Chromatography Forum

Hi Folks...

I hope someone can help me out... I am having major issues with a used HPLC machine I purchased. Everything works great on it , except for te fact that I am always getting a negative drift. Even with no sample being injected, te baseline will drift down.. If it is helpfull I can post a picture of what I mean.. please let me know if you need one.

I am using a Bio-Rad column, Aminex HP-87H, at 65°C in the column compartment and 50°C for the RI detector. My solvent is 0.02 N H2SO4, which is normal for this column.

I vaguely remember someone sometime talking about issues with the Bio-Rad amino columns with the Agilent column compartment and heating.. but I can't find out what it was.. anyone else hear this??

Anyways. any help would be much appreciated..

Thanks
- Hubert

Most likely one of two things:
- temperature issues (as you suspect)
- cationic crud bleeding off the column (e.g., if you ever injected a sample with lots of sodium, it can take days for the Na+ to bleed off.

I'd run the system for 24 hours (aquiring data the whole time). If it's temperature problem, there should be a diurnal pattern to it (what goes down must come up?). If its bleed, it should asymptotically approach a constant level.

Thanks for the post...

This is a brand new column (trying #2 as the first one showed the same problem, both were brand new). I have ran the system for a couple days before and I see the same thing over and over.. the baseline just drifts lower, and lower....sometimes it shows small humps (up and down) but the overall trend is definitelly down...

I guess one thing that kind of jumps into my mind.. I have a fairly long SS capillary from column outlet going to detector (~ 12") best I could do.. have a long column in there, might it help if I insulated it somehow?? most of it is in the thermostated column chamber, but about 5" is from the chamber to the inlet of the detector below it.. could it be colling down too much inbetween the 2 units?? How could I insulate it??

Any ideas??

- Hubert

Agree with Tom (as usual). Oftentimes we do insulate the transfer line to the 1100 or 1047 RIDs.

How would I go about insulating it?? it's a damn thin line.. what kind of material is best for that job??

- Hubert

OK.. I went creative and found some pipe insulation laying around the plant here.. I made a nice cover for the capillary.. we'll se if it improves over-night..

Thanks again for all the help .

- Hubert

I've used foam pipe insulation from the hardware store and also fiberglass insulation tape like Fisher #01-472A

Hubert,

All the previous commenst are good and valid. If there is no progress after you try some of them perhaps you should check for thermal gradients in your oven. This is a problem I have found in several HPLC systems, the temperature settings maybe inaccurate as well.

Good Luck,

josebenjamin

For insulation, I've just taken Tygon tubing, slit it lengthwise and slipped it over the capillary.

Hi Folks,

Thanks for all the info... I insulated the detector capillary and ran the machine overnight while acquiering data, here is what I got... still a very large negative drift... any other ideas?? How would I go about checking the thermal gradients in the oven??

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Thanks for all your help..

- Hubert

Hubert,

Hubert,

I have done it in the past with a thermocouple wire and a multitest meter. In many instances these meters already come fitted that way. The thermocouple wire is usually very thin and can be passed through the oven door slightly open. Just place the wire at several positions in the oven and take a look at the readings. It takes some times for readings to stabilize.

Recently I found an instrument with an oven set to 80C, the lower part of the oven was essentially at rooom temperature and the upper part at 60C, only the heating plate was close to 70C. Even after 24hrs the readings were still the same.

Good Luck,

josebenjamin

In the past, I have had negative baseline drifts that were caused by microleaks. Very tough to find and cure. Usually these leaks are internal to the detector often associated to the ref side of the flow cell.

Do you get a negative drift if you just run water or mobile phase through, without a column?

Yes.. the drift is there no matter what I do.. with a sample, no sample.. the run pictued above is just with a 0.05 N H2SO4 eluant, no gradients, no changes.. nothing extra.. just straight data acquisition..

- Hubert

No column?