Frits and filters are porous metal products used in virtually all
liquid chromatographic (LC) systems. Their function is to prevent
unwanted particles from entering the LC system. These particles may
come from the sample,
the solvent or debris generated by the LC system itself (i.e. pump
or injector). Particles entering the LC system may lead to clogging
of capillaries, interference with the chromatography by changing
chromatographic parameters or disturbance of the detector function.
most important characteristic of a frit, besides the diameter and
the thickness, is the porosity. When considering porosity, it
is not only the average size of the pore that is of interest, but
also the size distribution and the amount of pores available. The
size distribution should be a gaussian function with narrow tolerances.
At Applied Porous Technologies, we continuously monitor our manufacturing
process and maintain strict control of the powders used in our
process in order to provide the tightest pore distribution possible.
to pore distribution, it is important to consider the pore density,
or number of pores available. Take, for example,
a frit with 2 µm porosity and a surface of 0.25".
The theoretical maximum amount of pores with 2 µm average
diameter would be about 5,000,000. This frit would give you the
achievable. Based on standard bubble point methodology, the same
frit having just a few pores would also qualify as a 2 µm
frit. Although the porosity may be within specification, it is
that that this frit would provide adequate flow. Careful control
of our powders and process allows Applied Porous to offer the
best pore size distribution while maximizing flow properties.
Applied Porous Technologies, we understand the needs of the
chromatography industry. We work closely with many leading instrument
and column packing companies to provide the best products available.
Our employees are committed to continuously upgrading our products
through new product development and process improvement. We
always open to discussions and ideas on how to make our products
and most obvious filter you will see in a liquid chromatographic
system is the solvent inlet filter.
This is located in the solvent reservoir and prevents dust particles, general
debris and, to
some extent, bacteria from entering the LC system. These contaminants can interfere
with downstream in the system such as the low-pressure gradient former or the
pump. Particles entering the valves of either of these two instruments may interfere
with the proper function. The result could cause an increased baseline noise,
nonrepeatable gradient forming, unreliable flow rate or other interferences with
the LC system. In general, the solvent filter is a low-pressure filter and will
allow a high flow rate due to a large surface area and large porosity. The high
flow rate is necessary to guarantee that no extra force is needed for suction
of the solvent from the solvent reservoir to the pump. It is also often recommended
to have the solvent level slightly higher than the pump, helping to create a
slight prepressure on the pump head and valves.
Liquid Chromatographic System
parts within the LC system can generate debris. Abrasion from
the pump piston seals is one of the
most common sources. Despite the superior sealing
materials available today, small irregularities in the seal itself or on the
piston, dirt on the piston or an improperly installed seal will result in small
particles being removed from the seal and washed downstream towards the injection
valve. Regardless of whether your system has a manual or automatic injection
system, all injection valves have close tolerances (flat surfaces moving against
each other to provide sealing under high-pressure conditions). Improper valve
operation can occur as a result of debris interfering with proper sealing of
the valve. Alternately, debris entering the valve can destroy the sealing surfaces,
generating additional particles and making it necessary to repair the valve
or replace the rotor seal. To prevent this costly damage, a large-surface,
inline solvent filter is placed in line between the pump and the injection
valve. These solvent filters have easily replaceable filter discs
that can be changed
out for a fraction of the cost of repairing a damaged injection valve.
source of particles in the system is from the sample itself.
Whenever possible, the sample should be prefiltered prior to
injection into the system.
Once injected, the sample travels through capillary tubing towards the column.
Particles entering with the sample, or those generated by the injection valve,
can easily clog the capillary. Debris passing through the capillary will
collect in the column and can also affect the performance. To
prevent these problems,
it is recommended that a low-porosity, small surface area (hence small dead
volume) precolumn filter be added between the injection valve and the column.
the cost to replace the filter element in the precolumn filter is minimal
in comparison to the cost of the column or the time lost to replace
The column is
the one part of the LC system that always uses porous frits.
