Frequently Asked Questions

A. Syringe filters

What are syringe filters?


Also known as wheel filters because of their wheel-like shapes, syringe filters are attached to the end of a syringe and are frequently used for removing particles in samples. Removal of particles in the sample preparation step is very imporant for high performance liquid chromatography (HPLC) as large particles can clog or damage the column and also cause interference in the analysis results. For ultra performance liquid chromatography (UPLC), filtration of the sample prior to analysis is especially important due to their much smaller particle size columns. Syringe filters could be produced as sterile or non-sterile for use in different industries. They may also be used for general purpose filtration, especially for smaller volumes where sample losses from larger filters are significant. Syringe filters are also used for the filtration of gases and for the removal of bacteria from a sample.




What size syringe filter do I need?


Syringe filters are sized according to their inner diameter. Based on the volume of the samples, you can choose the appropriate sizes from the following;




Which syringe filter should I use?


With all the different types of membranes, it is best to choose the right one for the job. We recommend that you use the following table as a guide.





B. Membrane filters

What are the differences between "hydrophilic" and "hydrophobic" in the filtration industry?


Hydrophilic filters are easily wet with water. Hydrophilic filters can be wetted with virtually any liquid, and are the preferred filters for aqueous solutions, as appropriate by compatibility. Once wetted, hydrophilic 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 will not be wet in water but will get wet in low surface tension liquids, for instance, organic solvents such as alcohols. Once a hydrophobic filter has been wetted, aqueous solutions also will pass through. Hydrophobic filters are best suited for gas filtration, low surface tension solvents, and venting. In certain applications, hydrophobic filters are used to filter aqueous solutions because of compatibility requirements. Water or aqueous solutions can also pass through a hydrophobic filter once the water breakthrough pressure is reached.




What is a membrane filter?


A membrane filter is a matrix with channels which act as a screen and retains particles larger than the filter on the surface of the membrane. Membrane filters allow the retention of sub-micron particles and organisms.




What is the shelf life of membranes?


To test the membranes, we can perform a "wetting out" test by dipping a small portion of the membrane into water (for hydrophilic) or alcohol (for hydrophobic). If the membrane absorbs the liquid, it has not gone past the shelf life.




What membrane has been used to prevent water from passing, but would allow regular air to pass?


The common membranes used for gas/air filtration are hydrophobic Polypropylene (PP) and PTFE. They both inhibit the flow of water while allowing air to pass. The table below is to show the water intrusion pressure of PTFE membrane which is based on the principle of the smaller the pore size is, the greater intrusion will be.




How is pore size rated?


A pore size rating is determined by the diameter of the particle that can be expected to be retain with a defined, high degree of efficiency. The rating is stated in nominal or absolute terms.




What is the difference between nominal and absolute pore size rating?


Nominal pore size rating describes the ability of the filter to retain the majority of the particles at the rated pore size and larger (60-69%) and cannot be used to compare filters among manufacturers. Glass fiber filters and screen filters are a good example of nominally rated filtration. Processing conditions such as operating pressure and concentration of contaminant have a significant effect on the retention efficiency of the nominally-rated filters. Absolute ratings are used to represents the size of the smallest particle completely retained. Complete retention is within the experimental uncertainty of a standard test method consistent with the intended filter use. Among the test conditions that must be specified are test organism (or particle size), challenge pressure, concentration, and detection method used to identify the contaminant. Most membrane filters are rated as absolute terms.




What is a Polytetrafluoroethylene (PTFE) membrane?


Polytetrafluoroethylene (PTFE) is a synthetic fluoropolymer of tetrafluoroethylene that has numerous applications. PTFE is hydrophobic (neither water nor water-containing substances will wet PTFE), as fluorocarbons demonstrate mitigated London dispersion forces due to the high electronegativity of fluorine.

PTFE has one of the lowest coefficients of friction against any solid. Due to its properties, PTFE is often used as a non-stick coating for pans and other cookware. It is also very non-reactive, partly because of the strength of the carbon–fluorine bonds, and so it is often used in containers and pipework for reactive and corrosive chemicals. When used as a lubricant, PTFE reduces friction, wear, and energy consumption of machinery.
Its resistance to highly corrosion chemicals makes it popular for use in the laboratory as containers, magnetic stirrer coatings, and as tubing for highly corrosive chemicals such as hydrofluoric acid, which would dissolve glass containers. It is also used in containers for storing fluoroantimonic acid, a superacid.

