Repeatibility issues

[rivina]

I am running chromatograms of a standard mixture of 37 FAMEs in a Shimadzu GC with an RTx2330 column. The protocol can clearly separate 36 of the fatty acids (butyric elutes with the solvent) and the relative values of the peak areas are fairly repeatable in succesive runs but the areas are not, so that I cannot verify the linearity of the response by running several concentrations of the mixture. And I´m running out of ideas...

[gpronger]

Since your relative areas are consistent, then either you're most likely not reproducibly getting the same volume injected or you're not getting the same volume transferred from your inlet in your column.

For us to help you much more, post your instrument conditions, etc., and there's a lot of folks who can then comment with a bit more information.

Greg

[Consumer Products Guy]

Let us know if you're just injecting FAMEs, or FAMEs in solvent. We extract our FAMEs into hydrocarbon solvent, easier to control the amount injected.

[rivina]

Many thanks for the replies so far.
Our conditions are as follows: Shimadzu GC2010, column Rtx 2330 30m x 0.32 mm x 0.2 um.
Inj temp 260 ºc, FID 260 ºC; ramp 150 º C hold 5 min, ramp at 2º C/min, up to 250 ºC hold 5 min
Carrier H at 40 cm/sec (yes H, not He)
Split 1:20 (intend to go to 1:25 or 1:30)
Volume injected 1 ul of 720 ppm 37 FAMEs std mixture in solvent.
Regards to all

[rivina]

I forgot to add that the liner has no wool of any class.

[AICMM]

rivina,

The solvent is?

Does your curve go up or down from 720 ppm?

Try diluting your curve and running splitless to see if your split is discriminating (shouldn't be but....)

Best regards.

[Peter Apps]

Are you doing manual injections or using an autosampler ?

Peter

[Ron]

Try a liner with deactivated glass or quartz wool. You should see an increase in sensitivity and precision.

[leo1352]

Many thanks for the replies so far.
Our conditions are as follows: Shimadzu GC2010, column Rtx 2330 30m x 0.32 mm x 0.2 um.
Inj temp 260 ºc, FID 260 ºC; ramp 150 º C hold 5 min, ramp at 2º C/min, up to 250 ºC hold 5 min
Carrier H at 40 cm/sec (yes H, not He)
Split 1:20 (intend to go to 1:25 or 1:30)
Volume injected 1 ul of 720 ppm 37 FAMEs std mixture in solvent.
Regards to all

If you setup your initial temp at 30C holding for 5 min, you will be able to separate your C 4:0 at least, provided you use a DB-WAX. I found DB-WAX not very good for SCFA(short chain fatty acids). a non polar colum like DB-5ms will give you better resolution aas your compounds are nonpolar as well.