We are hoping for some help with a GC problem that has just developed.

 We have 2 Trace GC's (both the same age -about 20 months)- one linked to a MS252 (for d13C), the other to a DeltaplusXL (for d2H).
For over a year we have been successfully injecting n-alkanes mixtures from n-C7 to C30+ in splitless mode and getting nice peak shapes for all compounds including the C7.
After a couple of weeks of running different samples on a different column (on the MS252) we have gone back to our normal column to resume the n-alkane mixtures. The C7 peak has become broad and tails significantly, and with a gradual improvement to C10 and thereafter. An injection in split mode 20:1 gives nice sharp peaks, however this is not an option as often we don't have much sample. Changing the injector temperature (we inject at 250 deg. C), splitless time, pressure (i.e. pulsed splitless) doesn't help.  We have changed liners, septa, dismantled the injector and replaced the silver seal, washed the injector body with solvent, and injected manually and with an autosampler to no avail.  We inject with pentane at 30 deg. C.  It has also become increasingly difficult to get a good leak-tight seal with the injector nut which could be contributing to the problem (spraying the injector nut with argon verifies this).  We are starting to suspect metal fatigue, particularly as the exact same problem has now cropped up on both GC's.

We were wondering if anyone else has seen the same thing and hopefully managed to resolve the issue.



Janet Hope
Isotope and Organic Geochemistry
Petroleum and Marine Division

Geoscience Australia
ABN: 80 091 799 039

Tel: (02) 6249 9487 / 9594
Fax: (02) 6249 9961
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web site: http://www.ga.gov.au