Syringe Filters (Part 1 of 2)
Always the right syringe filter
For the preparation of complex samples including after successful derivatization of the analytes, a rapid filtration is required. This is necessary to protect the instrument from damage by particulates. Most analysts simply use the first syringe filter they find in the lab. However, often that syringe filter is not the right one and the membrane tears after a few milliliters passes through. This is often due to inadequate of the filter to the sample solvent. Sometimes larger signal is generated by samples without filtration – usually due to adsorption of the analytes on the filter. All the above demonstrates the fact that knowledge of different filter types is important for choosing the right syringe filter instead of the first one you find in the lab drawers.
Factors to be Considered
There are four factors to be considered for a liquid sample that needs to be filtered:
- sample volume
- particulate load factor
- polarity of the liquid being filtered
- adsorption of the analytes on the filter.
Syringe filters are used for the elimination of insoluble material from samples. They consist of a polymer housing and a filter membrane.
The polymer housing is usually one of three types:
Polypropylene housings feature a good solvent stability and are characterized by a low content of extractable substances. Due to ultrasonic sealing instead of gluing (glue itself has extractable substances) of the two parts of the housing, the content is even more diminished. A Luer-lok fitting on the side of entry ensures safe connection of the filter on the high-pressure side – the syringe side. A Luer-lok exit can be used for a combination of several filters, e.g., for robots. The minispike (MS) Luer-lok of smaller filters (membrane diameter: 15 mm) ensures a filtration with a low dead volume.
The filter membranes must be matched to sample properties:
- Type of solvent
- Adsorption behavior of dissolved substances
- Size of particulate matter
- Viscosity of sample,
- and so on.
Commonly used polymers for membrane filters are:
- And other polymers
An optimal percolation through the whole area of the membrane is achieved with a star-shaped distribution device. The stream of sample liquid is broken into 4 lanes by the impact plate and then further distributed to 8 slots in the form of a star connected with 5 circular channels for 13mm and 15mm or with 8 channels for 25mm filters.
Membranes with pore size 0.45 µm are suitable for elimination of fine particles, which can (and will) plug up liquid chromatographic columns. The smaller pore size of 0.20 µm is recommended for filtration of samples in UHPLC or other techniques requiring high purity samples. The greater pore sizes of 1.0µm to 5.0µm are applied for samples with a high load of particulate matter.
Filters with very small membrane diameters (3 mm) are used for very small samples, which need an extremely low dead volume.
|Sample volumes||Recommended filter diameter||Dead volume|
|≤ 1 mL||3 mm||5 µL|
|1–5 mL||13 / 15 mm||30 / 35 µL|
|5–100 mL||25 mm||80 µL|