Millistak+HC AppGd_textC.qxd - Millipore

Evaluation and
Optimization
Separation Mechanisms
The particle removal achieved by
Millistak+ HC filters is the consequence
of two distinct separation mechanisms.
The first of these mechanisms is
mechanical sieving, in which particles
are captured within the prefilter due to
their physical size and/or orientation.
In the case of a depth filter, the path
through the media matrix is tortuous
and varies greatly in effective pore
dimensions, providing ample opportunity
for particle retention by entrapment.
The outward effect, typical of
most filters, is a build-up in pressure
differential in the direction of flow.
The second particle capture
mechanism for Millistak+ HC filters
is adsorption: the physical/chemical
binding of particles to the filter media.
This process is aided by the net
positive charge on the surface of the
Millistak+ HC filter media. Colloidal
particles in a biological suspension
(cell wall fragments, agglomerated
proteins, polynucleic acids, etc.)
exhibit a net negative electrostatic
charge on their surface at typical
near-neutral pH conditions. The
cationic charge on the Millistak+ HC
filter media allows these particles to
be captured, when otherwise they
might escape retention due to their
small size.
This electrostatic retention mechanism
allows the Millistak+ HC prefilter
to be effective in removing particles
well below the smallest pore size
of the media and thereby provides
additional protection to the sterile filter.
This capture mechanism does not
however reveal itself as a pressure
build-up on the prefilter. Instead, the
user must carefully monitor the quality
of the filtrate, to detect the point at
which all available adsorption sites
within the media have been used.
This can be done either by monitoring
filtrate turbidity in relation to process
throughput or by measuring the
capacity of the downstream sterile
filter in situ or off-line (with a post-run
Vmax SM test).
Depending on the properties of
the feed stream (and the consequent
relative contribution of the two separation
mechanisms described above),
two distinct types of behavior may
be observed for the Millistak+ HC
prefilter. These two distinct profiles
Millistak+ HC Filters Application Guide 15