Chromatography Forum

Hi guys,

I have a question: Why do the compounds vaporize in the inlet and elute at temperatures well below their boiling points? For example, boiling point of eicosane is 343 degree Celsius but it shows up in chromatograms well below this temperature (for example, at 150 degree). And why do we assume that all eicosane gets vaporize completely at inlet temperature of, let's say 270 degree?

Anybody knows whats the vapor pressure cut off value for the stuff to be GC amenable?

kaka

The key difference is between vaporize and boil. Open a box of fruit flavored candies and you will notice as the vapors of the fruit flavor reach you.

In a GC separation we make use of the partitioning between vapors of a compound and the compund partitioned into the stationary phase of the column.

For transfer from the inlet, we assume that the vapors from the evaporating compund are swept from the inlet. You must take care to be sure that for a given sample type, inlet pressure and temperature, and GC flow rate that there is sufficient time to transfer all of the vapors for a compund of interest from the inlet to the column.

I have a question: Why do the compounds vaporize in the inlet and elute at temperatures well below their boiling points? For example, boiling point of eicosane is 343 degree Celsius but it shows up in chromatograms well below this temperature (for example, at 150 degree). And why do we assume that all eicosane gets vaporize completely at inlet temperature of, let's say 270 degree?

There is a difference between injecting a neat chemical and a chemical solution into a GC inlet. If you want to analyze pure eicosane by GC, the inlet temp need to be set way higher than 343 degrees which is beyond the capability of a GC inlet. If you analyze an eicosane solution in a solvent, 270 degree probably work. At 270 degree, the solvent vaporizes instantly and that leaves eicosane in the vapor phase.

Messeurs Hilton and 2002 summed it up well.

My favorite example of vaporizing without boiling is the disappearance of ice into vapor, or, the fact you can smell candle wax even below its melting point, much less its boiling point.

What is the boiling point of a C40 alkane? That is close to the limit of practical GC elution.

best wishes,

Rod

If you plot vapour pressure vs temperature you get a smoothly rising curve - there is nothing magical about a boiling point; it just happens to be when the vapour pressure matches the pressure on the liquid so that bubbles of vapour are able to form in the body of the liquid and rise to the surface - there is no sudden change in vapour pressure at that point.

There is no need for analytes to be transferred to the column as very narrow bands - under normal temperature programmed GC runs the late eluting anlytes can come out of the inlet as wide bands and still appear as sharp peaks due to their being focussed into the stationary phase at the relatively cool start of the programme. It is only if you are trying to analyse compounds that elute very early then the width of the inlet bad is important.

The Grobs published a lot on what goes on inside a split-splitless inlet - flash vapourization of solvent leaving heavy analytes in the gas phase does not even come close.

Peter

thanks everyone for the valuable feedback!