A typical column will have two frits, one at the inlet
and one at the outlet.
frit on top of the column, or inlet, prevents particles from entering the
bed of the column. This frit has a protective function. Any debris that
enters the column will be trapped on the inlet filter. Even though
the frit can
become clogged, the expensive column bed will remain intact. The inlet
frit also aids in the distribution of the solvent/sample over
the column. A column with 4.6mm I.D. has a surface area that is 330 times
1000 times (0.13mm I.D.) larger than that of the capillary. The solvent
stream has to be distributed evenly over the column surface to give the
best results for the separation.
is true for the outlet frit, where the solvent stream has to be concentrated
from 4.6mm into the small capillary I.D. without band broadening. The ideal
frit for the inlet would have to have a larger porosity and minimal thickness
to the outlet frit. This would minimize pressure losses and reduce the
amount of dead volume. Often, however, the same frit size is used on both
of the column to prevent the user from reversing the column and switching
function of the outlet frit is to retain the packing material.
Here it is important to use a frit with a porosity smaller
than the packing
(i.e. a 5 µm packing material would need a 2 µm frit). Care
must be taken not to choose too small of a porosity for the outlet frit.
material contains some smaller particles and through attrition will also
break into smaller particles. If you choose your frit with a very small
small particles contained or generated in your packing material will
eventually work into the pores and clog the frit, resulting in increased
It is generally safe to choose a frit for your column outlet with a porosity
half the size of your packing material.
sample has left the column it will enter the detector.
It is desirable to keep the void volume
between the column outlet and the detector
short as possible. Small particles of the packing material can penetrate
frit and will then enter the detector. The use of a large-volume filtering
device between the column outlet and the detector can result in band
the same time, the detector cell has to be protected from column packing
material particles. A
thin frit with small pores and small
surface area might work well here to filter out particles while still
keeping void volume low. This will not work for all separations
and is very
unusual to find in practical work.
Due to pressure
differences and the relaxation of the solvent, small
gas bubbles can form. The formation of bubbles can be prevented
by the use
of a backpressure
regulator. A porous metal frit in an in-line filter can be used
as a backpressure regulator, but care must be taken since a clogged
increase the backpressure in the detector. Most detector cells
not designed for
high-pressure applications, therefore we recommend the use of a
standard backpressure regulator.
Advice: Choosing the Right Frit
Frits and Polymer Ring Assemblies
There are a number of things to consider when deciding on what type of frits
and filters to use in an LC system. The choice of material is very critical to
the function of the frit. Our standard material is 316LSS, which is suitable
for many applications. We also offer our porous frits in Titanium, Hasteloy and
a number of other materials for applications that require greater corrosion resistance
are also supplied with a press-fitted polymer ring. This ring
has two functions. First, since a stainless steel frit will not
seal well against a stainless fitting, the ring acts as a gasket. Additionally,
the ring fills up a void in the fitting thereby reducing dead
volume. Applied Porous offers frits with rings
in a wide variety of polymers including PTFE, ETFE, PEEK and
Kel-F. The end user must choose the combination of porous metal
and polymer that will perform the best with the intended sample
and buffer chemistry.
geometry is another consideration. Frits can be produced with
or chamfered edges, depending on the preference of the end user.
edge facilitates assembly in applications where the frit is pressed into
Some column builders, however, prefer the straight-edged frits because they
minimize dead volume. For more demanding applications, we offer a variety
frits such as a dual-density frit with a 5 µm porosity in the center
and 1µm around the periphery. For preparative columns, we manufacture
a multi-porosity frit that has a coarse support layer mated to a very thin
fine filtration layer.
This combination provides excellent filtration with minimal pressure drop.
Frits are also available with embossed star patterns for distribution, special
designs, or assembled into custom fittings and hardware. One of the more
unique frits manufactured was parabolical. In tests at the Technical University
Berlin, this frit was shown to provide extremely uniform flow distribution
in a preparative
column. It is this type of effort that will allow us to produce the next
generation of filter products for chromatography.
Products for the chromatography industry are only a small part
of what we do at Applied Porous Technologies, Inc. However, our dedication
the widest variety of options to our chromatography customers is just one
example of our commitment to be the number one supplier of porous metal
the industry. Working alongside our customers, we are constantly developing
innovative solutions. The experience and knowledge of our employees, combined
with the rigid control of our manufacturing processes, strict quality control
and wide variety of available base materials, guarantees that we can supply
the right solutions for your chromatography applications.
was provided by Applied Porous Technologies, Inc. It was authored
by Dr. Christian Wegner and co-authored by
Ed Swiniarski. Applied Porous is a world leader in the design,
development and manufacturing of sintered porous
metal media, sintered
porous metal components and engineered
sintered porous metal assemblies.
They also offer custom-engineered porous metal products and OEM
assemblies. Please contact Applied Porous for any of your porous