I have to determine composition of mixture of fatty acids (stearic acid at the beginning) with both saturated and unsaturated long chain hydrocarbons. At this time I have at disposal for analysis of this mixture an HPLC with RP18 column and an UV-Detector. But I don't have any experience with it. Could you please suggest me some pretreatment or approach. Or is it better to use another detector or column?

Thank You, Iva

Dear Iva;

HPLC has never found a good acceptance for fatty acid analysis. They do not have good chromophores, and the selectivity required to separate closely related ones is difficult to achieve.

A far better approach, and something of a time-honored traditions. Is to do GC analysis of their methyl esters. You will get far better results this way. The Hydrocarbons you have will also be much better suited for GC.

Good Luck;

Benjamin

Iva - use GC with a polar capillary column such as Restek RTX-2330 or PEG-carbowax type for which the saturated fatty acid methyl esters elute before the unsaturated ones with same carbon number. Suppliers usually have bulletins on FAMES analayses very common, check websites of Restek (restekcorp.com) and Supelco (sigma-aldrich.com). I work for a soap company, this is very routine by GC. Use boron-trifluoride in methanol (Aldrich or Acros) or H2SO4 in methanol to make the esters from either fatty acids or soaps.

UV detector isn't suitable for saturated hydrocarbons. If you should stay with HPLC,consider another detectors (Refractive index - if the requred sensitivity can be reached;ELSD - probably the best choice).

Dr.Alex Weisman