PTFE membrane filters are among the most efficient filters used in industrial applications. Filters coated with a PTFE membrane are often used within a dust collection system to collect particulate matter from air streams in applications involving high temperatures and high particulate loads such as coal-fired power plants, cement production, and steel foundries. Its broad chemical compatability also makes it a very versatile filter to use in the lab. PTFE's durability and properties helps it to filter harsh liquids in experiment and gases for protecting both people and expensive equipment.




What is the maximum temperature for the different filter membranes?





What variables affect the performance of a filter?


*Viscosity: The viscosity of a liquid determines its resistance to flow; the higher the viscosity, the lower the flow rate and the higher the differential pressure required to achieve a given flow rate.

*Porosity: The flow rate of a membrane is directly proportional to the porosity of a membrane, eg. the more pores, the higher the flow rate.

*Filter Area: The larger the filter area, the faster the flow rate at a given pressure differential and the larger the expected filter throughput volume prior to "clogging for a given solution."




Can I filter aqueous solutions through a polytetrafluoroethylene (PTFE) hydrophobic membrane?


Yes. However, the membrane requires pre-wetting with an alcohol (such as Isopropanol or Methanol) to establish flow with reasonable pressure differentials.




Which membrane is better? PVDF or Nitrocellulose?


To compare which one is better, we need to take membrane type, pore size, and membrane format into consideration. Physical characteristics: While nitrocellulose is brittle and fragile, PVDF is more durable and has higher chemical resistance making it ideal for reprobing and sequencing applications. Nitrocellulose can prove to be difficult to strip and reprobe without losing signal. However, supported versions of nitrocellulose membranes are considerably more durable and resilient than standard nitrocellulose membranes so they can be stripped, reprobed, and subjected to harsh chemical treatments. Pore size: Both membranes come in typical pore sizes of 0.1, 0.2 or 0.45μm. The 0.45μm membrane is suitable for most protein blotting applications but for smaller peptides or lower molecular weight proteins (less than 15 kD), you should use 0.1 or 0.2μm pore size membrane. Note: When you are detecting a protein loaded at low levels or when quantification is considered critical, you should always choose the smaller size membrane. Membrane format: There are several factors that you need to take into account when choosing the most suitable membrane format, including transfer system (semi-dry, wet or fast), convenience, price and flexibility. Pre-cut and pre-wetted membranes are the ideal choice when convenience, reproducibility and high throughput are of highest importance while rolls offers more flexibility since you can cut the membrane to the specific size of your gel. The only problem is that this can add extra time to your workflow and introduce variability in the membrane size. Pre-cut membranes, on the other hand, are available in a range of sizes suitable for all gel types. Using a precut membrane may result in better transfer reproducibility.




What is a Glass Microfiber filter?


Glass microfiber filters are made of microfiber borosilicate and may or may not have binder. These filters have high filtration efficiency, good chemical stability, and low resistance. Glass microfiber filters can help to increase throughput by filtering out the larger particles before it reaches the main membrane. For high efficiency air filters these become the material of choice. Glass Microfiber filter can be used to produce a separator without clapboard air filters for a variety of fields.




What is the difference between PVDF and PTFE?


As we all may know, PTFE and PVDF membranes are normally hydrophobic. At Finetech, we also carry hydrophilic PTFE membrane filters and syringe filters. The big difference between PTFE and PVDF is that PTFE membranes can be used in the filtration of strong acids and aggressive solutions, but PVDF membranes are more suited for the filtration of non-aggressive aqueous and mild organic solutions. Also there are some similarities between these two membranes. Both membranes have low protein binding ability, can be used for the filtration of gases and air, and they both have good chemical compatibility.




Which chemicals could be used for each membrane?


Please refer to the Membrane Chemical Compatibility Table.





D. Quality Control

What is the integrity testing method?


Intergrity testing sterilizing filter is a fundamental requirement of critical process filtration applications in the pharmaceutical industry. There are two classifications of integrity testing, destructive and non-destructive. Finetech's practice is to perform destructive testing as a lot release criteria on samples from each manufacturing lot of all fabricated sterilizing-grade filter products, and non-destructive testing on each sterilizing-grade filter prior to sale to insure its integrity. Destructive challenge testing is the best way to determine a sterilizing filter's ability to retain bacteria. While Non-destructive testing may be done on filters before and after use. Integrity testing sterilizing filters before use monitors filter integrity prior to batch processing, preventing use of a non-integral filter. There are three types of non-destructive testing which are the bubble point test, the diffusion test, and the water flow integrity test for hydrophobic filters.




What is bubble point test?


