Chromatography Forum

I'm having trouble with my Agilent gc/ms. I have a standard containing 40 compounds. Ten days ago I cleaned the ion source and since then the responses for all of the compounds have been increasing steadily, but at different rates relative to the internal standard. I can't find anything written regarding this anywhere, but have done every maintenance thing I can think of. After cleaning the GC inlet port, the responses dropped but immediately began rising again. The same thing happened when I installed a new column. Air and water check look fine, vacuum is fine, tune looks fine, the peaks look great except they keep getting bigger. Immediately following the ion source cleaning I didn't have any baseline but it looks fine now. I would appreciate any theories or suggestions!

What compounds are we talking about here ? Are the peak areas increasing, or only the areas relative to the internal standard - or in other words is the standard peak actually getting smaller with time ?

Peter

The solution is a mixture of small phenolic compounds resulting from the acidic treatment of plant biomass. The internal standard is 4-isopropylphenol. All responses, including the ISTD, are getting larger, although some faster than others. Later eluting compounds seem to be more affected, although it isn't a completely consistent trend.
So far I've changed the insert, gold seal, syringe, septum, and column without improvement, in addition to cleaning the inlet port. If you have any ideas I would really appreciate it.

The classic explanation for this is that you have adsorptively active sites that are being progressively masked by repeated exposure to the phenols. What quantities of analyte are involved ? How do the peak shapes look when the peaks are small, and do peak shapes change as the peaks get bigger ?

Please post all the detials of your hardware and operating conditions.

Peter

Hi Peter,
Thanks for your help. I had thought active sites might be the problem too and I thought I had cleaned or replaced all of the potential problem areas. But I had forgotten about the QuickSwap, removing it entirely seems to have greatly improved our results. The responses are still increasing, but only slightly and all at the same rate. An engineer from Agilent said that a newly cleaned ion source will have decreased responses relative to one with a light coating of sample, so hopefully our responses will stabilize further as the ion source "seasons". Again, thank you for your time.
Valerie

Er, I recently removed our quickswap and our sensitivity and reproducibility has improved a lot :-/
I wonder if there is anyone else out there who have the same problems with quickswap?
cheers
Liz

Hello, it turns out that I still have a problem. We removed the Quickswap and while the responses continued to increase with every injection they were increasing at the same rate so it wasn't really an issue. Yesterday, we had a service call and installed a new quickswap unit, and now the responses are increasing at different rates again. If anyone has any suggestions I would really appreciate it. We have multiple users on this machine, so the quickswap is necessary although it is obviously the problem. We have Aux EPC 3 set for constant pressure at 4 psi, which I am going to adjust to see if it helps.

Try setting the quickswap to zero and turning it off. I used to do this before we gave up and removed the quickswap.
Even better, remove the quickswap and replace it with a new Agilent backflush kit.