General Recommendations for Using an Agilent RID

Answer

This Information Applies To: Agilent Refractive Index Detectors, G1362A and G7162A/B.

Issue:

The RID is sensitive to its environment and can experience stability problems when exposed to contamination, vibrations, mechanical shocks, or changes in temperature or pressure. 

Resolution: 

Considerations on the general set-up:

• To avoid cross-talk between different solvent channels, preferably use an isocratic pump when operating a RID. If it is not possible to use an isocratic pump, consider the following: 

  • Binary pump: Use channels with a 50:50 solvent ratio for Channel A and B. Both channels should draw from the same, premixed solvent out of the same bottle. Bypass the solvent-selection valve if installed.

  • Quaternary pump: Bypass the multichannel gradient valve (use the PEEK adapter part number 0100–2298 PEEK Adapter), so that the solvent tube comes directly from the degasser directly to the inlet valve of your pump head.

  • Take notice of the importance of how solvent mixtures are prepared and their potential influence on retention times: Mobile Phase Preparation for Isocratic LC Systems.

• Never ever operate a RID without a proper degasser. All Agilent degassers can be used except for G1379A and G1379B.

• Always have a thermostatted column compartment (TCC) / multicolumn thermostat (MCT) in your stack.

• When the solvents have been exchanged or the pumping system has been turned off for a certain time (for example, overnight), oxygen will rediffuse into the solvent channel between the solvent reservoir, vacuum degasser, and the pump. Solvents containing volatile components will slightly lose them. Therefore priming of the pumping system is required before starting an application. As per recommendation in the manual, pump for at least 30 minutes before the operation to ensure a stable signal.

Considerations on the pressure:

• The solvent and waste bottles should be stored above the module to have a little backpressure in the waste line, stabilizing the RID signal.

• The tubing should have no extra restrictions such as bends or curves.

• The maximum pressure of the flow cell is 5 bar. If multiple detectors are used the RID should always be the last module in the stack.

Considerations on the temperature:

• The setpoint of the RID Heater should be set to the same temperature as the Column Oven (considering limitations given by the column's temperature resistance). The RID optical unit's temperature can be operated between 5 °C above ambient and 55 °C while the recommended setting is at least 5 °C above ambient temperature. Adhering to any limitations given by the column, solvents and sample, a higher temperature setting will typically deliver better results.

• If the temperature of the solvent or waste line or deposits varies a lot, measures should be taken to stabilize it or shield the solvent lines and bottles against external influences. Measures include stable air conditioning, shielding the tubes and solvent reservoir from sunlight and against evaporation. While using air conditioning, avoid placing the RID directly below the air current.

• Let the temperature inside the RID reach its setpoint and stabilize for at least two to four hours. Even if the sensor shows the set temperature, this does not mean that the whole optical unit has reached this temperature or that the baseline is stable. Before conducting any operational qualification or performance verification, it is recommended to let the system equilibrate over night.

• If a baseline-wander persists, consider purchasing the Agilent Thermal Column Insulation Enclosure kit for your TCC (or the Agilent InfinityLab Thermal Equilibration device for your MCT). The part numbers are G1316-60001 and G7116-60013, respectively. Composition fluctuations in the eluate may cause baseline-wander, especially in water-acetonitrile mixtures with high acetonitrile content. These fluctuations are induced in turn by very minor temperature fluctuations on the column. The temperature fluctuations in this range are difficult to control, because the ramps of 10-3 ºC/min or more are already critical and can cause baseline offset of over 100 nRIU. Ask your local Sales or Support representative for further information.

Considerations on the correct use and housekeeping:

• Use the recycle valve to allow automatic recycling of the mobile phase delivered when no analysis is running. The pump flow can therefore continue uninterrupted until the next analysis without wasting mobile phase solvents. In addition, the RID is always stabilized and ready for immediate use (Right-click your RID and select Recycle: On or check Automatic recycling after analysis in your method).

• If the flow is stopped and the mobile phase remains inside the online vacuum degasser, the solvent composition will change. When restarting the flow or when using the new mobile phase, flush each degasser channel used for 10 minutes. Use the maximum flow rate of the pump (with the purge valve of the pump open to avoid a potential overpressure in the RI detector’s flow cell).

• The optical balance, which is calculated based on the signals of the two light-receiving diodes as (diode₁ - diode₂) / (diode₁ + diode₂), should always be zero. To check this, the optical balance procedure should be followed to adjust the amount of light falling onto each diode.

• If you encounter issues with your signal, check your system for correct function (leaks, clogged filters/frits, solvent degassing, solvent changes over time, correct function of the RID purge valve).

Learn how to effectively operate your Agilent Refractive Index Detector: G7126A - Agilent 1260 Infinity II RID user manual on Agilent.com