Syringe filters

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]

Filter material

There are many different filter materials used in filtration assembly. 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 filter based on some properties given below [1,2]:

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 (RN): 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.

Polytetrafluoroethylene (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.

References

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

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