Clarification on Chromatograms for Formic and Acetic Acid

[sribala]

When I am analysising the samples using GC using Capillary Columns which Peak should come first i.e Formic or Acetic acid i.e Based on Molecular weight or not. But the Stanadard supplier has given the chromotogram as first Acetic acid Follwed by Formic acid. Wheras I am under the impression that it should be in reverse order. Please clarify

[Don_Hilton]

It is all about how the analyte partitions between the stationary phase and vapor. So which stationary phase?

Even thought formic acid boils slightly lower than acetic acid, formic acid is the stronger acid (lower pK a). So, expect polar interactions to be important, inlcuding the formation of a dimer by hydrogen bonding.

If you look at flavornet.org you can see retention indicies for a number of compunds on four different stationary phases- and while I did not see formic acid in the list, you will notice that butiric acid has a lower retention index than acetic acid on an OV101 column. (Methyl silicone, like DB-1 or HP-1, SP-1, etc.) On the other stationary phases, the acids are in molecular weight order.