Avoiding Refractive Index Detector Problems - - Chromatography Online
Avoiding Refractive Index Detector Problems

LCGC North America
Volume 30, Issue 12, pp. 1032-1037

Temperature Problems

As described above, the RI detector is constructed to shield the detector cell from external changes in temperature, both through use of a thermostated cabinet and a heat exchanger for the incoming solvent. However, both of these features are compromises between effectiveness and expense. The cabinet cannot protect against all environmental temperature changes, so it may be necessary to take additional action to protect the instrument from local environmental temperature fluctuations. Although the laboratory temperature control may be quite good, at least as indicated by the thermostat on the wall, the local temperature may vary. Perhaps a heating vent blows hot or cold air directly at the instrument or direct sunlight may cause local warming of the laboratory. In some laboratories, a different temperature is maintained at night than in the daytime. Any of these factors can result in a change in the temperature of the local environment. You may need to block a heater vent or redirect it. In one laboratory I visited recently, the staff had built a cabinet around the LC system to shield it from local temperature fluctuations. It may be necessary to move the instrument to another location with better temperature control.

The heat exchanger's job is to change the temperature of the incoming mobile phase to match that of the solvent in the detector cell. Because the heat exchanger adds extracolumn dead volume to the system, it is a compromise between efficiency of temperature adjustment and minimizing extra volume. To minimize the temperature adjustment requirements, a column oven should be used and set to the same temperature as the detector (or vice versa) so that little or no temperature change is necessary. Also, be sure to insulate the tubing that connects the column to the detector. Some detectors come with insulated connecting tubing, but a simple homemade insulator can be made by slipping a piece of heavy-walled plastic or rubber tubing over the connecting tubing.

It may take several hours for the detector to warm up and equilibrate with the column temperature, so most RI detectors are equipped with a valve that can divert the waste stream back into the mobile-phase reservoir. In this manner, the mobile phase can be recycled and the system can be left with the flow on for several hours to warm up or left pumping continuously, so it is always ready to use. When samples are run, the valve is switched so that the solvent from the detector is directed to waste. If you do recycle the mobile phase, be sure to replace it once in awhile. For a mobile phase that has more than approximately 70% buffer or aqueous component, I recommend changing the mobile phase once a week. When the mobile phase contains at least 30% organic solvent it can be used longer, but it should be replaced every few weeks. Mobile phase that is used for too long can gradually change composition because of evaporation of a more volatile component or may grow bacteria that can block frits in the system. Be sure to replace the reservoir with a clean one instead of refilling the reservoir to prevent passing any contaminants from the previous batch of mobile phase on to the new one.

Temperature-related problems usually show up as baseline drift. Depending on the magnitude of the temperature change and the sensitivity setting on the detector, this may be a gradually sloping or steeply sloping baseline. When baseline drift is a problem, review the preventive steps listed above and see if there is something you can modify to reduce the problem.


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