I have a different experience. There are some publications describing similar situation and IR was used a a method of their choice. E.g. http://journals.tubitak.gov.tr/chem/issues/kim-02-26-1/kim-26-1-12-0010-14.pdf
You could try to determine them by GC(MS) after derivatisation with an appropriate agent, for example BSTFA. Alternatively, they could be separted using HPLC...