Baseline Flactuations

[Omar]

Hi All,

when we run several isocratic HPLC methods with a variety of mobile phases and on different systems, sometimes we observe baseline flactuations similar to the one shown in the above chromatogram, with a periodicity of 5 to 10 minutes. The strange thing is that when we switch off the air conditioners of the lab the flactuations disappear. In most cases the columns are thermostatted and so I do not think that its a temperature flactuation. Do you think it could be an electronic interference? Has anyone ever seen something like this? Do you suggest any solutions for such an occurance other than swiching off the A/C s?

Thanks very much for your help,
Omar

[zokitano]

Dear Omar,

I had a similar situation with my Agilent's 1100 DAD last week. We also used air conditioning in the lab to maintain a room temperature (cca 25 degrees Celsius). I observe this baseline drift during the column equilibration with a new mobile phase (in the morning). I used column thermostating too.

The problem seemed to go away after I "balanced" the detector. I simply run the balance function for the detector and the signal became as it supposed to be-flat. I suppose that it seemed like a electrical interference that caused the baseline to have toothy appearance.

Hope this helps
Regards

[Peggsy]

just a thought, this type of sinal/wavy baseline is often a sign of pump/flow related problems. Do a check valve test, its quick and easy to do and will tell you if all the checkvalves (both inlet and outlet) are operating fine.

Please let us know the outcome.

Greg

[Omar]

I am pretty sure its not a check valve problem as it disappears as soon as we switch off the A/C

regards
Omar

[JGK]

In our Lab the LC equipment is connected to the power supply through UPS units to bridge the gap between power failure and our generator unit coming up to speed.

The units we have also contain a current smoothing facility. If your problem is related to the power supply this may work.

[Bryan Evans]

Get a multimeter and check the voltage at the receptical - with the A/C unit on and off and see if
there's a difference. It's best if you have a multimeter that can determine voltage max / min over a period of time.

Use fans instead of the AC (sorry)!

[GoldLeader]

Also, try searching the forum for a thread " Humidity Affects Baseline." This topic has been discussed a couple of times before. I have seen a similar phenomenon in my lab - baseline wave cycling with A/C on/off cycle - and it seemed more related to humidity fluctuations caused by AC rather than actual temperature. Not sure how to mitigate this other than not running A/C during critical sequences.

Does this also occur when simply monitoring the baseline with no flow? This should rule out pump issues (power cycling/bubble, etc.)

[mardexis]

If you have an online degasser I would try bypassing it to see if the problem goes away. I have seen the degasser cause a fluctuation at that frequency a few times.

[zokitano]

In our Lab the LC equipment is connected to the power supply through UPS units to bridge the gap between power failure and our generator unit coming up to speed.

The units we have also contain a current smoothing facility. If your problem is related to the power supply this may work.

From my personal lab experience I agree with JGK.
See if you could eliminate any electrical/magnetic field interferences nearby your instrument. Are there any electrical devices that haven't being electromagnetically insulated? I wonder if cell phones could trigger such interferences.

Best regards

[Omar]

Get a multimeter and check the voltage at the receptical - with the A/C unit on and off and see if
there's a difference. It's best if you have a multimeter that can determine voltage max / min over a period of time.

Where exactly would you check the voltage? I am pretty sure that the power supply is stable. Would you check it at the detector end?

Furthermore the system is on a UPS and on a different phase from the A/C system. Also the flactuations are also seen with a switched off pump.

Thanks and regards

[Bruce Hamilton]

Si,ilar effects have been discussed before. The turning off of the AC probably may have just changed a controller from cycling to full on. Systematic investigation ( by turning off modules, other equipment ) may be the fastest way of finding the culprit.

I'd suspect a temperature controller, probably in the detectors ( if there is one ), or column heater. Also, is the flutuation size/period wavelength and mobile phase absorbance dependent?. That would point to the detector as well. I assume all instriments show the same periodity and peak times?.

If you have a laser printer, make sure it is not downstream of your UPS with the HPLC. Same applies with any other instrument/detectors that have internal thermal/fan speed controls including some modern computers, as the UPS can't fix downstream sources.

Please keep having fun,

Bruce Hamilton