the glass test tubes are for one time use. so everytime i used a brand
new test tube. for the plastic cuvette, they are also new. so i do not
think it is the problem for the test tube or cuvette themselve.



do you wash your glass test tubes in a dish washer? our old one didn't
rinse well. so glass often had a bit of a detergent residue. your
plastic cuvettes would be more disposable. have a look if you get
percipitation on a new plastic cuvette.
are the tubes or PPi chilled? i think some surfactants percipitate when
cool - SDS will very quickly in the fridge don't know about triton.
is it a large flaky percipitate (probably surfacant), white snotty
(protein/sugar) or clear (could be DNA)?

http://www.pubmedcentral.nih.gov/art...i?artid=102300
these people used more buffering 100mM HEPES?

craig