-phase spectra

The success of spectral identification depends on the appropriate reference spectra for comparison. IR measurement of eluates that are at slightly subambient temperature is advantageous considering that the large databases of condensed-state spectra may be searched. Spectra measured by matrix-isolation GC-FTIR have characteristically narrow bandwidths compared with the spectra of samples in the condensed phase near ambient temperature or in the gas phase. In addition, the relative intensities of bands in the spectra of matrix-isolated samples often change compared with either gas- or condensed-phase spectra [195]. GC-FTIR spectra obtained by direct deposition match well with the corresponding reference spectra in standard phase... 

IR spectra of the fundamental vibrational band of small gaseous diatomic molecules, such as CO and NO, contain a large number of absorption lines that correspond to these vibrational-rotational energy transitions. Since many different rotational levels can be populated at ambient temperature, many different transitions at different energies may occur (Fig. 1). Vibrational-rotational lines are evident only in gas-phase spectra collected at sufficiently high resolution. These lines are not resolved in condensed-phase spectra because of frequent collisions between molecules hence, condensed-phase spectra are characterized by broad absorption bands occurring at the vibrational transition energies. 

The difficulty in gas sampling is that the spectra differ from the condensed phase spectra. Therefore, reference spectra of gas-phase spectra are required for accurate identifications. 

Cryodeposition is the newest interface type for a GC/FTIR instrument. In this system, the eluents in the GC effluent are frozen on an IR transparent slide, which is cooled using liquid nitrogen. The carrier gas evaporates in the process so that the chemicals are directly deposited on the slide surface. Transmission spectra are then measured through the slide. These spectra are like normal condensed phase spectra, with rare exceptions. The sensitivity is five times better than in light-pipe, and the same or even slightly better than in GC/MI/FTIR. 

The phases of the chemicals measured in lightpipe, matrix isolation, and cryodeposition instruments are different vapor phase, matrix-isolated, and condensed phase respectively. The intermolec-ular interactions are missing in the vapor phase and matrix isolation. Therefore, for example, all hydrogen bond-related vibrations are missing or different. Also, the vibration bands are narrow in the gas phase and even narrower in matrix isolation. Thus, the spectra cannot be compared with each other. The traditional IR spectra measured using salt pellets or windows produce also condensed phase spectra, which are therefore comparable with cryodeposition spectra (see example in Figure 4). There are other differences because of factors of more practical nature lower sensitivity and resolution. Owing to all these differences, separate sets of reference spectra have to be measured for each interface type. 

Figures 5-8 show further that the two features of the condensed phase spectra which differ from their gas-phase counterparts are the energies and widths of the individual ionization peaks or optical-absorption lines. In both the UPS and UAS spectra the lines in the solid state are shifted to lower energies relative to the corresponding ones in the gas phase by a relaxation energy, 1 eV for ion states and % 0.1 eV for exciton states, associated with the intermolecular polarization induced by the ion or exciton (1, 2, ). In addition, these...

Infrared Spectra.—A gas-tight Kel-F cell fitted with AgCl windows was used for all condensed-phase spectra. The powdered solids were dusted on to the inner surfaces of the windows, the same precautions being taken as for X-ray sample preparation. A Perkin-Elmer 137 Infracord was used over its full range of 4000-400 cm". ... 

The thermal functions which would result from the use of the available structural and vibrational Inbformation for the remaining polyatomic species, are biased In the same way as for Sg(g). Some of the necessary Information is derived from condensed phase spectra (rather than gas phase spectra), as with the octamer, the low-valued vibrational frequencies (and possibly the structure) may need to be adjusted. In any event, the use of the existing spectroscopic data (unaltered) leads to calculated entropies which are unreasonable when used in a 3rd law analysis to mesh in the vapor pressure with these thermal functions. 

GC-Fourier transform infra-red (GC-FTIR) spectroscopy is less frequently used than GC-MS, but involves a similar principle in which the outlet from the column is coupled to an infra-red spectrophotometer. The technique currently suffers from a lack of library spectra, as the IR spectra taken in the vapour phase can be subtly different from condensed-phase spectra or spectra collected using the well-established KBr disc method. 

Another spectroscopic technique which can be coupled to GC is infrared spectroscopy. It is particularly valuable when isomer identification is required. On its own, however, infra red (IR) spectroscopy is not very useful because it cannot differentiate homologues without additional molecular weight information. Besides, the gas phase spectra differ from the widely available condensed phase spectra, which means that new libraries would have to be built to enable analyte identification based on library searches. Thus, IR spectroscopy found its niche application in GC in combination with mass spectrometry. 

