Chromatography Forum

For the few normal-phase people out there, which weak solvent do you prefer, Hexane or Heptane? I typically use hexane because of availability, but am not partial either way. I do however need to choose one because I am setting up my NPLC instrument to do screening (chiral and achiral) and I have only so many solvent inlets.

Are there any significant retention/selectivity differences with one CH2 unit?

Both are essentially non-polar, so any selectivity differences should be minimal. Hexane is more volatile (bad) and should have a lower viscosity (good) compared to heptane. It would be a "coin toss" in my book.

Thanks Tom. Since more often than not my purpose will be meth dev for prep, the volatility will work to my benefit in dry-down.

I used to use hexane as it was cheaper but switched to heptane as I had problems with noise in my FLD detector with the hexane and I only had FLD-grade heptane... As it worked I stuck with it..

But chromatography-wise there was no significant difference..

I prefer n-hexane, as opposed to hexane fraction, for most NP work, and also n-hexane over n-heptane - because the non-volatile matter is usually lower, and UV transparency may be slightly better - which both are helpful when scaling to preparative. Non-volatile matter is often the critical parameter for preparative work.

Product quality and batches can vary quite a bit, and most suppliers specifications will give an indication, but their CoAs often also just show the product has passed specification, rather than actual numbers.

Product, grade and supplier comes down to what works for you, but the chromatography will not change much, however some detectors can be affected by the different properties, especially if using RI - hexane can be quite noisy on some.

Normal phase hydrocarbon solvents pick up non-volatile matter really easily, so I try to minimise transfers. Note that popular higher-grade solvents can be cheaper than less popular lower grades, consider Chromasolv Hexane Fraction example below, so always ask your suppliers about popular stock grades, container sizes, and prices.

Example specifications - ( Non-volatile matter %, UV@wavelength, cost/L ), and please note that some < will also be <=). As you see, general rules don't cover all products/suppliers, and there are plenty of exceptions.

Ex Aldrich - Chromasolv Grades.
Hexane
Plus n-C6 ( <0.0001, 0.20@210, 0.05@220, A$103 )
>95% n-C6 ( <0.0005, 0.30@210, 0.10@220, A$80.50 )
Plus >98.5% C6 Fraction ( <0.0001, 0.05@225, A$72 )
>98.5% C6 Fraction ( <0.0005, 0.30@210, 0.10@220, A$73 )
Heptane
Plus 99% nC7 ( <0.0001, <0.10@225, A$86.50 )
>96%, nC7 ( <0.0003, 0.40@210, 0.10@220, A61.50)

Ex Merck - Lichrosolv grade,
Note the residue is % mass/volume, and so will be reported as about 50% lower than equivalent % mass/mass, UV is % transmission.
Hexane
98.0% nC6 ( <0.0001, 50%T@210, 80%T@220 )
99.0% C6 Fraction ( <0.0002, 60%T@210, 80%T@220 )
Heptane
99.3% nC7 ( <0.0002, 50%T@210, 80%T@220 )

There are disadvantages with using n-hexane, especially with regard to much greater toxicity and volatility ( samples in vials evaporate faster, some RI detectors are noiser ), and also in ambient pressure silica columns, where the exotherm can cause bed disruption on larger columns.

Please keep having fun,

Bruce Hamilton