Consistent decrease in response

[Rosie]

Hello,

I am experiencing a consistent decrease in response (pk area) in the determination of methoxypyrazines. I am injecting a 150ppb solution (1-2µL injections) in splitless mode in a PE Autosystem XL/Turbomass Gold GC/MS system.

I have seen this problem before, but in the past the issue has been resolved through cleaning the injector liner and changing the glass wool in the liner. But over the past week, I've tried clean liners, new liners, new wool, no wool, trimming the head of the column, baking out the column and source, and finally cleaning the source.

The injector temperature is held at 250°C. Any ideas? I feel that I've tried everything I can think of.

Thanks!

[Rosie]

I forgot to say the injection solvent is chloroform. I am going to try using DCM instead to see if there's any change.

[Ron]

I would suggest thoroughly cleaning the inlet port after removing the liner with either cotton swabs moistened with solvent or a nylon brush and solvent. Since the problem went away in the past with injection port maintenance I suspect a thorough cleaning will resolve the issue.

[Peter Apps]

Hi Rosie

You are injecting tiny quantities 0.15 - 0.3 ng if my calculations are right. At this level you might have activity on the column - how are the peak shapes ?, any tailing ?

Peter

[Rosie]

Ron - I will try to thoroughly clean the injector, as you say. I haven't done this in a while. I usually focus on cleaning the liner only. A thorough cleaning won't hurt it.

Peter - The peak shapes are reasonably symmetrical. No fronting or tailing noticed. The column is a DB-17, and is relatively new (6-8 months old and was only used routinely for a couple of months).

[Peter Apps]

If there is no tailing the column is very likely still in good shape. How much of a decrease are you seeing, and over how long a period ? Chloroform can deterioate in storage, maybe there is some interaction with the pyrazines that makes your solutions unstable ??

Peter

[Rosie]

Peter - I am seeing a loss of about 10% (based on peak areas) from run to run. I have been allowing 3 injections usually before trying something to resolve the problem. Seeing as the decrease is constant, I know that I'm losing the MPs somewhere - probably due to absorbtion as I'm performing splitless injections. The injector is suspect.

I have thought about using other solvents as well. I thought maybe there might be some unfavourable interaction between chloroform/MPs/glass wool. I will inject my MPs in DCM and Et2O today.

Thank you.

[Peter Apps]

Hi Rosie

Just so I'm sure, the response on injection 2 is 90% of that on injection 1, on injection 3 it is 90% what it was on injection 2. Then you do some maintenance.

Then what happens to the signal ?, does it go back up to what is was on injection 1 ?, or does it continue the downward trend (from your first post it continues downwards but if it did, by now you would have no response at all).

Is the MS in full scan or SIM ?, if in full scan are you seeing any other peaks that get bigger with time ?

Peter