Chromatography Forum

Dilute your sample by 1:30 or 1:40 or something, then run it again. Should be within the linear range of the MSD.

Dilute the hexane fraction to get an appropriate concentration. The concentration listed was applicable to packed-column GC with FID (no inlet split). Capillary GC column and detector capacity is much less, as your instructor noted.

'We saponify 1 gram fat/oil in a 100 ml volumetric flask using 10 ml 0.5 N KOH or NaOH in methanol on a steam bath 20-30 minutes.

Just curious but how do you prevent the methanol from evaporating during the 30 min KOH digestion?

'We saponify 1 gram fat/oil in a 100 ml volumetric flask using 10 ml 0.5 N KOH or NaOH in methanol on a steam bath 20-30 minutes.

Just curious but how do you prevent the methanol from evaporating during the 30 min KOH digestion?

I never gave it much thought. The methanol (or most) condenses on the inside of the volumetric flask and drips down. I was working from memory when I wrote that above, so I just checked: 15 minutes. It doesn't go dry, at least for us. We use VFs with size 13 stoppers, haven't tried this with the wider-neck ones (which I really like for other assays).

Makes sense.

These user observations are very useful.

Thanks

ukstudent please post your sample prep procedure with all volumes, we need to calculate how much sample you are actually delivering. Please also post your injection volume and split ratio.

Don't despair, you can easily add a dilution step in your sample preparation, or inject a smaller volume into the GC, and also run a higher split ratio. The first two would insure you're not injecting too much sample into the inlet (can cause contamination or rapidly dirty the liner) and the third would insure we are not delivering too much sample to the column and thus the MS.

Your supervisor is right, injecting too much sample can cause lots of problems both in the inlet and MS. Highly concentrated samples can contaminate the walls of the inlet, the split vent line, and can possibly flash up into the gas supply lines and damage the pressure controller in the worst case. Highly concentrated samples may suffer chromatographic problems (tailing, fronting, weird chair shaped peaks, the whole gamut.) Finally, highly concentrated samples may dirty the MS source more rapidly, may diminish the lifetime of the filament, and will definitely diminish the lifetime of the electron multiplier. Even the rough pump oil gets dirty faster, which can cause its own set of problems.

It is very important in any method development to do this type of calculation, it will help you avoid hours of frustration haha.