Light pipe interface

Table 7.46 shows the LC-FTIR interface detection limits. Detection limits approaching those for GC-FHR light-pipe interfaces have been reported for flow-cell HPLC-FTIR when IR-transparent mobile phases are employed. For both the moving-belt and thermospray LC-MS couplings the detection limits are in the ng range. Selective evaporation consisting of fraction collection followed by DRIFT identification achieves a detection limit of 100 ng. 

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. 

Light-pipe interface is the easiest method for connecting a GC and an FUR spectrometer. The lightpipe is a long flow cell with reflective inner coating and IR transparent windows at both ends of the cell. It is very easy to operate and to maintain. The problem of light-pipe interface is that it is not very sensitive compared to a normal bench-top mass spectrometer, which makes the analysis of the same samples with both instruments difficult. 

Spectram of Linalool from a Light Pipe Interface Courtesy of the Perkin Elmer Corporation... 

Figure 3.5 shows a schematic diagram of a typical GC-FT-IR system. The apparatus consists of an FT-IR spectrometer, a gas chromatograph, a light pipe, a detector and a computer for analysis. The light pipe interfaces the gas chromatograph with the spectrometer, and is usually made of quartz it should be... 

Instrumental Interface. Gc/fdr instmmentation has developed around two different types of interfacing. The most common is the on-the-fly or flow cell interface in which gc effluent is dkected into a gold-coated cell or light pipe where the sample is subjected to infrared radiation (see Infrared and raman spectroscopy). Infrared transparent windows, usually made of potassium bromide, are fastened to the ends of the flow cell and the radiation is then dkected to a detector having a very fast response-time. In this light pipe type of interface, infrared spectra are generated by ratioing reference scans obtained when only carrier gas is in the cell to sample scans when a gc peak appears. 

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... 

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... 

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. 

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. 

GC resolution degrades slightly in all of the available GC/FTIR interfaces. In the light-pipe type interface, some broadening of the chromatographic peaks occur because the diameter of the light-pipe... 

Instruments IR-85 Fourier Transform infrared spectrometer, through an IBM GC-IR interface. The interface consisted of a gold-coated Pyrex light-pipe with potassium bromide windows. A scan rate of 6 scans/sec and a spectral resolution of 8 cm- - were used for data acquisition. Samples were introduced into the system via splitless injections. A fused silica capillary column, 30 m x 0.32 mm i d DB-WAX (dj 1.0 pm), was employed with the outlet end connected directly to the GC-IR light-pipe entrance. Helium was used as the carrier gas at an average linear velocity of 41.4 cm/sec (35°C). No make-up gas was employed in the system. The column temperature was programmed from 35°C to 180°C at 2°C/min. The GC-IR light-pipe assembly was maintained at 170°C. 

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-. ... 

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 between the sample and the spectrometer is vital wherever a spectrometer is sited. The sample can be piped into the spectrometer or, in some cases the radiation used by the spectrometer can be transmitted to a convenient sample or probe location point using optical fibres or other light-pipe devices. The sample presented to the spectrometer must be representative of the material from which the measurement is required and the interaction between the radiation of the spectrometer and the sample must be suitable for the measurement to be made (sufficient power and suitably clean interface). 

There are two main types of FTIR detection for GCs, in the gas-phase using an in-stream optical system and through vapor deposition with detection being away from the GG flow stream. In the first, a light pipe that can transmit IR radiation is positioned on either side of a detection cell. Transparent windows pass the IR radiation into the flow ceU. The whole assembly is maintained at temperatures of 250 °C to 350 °C to prevent deposition of sample molecules. Most interfaces for this type of GC-FTIR also have heated transfer lines to and from the flow cell to ensure that no deposition occurs before introduction into the spectrometer. 

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