Chromatography Forum

I have some trouble with my Agilent 1200 RID. CS A.10.02. Running 0.6 mL/min with 10 mmol sulfuric acid. Regardless of what pumps there seems to be some electronic
problem. Take a look at the pic below.

The RID signal fluctuates by about 60,000 units. Turning the RID heater on and off seems to make no difference. Note that the temp of the optical unit seems to spike at
the height of it's ripple but the periodicity of this is constant whether optical temp is on or not. Diode1 and diode2 both spike right after the temp spike. Any ideas?

Thanks,
Marc

Hi Marc,
it looks like there are air bubbles in your system. Please check the RI flow cell first.
Good luck.

Run with pump at zero flow, to confirm that the issue is with the pump.

What do you mean by regardless of what pumps? Do you mean different pumps or just different channels of a quat pump? Are you bypassing any solvent mixing valves for your mobile phase?

And what about basics, purging, degassing, etc.?

Just wanted to add that the pressure is perfectly constant at 60 bar, there is no injection and the UV is monitored
at 200 nm (measured just before the RID) and the UV is also completely flat (<0.01 AU noise and flat and random)

My wild guess is that you have a bit of outgassing (or, depending on the flow resistance of the RI cell, maybe a small leak at its inlet such that it can aspirate air), with a small bubble hanging in the flow cell and accumulating friends to gradually grow larger until it finally breaks loose to start the cycle anew. The fact that you don't see it on a UV detector *before* the RI may simply be that either a) there is enough back pressure on the UV cell that the bubbles don't form until the outlet or b) the bubbles are small enough not to show up as spikes and the UV flow cell geometry is such that they don't hang up and accumulate.

Following up on Gerhard & KM's comments: cut the flow in half and see if the period (approximately) doubles.