Is It Clean? – Particles – Evaluation – Millipore Testing

In preceding blogs we have talked about harvesting particles for evaluation and talked about specifications.  Specifications may or may not dictate the harvesting process but almost always provide detail on how to evaluate the particles harvested in a process involving re-washing.  The first step is to concentrate the harvested particles for evaluation.  This usually involves a filtration and evaporation process.  By far the most-used method involves filtration or a procedure more commonly known as, “millipore testing.”  The test got its name from a maker of filter media, the Millipore Corporation, who’s filters are often used as the filter media in the test.  More information about Millipore products is available at  The filters are small, flat discs with varying properties depending on size and nature of the particles to be evaluated.  Details of the filter media should be included in the test specification.

Illustration of vacuum filtration apparatus.
Illustration of vacuum filtration apparatus used to collect particles harvested by re-washing.

Since the filter media is usually quite resistant to flow, most filtration is accomplished using a vacuum assist.  The basic arrangement if the apparatus is shown in the accompanying illustration.  There are devices made specifically for the purpose of vacuum filtration which include a means to support the filter (which is necessary once vacuum is applied) and conveniences such as a spring-loaded clamp to connect the collection funnel to the collection flask.  It is a good idea to use equipment intended for the purpose if possible.

There is an excellent video demonstrating the proper way to prepare testing equipment, re-wash the sample parts and harvest and evaluate the particles under the Technology Library tab on the Cleaning Technologies Group website,  The video can be accessed directly at the following link – Millipore Test Procedure Video.

Millipore evaluation is done in two distinct ways – gravimetric (weight) and particle counting.

Gravimetric Evaluation

Gravimetric evaluation requires careful preparation of the filter (frequently called a “patch”) to remove any moisture or other volatile material prior to the filtration process.  The filter may be pre-washed with the solvent used to harvest the particles and then dried in an oven at a specific temperature for a specific time.  Once prepared, the filter is carefully weighed using a very sensitive balance and the weight is recorded.  In some instances a second solvent washing is performed followed by a second drying to assure that the weight of the filter is stable.  The filter is then placed in the vacuum filtration device and the solvent and harvested particles are filtered through it leaving the harvested particles on the filter disc.  Once the filtration is complete, the filter is carefully removed from the vacuum filtration device and again dried in an oven to evaporate any residual solvent.  The dried filter is then weighed again.  The new weight (which includes the particles filtered out of the solvent) is recorded and the starting weight previously recorded is subtracted to determine the weight of the particles collected.

Evaluation of Particle Counts and Sizes

The filtration procedure for this type of evaluation is essentially the same as that described above except for the fact the pre-weighing of the filter is not necessary.  The goal is to determine the number and size of particles, not their weight.  Filter discs used for particle counting analysis may have lines printed or scribed on them to facilitate counting particles once the filter is placed under a microscope or other magnification device.  The sizing and counting process can be accomplished using manual or computer imaging techniques.  The computer automatically sizes and counts the particles collected on the filter patch.

Gravimetric and Particle Counting Combined

There are some specifications that combine gravimetric and particle sizing/counting.  In this case, the gravimetric test is conducted to determine the total weight of particles collected but following the gravimetric analysis, the filter disc is examined to determine the number and size of particles according to the specification.

An Extension of Evaluation by Filtration

In some cases, it is desirable to evaluate not only the particles left on parts but the “non-volatile” residues as well.  Non-volatile residues include oil, grease, soap and other contaminants that will not evaporate at temperatures and under use conditions for the part.  To determine the amount of non-volatile residue present, the solvent that collects in the vacuum flask during filtration is evaporated, usually in several steps, until just the non-solvent residues remain.  These are carefully weighed to determine if the specification for non-volatile residues has been met.

In upcoming blogs, we will talk about the evaluation of numbers and sizes of particles and try to make some sense of how these specifications apply in different applications.

–  FJF  –

Leave a Reply