The M-Vac Pre-filter and Shield are used to "clean up" an M-Vac sample to enable the lab to maximize the DNA recovery of the sample.  The pre-filter is a 40 micron, 50ml unit that can filter out some of the debris, dirt and other items that might hinder the extraction process, so it is used whenever the M-Vac system user determines it is a necessary step.  Prefilters come in individual units and cases of 25.  The shields, which are used to protect the M-Vac SEC, come in individual units only.