10 fold response reduction in GCMS

I am analyzing water samples for EPA 525.2 on a GC 6890 and a 5975 MSD. I have encountered an almost 10 fold decrease in response over all analytes a few months back after a power surge in the lab and have not been able to "fix" it. Since then, I have cleaned the EI source, changed the pump oil, and replaced the GC column, (Restek RTX-5MS). The analytical acquisition method have not been changed. The tune files do not have any glaring differences before and after this power surge. I have included autotune, DFTPP tune and a low level comparison from before and after.

I was told I should attempt to clean the HED as it might be dirty as a result of the power surge. Any other suggestions would be appreciated.

  • Are your DFTPP runs also lower?  Are you certain that your acquisition method points to the current tune file? Both the autotunes and the DFTPP Target Tunes are quite nice looking even with the temperature differences and different filaments.

    Have you tried a new syringe? Done inlet maintenance?  Has a PM been done lately and the split vent trap changed?

    The HED does not need to be cleaned with only 118 peaks in the last DFTPP tune spectrum.

    Also - there is no reason to run the quad at 180°C even with the ion source at 300°.  Higher quad temps reduce the quad's lifetime.

  • The signal count is lower between the DFTTP/Degrad check with each analytical run. The analytical run acquisition is pointing to the dfttp tune file.

    A new syringe has been replaced. Inlet maintenance is done every 4-5 analytical runs; this instrument runs EPA 548.1 and EPA 525.2 samples and DDD and Endrin breakdown (20% breakdown) conditions fail after about the 5th run. The split vent trap hasn't been changed in the last year. Rough pump oil is changed every 6 months.

    Ion source was increased to 350C to "burn off" all the PAH's. Our lab started to analyze EPA 625.1 samples and the higher ion source temp helped to prevent excessive build-up. Another GC-MS has been purchased for the sole analysis of EPA 625.1 samples. Going forward, the quad temp will probably be lowered back to 150C.

    I changed the tune parameters to tune ion 69 at 1,000,000 counts instead of 500,000 counts, increasing the EM gain. This has resulted an increase of responses overall. The latest low-level spike is now about 1000-1500 counts. It's better but I think improvement could still be made.

  • How many total runs on this system?

    • 8890 and Intuvo: Service Mode, Instrument, Usage
    • 7890: Service Mode , Diagnostics, Instrument Status
    • 6890: Options, Diagnostics, Instrument Status

    A 6890/5975 probably has many, many runs on it and passed the end of guaranteed support some years ago.  There are some more remedial maintenance things to do and parts to replace.

    Replace the copper split vent line between the inlet and the split vent trap with a new piece of copper tubing. While that piece of tubing is off remove the gold seal, too. Break a cotton swab stick in half and use that to clear the side arm of the inlet body. You may need to use a tool like a small allen key to clear it.  While the gold seal is out, take three cotton-tipped swabs, hold them together, dip them in solvent, and scrub the inlet body. Do this three or more times.  Replace the split vent trap at the same time. 

    Vent the MS. While the ion source is being cleaned take a solvent-dipped lint-free cloth and wipe the inside of the manifold, the entire source radiator, the outside of the quad radiator, and the side plate.  This can be done every source cleaning, too. 

    Replace both repeller insulators, both filaments, and the lens stack insulator if they are more than a couple of years old.  Replace the entire ion source if it is original to the 5975 as it has probably been cleaned so many times that the physical lens shapes are not the same as they were, the corners are not crisp and the faces are not flat. A rounded off repeller face, for example, will still tune but the maximum response is reduced.

    If the electron multiplier is more than a couple of years old it should be replaced, even though the tune EM voltage is in the acceptable range. They do not last forever.

    You could change the gain setting in the acquisition method rather than in the tune.


Was this helpful?