[gaschroking]

Hello everyone,

I am just curious about the gas chromatogaphy conditions that many researchers are using. For example for the analysis of PAHs- phenanthrene, napthalene and pyrene I  read several conditions which are entirely different. In one journal  an oven temperature of 60C to 300C with a ramp of 10 minutes; in another journal an oven temperature of 200-320C with a ramp of 10 minutes, and others.Although there are no such big difference among the injection and FID temperatures. May I know  if we can get a desired accurate results even though we use different GC conditions? Using different  oven temperature program will result to different retention time.  Will it yield similar results using other condiitons provide that you are consstent with your procedure?

In my case for example I use 60 C-300C with a ramp of 10 min, FID of 300C and injection of 280C.Can I use  similar conditions for other hydrocarbons say C10-C28?


Many thanks 

[enahs]

Different GC's have different stationary phases; from completely different materials, to different lengths and surface area, etc etc. And also the detector at the end of the stationary phase. The quality and the type of detector will also influence the rate at which you can pass the mobile phase over/through the stationary and get a good separation that is detectable.

[Dude]

Some generalities:

1.  With the same type of GC column (i.e. polysiloxane stationary phase or DB-1), you will never be able to change the elution order of the compounds of interest no matter what funky temperature ramp you select.  For the aromatic compounds listed, I suspect that you would never be able to get pyrene out before naphthalene due to the huge difference in boiling point and the fact that both have simliar polarities.  The temperature ramp is selected to optimize separation and to produce the best peak shapes.

2.  The way all GC's are run is to make mixtures of the pure components (ie purchased from Aldrich Chemical) using an analytical balance and then dissolve the compounds in toluene or something and delete toluene from the integration.  The response factors are then calculated from three the average of three runs.  If there is a question about whether the GC is operating correctly, run the standard as an unknown to confirm that the measurement is accurate.

3.  One thing to watch for if you have a "split" inlet (as opposed to a direct or on-column injection) and a mixture of chemicals with both low and high boiling points is to make sure that your inlet temperature is high enough to volatile all of the compounds.  Otherwise, systematic errors may result from an excessive precipitation of the heavy component at the inlet.  Keep in mind that the boiling point of the chemical does not necessarity reflect its volatility at the split inlet.  There is a large gas flow that serves to significantly reduce the pressure.