lactic acid standard shows 2 peaks on Aminex HPX 87H

[nasirpour]

Hi
I recently bought an Aminex HPX 87H for analysis of organic acid. I have the good peaks for many OA but in the case of lactic acid, when I inject the standard, it shows 2 peaks separatly. but in cheese samples I have only one of the peak (the bigger one). I changed the pH of mobile phase, it is sulfuric acid, from 2.15 to 2 and the 2 peaks came closer to each other. I added 10 % acetonitrile in mobile phase but I have 2 peaks. I used another source for acid lactic from Merck, the same problem I have. I saw in this forum that maybe lactic acid became dimer, trimer to oligomer. So can somebody help how I can resolve the problem.

[carlo.annaratone]

If the acid can dimerize, maybe you just have an old standard. Buy a fresh one and see if the problem solves

[nasirpour]

Just is it possible to convert the esters or dimers to lactic acid?

[Consumer Products Guy]

Lactic acid is known to dimerize, discussed in USP-NF Monograph, where saponification is used in the assay of "pure" lactic acid solution.

When we do lactic acid here, we use sodium lactate as the analytical standard and saponify the sample into sodium lactate, then assay using non-suppressed ion chromatography using a dilute sulfuric acid mobile phase. We completed a GxP validation doing that.