In accordance with the normal practice in
spectrophotometry, the initial DPPH concentration
in the cuvette should be chosen to give absorbance
values less than 1.0 (which corresponds to the light
intensity being reduced no more than tenfold in
passing through the sample). This implies a concentration for the stock solution in the range 50 to
100 µM ( 7). In the originating paper (Blois,
1958) it was noted that the stock solutions of this
“stable free radical” do slowly deteriorate; it
was therefore recommended to use an automatic
burette with a nitrogen atmosphere and covered
with aluminium foil, whereby the loss of free
radical activity may be reduced about 2-4 per cent
per week.