1. Ammonia in plasma exists in equilibrium with the ammonium anion. Ammonia is also present in the atmosphere. When the sample is collected there is the possibility of the equilibrium being disturbed by the addition or loss of ammonia to the air space.
To mitigate against this affect we only analyse samples that arrive in the lab within 15 minutes of collection. You could reject grossly under-filled specimens but my experience is that such specimens are likely to be haemolysed and so would be rejected on that basis - ammonia is present in red cells at ~3 times the concentration of plasma.
2. The key step is to separate the plasma from the cells. Once that has happened ammonia still increases due to deamination reactions but at a slower rate. Low temperatures retard these reactions and so increase the stability.
We allow promptly separated plasma to be refrigerated if it can be analysed within 2 hours, otherwise it should be frozen.
Gavin