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Methamphetamine - poor peak shape

vadfs1

After a failed methamphetamine quantitation, it was noticed that the methamphetamine peak shape on our GC/FID instruments has been consistently poor or deteriorating through out a run.  This has been happening on both of our GC/FID instruments (all 3 columns).  I've been hard at work on the problem for over a week now and have tried a multitude of fixes, to no avail.  Any help/insight is appreciated.  Below is a synopsis of the things I've done as well as data.

 

Instrumentation:

7890B - 15meter HP-1   - labeled as GC1 on data

6890N - 15meter HP-1 - labeled as GC2 on data

 

In the past week, I have:

1.  replaced GC columns with brand new columns

2.  GC1 -

      changed the syringe

      changed merlin

      changed liner and o-ring (multiple times)

      changed gold seal x 2

3.  GC1 - swapped out autosampler towers (no change)

5.  GC2 - changed the liner and o-ring  (this instrument gets little use, so the gold seal is new)

 

Data

On GC1 - the GCQA06 standard methamphetamine peak shape looks fine on the printout, but when you zoom in it shows significant fronting.  This is the same QA std that we run routinely on our GC/MS instruments with no problems.  (see page 1-3 of attachment 1 (GC/MS instruments also have an FID))  All other peaks in the standard are symmetrical.

 

In attachment 1 see also data for our methamphetamine std, which is also used routinely on our GC/MS instruments with no problems.  Again, poor peak shape.  This standard was run multiple times on GC1 and that data is in attachment 2.  The peak shape continues to broaden to the point of splitting in Run 9, then actually "improves" to a shoulder for runs 10 and 11.  The peak area counts within this run also tend to vary.

 

On GC2 - the same methamphetamine standard was run multiple times.  As can be noted in the data from attachment 1 the y-axis abundance continues to decrease while the area count stays consistent between runs, indicating peak broadening.

 

*GC1 also houses a 2nd column, which is a 30meter HP-5 column, also new. The fronting meth peak in the QA standard is also seen in this system.  

* the methamphetamine in both standards is extracted

attachments.zip
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  • 0

    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

Reply
  • 0

    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

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