Methylene Chloride shoulder peak

[moser]

Methylene Chloride interference at 8.4 minutes occurs only on one GC (see image link at end of message). When I switch the columns and inlet liner to another system (6890N) the interference at 8.4 minutes is gone. What could be causing this peak to appear on one system and not on the other?

Chromatographic Conditions:
System: Agilent 7890A
Columns: Stabilwax-DA 30m x 0.32mm x 1.0µm in tandem with Rtx-5 15m x 0.32mm x 1.5µm
Flow: 2.5 mL/min for 28 min \ 7.5 mL/min² to 10 mL/min hold 10 min (Helium)
Oven Temp: 35°C for 2 min \ 25°C/min to 60°C hold 7 min \ 15°C/min to 130°C hold 3 min \ 5°C/min to 180°C hold 0 min \ 30°C/min to 250°C hold 10 min
Splitless Inj: 1µL inj volume \ 50 mL/min purge flow \ 0.25 min purge time \ 4mm cyclo dbl gooseneck inlet liner (Restek Cat# 20895-214.1)
Inj / Det: 120°C / 250°C (30 mL/min Hydrogen 300 mL/min Air 27 mL/min Nitrogen make-up)



http://i.imgur.com/mbmwc41.png

[Peter Apps]

One possibility is that the extra peak is a contaminant that is picked up during flashback of solvent vapour into parts of the inlet plumbing outside the liner.

Another is that it is peak splitting due to solvent effects on the column; your starting temp is very close to the BP of methylene chloride.

You might also have some dirt in or on the autosampler of one instrument

Is there a reason for the multistep temperature programme and the programmed and very high flow rates - they will test the reproducibility of the GCs control systems and might be the cause of the difference in the chromatograms. 8 minutes seems like a very long retention for MeCl2 under the conditions that you give.

Does the extra peak actually interfere with an analyte peak ?

Peter

[moser]

The Methylene Chloride peak elutes at 6.5 minutes. I haven't put much thought into whether that is expected or not. All I can offer is that it is consistent for this method.

The complexity of the separation is due to the application and the high flow rates are in place to expedite analysis. The 8.4 minute shoulder peak is nearly a co-elution for the analyte.

I will try to vary the conditions to address the issues you suggested. It might prove useful to bake out the inlet if the interference is due to a contaminated inlet.

Thanks for the suggestions.
Don