A bubble point test is a test designed to determine the pressure at which a continuous stream of bubbles is initially seen downstream of a wetted filter under gas pressure. To perform a bubble point test, gas is applied to one side of a wetterd filter, with the tubing downstream of the filter submerged in a bucket of water. The filter must be wetted uniformly such that water fills all the voids within the filter media. When gas pressure is applied to one side of the membrane, the test gas will dissolve into the water, to an extent determined by the solubility of the gas in water. Downstream of the filter, the pressure is lower. Therefore the gas in the water on the downstream side is driven out of solution. As the applied upstream gas pressure is increased, the diffusive flow downstream increases proportionally. At some point, the pressure becomes great enough to expel the water from one or more passageways establishing a path for the bulk flow of air. As the result, a steady stream of bubbles should be seen exiting the submerged tubing. The pressure at which this steady stream is noticed is refered to as the bubble point.





D. Quality Control

What is the integrity testing method?


Intergrity testing sterilizing filter is a fundamental requirement of critical process filtration applications in the pharmaceutical industry. There are two classifications of integrity testing, destructive and non-destructive. Finetech's practice is to perform destructive testing as a lot release criteria on samples from each manufacturing lot of all fabricated sterilizing-grade filter products, and non-destructive testing on each sterilizing-grade filter prior to sale to insure its integrity. Destructive challenge testing is the best way to determine a sterilizing filter's ability to retain bacteria. While Non-destructive testing may be done on filters before and after use. Integrity testing sterilizing filters before use monitors filter integrity prior to batch processing, preventing use of a non-integral filter. There are three types of non-destructive testing which are the bubble point test, the diffusion test, and the water flow integrity test for hydrophobic filters.




What is bubble point test?


A bubble point test is a test designed to determine the pressure at which a continuous stream of bubbles is initially seen downstream of a wetted filter under gas pressure. To perform a bubble point test, gas is applied to one side of a wetterd filter, with the tubing downstream of the filter submerged in a bucket of water. The filter must be wetted uniformly such that water fills all the voids within the filter media. When gas pressure is applied to one side of the membrane, the test gas will dissolve into the water, to an extent determined by the solubility of the gas in water. Downstream of the filter, the pressure is lower. Therefore the gas in the water on the downstream side is driven out of solution. As the applied upstream gas pressure is increased, the diffusive flow downstream increases proportionally. At some point, the pressure becomes great enough to expel the water from one or more passageways establishing a path for the bulk flow of air. As the result, a steady stream of bubbles should be seen exiting the submerged tubing. The pressure at which this steady stream is noticed is refered to as the bubble point.





E. Vials

What is the use of Scintillation Vials?


The main purpose is for an analytical technique, liquid scintillation counting, which detects the activity of a radioactive material in a sample. Before use in a liquid scintillation counter instrument, first, the sample is disolved or mixed into a mixture of a suitable solvent, a surfactant, and a scintillator.




What is the difference between Soda Lime and Borosilicate Glass?


Vials can be made with two types of materials, soda lime or borosilicate glass. Soda lime glass is the most commonly used glass in the world. It is composed of silicon dioxide (SiO2), sodium oxide (soda), and calcium oxide (lime). Borosilicate glass was developed to be physically and chemically stronger than soda lime glass. This type is used in laboratories when soda lime glass would not be able to handle the rough treatment of daily lab use. -Borosilicate glass if more durable and is able to handle drops better than soda lime glass -Borosilicate glass much lower coefficient of thermal expansion, so it can handle changes in temperature without shattering. -Borosilicate glass is more resistant to chemical attacks such as from acids. Soda lime glass would be more susceptible and would cause substances to leach from the glass into the sample.





A. Syringe filters

What are syringe filters?


Also known as wheel filters because of their wheel-like shapes, syringe filters are attached to the end of a syringe and are frequently used for removing particles in samples. Removal of particles in the sample preparation step is very imporant for high performance liquid chromatography (HPLC) as large particles can clog or damage the column and also cause interference in the analysis results. For ultra performance liquid chromatography (UPLC), filtration of the sample prior to analysis is especially important due to their much smaller particle size columns. Syringe filters could be produced as sterile or non-sterile for use in different industries. They may also be used for general purpose filtration, especially for smaller volumes where sample losses from larger filters are significant. Syringe filters are also used for the filtration of gases and for the removal of bacteria from a sample.




What size syringe filter do I need?


Syringe filters are sized according to their inner diameter. Based on the volume of the samples, you can choose the appropriate sizes from the following;




Which syringe filter should I use?


With all the different types of membranes, it is best to choose the right one for the job. We recommend that you use the following table as a guide.





Constantly researching and innovating...

Since 1999, Finetech is the leading Taiwanese manufacturer of syringe filters and offers a wide range of products for environmental, pharmaceutical, life science, and other laboratories.

*All images used are for illustrative purposes only. Actual color may vary due to lighting, enhancements, or monitor setting.