Signal periodicity

[lcrandall]

Hi,
I've been getting this strange periodic signal in my baseline the last two days and I can't figure out where it's coming from. It seems to go away when I have a peak eluting. The frequency varies.
It's also mixed in with non-periodic noise that looks like contamination. I've never had carryover or contamination with this method in the past. I'm thinking the two problems aren't related.

Agilent 1100 with DAD
MPA: H2O with .5% TFA
MPB: ACN with .5% TFA
gradient run 85:15 to 25:75 with a 5:95 wash
220nm detection reference wavelength off

I've tried a zillion things. Could this be a lamp problem?
Thanks,
Lisa

[lcrandall]

Oh, this instrument is on it's own circuit. There is nothing new sitting near it that could be giving elecronic noise.
Could the building AC do this? It's hot out the last two days. But, we had a hot wave a few weeks ago as well, and I didn't see this problem then.
Thanks

[tom jupille]

As you've indicated, there are a lot of possibilities. It would help if you could post a chromatogram showing the problem. Posting instructions are here:
http://www.sepsci.com/chromforum/viewtopic.php?t=2617

[lcrandall]

Neat, I love learning! Especially on a bad day.
Lisa

[mardexis]

Hey,

Looks like a bad check valve to me. Probably the organic side. I have a lot of problems with check valves on the organic side on my Agilent systems.

I would try isocratically eluting your major component a few times with just your 'A' pump... maybe with 65% ACN or something like that, and see if the fluctuations go away.

[tom jupille]

Apparently, ACN is known to cause problems with some check valves. See the John Dolan LC Troubleshooting article in the May LC/GC (North America):

http://tinyurl.com/5h6xbb

[juddc]

If it is a sticky check valve caused by high concentrations of ACN (and you know your DI water is clean), a ceramic check valve will in all probability help. I use them exclusively in my LC's and very rarely have any problems with ACN.

[sfe-co2]

Hi lcrandall,

This ripple effect has been covered elsewhere in this forum (look up responses from Mark Tracy, Uwe Neue and Bruce Hamilton, among others). Basically, TFA and 220nm (low wavelength) have been known to cause what you are seeing now--the TFA is amplifying fluctuations of the pumping system. Good idea to have an additional mixer in this case.

Hope this helps.

[lcrandall]

Hi all,
I changed the UV lamp (it had 3000 hours on it) and the problem seems to have disappeared. I know I've read about acn causing problems with Agilent 1100s, but have never seem it myself after running these instruments with ACN for years.
I was also thinking the TFA might be a problem, but I haven't seen the problem since I changed the lamp. Fingers crossed.
Thanks,
Lisa

[Consumer Products Guy]

I also have used straight ACN for years without having any "assigned" hardware issues due to it. Eeven if, I would say for us it would continue to pay for us to continue doing as we have been...

[juddc]

I have seen ACN related hardware issues on a number of LC's, but they're rare and easily fixed.

Glad the problem's solved!

[sfe-co2]

Good to know the problem has been solved. lcrandall, thanks for sharing this solution--I didn't even think about the lamp, presuming one would, periodically at least, monitor its usage hours and/or energy output. However, you are using 0.5% TFA in the method. Do you mean 0.05% TFA? Just thinking 0.5% might be a bit excessive, that's all.

[lcrandall]

Oops, why do I keep writing it incorrectly? Yes, it's 0.05% TFA
Thanks for all the help everyone.
Lisa