Heat exchanger, detector cell

Detection Inlet heat exchangers Flow cell volume and geometry MS ion sources Sprayers (e.g., in evaporative light scattering detectors) Data filtering effects in high speed applications... 

The separation of a mixture of aromatic compounds (benzene, naphthalene, anthracene, chrysenes, and benz(a)pyrene) at 31 bar is shown in Figure 3. This chromatogram was obtained with a Perkin Elmer Model 250 ultraviolet detector with the high-pressure cell placed after the cooling heat exchanger and before the flow control valve. A similar chromatogram is obtained with an Isco Model UAA with a 10 mm micro cell placed after the flow control valve. 

The detector inlet usually passes through a coiled stainless steel tubing heat exchanger and into the flow cell. The flow cell is the most complicated part in the system. The body is stainless steel or quartz, windows are quartz, and, if it can be taken apart for cleaning, there is usually a Teflon gasket between the stainless steel body and the quartz window. Finally, we move out of the flow cell into the wide-diameter Teflon tubing of the outlet tubing and into a backpressure device in the waste vessel. 

Figure 21-23. Flow scheme of an on-line HPLC system developed in-house (1, mobile phase 2, HPLC pump 3, heat exchanger 4, injector with 2-p.L loop 5, HPLC-column 6, heating oven 7, UV cell 8, LFV lamp 9, UV detector 10, reference sample 11, restrictor 12, valves).

If the inner diameter of the connecting capillaries is < 0.2 mm and there is no heat exchanger capiilary hidden in the detector and your fittings are OK. your only option to significantly reduce your dead volume is by a smaller detection cell. 

All volumes within an HPLC instrument which affect the separation are termed extra-column volumes. (The term dead volumes should not be used because sometimes the retention volume at fo is also called dead volume.) This concerns the volume of the injector, the capillary between injector and column, the capillary between column and detector, the volume of the detector cell, the volume of the connecting fittings and, depending on the instrument design, also additional parts such as heat exchange capillaries (often hidden within the detector) or switching valves. Volumes of the system which are located before the injector or behind the detector are not extra-column volumes. 

By using the deflection principle of refractometry, it is possible to use one sample cell throughout the entire refractive index range from 1.00 to 1.75. An optical block and heat exchanger are provided to bring the liquid temperature to the temperature of the cell at all flow rates normally encountered. An example of a chromatogram obtained from this type of detector is shown in Figure 2. 

In order to eliminate thermal noise and flow sensitivity, the inlet flow of solvent from the column must be brought to the same temperature as the detector cell before it actually enters the cell. The heat exchanger used for this is shown in Figure 8 which embodies many of the standard features of modern LC detector heat exchangers. The inlet tube is first coiled tightly round the exit tube to make good thermal contact and then wrapped round an... 

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