Methamphetamine - poor peak shape

Hi vadfs1, 

I think you are correct in approaching the inlet in your troubleshooting.  The one piece I haven't seen addressed is the split vent filter.   As drug analyses are often performed incorporating high splits, this filter can become partially or completely blocked, which affects the pressure control inside the inlet.  Amphetamines are early eluters on non-polar columns, they will be more impacted than later eluting components.  

If you don't have a spare filter, you could measure your split flow exiting the top of the GC and compare to the calculated amount to see if your issue is here.   If you are unfamiliar, just follow the 1/8" copper line back from the inlet.  

Other things to consider are ferrule type in the inlet and column cut quality.  If either of these have changed, it could have an effect on the sample transfer from the inlet to the column. 

This link has a video with instructions on how to replace the trap, should you need to perform the maintenance action.  

Good luck and keep us posted!

Abbey

Thank you Abbey!!

On GC1 I have uninstalled/reinstalled the column several times throughout this ordeal paying special attention to the column cut and even changing from graphite to vespel ferrules.  This instrument is scheduled for an annual PM visit in a couple of weeks so the split vent filter is due for a change.  I don't have a spare one on hand currently, but will report back after the PM to see if peak shape has improved.  

As for GC2, this instrument gets little use and I changed the split vent filter back in March of this year, so I don't think that's the cause of our broadening for this one.  

Hi vadfs1,

I'm assuming that you are analysing methamphetamine as the free base and not as a derivative?

Generally if a peak is fronting, it is almost always caused by overloading the column. Your HP-1 column is very apolar (polydimethylsiloxane) and your methamphetamine analyte is polar (amine). Your 0.25mm 0.25u HP-1 column has a maximum sample capacity of ~50-100ng on column for each (apolar) analyte  - The capacity will be much less for very polar compounds (the analyte is not very soluble in the dimethylsiloxane stationary phase) so you could expect to see signs of fronting at perhaps 10ng for a polar compound (compared to ~100ng for something like dodecane). I would expect the results to be somewhat better for the HP-5 column, but high levels are still likely to cause fronting (It is still relatively apolar as it has a polarity equivalent to 5% phenyl/95% dimethylsiloxane phase).

A simple way to determine if overloading is the cause of the problem is to dilute the sample 10:1 and try again, the fronting should reduce or disappear. Obviously, diluting the sample will reduce the range of your method - You may have to work within 100pg to 10ng on-column (100:1 dynamic range).

Other alternatives might be to derivatize the methamphetamine - Trifluoroacetic anhydride, Heptafluorobutyric anhydride, or N-methyl-bis(trifluoroacetamide)? Or change the column - A more polar HP-17 (50% phenyl) column might be a better choice, but will have a lower high temperature limit (260/280C vs 325/350C for HP-1).

Regards,

Tim

The 11 injections are made with the same vial? I suggest to perform 11 run with 11 different vials, once the vials is perforated the solvent evaporate. I don't analyse drugs but 100:1 split lead to higher error...maybe you can inject less than 1uL and less split factor?

The bad peak shape suggest phase activation, it'ìs better to add a precolumn made of simply silica glass.

Hello,

I do not see any mention of column trimming as part of maintenance.  Try trimming the column by a full loop and see if that improves anything.  I recommend trimming the column 10-20 cm or more every time you change the gold seal. 

-Mark

Hi vadfs1,

Any update to your issues? It would be good to share the resolution with the community in case others have the same issues. 

Regards

James