I began this post with the intention of discussing the most popular GPC/SEC consumables available from the Malvern Panalytical eStore.  As with my previous posts on columns and standards, my goal was to provide a list of consumables to keep your GPC/SEC system running smoothly.  But as I was gathering information for this post the same item kept coming up over and over: the post-column filter.

post-column filter components

What is it?

The post-column filter is exactly what it sounds like: a filter located immediately after your column set.  It consists of a filter membrane with 0.2 µm pores sandwiched between two frits fitted with O-rings squeezed inside a metal housing.  In OMNISEC it is located inside the column compartment (below left).  It is in a clamp above the light scattering detector in TDA units (below right).  In modular systems it may be visible in the tubing connecting your column set to your detector(s).

location of post-column filter in OMNISEC & TDA

Why is it there?

In short, to save you time and money!  The post-column filter does this by serving as a last line of defense for your detectors against particulates and even poorly soluble samples.  Your column set is a major source of particulates, as the gel particles can break into pieces and shed from the column housing.  In addition, your mobile phase and even samples can introduce contaminates into your system that can affect data and system health. 

The effects of these particulates will be most noticeable in the light scattering detector, as that detector responds most strongly to high molecular weight materials.  A constant stream of particulates will generate a lot of spikes, which can distort otherwise quality data and render it unusable.

In extreme cases, the capillaries of the viscometer detector can become plugged and will require replacement. Additionally, the contaminants can get stuck in the light scattering flow cell and send the detector signal off-scale.  At best, this is fixable through a tedious flushing process; at worst, it necessitates a service visit. 

Wouldn’t it be more time- and cost-efficient to simply replace the post-column filter?! 

When does it need replacing?

It’s time to replace your post-column filter whenever you change your mobile phase, or you observe one of the following two symptoms. 

Firstly, your light scattering signal becomes full of spikes.  As you can see in the example below, this symptom is hard to miss.  The spikes are often more pervasive in the low angle signal due to that detector’s sensitivity to high molecular weight materials.  The spikes don’t show up in the refractive index response because the contaminant (usually) exists in low concentration. The post-column filter is sometimes referred to as the light scattering filter because of its ability to improve light scattering data.

spiky data, time to change the post-column filter

Secondly, your system pressure will increase over sequential injections.  This can be monitored in the OMNISEC software by adding Pump Pressure to your Record View.  To do so, right-click on an existing column heading, select Column Configuration, then add Pump Pressure under Run Conditions. 

pressure increase due to clogged post-column filter - time for a change!

Which membrane do I need?

We offer three different types of post-column filter membranes.  All have 0.2 µm pore size.  The most appropriate choice for you will be dictated by your application:

  • Nylon
    • Use these for all organic solvents except HFIP and formic acid
    • Use these for all water-soluble polymers (PEO, polysaccharides, DNA, etc.)
    • Second choice when running proteins
  • PTFE
    • Use these when running HFIP or formic acid
    • Second choice for other organic solvents
  • Cellulose acetate
    • First choice when running proteins

How do I replace it?

Replacing the post-column filter is a straightforward process.  The procedure is summarized in the bullet points and the video below.

  • Turn off system flow & remove the post-column filter housing from the system
  • Open filter housing and remove used filter membrane, frits, & O-rings
  • Snap new frits into new O-rings
  • Place one set of new frits & O-rings into filter housing; use smallest housing piece to seat
  • Place new membrane in filter housing and gently push into place using smallest housing piece
  • Place other set of new frits & O-rings into filter housing
  • Reassemble filter housing & connect the inlet
  • Turn on system flow to slow flow rate; wait until solution elutes before connecting outlet
  • Please note that the light scattering signal will be noisier upon filter change and should begin to quiet down immediately

Where can I get spare parts?

I’m glad you asked!  Our eStore is conveniently set up to allow you easy access to replacement parts.  The Nylon and PTFE filter membranes come in packs that include frits and O-rings as well. 

Will the post-column filter affect my sample?

In almost all cases, the answer is no. Remember, the pores in the post-column filter are 0.2 µm. Hopefully your sample (and mobile phase) has already been filtered through a 0.2 µm filter prior to injection. Therefore, the post-column filter should change nothing about your sample.

We strongly encourage you to use a post-column filter in your system.  However, there are two situations in which we recommend NOT using a post-column filter:

  • You are using an APC-REVEAL system for UPLC analyses
  • Your sample is sticking to the filter membrane

In the first scenario, the volume of the post-column filter is large enough to increase the band-broadening and affect your sample’s chromatography.  With the second situation, I’d suggest trying an alternate membrane before you decide to use no filter at all.

Final thoughts

In conclusion, the post-column filter plays a critical role in obtaining quality data.  Furthermore, its maintenance is easy and can save you a lot of time and money.  If you have any questions please don’t hesitate to contact us or email me directly at kyle.williams@malvernpanalytical.com.

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