Matrix isolation interface

The second issue is to interface two detectors to a single GC. Since both the MS and the matrix isolation interface are destructive detectors, the sample is split in a 1 1 effluent splitter and half the sample is routed to each detector via a specially-designed open split interface (18). 

Reedy Scientific sells a ClearIR GC/IR/MS system, which splits the GC (Thermo Finnigan Trace GC) effluent between an IR spectrometer (Thermo Nicolet Nexus 670), with a matrix isolation interface, and mass spectrometer (Thermo Finnigan s PolarisQ quadrupole ion trap or TRACE DSQ single quadrupole instrument). The GC effluent is mixed with 2 % argon. The mixture is directed to the surface of a cryogenically cooled rotating cylinder. 

For microbore HPLC, with a flow of less than lOOpLmin-1, off-line LC-FT1R has been developed using matrix isolation techniques. The solutes are deposited on a moving IR salt window [504] or on a rotating plated disc [486], and are measured afterwards with the aid of a FITR microscope or a reflectance accessory. FTIR detection was first applied to the analysis of microbore HPLC eluent by Teramae and Tanaka [505]. In microbore HPLC-FTIR the amount of mobile phase required for separation is much less than for conventional scale HPLC. This simplifies both flow-cell and mobile-phase elimination interfaces. Flow-cell... 

Dehydrohalogenations (elimination of HX) can in turn be effected by passing the halide over a solid base such as vacuum-dried hot KOH or f-BuOK adsorbed on some carrier material. Recently, Billups et al. showed that elimination of Mc3SiBr may even be effected at room temperature over solid CsF. Several very interesting strained alkenes have been generated by such techniques, but although these techniques would lend themselves very well for interfacing to matrix isolation we know of no example where this has been achieved. 

Commercial GC/IR/MS instruments are available from Mattson Instruments (using a matrix isolation GC/IR interface) and from Hewlett Packard (using a highly optimized light pipe GC/IR design). Each uses a Hewlett Packard Mass Selective Detector to obtain electron impact MS data. The instrument in our laboratory is a prototype version of the Mattson instrument, built in collaboration with Mattson Instruments. 

There are three different types of GC/FTIR interfaces light-pipe, matrix isolation (MI), and cryo-deposition (also direct-deposition, cryotrapping). In the two latter techniques, the sample is deposited on a surface before measurement of spectra. All three techniques have been used for the analysis of CWC-related chemicals. Light-pipe interface has been the most popular, even though the usage of cryodeposi-tion in this type of analysis has been increasing over the years. 

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. 

The other interface is a type of matrix isolation in which the analytes are frozen. A comparison of these two types has recently been published.17 Both are listed in Table 3 along with the other interfaces. 

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... 

The interface for matrix isolation consists of a high vacuum chamber housing a motorized gold-platted collection disk and a helium refrigeration unit. Figure 9.17... 

Figure 9.17. Schematic diagram of a GC-FTIR interface based on matrix isolation. (From ref. [193] Marcel...

Figure 2 Schematic of matrix isolation GC-FTIR interface (based on Mattson Instruments Cryolect).

The two IR interfaces in common use are the light pipe [8] and so-called matrix isolation [9]. In the former method, the column effluent is passed through a heated IR gas cell (light pipe), and in the latter, it is condensed and frozen into a matrix suitable for analysis by IR [10]. 

In GC/FTIR systems, three types of FTIR interfaces are currently in use light pipe, matrix isolation (MI), and... 

Matrix isolation was also used to identify the radical intermediates in the partial methanol oxidation reaction over a Pt/Si02 catalyst. Methyl peroxy radicals (CH3OO ) and methyl oxy radicals (CH3OO ) were discriminated between, based on their different stability to photolysis (142). By combining EPR and in situ IR spectroscopy, it was shown that the radical species form super equilibrium concentrations under reaction conditions at the platinum/support interface region (143). 

Supercritical fluid chromatography (SFC)/ FT-IR spectroscopy, generally with carbon dioxide as the mobile phase, bridges the gap between GC/FT-IR and LC/FT-IR, and is particularly useful for separating nonvolatile or thermally labile materials not amenable to gas chromatographic separation [109J. Flow cells, mobile phase elimination and matrix-isolation techniques are used as SFC/FT-IR interfaces. 

Figure 23.5. Cryolect matrix-isolation GC/IR interface. (Reproduced from [17], by permission of the American Chemical Society copyright 1985.)...

In practice, however, the matrix-isolation GC/FT-IR interface has three main drawbacks ... 

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