RID stabilization

Please follow Below Procedure for RID Optimization.

Follow these thirteen points to optimize the performance of your refractive index detector.

1 Position the solvent and waste reservoirs correctly

Position the solvent and waste reservoirs above the level of the refractive index detector and solvent pump. This maintains a slight pressure in the sample cell and will improve the performance of the detector.

2 Do not over pressurize the flow cell

Be aware to not exceed a 5 bar pressure drop after the flow cell when hooking up additional devices like other detectors or a fraction collector. If an additional detector is installed place upstream in the flow path it before the G1362A refractive index detector.

3 Use the correct solvents

To minimize baseline noise and drift solvents must be LC grade and filtered prior to use.

4 Check for leaks

Leaks within the LC instrument that the refractive index detector is connected to will cause problems with baseline long term noise or drift.Confirm that the instrument is free from leaks by performing the Agilent 1200 Series diagnostic pressure test (for the high pressure parts of the system between pump and column). Ensure that the connections from the on-line vacuum degasser to the pump and the detector inlet, waste and recycle connections are air tight.

5 Verify frit, filter and fitting quality

Partially blocked frits, filters and fittings can cause baseline long term noise. Verify that the pressure drop across all such parts is within expected limits.

6 Control the optical unit temperature

1200 Series RID User Manual 69 Optimizing the Refractive Index Detector 5 Always control the optical unit temperature (heater = ON) for maximum detector sensitivity or with samples that could precipitate in the sample cell at room temperature and set an elevated optical unit temperature at least 5 °C above ambient conditions.

7 Use an appropriate response time

For most applications a setting of 4 seconds is adequate. Only for high speed analyses (short columns at high flow rates) a lower setting is recommended. Bear in mind that even if the response time setting is too high fast peaks will appear a little smaller and broader but retention time and peak areas are still correct and reproducible. 

8 Recycle mobile phase

Use the recycle valve to allow automatic recycling of 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 refractive index detector is always stabilized and ready for immediate use.

9 Consider using a degasser

For many solvents you can achieve better baseline stability, when using a degasser. For some solvents a degasser might not lead to a better baseline quality.

10 Flush the degasser

If flow is stopped and mobile phase remains inside the on-line vacuum degasser the solvent composition will change. When re-starting the flow or when using new mobile phase flush each degasser channel used for 10 minutes at the maximum flow rate of the pump (with the purge valve of the pump open to avoid a potential over-pressure in the RI detector’s flow cell).

11 Use pre-mixed solvents, only

Don’t use a pump for mixing solvents. When operating the RI detector together with a quaternary pump, bypass the MCGV in the quaternary pump. You have to virtually convert the quaternary pump into an isocratic one, by directly connecting the solvent inlet tubing from degasser or solvent bottle to the active inlet valve of the pump (use adapter 0100-1847, which is delivered with the accessory kit of the detector).

12 Consider solvent changes with time

70 1200 Series RID User Manual 5 Optimizing the Refractive Index Detector Baseline drift can be caused by the tendency of certain solvents to change over time. For example the acetonitrile content of acetonitrile/water mixtures will decrease, tetrahydrofuran will form peroxides, the amount of water in hygroscopic organic solvents will increase and solvents such as tetrahydrofuran held in the reference cell may begin to regas.

13 Eliminate mobile phase/column combination problems

Certain mobile phases in combination with specific columns can generate long term baseline noise. For example acetonitrile/water mobile phases with certain aminopropyl bonded phase columns. To eliminate the combination of mobile phase and column as a cause of long term noise replace the column with the restriction capillary (G1362-87301)

All the important steps can also be found in the manual. No reaction on the baseline COULD point to a faulty purge valve. Keep pumping with fresh, clean water and switch the purge valve repeatedly. Maybe add an IPA flush in-between and also keep switching the valve. Once the valve is working correctly, thoroughly flush both the sample and reference side with water, then check the diode balance in LabAdvisor.

I disagree with that statement completely! By using a gallon of water, you create a nice place for algae to grow and then you pump it trough the RID. Not to mention the weight of such bottle.

I recommend using a large bottle of solvent, ie a gallon of water instead of a liter. This will allow the solvent to be more resistant to temperature changes.

Yeah notice I didn't say "..and keep that bottle exposed to light and let the water sit for weeks on end."

Consider RI for water v. temperature and  the y axis units of the RID. My opinion was developed over years of working on RIDs, including many successful RID OQs with less than ideal environmental conditions.

All of the previous advice is very good.  I do all of these things, too.

In addition:

1.  I place a box over the HPLC solvent bottles to provide a buffer against temperature fluctuations and air currents (HVAC off-on cycles).  The boxes that vendors ship 4x4-L solvent bottles works great.

2.  At another lab, I had to fashion a cardboard baffle, packing taped to ceiling vents to direct air currents away from blowing directly on the HPLC system / RID.

3.  I tightly swath all exposed flow path lines in small plastic bubble wrap.  This includes the waste line, as it can amazingly radiate temperature changes back to the detector.

4.  A vendor taught me how to coil metal HPLC line tightly around a pencil, attach it to column, then fit it inside the heater with the column (a pre-conditioner).  It took a couple of tries to determine the best i.d. and length.

I know you are expecting me to say that I stand on my left foot while chanting during the RID analysis. . . Tempting at times, but not yet.  These are the things that I had to add over a few years of working with RID analyses, older building / HVAC / electrical, etc.

I hope this helps.  Good luck with your RID.