peaks that come and go

[brinker]

Hello, I'm having a strange problem with an LCMS method that I've used for a couple years now. First, the retention times for standards in solvent are now about 2 minutes later than previously (in a 16 minute method). Second, I injected the same standard mixture three times in succession and saw peaks on the first and third injections but not the second. The peak areas are about the same as in the past and were the same for the first and third injections. The pressure trace for this gradient method is the same as usual. I checked the flows out of all lines separately and they were correct. I did an injection where I collected all the effluent from the column (instead of it going to the MS) and the final volume was correct. I checked the proportioning valve by pumping 50:50 mixtures with the inlet lines in measured amounts of solvent, and then checking whether both lines pumped the same volume of solvent. This also was fine. In short, I don't see anything obviously wrong except for this huge shift in retention time and the now-you-see-it-now-you-don't peaks. A colleague using this instrument with a different column and gradient is seeing the same thing. Any suggestions?

[millerk8561]

In short, I don't see anything obviously wrong except for this huge shift in retention time and the now-you-see-it-now-you-don't peaks. A colleague using this instrument with a different column and gradient is seeing the same thing.

My first thought upon seeing that another method on the same instrument is having the same issue is that it could be your autosampler. The autosampler could be drawing up inconsistently, or sometimes not drawing up sample at all even if it thinks it is.

Do you use an internal standard that you add to the sample after preparation but before putting on the instrument? If you don't detect that or your target analytes I'd say it's definitely a problem with the autosampler.

[brinker]

More troubleshooting today.....turns out that, in my case at least, the wrong sample vial was specified for some of the injections, which accounts for the peaks that didn't appear. Duh. But the retention time is still shifted, even with a different column. I'm now trying switching out the tubing.

[brinker]

Problem solved - the retention time shift was due to one particular solvent bottle. Not sure what happened, but at least it's an easy fix.