Refractive Index Detector questions

[itspip]

I recently started with a company and have been given the task of using the HPLC more for the analytical work. We have an Agilent 1260 Binary Pump setup with an Diode Array Detector and a Refractive Index Detector. The RID was not setup with the system at all. I have been able to hook it in line with the system and configure it. I have been able to get the RID to send out data.

Here's where things start to go awry. When I run blanks I notice that the baseline, which is acceptable in the DAD drifts and is generally awful in the RID. I do see a solvent front in both the DAD and the RID. When I run our products, malonate, the sensitivity is far higher than in the RID. I ran a curve of concentrations from 10µg/g -> 100µg and all I saw in the RID was noise.

I am not as familiar with the RID as the DAD. I do have the heater set to 30ºC. What other places should I look to help with the RID?

[ATAlR]

The only thin that comes to my mind is that reference cell is not purged enough. Try to purge it with IPA for 20 min on 1ml/min, then again with your mobile phase.

[Consumer Products Guy]

For sensitive RID stuff, very important to run the pre-mixed mobile phase from just one pump. So maybe bypass the other pump if necessary (and similar if using a quaternary pump), Let the RID warm up a long time with mobile phase pumping or recycling through column with column compartment temperature stabilized, through detector. Overnight is nice.

[tkubowicz]

Hello

First of all RI is very sensitive for temperature so:
1.Make sure that connection between DAD and RI is as short as possible
2.Always have front cover on RI detector
3.Make sure that RI is not close to air con or any source of air circulation area
4.Make sure that RI (LC stack really) has enough space from sides and back to have good heat exchange area.

As you described you see drift on RI so I'd check temperature first. Of course second thing is reference cell and good flushing.

I hope it will help a bit

Good luck

Regards

Tomasz Kubowicz

[Kreall]

I don't think that you will be able to see a malonate on RID with this low concentrations (10 - 100 ug/ml). I would go moe with concentations about 1 mg/ml.

Also RID detector is very vulnerable to pressure differences. See if there is no blockage after or before the detector.
Kreall

[JI2002]

Agree with Kreall. Generally UV is much more sensitive than RID. Try standards at much higher concentration than 10-100 ppm.

[itspip]

Thanks for the advice everyone.

Should I expect an order of magnitude difference in the detection between UV-Vis and RI?

[JI2002]

It's not uncommon to see an order of magnitude difference in the sensitivity between UV-Vis and RI. Samples are generally prepared > 1 mg/mL for GPC-RI.

[Sallybeetle]

All of the previous advice is very good RID advice.

In addition to the previous advice from others, I make sure my lab is tightly temperature controlled, place a box over the mobile phase reservoirs, and bubble wrap any exposed lines between the HPLC units. It sounds nuts to do all these things, but the cumulative effect is better RI baseline.

[gstaepels]

Hello,

I addition to the previous recommendations, I have found that degassing the eluent is very important. The best way to do this is to degas with Helium, with at least 10 times the volume of helium relative to the volume of the eluent. Also maintain a constant He sparge ( 10 - 20 ml/min).