Just discussing some things mentioned above!
Oxidation, though you are right iso240 in saying it does not NEED to involve Oxygen, when discussing organic compounds, oxidation USUALLY refers to the addition of an oxygen, or the removal of 2 hydrogens. By definition however, it is a change in the oxidation #, (or the number of electron's "owned" by an atom)
FOR EXAMPLE: Involving carbon (as the simplest alkane)
Methane is oxidized to Methanol (Oxygen added)
which is oxidized to Methanal (2 Hydrogens removed)
which is oxidized to Methanoic acid (Oxygen added)
which is oxidized to Carbon Dioxide (2 Hydrogens removed)
Oxidation requires something ELSE to be reduced (to conserve the # of electrons throughout). That is why they are called redox (pronounced ree-dox, or Oxidation-Reduction) reactions.
However, the oxidation of our oh-so-precious research chemicals, I would assume to be the result of oxidation w/ the small amount of oxygen and other gasses dissolved into sol'n. Tryptamines, in general, are more suceptable to this than the phenethylamines. The Oxygen in water, hydronium ion, and hydroxide ion are ALL with Oxidation # of "2-". Most likely this will not be reduced (to oxidize the other compound)! The same is most likely true with ethanol (it will not be redced).
Which draws the (quite presumptous) conclusion that:
ATMOSPHERIC GASSES ARE THE CULPRIT!
This is however acting on the assumption that one is not playing with tap water (or other non distilled water). The number of things to react in a ReDox reaction in tap/non-distilled water is countless.
MORAL OF THE STORY: use distilled water, or ethanol which does NOT contain other dissolved compounds (vodka or such). I would most likely choose vodka OVER distilled water in most cases, because of it's protection from bacteria and such, but the best would be "everclear", or dry alcohol. PROTECT THEM FROM AIR, LIGHT, AND HEAT! Block out the air, you block out the oxidizing agent. Protect it from light and heat, and you slow down the speed of reaction/decomposition.
I would not however understand why one would want to use IPA (isopropyl alcohol), unless they were planning on PROPERLY evaporating it all before consuming!
Anyways! I hope this made sense mostly!
A bientot!