Chromatography Forum

I can't get reproducibility within runs of the same sample. What troubles me the most is that in a mixture of flavor components I get varying RSDs for each component. The percentages are anywhere from 2 to 48% for an individual component within the overall flavor say for example something like orange. If I run the same orange sample liquid injection seven times, and then look at each peak, the separate components that I know make up the flavor since we made it in house don't have the same peak areas. For example, the orange has ethyl butyrate in it, ethyl propionate, and maltol among other things. My RSDs for peak area for ethyl butyrate is 38%, for ethyl propionate 3%, and for maltol is 9%. Why would I get such variation in peak areas from one run to the next of the exact same sample. I have tried it where I have the same vial for each run and where I make up separate vials so there isn't an issue with lowered volume affecting draw up in syringe with the autosampler (Gerstel MPS-2). I get the same issue regardless of how I handle the vials.

the operating conditions are:
inlet: He carrier gas, split injection 50:1 ratio, 54 mL/min total flow
(tried also 20:1 split ratio 24 mL/min total flow
and
100:1 split ratio 104mL/min total flow --neither of these were any better with the RSDs either)
19 psi in inlet @260 degC with a low PSI drop universal ultra-inert liner with glass wool.
column: capillary HP-1, constant flow, 19psi, 1mL/min flow, 26 cm.sec avg velocity
oven: 70degC, ramp is conservative with every 5 min adjustment by a few degrees.

Any thoughts as to why I'm getting strange RSDs for individual components and how I might troubleshoot it?

Thanks,
Eva

Are your injections with an autosampler or manual?
How often to you change the inlet liner and the septum?
How well are the components separated from each other - and other stuff present?
Can you post a picture of the chromatogram?
And what kind of detector? FID? MS?

What solvent are your flavour compounds dissolved in ?

Having the same problem in two forums is going to get very confusing - if you and don are agreeable, let's put it all in the GC-MS thread

Peter

Yes, I'll put it all over on the GC-MS thread. This is going to get too confusing.