Understanding syringe filters

First published at see.leeds.ac.uk on 30th October 2015. Updated 10th November 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.

The main two are types of filters are either those where you can recover your sample or not. The majority of syringe filters used do not allow you to reclaim the solid. They are often used before analysis to remove any solid, undissolved, material. Other, filter holder (in-line) types allow you to regain your filter (Figure 1).

Syringe filters normally use membrane type filters which have a specific particle size cut off (e.g. 0.45 μm). 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 questions I ask here are:

  1. Which syringe filter is right for you?
  2. Are syringe filters really as inert as you think?
  3. What is the best method of using a syringe filter?

Choice of filters

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

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

Filter body

The key choices are [4]:

Choice of syringe filter body.
Table 1: Choice of syringe filter body..

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 0.1 um 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.

Pore size and suggested application.
Table 2: Pore size and suggested application for syringe filtration.

* components passing through 0.45μm filter are often deemed to be “dissolved” or “soluble” This in an operational definition and, while practical, is arbitrary and inaccurate as colloids and polymers can penetrate through the filter together with truly dissolved substances. [3]

Filter material

There are many different filter materials used in syringe filters. Some of the most common are cellulose acetate (CA) and polyethersulfone (PES). The key differences are chemical compatibility, flow rate, and burst pressure (i.e. strength). The material will also give variation in the effective filtration area (EFA) and therefore the capacity of the filter.

A selection of the main options are [4,5]:

  • Cellulose Acetate (CA): Excellent flow rates. Very low protein binding, so they are suitable for protein recovery applications. Hydrophilic, so fine for aqueous and alcoholic media although they have limited solvent resistance. pH range ~4-8.
  • Cellulose nitrate: High mechanical strength, high flow rates, and low extractable levels. A good choice for trace element analysis applications. High protein binding. pH range ~4-8.
  • Regenerated cellulose (RC): made from pure cellulose without wetting agents. Chemical resistance to a wide variety of solvents. High wet strength. Hydrophilic, so suitable for aqueous and organic samples. Very low protein binding capacity. pH range ~3-12.
  • Glass Microfibre (GMF): chemically inert and available in higher pore sizes than other membranes. Mechanically extremely strong and tolerant to organic solvents. Not idea with strong acids (particularly hydrofluoric acid) or bases. Ideal for high particulates solutions, often used as a pre-filter before a membrane filter. Not a membrane filter and so has a slightly less exact retention efficiency than membranes. Will contribute extractables that interfere with ionic and metals analysis.
  • Nylon: Nylon membrane filters are hydrophilic, flexible, tear-resistant, and autoclavable. They are resistant to a range of organic solvents and suitable for use with high pH samples. Nylon binds proteins. Unsuitable for acidic solutions. pH range ~3-14.
  • Polyethersulfone (PES): Hydrophilic, stable in low pH, have low levels of extractables, and exhibit low protein binding, making them suitable for many aqueous and organic solvents. PES membranes allow higher liquid flow than PTFE. Temperature resistant. pH range ~3-14 (sometimes quoted as 1-14).
  • Polypropylene (PP): slightly hydrophobic, can be used with a very wide range of solvents including aggressive hard-to-filter solutions such as strongly acidic samples. High and uniform tolerance to heat and mechanical stress. pH range ~1-14.
  • Polyvinylidene difluoride (PVDF): designed for high tensile strength, high solvent resistance, and low protein binding, making them suitable for biomedical filtration, sterilization filtration, and HPLC sample preparation. pH range ~1-14.
  • PTFE: is perfect for the filtration of gaseous or organic solvent-based samples and highly corrosive substances. Hydrophobic so provides chemical resistance to aggressive media and excellent temperature stability allowing an extended sampling range. If used with aqueous samples, the membrane usually requires pre-wetting (normally by using a small amount of alcohol). Can also be used to prevent moisture passing through air vents. pH range ~1-14.

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.

Table filter diameter
Table 3: Choosing the correct diameter filter depends on the volume you are filtering and how much hold-up volume you can accept. Values will vary depending on manufacturer and filter material. * hold-up volume is measured after air-purge.

