Nasty NADH Nightmare

[Matt Purnell]

Hello,

I am having trouble preparing my NADH solutions for spectrophotometric assays.
I prepare a 1 mg ml-1 solution, without vortexing, in 100mM Tris pH 8.0.
Sometimes (but not always) I find that blanking on the NADH solution and then 
following the absorbance change at 340 nm doesn't give me a nice, straight 
line, but rather an absolutely crazy read (it jumps all over the place).  I've 
ruled out machine error, always use fresh NADH, and have confirmed the pH is 
8.0 (i.e., the pH is not too low).

Can anyone suggest what is going on and how to fix it?
Thanks.

Matt Purnell
Botany Dept.
University of Qld