Glutathion OD readout fluctuates

[Josmar]

Dear All,
trying to use 30mM glutathion as a blank in OD measurement seems
impossible because at both wavelenths 220 and 280 the readout
fluctuates heavily. This happens with pastic UVettes by Eppendorf
(220-1600nm) in two different photometers.
The other buffer components are: 50µM ZnAc, 50mM Tris pH 8, 150 mM
NaCl, 0,2% Triton
Any hints on why this is so are highly appreciated!
Josmar