First published see.Leeds.ac.uk on 29th February 2016. Updated 7th May 2017 by Andy Connelly.
Filtration is one of the first laboratory techniques we use as young scientists in school. It all seemed so simple then, a magical paper that lets water through and holds on to the solid. Fast forward to the start of a research project when you realise you need to filter your solution. You search the web for ‘filter papers’ and a nightmare world you never knew about opens up beneath you.
You could spend a long time trying to find the right filter paper for your project and still not be sure which you should use. In this blog post I explore some of the different filters available for non-biological sciences – hopefully giving enough information to help make the process of choosing a filter paper easier.
DISCLAIMER: I am not an expert in filtration. The content of this blog is what I have discovered through my efforts to understand the subject. I have done my best to make the information here in as accurate as possible but I do not guarantee accuracy. If you spot any errors or admissions, or have any comments, please let me know.
Types of filtration
A filter paper is a semi-permeable paper/plastic/etc. barrier which is placed perpendicular to a liquid or air flow and as such separates fine substances from liquids or air. There are four main types of filtration used in laboratories:
- Gravity filtration – The traditional form of filtration. Just like that used in a coffee percolator.
- Vacuum filtration – using a pump or similar to generate a vacuum and so pulling (arguably pushing) material through the filter (e.g. Buchner funnel). This is generally used to filter to low particle size (e.g. 1um).
- Syringe filters – these are filter papers contained within a plastic housing. They do not generally allow the solid to be recovered after filtration.
- Centrifuge filtration – uses the forces generated by a centrifuge to force solution through filter – they can achieve very fine filtration
- Chemically selective filters – e.g. for separating organic and aqueous phases or SPE.
In this blog post I am mainly looking at gravity and vacuum filtration.
Choosing a filter paper
From experience, I find that the important parameters when choosing a filter are:
- What particle size they will hold on to? – known as particle retention
- Is the filter chemical compatible with your solution? – this will also affect the contamination of your sample by the filter.
- Can you get the solid sample off the filter if you need to? – volume (or depth) filtration (lose much of the solid sample), surface filtration (easy to get the solid sample back)
- How much solid sample can the filter hold before it gets clogged up? – known as loading capacity.
- How quickly the solution will flow through the filter paper? – known as flow rate, the capacity is also important here.
- Will the filter break during filtration? – particularly vacuum filtration (to some extent this is measured as wet strength)
Types of filter materials
There are three main types of filter papers used for vacuum and gravity filtration. Details about these can be found in Table 1.
Cellulose filter papers (‘Whatman’ type)
The Whatman style filter papers are made from cellulose and are used for general filtration. They can remove particles down to 2.5 um depending on the grade (see Table 2). There is a wide choice of retention/flow rate combinations to meet the needs of numerous laboratory applications. Table 2 shows a selection of grades available and some of their properties.
Glass Microfibre Filters (GMF)
Glass microfibre filters are excellent for corrosive acidic liquids where you are not wanting to retain the solid sample after filtration. They will filter samples removing particles into the submicron range. They give fast flow rate and high loading capacity. They are usually made from borosilicate glass or quartz and most are completely binder-free so allowing high temperature treatment (up to around 550°C) and reducing possible contamination. There are various types available .
Membrane filters are usually classified as surface filters because particulates are retained on the smooth membrane surface. They can filter out particles down to 0.02 um depending on the filter (see Figure 2). They are used where people want to retain their particles from aqueous solutions; water microbiology; and air pollution monitoring.
These filters vary greatly in their chemical compatibility. This can be seen in Table 3 for selected solutions. Some general properties of a selection of materials used for filters can be seen in Table 4.
Membrane filters can be ”hydrophilic” or ”hydrophobic”:
- Hydrophilic filters: these are easily wet with water. Once wetted these filters do not allow the free passage of gases until the applied pressure exceeds the bubble point and the liquid is expelled from the pores of the membrane.
- Hydrophobic filters: these are not easily wetted by water only by low surface tension liquids (e.g. organic solvents such as alcohols). Once they are wetted aqueous solutions also will pass through. Hydrophobic filters are best suited for gas filtration, low surface tension solvents, and venting. The materials hydrophobic filters are made from (e.g. PTFE) are often highly resistant to chemical attach and so they are used to filter aggressive aqueous solutions. Water or aqueous solutions can also pass through a hydrophobic filter if the water breakthrough pressure is reached.
If you are still not sure what filter paper you need this decision tree might help:
Thank you Stephen Reid for reading through this for me.
 Access Feb 2016, https://www.prlabs.co.uk/news/downloads/wPjkKWhatman%20filter%20paper%20guide.pdf
 Accessed Feb 2016, http://www.advantecmfs.com/catalog/filt/membrane.pdf
 Accessed Feb 2016, http://www.chimica.unipd.it/nicola.tiso/pubblica/_private/Utile/catalogo%20whatman.pdf