The separation of aldehydes with GC is only practicable be knowing the substances and their chromatographic nature. A separation is possible with HPLC, too. For the detection the best is mass-spectrometry. But you can not quantify the total amount of the aldehydes. Only the sum of many single molecules.

The reaction of aldehydes and ketones with hydroxylammoniumchloride formes a colored solution, which can be titrated with NaOH and an indicator.

The titrated quantity give the equivalente amount at the changing point.

It may be possible to detect the amount of the colored solution with spectroscopy.

For quantitative determination of aldehydes can be used by HPLC. 2,4-dinitrophenylhydrazine is derivatizing agent.

2,4-dinitrophenylhydrazine is reacted with an aldehyde in the presence of perchloric acid with the formation hydrazones which are well separated in the chromatographic column. The detector was a UV detector is used (length 360 nm)