Extensive correlations of IR absorption frequencies against functional groups, and substituents on phosphorus, have been made by Thomas and Corbridge for condensed phase spectra. 

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... 

This collection of spectra includes both transmission and pyrolysate (condensed phase) spectra of a comprehensive range of technically important rubber and plastic based materials. Included are homopolymers, copolymers and blended materials, and pyrolysate spectra have by necessity been obtained for crosslinked rubbers and thermoset materials. Materials are indexed under both polymer type and trade name. Photocopies of this source are not available. The publication can be purchased from Rapra s Publication Sales group. 

Perhaps the most interesting fact revealed by the spectroscopic work on the ionic clusters is that they do not approach the condensed phase spectra in a simple monotonic and asymtotic manner. Indeed in all these cluster species X=He,Ne and Ar, the spectral... 

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. 

In condensed phases, spectra are commonly measured in absorption. Three main types of transitions are observed in the absorption spectra of the actinide ions (1) Laparte-forbidden f to f transitions, (2) orbitally allowed 5/ to 6d transitions, and (3) metal to ligand charge transfer. Of these, study of internal f to f transitions has found wide use in the investigation of actinide chemistry. These band usually in the visible and ultraviolet regions, can be easily identified because of their sharpness, and are sensitive to the metal environment. As discussed earlier, the 5/ orbitals of the actinide elements are more exposed than the lanthanide 4/ orbitals, and therefore, crystal field effects are larger in the 5/ series. The f to f transitions for actinide elements may be up to 10 times more intense and twice as broad as those observed for the lanthanides, due to the action of crystal fields. In addition, extra lines resulting from vibronic states coupled to / / states have been observed. 

Experimentally, free-ion spectra (both neutral and ionic species) are usually observed in emission, and the energy level structure is deduced from coincidences of energy differences of pairs of spectral lines, subject to verification by isotope shift, hyperfine structure and magnetic gf-factor tests. In condensed phases, spectra are more commonly measured in absorption. Relative intensities associated with parity-allowed and forbidden transitions are reflected in the nature of two processes transitions in which the initial and final states belong to electronic configurations of different parity (parity-allowed transitions, e.g. 5f - 5f 6d) and those in which both states belong to the same configuration (parity-forbidden transitions, e.g. 5f 5f ). The latter are weak and sharp. The... 

A great deal of progress had been made in interpreting condensed-phase spectra of the trivalent lanthanide ions in terms of a physical model before much was known about the transuranium elements [3], Since, except for Th and Pa, each member of the actinide series also exhibits a well characterized trivalent state, it is not surprising that the model developed in treating lanthanide spectra has served as the basis for interpretation of trivalent actinide spectra as well. 

Cell and gas-phase spectra The major advantage of this method is that IR spectra can be obtained on microgram quantities of materials and the spectra do not show the effects of interactions between molecules characteristic of condensed phase spectra. All that is needed to record gas-phase spectra is a sample with vapor pressure of a few mbar and a path length of about 10 cm. Cells with NaCl or KBr windows are commercially available or can be built easily. Crystals of KBr are transparent to 4000-250 cm" and are perfectly acceptable for most uses. They have the disadvantage of being hygroscopic and must be stored in a desiccator. Cells of NaCl are transparent to 4000-600 cm", less expensive and less hygroscopic. 

Cells and condensed phase spectra Condensed phase spectra can be obtained from samples that are either solid or liquid. Comparison of the IR spectra of the same substance in liquid and solid phases will differ. However, the major differences will be... 

The result that is reported as the peak wavenumber is frequently the wavenumber value of the maximum discrete point, which rarely corresponds to the exact center of the band. It is not uncommon for these values to be reported to four or five decimal places, as this is how well the wavenumber of each datum is recorded however this number of digits is well beyond the realistic accuracy of most experiments. The use of the maximum data point for the wavenumber position of a band is suitable only for noncritical information. The actual band center may be considerably displaced from the maximum data point. For example, in a typical spectrum measured at 4-cm resolution, data points are typically computed at 2-cm intervals. Under these conditions simple peak-picking algorithms would not readily detect band shifts of about 0.5 cm . In practice, the wavenumber of the maxima of bands in the spectra of condensed-phase spectra measured at a resolution of 2 or 4 cm should only be reported to 1 cm . ... 

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