UV MS delay drifting in isocratic run

Discussions about GC-MS, LC-MS, LC-FTIR, and other "coupled" analytical techniques.

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I’ve just found out the delay between my MS and UV signals in a daisy-chained LC/MS is not fixed even though the flow is isocratic (iso pump, neat MeOH). This is a flow injection analysis by the way, no column.
I’m strugling to understand how this is at all possible.
The run is very long, about 2h and the delay is about 0.015min at the begining and goes above 0.1min (7x longer) after 2h. Fairly reproducible from run to run.
There are no splits, just a straight line. Empower software, acquity kit.
I’m flow-injecting every several seconds into a continuous run.
I’m considering some weird temperature effects or software issues with time-stamping as the file file grows, but quite frankly finding it very odd.

Any comments on that?

? strange
any micro leaks?

What flow rate are you running at?
What dimension of transfer capillary?
-> What volume / "flowrate" would that drift be? (also in % of set flowrate)
--> then maybe this gives a hint what could be possible; leak, thermal or pressure effects (compression if pressure build up at source) or a combination of all (I don't say that's likely, just brainstorming)

For your suggested synchronsation issue in data acquisition:
export the raw data channels as ASCII file and examine the timestamps of each datapoint. Maybe you can plot the two time-axxis against each and find the same correlation/drift.

What about differences in sampling rates?
eg. if UV is at 20 Hz and MS best effort is some odd number like 8 Hz?
Hollow, many thanks for interesting comments.
I do not believe there are any leaks, at least not in the exposed parts of the line.
Just a few inches of red tubing from PDA to QDa ESI kit capillary, a good union between them. Flow rate of 0.5 mL/min MeOH.
I checked the timestamps on the TIC and UV extracts and they seem fairly stable, with only some small scatter around the mean which is stable over the whole run, i.e. no up or down trend either way.
The effective rate (based on timestamps) is 39.99 Hz UV and 3.54Hz MS (full scan 40-1200).
The only vaguely suspicious thing is a bit of a tend in the back pressure. It's roughly 200 bar rising to 202 with some undulations on the way.
It's not reproducible though from run to run while the delay drift is...
I'm going to talk to W to check what could be going on...
For the moment when I try setting the offset for the beginning of the run the MS alignment at the end is badly off. I can only fix it with progressive scaling, which doesn't seem to be available to 3D data though, only 2D extracts, so not a fix really.
Worth testing: if it's software, then it's almost certain that if you stop your 2-hour run and immediately start a new run, the time-delay will instantly drop to its initial value (because all parts of instrument will be synchronised on starting the new run). If it's hardware, then the delay will remain at its longer value.

I wouldn't be completely surprised by some sort of software thing. A year or two ago we had a Thermo LC system attached to a Waters mass spec, and found that if both started a 30min run at the same time, one was finished substantially before the other (the Waters minute wasn't the same length as a Thermo minute! We did get to the bottom of this: it was caused by one of the two instruments trying to send more information to the PC than the PC could handle, so things were waiting that shouldn't wait, and timings were all awry).
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