Chromatography Forum

I have isolated a compound which could be either a hydrocarbon or an ester of fattyacid as it is cleared from the NMR spectrum of this compound. I need to take mass spectrum of this and would it be helpful if I take GC-MS?

GCMS spectra of many fatty acid esters are distinctive for a computer search, even on NIST libraries.

In order to run on GC-MS the molecular weight has to be low enough for the ester to be volatile enough to get through the column. With more than about 30 carbons in the molecule you will need special conditions.

Peter

In order to run on GC-MS the molecular weight has to be low enough for the ester to be volatile enough to get through the column. With more than about 30 carbons in the molecule you will need special conditions.

Peter

Thanks for your good point. What could be the solution for this?

GCMS spectra of many fatty acid esters are distinctive for a computer search, even on NIST libraries.

Does it mean that it is not easier to interpret the spectrum of fatty acid esters?

gbalaji87
After your analysis of your hydrocarbon or ester, if you are unable to identify it you could communicate with me and send me your sample for our analysis. I will be happy to help.
We have the 5975-SMB GC-MS with Cold EI that uniqely provides molecular ions and high mass fragments to all hydrocarbons and esters, including the heavy and branched isomers up to the mass limit of m/z=1050
Amirav
amirav@tau.ac.il

In order to run on GC-MS the molecular weight has to be low enough for the ester to be volatile enough to get through the column. With more than about 30 carbons in the molecule you will need special conditions.

Peter

Thanks for your good point. What could be the solution for this?

You could try a short wide bore column with the thinnest film that you can get, and run at high temperature.

If you have your molecule as a pure compound you do not need to do the chromatography - which serves only to separate things, you could use a solid probe to introduce the molecule to the MS.

How big is the molecule ?

Peter

gbalaji87
After your analysis of your hydrocarbon or ester, if you are unable to identify it you could communicate with me and send me your sample for our analysis. I will be happy to help.
We have the 5975-SMB GC-MS with Cold EI that uniqely provides molecular ions and high mass fragments to all hydrocarbons and esters, including the heavy and branched isomers up to the mass limit of m/z=1050
Amirav
amirav@tau.ac.il

Thanks for your interest sir.

Differentiating between the mass spectrum of a methyl ester of a fatty acid and the mass spectrum of a hydrocarbon should be very easy. If the fatty acid has less than two double bonds, there will be a peak with a significant intensity at m/z 74. Regardless of the number of double bonds and substituents on the backbone of the aliphatic moiety, there will be a low intensity peak representing the [M – 31] ([M – OCH3]) ion. Finding a peak representing the molecular ion may be difficult.

The spectra of aliphatic hydrocarbons have peaks every 14 m/z units. The most intense peaks wil be at m/z 43, 57, 71, and 85. The spectrum will have different appearances depending on the presence and number of double bonds and or branches. Again, the molecular ion peak may be difficult to identify.

The spectra of aromatic hydrocarbons show ion current at far fewer m/z values than those of aliphatic hydrocarbons. Often there are peaks at m/z 91 and m/z 77. The molecular ion peak should be very easy to identify.


Good Luck