Syringe Filters (Part 2 of 2)
Filtration of polar samples
A polar aqueous sample requires a hydrophilic membrane. The hydrophilic membrane of the cellulose mixed esters (MV) syringe filter has very low adsorption of dissolved substances and is suitable for such sample. If the sample additionally contains water-soluble oligomers and polymers (especially biological macromolecules) a low binding capacity of the membrane for proteins of the membrane is important. Cellulose acetate (CA) shows a very low protein binding capacity (CHROMAFIL® CA / 25 mm: 21 µg). The polyvinylidene difluoride (PVDF) filter CHROMAFIL® PVDF with 25 mm membrane diameter also shows a low binding capacity of water-soluble proteins up to ~20 µg per filter.
If the sample consists of highly polar aqueous/organic liquid mixtures, then three filters can be successfully used:
- hydrophilic polyethersulfone (PES),
- polyester (PET)with a hydrophilic multipurpose membrane
- hydrophilized polytetrafluoroethylene (H-PTFE)filters
PES filters feature a good stability against acids and bases. H-PTFE filters also towards all kinds of solvents. The PET filter with its multipurpose membrane (CHROMAFIL® PET) is characterized by hydrophilic and also hydrophobic properties, can be used for medium polar and nonpolar samples as well. This is an advantage, especially for application of samples with different polarities, because only one filter type has to be stored.
Filtration of medium polar samples
For medium polar aqueous/organic liquid mixtures, weakly hydrophilic membranes of regenerated cellulose (RC) or polyamide (PA) are mainly used.
Filtration of nonpolar samples
The polytetrafluoroethylene (PTFE) filter features a hydrophobic membrane and is thereby suitable for nonpolar, organic samples. The membrane is very resistant towards all kinds of solvents as well as acids and bases. Flushing with alcohol, followed by water, makes the originally hydrophobic membrane wettable by water. Then it can be used for polar samples. The hydrophilized PTFE filter CHROMAFIL® Xtra H-PTFE is a good alternative with similar high stability, eliminates the extra conditioning step mentioned above for PTFE filters.
Also the hydrophilic to weakly hydrophobic PVDF filter as well as the multipurpose filter CHROMAFIL® PET can be used for the filtration of nonpolar samples.
Filtration of samples with a high load of particulate matter
Samples with a high load of particulate matter can be filtered with inert glass fiber (GF) filters. They facilitate higher flow rates by the nominal pore size of 1.0 µm compared to small pore filters. The filters are also suitable for highly viscous samples such as soil samples or fermentation broths.
Frequently, highly contaminated samples contain both coarse and fine particles. With the Combi filters CHROMAFIL® GF/RC, GF/PVDF and GF/PET, coarse particles are removed by the glass fiber (GF) membrane (pre-filtration) before they can block the second porous smaller pore membrane (main filtration). Thus, the back pressure is lower than for the single filtration of samples with a high load of particulate matter. The selection of the Combi filters is furthermore determined by the polarity of the liquid sample (GF/RC: medium polar, GF/PVDF: polar or possibly nonpolar, GF/PET: polar or nonpolar).
Hints for the right selection and use of syringe filters
The following chart shows the criteria for the proper selection of a syringe filter.
Selection chart for syringe filters
|Sample||Demand for membrane properties||syringe filters
|aqueous||hydrophilic||cellulose mixed esters (MV)|
|water-soluble oligomers and polymers (biological macromolecules)||hydrophilic, low binding capacity for proteins, if necessary sterile||cellulose acetate (CA)|
|water-soluble oligomers and polymers (proteins)||hydrophilic, low binding capacity for proteins||polyvinylidene difluoride (PVDF)|
|aqueous – weakly organic||hydrophilic, good stability against acids and bases||polyethersulfone (PES)|
|aqueous – organic||hydrophobic with additional hydrophilic properties, very good stability against all kinds of solvents as well as acids and bases||hydrophilized polytetrafluoroethylene(H-PTFE)|
|aqueous – organic (e.g., eluent mixtures for HPLC)||hydrophilic, but also hydrophobic (multipurpose)||polyester (PET)
|aqueous / organic – aqueous||weakly hydrophilic||regenerated cellulose (RC)|
|organic – aqueous||weakly hydrophilic||polyamide (PA)|
|water-insoluble oligomers and polymers (proteins)||weakly hydrophobic, low binding capacity for proteins||polyvinylidene difluoride (PVDF)|
|organic||hydrophobic, but also hydrophilic (multipurpose)||polyester (PET)|
|organic||hydrophobic with additional hydrophilic properties, very good stability against all kinds of solvents as well as acids and bases||hydrophilized polytetrafluoroethylene(H-PTFE)|
|organic||hydrophobic, very good stability against all kinds of solvents as well as acids and bases||polytetrafluoroethylene (PTFE)|
|high load of particulate matter|
|highly viscous (e.g., soil samples, fermentation broths)||inert, wide pore||glass fiber (GF)|
|aqueous / organic – aqueous||inert, wide pore pre-filter with hydrophilic, small pore main filter||GF/RC|
|water-soluble or water-insoluble oligomers and polymers (proteins)||inert, wide pore pre-filter with hydrophilic–weakly hydrophobic, small pore main filter||GF/PVDF|
|aqueous or organic||inert, wide pore pre-filter with hydrophilic–hydrophobic, small pore main filter||GF/PET|
|For sample volumes of ≤1, 1–5 or 5–100 mL membrane diameters of 3, 13/15 or 25 mm are recommended. Standard pore sizes of 0.20 µm and 0.45 µm are suitable for an elimination of fine particles; pore sizes of 1.0, 1.2 and 5.0 µm are used for coarse particles.|
With the links in the chart product pages of CHROMAFIL® filters with further information are provided. For every filter you can find a “HPLC contaminant test” download, which shows the high quality concerning possible impurities by sensible HPLC measures. The service page “Chemical compatibility of filter materials” offers the possibility to check the solvent suitability of a filter membrane before filtration.
The product range of color-coded CHROMAFIL® (3, 15, 25 mm) and labeled CHROMAFIL® Xtra (13, 25 mm) filters comprises. Color code or imprint for a direct identification of membrane type, diameter and pore size exclude a confusion of filter types. Additionally, CHROMAFIL® MULTI 96 filter plates and CHROMAFIL® filtration cartridges are available.
The “FilterFinder“ allows to find high quality filters from MACHEREY-NAGEL as an alternative for the filters used until now.
The selection of a suitable filter is determined by the sample properties (polarity and kind of solvent, adsorption behavior of dissolved substances, size of particulate matter, viscosity of sample) and the sample volume. The CHROMAFIL® product range offers “always the right syringe filter” for a reliable filtration.