Experiment

Introduction

Contamination of your sample by syringe filters 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 Type I deionised water and 2% nitric acid through a syringe filter.

First we drew 10ml of solution into the syringe and expelled into a labelled tube (no filter); this was sample 0. Then the same solution was drawn into the syringe then expelled through a syringe filter; this is sample 1. The filter was removed, the media drawn up again, the filter replaced, and the sample delivered into another tube. This is sample 2. This process was repeated to produce a sample 3 and sample 4.

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).

Results and discussion

The full set of results can be found in this file Filter_rinse_tests. We only carried out a very limited number of tests so the results are only a guide. More work is required to confirm the discussion below.

When rinsing with water, for some elements, such as Na and Ca, the first rinse contaminated the solutions with impurities in the ppb range (Figure 2 and 3). After that the levels dropped significantly. However, for other elements, such as Ba and Zn, the syringe introduces significant contamination (Figure 3). These elements then seem to “stick” to the syringe filter, only being released on later flushes.

When rinsing with 1% nitric, for some elements, such as Na and Ca, the first rinse contaminated the solutions with impurities in the ppb range (Figure 4 and 5). After that the levels dropped significantly back to the background level. However, for other elements, such as Ba and Zn, the syringe introduces significant contamination (Figure 3). Again, these elements then seem to “stick” to the syringe filter, only being released on later flushes.

More work is required to see if other elements behave in this way.

Conclusion

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

Syringe filter contamination caused by washing with water (Na and Ca)
Figure 2: Syringe filter contamination caused by washing with water (Na and Ca)
Syringe filter contamination caused by washing with water (Ba and Zn)
Figure 3: Syringe filter contamination caused by washing with water (Ba and Zn)
Syringe filter contamination caused by washing with 1% nitric acid (Na and Ca)
Figure 4: Syringe filter contamination caused by washing with 1% nitric acid (Na and Ca)
Syringe filter contamination caused by washing with 1% nitric acid (Ba and Zn)
Figure 5: Syringe filter contamination caused by washing with 1% nitric acid (Ba and Zn)

Best practice – using a syringe filter

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.

Step by step syringe filtration
Figure 4: Step by step syringe filtration – see text for details

Basic method

This is the most basic method of using a syringe filter.

  1. Load the sample into the syringe.
  2. Attach the filter securely with a twisting motion. With a luer slip syringe, this is about one quarter turn as the filter is pushed on. If the syringe has a luer lock (as in this example), fix it firmly but do not over-tighten.
  3. Hold the assembled syringe and filter vertically to wet the membrane evenly. This prevents air blocks and promotes high flow rates as the sample is spread evenly over the membrane surface.
  4. Press the syringe plunger gently to push sample through the filter. If possible, discard the first 0.25-0.5ml of sample because if there is any contamination present, it is likely to be higher in those first few microlitres of sample. If the back pressure ever increases significantly, change the filter as it may have plugged. Avoid pressing excessively as this could cause the filter housing to burst.
  5. Change filter and repeat for next sample

 Variations

  • If you have a limited amount of sample – you can use the “air-purge” method to reduce the amount of sample lost in the syringe. To do this draw a small amount of air (about 1ml) into the syringe before filling with the sample solution. This air is used to purge the filter at the end ensuring the minimum sample is left in the filter.
  • If you are concerned about contamination (ppb level) – as results above show syringe filters can produce contamination at low levels. To reduce the amount of contamination you should wash the filter through with ultra clean water or weak (1%) acid solution before filtering your sample through same filter. The results above show that you may have to do this multiple times to truly “clean” the filter. The wash solution can be ejected from the filter using air purge method described above.

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 on the piece.

[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).

[3]  Radojevik & Bashkin, Practical Environmental Analysis, RSC, 1999

[4]  http://www.gelifesciences.com/webapp/wcs/stores/servlet/catalog/en

[5] Sartorius, Laboratory filtration products, SLU0006-e160712, 2016

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