We have been experiencing very large linear basline drift with our VSD Optilab refractive index detector. System is 0.3% NaCl MQ water at 0.5 ml/min over a HPSEC Shodex SB-804 column with Kontron pump. Drift is order of 100 mV in 30 min. The supplier's technicians don't find the problem. (UV is working fine)

Can somebody comment on that ?

Most drift with RI detectors is caused either by temperature or a slight leak in the reference valve. I would certainly check out the temp issue first.

Have you looked into a temperature issue yet? Does your lab experience shifts in temperature from morning to afternoon? Does your detector have an internal temperature controller?

The RI detector doesn't have a temperature controler.Temperature is an issue at some times (especially during warmer summers), but drifts are never near as large as they are now. We'll check the reference cell again, although it looked ok at first sight.

In the absence of a leak in the reference cell, another common cause of baseline drift is that the RI of the mobile phase is changing relative to that of the solvent in the reference cell. We always degas our mobile phases by helium sparging then maintain a helium pressure of about 5 psi in the mobile phase container to prevent reabsorption of air. You will need to purchase an apparatus designed for this purpose with an appropriate safety valve. I believe Alltech sells one. Inadvertent overpressurization of the mobile phase container can be very dangerous.

Best Regards,

Michael Hinsberg
http://www.hinsbarlabs.com