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Polypropylene generally contains relatively large amounts of leachables, especially weakeners or "plasticizers" (e.g. phtalates), whiteners, heavy metal (e.g. cobalt). For this reason, Enzyscreen "detoxifies" the polypropylene MTPs that it sells, by a procedure involving boiling in alkaline and acid solutions. We have been doing that for 10 years already, and feedback from our customers learned us that growth (both for microbes and mammalian cells) in these plates is  more reliable and reproducable than in untreated polypropylene microtiter plates, and also better than in non-treated plates that are marketed as made from "ultraclean" or "virgin" polypropylene (without addition of weakeners, etc.).   Polystyrene does not need any weakeners or heavy metals for the extrusion process (the production method for MTPs), and so in general  "leachables" are less of a problem than with polypropylene MTPs. However, regularly, growth inhibition problems occur  with certain batches of polystyrene MTPs as well. For this reason, we also supply our own-made MTPs in a washed version (soaked in warm ultrapure water for 24 hours).
Background information: types of plastic & leachables:   Microtiter plates are either made from: polypropylene (milky white, relatively soft, autoclavable, re-usable after cleaning      and autoclaving) polystyrene (transparent, relatively hard, non-autoclavable, often supplied in sterile package, disposable)   For both types of plastic, contaminants (also known as "leachables") may leach from the plastic into the growth medium, and affect the growth characteristics, and - in the worst case - totally inhibit growth. In practice, the inoculum size often appears to be an important factor: leachables are most harmful in projects where the inoculum only contains a small number of cells.
example of a (autoclavable and re-usable) polypropylene microplate
example of a (disposable) polystyrene microplate