Several peaks from n-pentane and n-hexane

[evaporator]

I obtained 2 peaks in my chromatogram when I injected a pure sample of n-pentane while 3 peaks using pure n-hexane. I am sure that they dont have contamination because I just acquired them from the company. That of pentane is a big peak that appears at RT 1.03 and a smaller one almost 1/3 the area of the first which appears at 1.15. While that of hexane is a big peak at 1.20 and two similar peaks each almost half that of the previous that appeared at 1.31 and 1.46 respectively. I ordered another bottle of chemicals but I got the same result. How can I get rid of this? Does it happen sometimes? What are the possible causes. By the way I am using GC-FID.

[Terry]

Evaporator,

n-Pentane, n-hexane and n-heptane usually have some hydrocarbon isomer impurities in them. The peaks in your chromatogram are typical impurity peaks.

What is the claimed purity on the label of reagent bottle?

You may buy these reagents from some other manufacturers like Sigma-Aldrich.

Regards

Terry

[evaporator]

I am using 96% hexane and 98% Pentane. are the hydrocarbon impurities of hexane, hexane isomers also? I haven't tried n-heptane yet. would a purity of 99% for n-heptane yield this impurity peaks also?

[Bruce Hamilton]

96% n-hexane isn't very pure, ther are plenty of more suitable grades. I wouldn't use anything <99% of the n-alkane.

You can purchase pure grades of hydrocarbons from Chevron Phillips Chemical Company.
http://www.cpchem.com/enu/specialty_che ... arbons.asp

Their 99.0% n-hexane has typical composition of:-
n-Hexane 99.4%
3-Methylpentane 0.1%
Methylcyclopentane 0.4%
2,4-Dimethylpentane 0.1%

If your compositions are different to the CoA, you could be overloading the main peak, causing the impurity concentrations to appear larger, ir you could have some discrimination ( much less likely with close boiling compounds).

You can check for overloading by diluting your sample with a different solvent, and comparing peak ratios. Those retention times also seem very quick.

Please keep having fun,

Bruce Hamilton

[Terry]

I am using 96% hexane and 98% Pentane. are the hydrocarbon impurities of hexane, hexane isomers also? I haven't tried n-heptane yet. would a purity of 99% for n-heptane yield this impurity peaks also?

Definitely there are plenty of C5 isomers in pentane and C6 isomers in hexane. I guess if you chromatograph the 99% heptane, your will get more but samller impurity peaks. Try some sample of higher purity grade, then check the difference.

Good luck

Terry