Piping Light

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. 

The LaTIS laser device (LaTTS, Minneapolis, MN) uses the slow injection of contrast material as a light pipe to carry the energy from the catheter to the embolus. The device was evaluated in a safety and feasibility trial at two U.S. centers. A preliminary account reported that the device could not be deployed to the level of the occlusion in 2 of the first 5 patients, and enrollment stopped at 12 patients. Although the catheter design was changed, an efficacy trial has not been pursued. ... 

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... 

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. 

For sensing applications, high bending losses restrict the applicability as (flexible) light pipes. Practical applications are mostly restricted to gas cells, where the hollow waveguide acts as a compact multi-reflection cell to increase the sensitivity in comparison to single-pass cells. 

The secondary electrons emitted from the sample are attracted to the detector by the collector screen. Once near the detector, the secondary electrons are accelerated into the scintillator by a positive potential maintained on the scintillator. Visible light is produced by the reaction of the secondary electrons with the scintillator material. The emitted light is detected by a photomultiplier tube, which is optically coupled to the scintillator via a light pipe. The PMT signal is then transferred to the grid of a cathode ray tube (CRT). Data collection... 

Gaseous samples require long path length cells to produce absorption bands of reasonable intensity up to several metres of optical path are obtainable from cells incorporating mirrors which produce multiple reflections. For GC-IR, light pipes provide the best sensitivity (p. 117). 

Figure 1. Diagram of Teflon cell (1) platinum electrode (2) glass scintillator (3) Macor ceramic disk cell bottom (H) Teflon O-ring (5) flexible elbow (see insert) (6) cell ports (six of them around cell body) (7) light pipe. Inset shows the details of the flexible elbow (8) stainless steel sphere (9) concave Teflon spacer (10) platinum wire for electrical connection across elbow (11) lock nut.

Both the GC-MS and GC-IR instruments obviously require that the column effluent be fed into the spectrometer detection path. For the IR instrument, this means that the IR cell, often referred to as a light pipe, be situated just outside the interferometer (Chapter 8) in the path of the light, of course, but it must also have a connection to the GC column and an exit tube where the sample may possibly be collected. The infrared detector is nondestructive. With the mass spectrometer detector, we have the problem of the low pressure of the mass spectrometry unit coupled with the ambient pressure of the GC column outlet. A special method is used to eliminate carrier gas while retaining sufficient amounts of the mixture components so that they are measurable with the mass spectrometer. 

The integrated fluorescence signal //was collected with a g-in. glass light pipe and detected through a combination of dielectric and colored glass filters with a photomultiplier tube. Fluorescence excitation and elastic scattering spectra were recorded simultaneously, in order to identify the type (TM or TE) of resonance responsible for the peaks seen in the excitation spectrum. 

Figure 2. Laser reflectance system. The light pipe is a 3-mm-diameter quartz rod.

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