I'm seeing a "wavy" baseline from my uv detector. The signal looks like a sine wave, with a period of about 2 minutes, and an amplitude of 0.1-0.2 mV. It is independent of flow rate, and uv wavelength (220-260), although at longer wavelengths the behavior is less evident. Over the course of this investigation, I've performed maintenance on the pump, changed uv lamps, even eliminated pieces of hardware (e.g. autosampler/ degasser) but have not met with success. I decided to change mobile phase, and the wavy behavior disappeared. Any ideas what's happening here? ("problem" mobile phase: THF:MeOH:buffer, 27:24.5:48.5; buffer=TEA phosphate; "good" mobile phase: 20:80 ACN:buffer, 50 mM phosphate, pH5). Is this related to a temperature or refractive index phenomenon or just bad electronics? We don't see this problem when running this mobile phase on other systems. Any help would be greatly appreciated. Note: column (C18) is heated to 40 C.

Ray
Your problem doesn't sound like electronic noise since that would show up with your other, "good", mobile phase. Anytime I have seen a regular sine wave it has been due to a tiny bubble. It could be that the "bad" mobile phase is more susceptible to this problem on your instrument. Purge thoroughly and increase and decrease your flow to see if you can get rid of the sine wave that way. Also tighten all your connections.
Wish you luck.
Thanos

You did not tell us how you mix the mobile phases. If you let your instrument mix it, there could be an issue associated with the mixing that shows up with the mixing of the three components and does not show up when you mix only two components.
Another possibility is that the column heater is the issue. Column heaters switch off and on, and this could be the cause of the sine wave. You would get different effects when you start running to when the temperature is in equilibrium or if you change the temperature slightly.

We have uncovered similar wandering (~50C) - less of a regular sine wave and more of irratic wandering. Our theory is it is due to inadequate heat dissipation btwn the col htr and the detector -- apparently based on mob ph additives with some UV background (TEA, octane sulfonate) or RI effects. It is vendor specific with HP1100 and ABI 783series no problem, Waters 486 showing the problem. Looking at tubing and heat sink differences was a major clue (we had chased pump and other problems for months!). Our experiment was: no column, normal flow rate, 210-220 nm, col htr off for 10min, ON, off, on.... Wavelength - we observed a significant dependence on nm, with longer (eg 240+) not having such a problem (indicative of solvent related cause).
Per Thanos' comment - we tried Waters, Eppendorf solid state heaters and Shimadzu forced air - same result.
Uwe - I'm meeting with Richard Cone on Monday and will discuss, need to talk to Waters Tech Rep.
Steve