Optical Balance Procedure for the Refractive Index Detector

Answer

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

Issue:

The amount of light falling on each of the light receiving diodes is out of balance. You may get the system message "unbalanced diodes".

An incorrect signal, which cannot be eliminated by purging the sample and reference cell with the same solvent nor by equilibrating the system, may be caused by unbalanced diodes. The optical balance procedure is performed to correct such a permanent misalignment of the light beam.

Steps to follow:

First, perform a general cleaning on the instrument, based on your last analysis:

1. Keep the analysis temperature on the detector (follow the method temperature).
2. Prepare the mobile phase/solvent to be used in your analysis and purge the pump normally.
3. Flush the solvent into the equipment with the usual operating flow for at least 15 minutes.
4. Turn on the RID purge valve and purge the reference cell for 10 minutes.
5. Turn off the RID purge valve, but keep the operating flow.
   
   
   Optical alignment
   
   
6. Close the acquisition software and openAgilent Lab Advisor software.
7. Connect to your instrument of interest.
8. After that, select the Service & Diagnostics option from the left menu.

9. Select the RID module and then select the RID Tool Screen option. The following screen should appear (Figure 1). Depending on your instrument configuration and version of Lab Advisor, it might look slightly different:

   
   
   
   Figure 1. Screen for the optical balance procedure (Lab Advisor version 2.17)
   
   
10. In you have an older Lab Advisor version: In the Optical Balance section, click On.
11. You'll see a bar on the scale from -1 to 1, as shown above, and a value corresponding to that position will appear on the screen.
12. Unscrew the screws on the service door (Figure 2, steel cover, marked in blue) to open the RID front cover to access the valves: 
    

    
    Figure 2. Opening the service door 
    
    

13. Use a flat screwdriver long enough to reach the setting point shown in blue (Figure 3): 
    

    
    
    Figure 3. Accessing the zero glass adjustment screw
    
    

14. Rotate the adjustment screw lightly until you notice that the value shown in the Lab Advisor is modified. This turns the zero glass in the light path to adjust the amount of light falling on each diode.
    1. The direction of movement will increase or decrease the unbalance value.
    2. Rotate to the direction that approaches to “0” (balance).
15. You should find the position that is closest to 0 (green region of the scale).
16. After zeroing, reinstall the metal and plastic covers.
17. Return to the software and check if the unbalance message is still present.

Learn how to effectively operate your Agilent Refractive Index Detector: Agilent InfinityLab LC Series: Refractive Index Detector user manual on Agilent.com