Orest,

I think I have seen this problem as well - it seemed like a flat 
shoulder on the peak, which seems to match what you describe.

I attributed the effect to the pyrolysis oven. Specifically, it 
occurred intermittently if I used a bare aluminum oxide tube and 
seemed to disappear when I carbon-loaded the tube. To carbon load, I 
pass pure methane through the tube as I move it slowly through the 
hot zone. This deposits a layer of carbon on the inside of the tube 
that prevents interaction with the ALO.

Another thought is that you can eliminate the injector as a potential 
source of trouble by dipping your guard column in LN2 while you 
inject. This would focus your sample and remove any injector influence.

Paul Eby
University of Victoria

At 07:09 PM 20/03/2007, you wrote:
>We have recently switched to an on-column injections on our Thermo 
>Trace GC Ultra - Delta
>V Plus measuring hydrogen isotopes. We have encountered many 
>problems (including
>septum material ending up in the 0.53 mmID pre-column) but have 
>reached a point where
>the biggest problem now appears to be a baseline change or peak tail 
>after the compounds
>of interest (> n-C34 n-alkanes). It really appears more like the 
>baseline stays slightly
>elevated for a while after a peak comes out and then drops 
>relatively quickly. It is not a long
>sloping tail. We thought of some type of degradation/decomposition, 
>but can't determine
>where/what it could be. We would appreciate any input/suggestions. 
>Here are our conditions:
>
>GC CONDITIONS:
>Oven temperature: 100C and 120C, programmed up to 325C
>Cold inlet
>Guard pre-column deactivated 0.53 mm ID (0.75-1.5 m in length)
>Analytical column HP-5MS 30m x 0.25 mm ID (0.25um film)
>Using a press-fit connector to connect above
>Flow rate 1.0 - 1.3 mL/min
>Syringe 80 mm needle cone tip, (currently using airgap above sample 
>to ensure full
>emplacement on column)
>Injection Volume 1.0 uL  Conc (200 - 300 ng/uL)
>
>H/D analysis - relatively new reactor
>Ar background with GC (backflush off; open-split on): 120-150 mV
>H2 Background with same conditions: 400-800mV (oven temp ~110C - 325C)
>
>By the way, any suggestions on how to keep the septum material 
>(Thermo on-column
>translucent septa with teflon face) from getting into the guard 
>column would also be quite
>helpful. We found pieces of septum material in the guard column and 
>in the press-fit
>connector.
>
>Thanks,
>Orest