Hi Orest,

It sounds to me a flow issue or problem. Are you ramping the flow rate
right after the peak? I am analyzing stable carbons, and seeing that
increasing flow rate in the middle of chromatogram lowers baseline.
Another thing I came up with is that a leak at the press-tight
connector. As long as I know polyamide coating cannot seal well at high
temp, like 300C. I played around with it for PAH analysis before, and
encountered this problem. It may work temporary, but doesn't last for
long. Sensitivity check using a standard solution may tell something.

Good luck, Satoshi 


****************************************************************
Satoshi Irei
Stable Isotope Research Laboratory
Atmospheric Science and Technology Directorate
Science and Technology Branch
Environment Canada
4905 Dufferin St., Toronto, Ontario, M3H 5T4, Canada
****************************************************************

-----Original Message-----
From: Stable Isotope Geochemistry [mailto:[log in to unmask]] On
Behalf Of Orest E. Kawka
Sent: March 20, 2007 10:10 PM
To: [log in to unmask]
Subject: [ISOGEOCHEM] Baseline change (tail?) problem: high MW alkanes,
on-column inj

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