That's an ingenious way to look at palmitic acid in a complex mixture.
The other compounds end up being your source of H and the D from the
per-deuterated palmatic acid gives a nice bump in m/z 3. Standard GC-MS
is more sensitive, but that also means louder noise unless the per
deuterated material generates unique resolvable fragments.
It sounds like a problem with the chemistry. If you can, do a test run
on a plan GC-FID or GC-MS. I'm guessing it will look the same, broad
peaks indicating that the methyl ester didn't form, or if it did, it's
broken down already.
IF you were having the same problem on the IRMS with other methyl ester
standards, then I'd check out the "T" the GC column goes into, the
backflush valves, etc. A leak at the "T" means much less material makes
it thru to the reactor, and the flow rate drops off. Unfortunately
leaks can open up and close as the oven temperature ramps up. IF that
"t" is connected to a vent/backflush valve, that valve can leak too and
you'll see the same sort of results.
A flow controller problem, e.g. the flow slows down/stops as the GC
ramps up may give similar problems--so make sure it it set correctly
using correct column length and ID. IF the GC is not reporting correct
flow rates, then start at the injection port and connections to it.
There can be problems in the GCC too, but that is less likely. Depending
on the setup, the "T"'s in the flow path in the GCC have a bleed out
line (pig tail) on the valves. You can see it bubble out slowly if you
stick the end in a vial of water if you have enough back pressure. If
you typically see it bubbling out there slowly, and it now drops off
during the GC temp ramp, you have a leak opening up. You can also check
the vent line from the GC. Depending on the setup, that will show some
bubbling as the GC ramps up since the gases in the line heat up too, but
not 2 mL/min range of typical GC run. IF the bubbling takes off, it's
not working. Unfortunately, that valve leaks out the stem or out loose
connections inside the oven (new ones have valve outside oven, so
connects don't cycle with oven and you can check its bleed out line).
You can also check flow rates. Disconnect the GCC end of the line from
the GC/TC and check flow rate during the temp ramp. IF not ok, problem
upstream. IF ok, problem is down stream. ...
p.s. and i really hate it when a piece of the glass capillary breaks off
inside the reactor. Sure, it's ok right after it happens, but
eventually it works it's way into the hot zone and melts...
On 5/21/2016 9:49 AM, SUBSCRIBE ISOGEOCHEM Isabelle Chery wrote:
> Dear Marylin, Willi, Bill and Brian
> Many thanks for your answers.
> In fact the D labeling is not 100% and with derivatization there are some H added to the mixture. So the mass 3 is significant and is directly proportional to quantity injected (standard curve over 6 concentrations linear at R2 0.99), even if not representing a 100% of the molecule. Moreover the final analysis will be done on human muscle lipid extracts in which the D31 is really at low concentration in a complex lipid matrix.
> The problem Isabelle faces is that the peak she which obtain is not sharp any more and is tailing. It is tailing only for D31 and for non of the other fatty acids of the mixture.
> Few weeks ago the peak and shape were perfect. We have changed injector, cut the column, change the pyrolysis oven, the syringe. We are stuck.