Intermittent Detector Noise

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Hi everybody,

I'm running an Agilent 1100 with a G1315A DAD at 205 nm. Method uses 93:7:0.1 MeOH-H2O-TFA, 1 mL/min @ 60°C. Every so often (intermittent problem) this little bit of baseline noise comes up (11-12 minutes):

Image

Sometimes the noise overlaps with analyte peaks. The peaks are present, which indicates that the detector is still working, but their shape is jagged rather than smooth. Also, I use the same instrument for a method with DAD at 260 nm, and I never see this type of noise.

My instincts tell me this could be related to the lamp (Sonntek 2000 hr bulb, not an Agilent lamp), or it could be an air bubble. I degas my mobile phase by sonicating under vacuum for a few minutes, and have a working G1322 degasser, but there is always room for improvement.

I welcome any and all feedback!
I love deadlines. I like the whooshing sound they make as they fly by.
When the problem shows up, is it always at (about) the same retention time? If so, that points at the chromatography rather than the detector hardware. If it *is* random, then it is most likely a hardware issue. I would expect a lamp problem (something like arc wander) to show up at all wavelengths but if the lamp has a fair amount of time (say, 1,000 hours or so), it might be worth changing.

If you are acquiring spectral data at the same time, you might take a look at the noise as a function of wavelength and see if any obvious pattern emerges (I'm thinking of a dicey detector array with a few dead-or-dying elements).
-- Tom Jupille
LC Resources / Separation Science Associates
tjupille@lcresources.com
+ 1 (925) 297-5374
Hi Tom-

It shows up at different retention times, so it appears to be random.

It's a 2000 hr lamp and I have the EMF set to alert me at 1900 hr but it's not on yet. Hmmm...any way to check the maintenance logbook/instrument counter in Chemstation (A) while a sequence is running?

In the meantime, I'll store 190 - 400 nm, hope (half-heartedly) that it happens again, and report back. Maybe it is a PDA problem; the bandwidth for 205nm is set to 4.

K.
I love deadlines. I like the whooshing sound they make as they fly by.
I think I have seen something similar and it was the lamp. I ssupect the lamp is starting to fail.
I see you have ChemStation so i would be tempted to run the built in lamp intensity test. I suspect it will fail < 220nm. Note it can fail also if the flow cell windows or the optic are dirty.
Although the lamp is rated for 2000 hours, their performance often starts to degrade before this.

Mike
Insanity: doing the same thing over and over again and expecting different results.
Albert Einstein, (attributed)
US (German-born) physicist (1879 - 1955)
Hi guys-

Thanks again to all for your suggestions.

@Tom: It took a while but the short-term noise appeared later in my sequence. Here is the chromatogram for the run, noise is at 16.2 min.:

Image

UV from 190-400nm indicates that the noise is visible throughout the spectrum, but it is proportional to the baseline noise. For example, at 205nm the short-term noise is ± 2 mAU, whereas at 300nm it is ± 0.2 mAU...but is always about 3X the intensity of the baseline noise. Hmmm.

@DawnRazor: Thanks, I took your suggestion of running a 60 min acquisition with everything else off. The noise appeared 19 minutes in and is similar in appearance/duration to the above chromatogram. So this rules out the pump, other hardware, etc.

@Bluesman75: I ran the DAD intensity test today and it passed. It was also run last month as part of PM and it passed then too.

A final note: the UV lamp "on time" is only 700 hours (a 2000 hr lamp). It's been doing this for a while, too.

Again, I welcome your suggestions and comments :)

K.
I love deadlines. I like the whooshing sound they make as they fly by.
If memory serves, Agilent DADs allow you to set a reference wavelength (sort of the software equivalent to double beam). I don't know if you can do that ex post facto, but if you can, go back and re-process one of your traces with reference wavelength set. If you can't, then set a reference wavelength, start aquiring data, and wait for the bug to bite. If the signal looks better with reference wavelength set, that points the finger at lamp fluctuations, since the use of a reference should cancel those out. On the other hand if it doesn't look better, that points to an intermittent electronic problem. Might be something as simple (and as devilishly hard to track) as a bit of corrosion on a connector acting as an antenna and picking up RF interference (any ham radio operators or police radios in the vicinity?).
-- Tom Jupille
LC Resources / Separation Science Associates
tjupille@lcresources.com
+ 1 (925) 297-5374
@Tom I should have mentioned, referencing wavelength is 360 with bandwidth 100. This leaves us at an intermittent electrical problem. I will try to track it down and diagnose it.

I would like to rule it out so I'll see what effect changing the lamp has, if any...and report back here if anything comes up. I have a 1000 hr Agilent lamp, maybe it is worth the extra $.

Thanks (again) to all for your ideas!

Kenny
I love deadlines. I like the whooshing sound they make as they fly by.
As a follow-up, I recently changed the lamp as it reached 2000 hours, and the noise is still present with a brand new Agilent lamp.

Image

Next chance I get, I will inspect the unit for corrosion or any physical damage.
I love deadlines. I like the whooshing sound they make as they fly by.
ksharp, all your chroms show that there was an injection and that there was flow. One of your statements appear to describe an experiment without flow which showed this noise also. During this "no flow" experiment was there any injection or other activation of the inj. valve? Did you ever see this noise twice during one aquisition?
Hi

Out of interest what is the absorbance of 0.1% TFA at 205nm. Try a 60 min run with just water at 205nm with the reference off.
Have you the possibility to get a chromatogram of the pressure as well?
if there is a disturbance there as well in some sort of corroletion to this noise then it could be your check valves also
they can behave randomly as well

I had somehting similar on a hitachi system once.
it made bad RSD of the standards or noise in the baseline.
the weird part was that we only saw the pressure problems when I used a short column with 100% AcN at 30 bars.
then the pressure fluctuations showed up well enough to point us toward the check valves
I was wandering if the intermittent noie problem was solved, detector or check valve?

We distribute systems in Canada as well, I will soon be in Halifax, if interested, send me an email richard7777@videotron.ca.

Regards

Richard
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