The Wrench
Vol. 5 Issue 1
Winter 2008
Sample and Mobile phase filtering
Instituting a preventative maintenance program, ensuring chemical compatibility amongst
samples, solvents, and system components, examining buffer composition and concentration
are all important aspects in keeping your HPLC system running optimally. There is another
simple, easy and inexpensive precaution that can be taken that can help minimize
instrument downtime, filtering your mobile phase phases and samples.
Filtering will help prevent blockages of tubing and frits. It can extend the lifetime
of in-line filters. It can result in less wear and tear to the injection port, plungers,
and seals. Finally, it may help improve chromatography by removing contaminants.
Sample filtering
Passing the sample through a syringe tip filter is the most common way to clean up a
sample. When choosing a filter there are a number of material choices available. It is
important to choose a filter with the proper physical characteristics required for your
analysis and is made of material compatible with your sample and solvent. Figure 1 shows a
typical syringe filter.
Figure 1.
Syringe filters are composed of 3 wetted parts; a housing, the filter membrane, and a
pre-filter. Housings are predominately made of a polypropylene material. The filter
membranes can be made of a variety of materials including Cellulose Acetate, Regenerated
Cellulose (RC), Nylon, Polypropylene, PTFE, and PVDF. The pre-filter is typically
comprised of either polypropylene, polyether sulfone, or glass.
PTFE is a hydrophobic material that has the widest range of solvent compatibilities
including aqueous and organic solvents as well as acids and bases. Nylon is hydrophilic
and is compatible with most aqueous and organic solutions, however it is not compatible
with many acids or bases. Regenerated Cellulose is a hydrophilic material that is also
compatible with organic and aqueous solutions, and has the added benefit of exhibiting low
protein binding.
Beyond the chemical characteristics of the filter, the physical characteristics must be
considered as well. Pore size, membrane thickness, burst strength, and extractables are
other characteristics to account for. Typical HPLC applications should be filtered through
a 0.45um filter, a 0.2um filter may be necessary if there is the need to remove bacterial
growth. 13mm diameter filters can handle sample volumes ranging from 1 to 10mL and are
suitable for most HPLC samples.
Mobile Phase filtering
Similarly, filtering mobile phases will benefit the HPLC system and the column. This is
especially true in the case of mobile phases prepared with buffers and should be standard
practice in these cases.
Filtering is done off-line, prior to pouring the mobile phase into the solvent
reservoir. Typically, the mobile phase is vacuum filtered; either a 0.2um or 0.45um filter
disc membrane is used (again depending on whether or not there is the need to eliminate
microbial growth). The same precautions or awareness must be taken with choosing the
membrane filter material as was outlined for the sample filters above.
The typical filter set up is pictures below in Figure 2. It comprises a vacuum flask
with a ground glass opening, a fritted glass support base, a filter membrane (typically
47mm in diameter), a clamp, and a funnel.
Figure 2.
By taking the time to introduce these simple additions to sample and mobile phase
preparation you can help ensure the reproducibility and quality of your sample results,
and minimize unplanned downtime of your HPLC.
(For past issues of The Wrench, see our archives)
