Syringe Filter Selection Guide - Part 4 of 4

Consider the following when choosing the best filter for your analytical application:

Part 4: Optimal Pore Size Rating

Assumption: it is desirable to extend the life of your column and reduce maintenance due to particulate in the pumping system thereby giving more analyses per dollar spent. Given this assumption, the filter pore size should be determined based on the column packing size. As you can see below, the column packing particles touch each other. Ideally, you would not want contamination to fit into the space between the particles of packing. This space (labeled Flow Path) is identified below. The idea is to find out how large that space is and remove particles that size.

For example, start with a 3 µm packing size and draw a few well-placed equilateral triangles as suggested in the figure below. Determine the largest particle capable of fitting through the column by circumscribing an equilateral triangle with a side length of 0.75 µm.

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Now enlarge the diagram and look further. Visualize a series of equilateral triangles whose side length gets down to 0.75 µm. The figure below illustrates a right triangle whose short side describes the radius of the sphere. The angle is one half of 60º, or 30º. The horizontal side of this new right triangle has a length of 0.375 µm (half of 0.75 µm). Calculating the tangent of 30º gives the ratio of the length of the opposite side over the adjacent side, in this case 0.58. This means that the short side of the triangle is equal to 0.58 x 0.375 or 0.217 µm. Coincidentally, this is also the radius of the particle. So, if the column packing is 3 µm in diameter, the flow path is 0.43 µm.

When an HPLC column has a packing size of 3 µm or smaller, you should use a 0.2 µm filter because a 0.45 µm filter may let particles through that will plug the column.

Are You Concerned About Accurate Retention of Particulate?

For liquid chromatography systems using columns with larger than 3 µm packings, the filtration industry standard is 0.45 µm for syringe filters and mobile phase membranes. For columns with 3 µm or smaller packings, microbore columns, or when concerned about microbial growth, a 0.2 µm filter is recommended. Once the best pore size rating is chosen for the application, you must rely on the filter manufacturer to provide an accurate pore size rating. As described in, “Prolonging HPLC Column Life,” accurate pore size rating is important to the economics of the appropriate filter choice.

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