By what reaction does (–)-sparteine decompose in ambient conditions? In its pure form (–)-sparteine is an only-slightly-yellow clear, viscous liquid. Yet, after only a week of being kept in dry, normal atmosphere (in a round bottom flask covered by a septa), some of it reacts with something to form a sticky, orange-brown substance. I've had to distill the sparteine I have twice due to whatever reaction is occurring, and I would like to know exactly what the reaction is so I don't have to distill every time I need it. The literature I found mentioned that it readily absorbs carbon dioxide in a 1-1 molar ratio, and I know that tertiary amines do sometimes react with CO2, but I haven't been able to piece it together as something that could happen in ambient conditions.
Amines left out in the atmosphere will air-oxidize to give N-oxides, which typically have a yellow color. These N-oxides can decompose to give further byproducts. I would store your sparteine under nitrogen or argon atmosphere.

[edit to give examples and mechanism]
The reaction proceeds through a radical chain mechanism with molecular oxygen. Here is one study of the mechanism [from first page of Google search for amines air-oxidation] . 

From wikipedia article for triethylenetetramine. "This oily liquid is colourless but, like many amines, assumes a yellowish color due to impurities resulting from air-oxidation."

"It can be observed that arylamines develop a yellow or black color if left exposed to air for prolonged periods". Ref Note - I realize that sparteine is not an aromatic amine, but the result is the same. 

If you have access to triethylamine or other amines in your lab, do a control experiment: leave one vial open to air, and leave the other under identical conditions except sealed under an inert atmosphere. A few days worth of observation will convince you of the role in oxygen in the yellowing of amines.