Collection and storage of urine in N-balance trials with mink

Traditionally the urine is collected daily and stored frozen until analysis in N-balance trial with mink.
The purpose of this trial was to evaluate if 1 collection for a period of 4 days, preserves the N-amount in the urine.
Furthermore it has been evaluated how pH and N-percent in urine is affected by storage temperature, -time and sulphuric acid preservation.