Chromatography Forum

Has anyone tried using n-heptane as a final solvent when blowing down hexane to lower volume. Also, who prefers n-heptane for GC-FID DRO analysis and why?

Perhaps you are not aware of the fact that these two different solvents have very different vapor pressures and boiling points? n-Hexane's bp ~ 69C and n-Heptane is close to water at 98C. Using heptane to dry something down would be a step in the wrong direction.

Link to "HPLC Solvent Properties Table (e.g. Polarity, Density, Boiling Point...)"; https://hplctips.blogspot.com/2011/11/h ... table.html

Multidimensional wrote:
Perhaps you are not aware of the fact that these two different solvents have very different vapor pressures and boiling points? n-Hexane's bp ~ 69C and n-Heptane is close to water at 98C. Using heptane to dry something down would be a step in the wrong direction.

Link to "HPLC Solvent Properties Table (e.g. Polarity, Density, Boiling Point...)"; https://hplctips.blogspot.com/2011/11/h ... table.html

Yes, I am aware. I was thinking that if one dried down at modest temperature 40-50C. Then you could add 1-2 mL of n-heptane to larger volume of (n-hexane and DRO) mixture for example. Blowing that down would effectively transfer the DRO into 1-2 mL of n-heptane. That in turn can be dried and transfered to capped vial for analysis.

The point of this escapes me completely. The only effect would be to delay the evaporation to dryness. If you are evaporating to a fixed end volume the heptane does no good, and still slows things down.

Peter

I thought perhaps I could avoid accidentally having my whole sample evaporate to dryness while I was attending to other things in the lab. I thought maybe evaporation would slow greatly as the volume approached that 1mL of solvent that had twice the B.P..

LALman wrote:
I thought perhaps I could avoid accidentally having my whole sample evaporate to dryness while I was attending to other things in the lab. I thought maybe evaporation would slow greatly as the volume approached that 1mL of solvent that had twice the B.P..

It would, but it was not clear whether you wanted to evaporate to dryness, or to a fixed volume.

Peter

Similar to the Drinking water method that uses Ethyl Acetate as the final solvent. The DCM will evaporate quickly and the EtOAc takes a long time to come to final volume of 1ml at 40C-45C.

As long as the Heptane peak will separate well from the lightest component of the DRO then it should work well, and you don't have to worry so much about a vial concentrating if it sits on the instrument overnight after the septa is punctured.