Flow cell interfaces

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 flow-cell interface in HPLC-FTIR, first reported in 1975 [492], is the most straightforward. Flow-cells consist of two IR-transparent windows (KBr for... 

Figure 7. (a) Flow diagram of the optical fibre continuous-flow system for bioluminescence and chemiluminescence measurements S, sample C, carrier stream PP, peristaltic pump IV, injection valve W, waste FO, optical fibre FC, flow-cell, (b) Details of the optical fibre biosensor/flow-cell interface a, optical fibre b, sensing layer c, light-tight flow-cell d, stirring bar. 

All experiments utilized a variable wavelength uv detector. Analysis of the hexane fraction was carried out using both the UV detector and an on-line Nicolet 6000 FTIR. The FTIR flow cell interface has been described in detail elsewhere.(9) This same FTIR was also used to gather static spectra on the various fractions, utilizing a liquid cell with KBr windows. 

One approach has been the use of a low-volume, internally reflecting, heated lightpipe (flow cell) interface between the gas chromatography and the infrared spectrometer (see Figure 3.37). The lightpipe has IR-transparent alkali halide windows at each end so the IR gas phase spectra of the eluent are recorded as they emerge from the column. 

In a similar manner to the lightpipe in GC-IR, a flow cell interface can be used where the eluent flows into a transmission flow cell with IR windows at each end and the IR beam passes through the cell perpendicular to the flow and continuously records IR spectra. This only works if the solvents do not absorb so strongly in the IR region of interest and mask the analyte transmission/absorption, or if the mobile phase absorption spectrum is subtracted (which only works well when the LC method is isocratic). In fact, the solvents used in normal phase HPLC are more compatible with IR than the more polar ones used in reversed phase HPLC. 

Because of the poor sensitivity of FTIR, the flow-cell interface has been mainly used with packed-column SFC for analysing sample mixtures [22]. Unfortunately, the identification capacity of this type of interface is limited by interfering mobile-phase absorptions, which reduce the sensitivity of measurements. Solvent elimination has therefore been applied to real-life applications such as polymer analysis [24]. 

Fig. 5.15 Schematic representation of the flow cell for electrochemiluminescence measiurements and detailed representation of the sensor/flow cell interface. Reprinted with permission from Ref. [77]. Copyright 2000 Marcel Dekker, Inc...

Norton and Griffiths " studied the comparison of flowcell and DD interfaces between GC and FTIR spectrometers. Seven barbiturates were separated on fused-silica GC capillary columns. Infrared spectra of the separated barbiturates were measured in real time by either a Hewlett-Packard infrared detector (flow-cell) interface or a Digilab Division of Bio-Rad Tracer (DD) interface. Without losing chromatographic resolution, the GC/DD-FTIR interface gave both detection limits and minimum identifiable quantities nearly two orders of magnitude lower than the flow-cell GC/FTIR interface. 

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