Chromatography Forum

I've been having an issue with sensitivity ever since I did source maintenance 2 days ago.
Instrument conditions:
Splitless mode
Injector: 280'C
Detector: 280'C
Injection volume: 1.0uL
Column Flow: 1.0mL/min
Total Flow: 23.3mL/min
Starting pressure: 7.7psi
Purge Flow: 20.0mL/min @0.0 min
Source:280'C
Quad:180'C

Instrument was running fine (m/z 69~500,000 in autotune, 1,4-DCB-d4~180,000 IS response, no leaks) until source maintenance. After source cleaning the autotune and DFTPP tune looked fine (same abundance for m/z 69 and no leaks) but my 1,4-DCB-d4 IS response has decreased 10 fold (lucky to break 20,000). I can get more sensitivity out of the instrument by increasing the purge time to 0.75 minutes but the chromatography for the early eluters suffers greatly (mostly blobs rather than sharp peaks). I've also noticed that the ramping for my repeller, entrance lens and entrance lens offset look a little funny (not exactly Gaussian shaped and reppeler is maxing out) Could this be a source issue or something with the injector? I've done injector maintenance twice to make sure there were no loose connections, but I did just start using a bore brush to clean the inside of the port. Possible active sites? Thanks for your time.

Have you checked how long the column end protrudes into the MS ?

@thohry-Yep. It seems to be the correct length. Just opened the source up again. Going to re-sonicate and reset the column in the transfer line just to be sure.

If the repeller voltage is unusally high, check the condition of the insulators. The autotune parameters may limit the repeller voltage to 20 but if you set the limit to 40 volts you will gain diagnostic ability. I find that if the tune program wants to run the repeller at >25 volts or the ion focus at > 90 volts I need to do some more source maintenance. I run either a BFB or DFTPP tune depending on instrument and analysis.

This isn't likely related to the early chromatography problem, I think you are right to start looking at the inlet for that.

If you are getting the signal for PFTBA at a good detector voltage, the mass spectrometer is working.

This then takes us to the question of getting compunds to the mass spectrometer.

If the DFTPP looks normal, then there may be somethign specific to the DCB.

And, if DFTPP looks normal, I would assume that it is likely that the inlet liner is in good shape - but it may be worth a look (you are using a new liner after doing maintanance on the instrument, I assume?). Also, how does the chromatogram look in full san mode - are there large humps showing in the trace where the DCB should elute? I have see column degredation elute close to the expected retention time for DCB (2 pg on column, DB-5 type column) - and while it does not show the same mass that DCB shows - and does not appear in the SIM trace, it does distort the peak and prevent good detection of the peak. I have seen a plastiszer obscure the DCB peak - as, apparently, someone had used a plastic pipette tip in the solvent bottle before we got there to use it for diluting the standard mix to test the GC/MS...

What does it look like using a direct injection of your IS?
I gather you are doing P&T. Water is a major issue using Agilent 5973 and 75's. Anything you can do to reduce water will help.

We just had a 5973 here that was having stability problems, finally the E2M1.5 rough pump died, once replaced with an old E2M2 the instrument was stable again. But now the turbo pump has gone out at about 12 years old. It could easily be a vacuum problem even if you are reading proper vacuum levels. This one was always reading low 10^-5 or high 10^-6 torr, so we never expected any vacuum problems.

We had the same problem on an ICP/MS last year, all vacuum readings were within spec, but after changing the vacuum pump we gained almost 10X sensitivity without much lower vacuum readings.

Thanks for the input guys. The tech at Agilent had me looking at the injector just b/c it tuned ok, but I had a feeling from the way the lenses were ramping that something wasn't right with the source. There was a slight haze of microgrit left on the repeller and after another round of sonication the ramping returned to normal and my sensitivity came back. Thanks again.

You have to set purge flow at inlet to after 1 min