Understanding syringe filters

First published at see.leeds.ac.uk on 30th October 2015. Updated 6th May 2017 by Andy Connelly

Introduction

Syringe filters are generally used to remove particles from a liquid sample prior to some kind of analysis to avoid damage to equipment (e.g. ion chromatography, ICP, etc.) They are relatively affordable, can be used for small volumes, and avoid the difficulties involved using Buckner filter set ups or similar. However, syringe filters do not allow you to reclaim the solid part of your sample unless they are of the removable filter type (Figure 1).

Syringe filters are also available for the filtration of gases and for the removal of bacteria from a sample. They are also used in illicit drug harm reduction (although not in the lab please!) [1]

The question I ask here is: Are syringe filters really as inert as you think?

Choice of filters

There are several aspects of choosing a syringe filter. Some of the key ones are:

  • Pore size
  • Filter material
  • Filter capacity (Effective Filtration Area (EFA), etc.)
  • Hold-up volume- volume retained on filter after filtration

Pore size

Filters come in a variety of pore sizes. The most common ones used in physical chemistry laboratories are 0.2 um and 0.45um. Generally, 0.45um is sufficient for the majority of procedures. However, where smaller particles may be present in the sample 0.2 um or 0.1um might be more appropriate. If you need to filter a smaller particle size (for example, to remove colloids) other types of filtration may be more appropriate; for example centrifugal filters.

Filter material

There are many different filter materials used in syringe filters. Some of the most common are cellulose acetate (CA), polyamide (PA), and polyethersulfone (PES). They all have different compatibility issues (see Table 1) and so a careful choice is needed. The material will also give variation in the effective filtration area (EFA) and therefore the capacity of the filter.

csm_material_properties_aac905e852
Table 1. Properties of various filter materials (majority of information from http://www.sartorius.co.uk; however, data compiled by the author so is for guidance only).

Filter size

The diameter of the syringe filter is a good indication of the EFA and the hold-up volume. As particles are removed from the fluid, they block pores of the syringe filter and reduce the usable portion of the filter eventually causing the filter to block up. Particulate-laden fluids generally plug a filter more quickly than “clean” fluids. Increasing the size of the filter (and/or the EFA) allows dirtier samples to be filtered. If the pressure required to push liquid through the filter becomes very high it is likely blocked and needs replacing. If you push too hard you may damage the filter and so let particles through.

The larger the diameter of the filter will also increase the hold-up volume. This is the volume of liquid remaining in the filter after use. A filter with a low hold-up volume is recommended for use with expensive fluids or those with limited availability.

The table below outlines general guidelines to the appropriate filter size for different volumes of fluid.

csm_filters_hold-up_bc3cfdc1e1
Table 2: Choosing the correct diameter filter depends on the volume you are filtering and how much Hold-up volume you can accept.

Analysis

Contamination of your sample by the syringe filter is an issue that is not commonly discussed. To find out the effect we ran some experiments looking at how much contamination we received on passing Grade I deionised water (milli-Q) and 2% nitric acid through a syringe filter 10ml at a time. We passed 4 lots of 10ml through the filter collecting the filtered liquid each time. The solutions were then analysed in-house using ICP-MS for the following elements (Li, Na, Mg, Al, K, Ca, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, As, Rb, Sr, Cd, Cs, Ba, Pb, and U). A summary of the results are shown in Tables 3 to 6. The LOD and uncertainties can be found in Table 7.

The clear trend for many of the elements is shown in Figure 2. This shows that the first rinse contaminated the solutions with impurities in the ppb range. Since we have only have results from two filters it is difficult to draw statistical significance from the results. However, the main elements that seem to come into the DI water samples are Na, Mg, K, and Ca and these come in at the 0-4ppb level (see Table 3&4). Looking at the results in full suggests that the PES filters may contaminate the samples slightly less than the SFCA samples; however, more work is required to confirm this.

For the 2% nitric acid there is significantly more of these elements already present within the solution (see Table 5&6). However, there is also a significant increase in the contamination of up to 35 ppb (Na). Looking at the results in full suggests that there is little difference between the PES and SCFA filters; however, more work is required to confirm this.

Another interesting observation is that certain elements seem to “stick” to the filter. Both Ba and Zn see a decrease in concentration after the initial filter and then a subsequent increase in many of the samples. (see Figure 3). Zn is a common background element and Ba was present within the caps of the centrifuge tubes used in this experiment so these elements are common around the lab. More work is required to see if other elements behave in this way.

csm_graph2_b7713abc17
Figure 2: The contamination present in samples passed through a single filter. See text for details.
csm_graph1_6568ec3dd5
Figure 3: The contamination present in samples passed through a single filter. See text for details.

Summary

From the work done here it is clear that the syringe filters trialed in this piece of work were not inert and released potential contamination at the ppb level. If such contamination is potentially an issue you may need to adapt your filtering technique appropriately.

Best practice

From the discussion above and the observation of low level impurities in filtered solutions we suggest the following methods for using syringe filters (some of this taken from [2]). This is a suggestion only and in no way replaces thorough trials with your own samples. This is not a recommendation.

For samples with high concentrations (>1ppm) of analytes or where low level contamination (see above) is not a significant concern:

  1. Ensure the appropriate filter has been chosen (see above)
  2. Draw up 1ml of air and then your sample into a clean syringe
  3. Draw up your sample into the syringe
  4. Eject around 1ml of sample into an appropriate waste container†
  5. Push through the rest of your sample into an appropriate vessel for storage.
  6. Push the air cushion initially created through into same vessel. This will discharge remaining liquid and so reduce the volume held on the filter.
  7. Dispose of filter and syringe appropriately.

† Note: This step is designed to remove the worst of the impurities – this step can be missed out if you are not at all concerned about impurities in the ppb region. The author has yet to establish the efficacy of this step.

Alternative, if low level contamination is a significant concern.

  1. Ensure the appropriate filter has been chosen (see above)
  2. Draw about 1ml of air into a clean syringe,
  3. Draw 10ml blank matrix (e.g. 1% nitric, milli-Q, etc.) into the syringe;
  4. Eject through the filter sending it to waste*
  5. Eject the air cushion initially created through the filter to discharge remaining liquid and so reduce the volume held of the filter.
  6. Draw up 1ml of air and then your sample into a clean syringe
  7. Draw up your sample into the syringe
  8. Eject around 1ml of sample into an appropriate waste container. You may be able to waste less of your sample depending on the hold-up volume of the filter.‡
  9. Push through the rest of your sample into an appropriate vessel for storage.
  10. Push the air cushion initially created through into same vessel.
  11. Dispose of filter and syringe appropriately.

* Note: while filters are bidirectional, once used in one direction do not reverse.

‡ Note: this step is to remove any remaining blank matrix held in the filter.

References and acknowledgments

Thank you to Fiona Keay for producing the samples and her feedback on this blog post. Thank you also to Stephen Reid for giving up his time to analyse the samples and giving feedback.

[1] Sundaram, S; Auriemma, M; Howard Jr, G; Brandwein, H; Leo, F (1999). “Application of membrane filtration for removal of diminutive bioburden organisms in pharmaceutical products and processes”. PDA journal of pharmaceutical science and technology / PDA 53 (4): 186–201. PMID 10754712.

[2] Manual_Minisart_RC_SRP_NY_PES_SL-6193-p.pdf, www.sartorius.co.uk/fileadmin/fm-dam/DDM/Lab-Products-and-Services/Lab-Filtration/Filtration-Devices/Minisart/Manuals/Manual_Minisart_RC_SRP_NY_PES_SL-6193-p.pdf, Sartorius (2015).

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