Syringe Filters for Aqueous Solutions Hydrophilic syringe filters are the most popular choice for filtering aqueous solutions. Nylon, polyethersulfone, and cellulose acetate syringe filters, to name a few, will efficiently and naturally filter water based solutions due to their hydrophilic properties. There’s also the option of using a hydrophobic syringe filter and wetting the filter’s membrane with alcohols to make it hydrophilic. This is sometimes done, for example, with PTFE and PVDF materials. This will allow you to push an aqueous solution through. However, keep in mind that it is often impractical to do this as the membrane must first be compatible with the alcohols used. There is also the possibility that the alcohols will end up in your filtered solution, so this should not be considered if this is a concern for your application. When choosing syringe filters for aqueous solutions, it’s important to consider how they are being used. The main factors to keep in mind are: filter connection, filter material, pore size, diameter of the filter, and sterility. This article reviews each factor to help you determine which syringe filters are best for your aqueous solutions. Choosing a connection type Syringe filters generally have female luer lock inlets and male luer slip outlets. However syringe filters can also be fitted with what is called a double luer lock connection which is a female luer lock inlet and a male luer lock outlet. This type of threaded connection is designed to provide a tight and secure seal. Choosing a membrane material A syringe filter’s membrane can be produced of various materials that can handle different applications and are compatible with a particular range of solvents used in aqueous solutions. Considerations when choosing a syringe filter for an aqueous solution are:: Chemical compatibility – whether the filter’s material can hold up to the chemical nature of the aqueous solution; things to consider are acids and bases, and the liquid’s pH Flow rate – how long it takes for the aqueous stream to pass through the filter Binding properties – whether components in your sample will bind to the filter and compromise the filtered molecular composition of your sample Extractables – the amount of contaminants that end up in the final filtering due to residual or secondary chemicals in the filter or shredding of filter materials Wettability of the membrane – whether or not this membrane is naturally hydrophilic or hydrophobic. Filters that are hydrophilic attract water by pushing away other molecules and allowing water access to the filter’s membrane. By doing so, the filter’s membrane stays free of contaminants. Common hydrophilic membrane materials for use with aqueous solutions: Polyethersulfone (PES) – mechanically strong membrane that can handle high temperature liquids, has excellent flow rates, and is low protein binding. Should not be used with acetone, ketones, halogenated or aromatic hydrocarbons, chloroform, hexane, esters, chloromethane, and concentrated acids Nylon – compatible with most aqueous and organic solvents, high throughput loading, excellent flow rates, not recommended for protein samples with high nonspecific binding, cannot be used with strong acids or bases, methylene chloride, aggressive halogenated hydrocarbons, and dimethylformamide (DMF); Cellulose Acetate (CA) – ideal for aqueous solutions, but not compatible with organic solvents, best usage is for proteins and protein-related applications, usable with heated liquids, really low protein binding, excellent flow rates, cannot be used with organic solvents or benzyl alcohol; Regenerated Cellulose (RC) – can tolerate organic samples and aqueous samples with pH range of three to 12, good resistance to solvents, very low protein binding, excellent flow rates, minimal extractables, cannot be used with nitric acid greater than 25 percent, dimethylformamide (DMF), tetrahydrofuran (THF), phenol, chloroform, hydrochloric acid, phosphoric acid, and sulfuric acid; Anopore – can be used with organic and aqueous samples, accommodates 100 ml sample volumes, high pore density, high flow rates, low protein binding, very efficient particle retention, low holdup volume, cannot be used with sodium hydroxide (NaOH), sulfuric acid, ammonium hydroxide, and hydrochloric acid; Glass Fiber – good for filtering large particles or viscous samples, good throughput, cannot be used with benzyl alcohol. Polypropylene (PP) – can be used for aqueous or organic samples, low protein binding, chemically resistant, do not use with organic solvents, including toluene, hexane, benzene, and aromatic or chlorinated solvents. Treated PTFE, PVDF, and Polypropylene With some water-based applications, a naturally hydrophilic filter is not sufficient to handle the acid content in your sample. In these instances, you may need to use a hydrophobic filter that has been pretreated in the manufacturing process to make it hydrophilic. The following membranes are naturally hydrophobic but can be manufactured as hydrophilic to assist with these more acidic samples: Hydrophilic PTFE (Polytetrafluoroethylene) – strong membrane can handle high temperature liquids, excellent flow rates, low protein binding, high loading capacities, compatible with most bases, acids and solvents, PTFE ideal for aqueous, organic, or gaseous samples, PTFE multilayer filter ideal for aqueous and organic samples, cannot be used with formic acid greater than 50 percent, dimethylformamide (DMF), perchloric acid, methylene chloride, and dioxane; Hydrophilic PVDF (Polyvinylidene Fluoride) – ideal for protein based samples with high nonspecific binding, high performance liquid chromatography (HPLC) and ultra high performance liquid chromatography (UHPLC); highly defined pore structure, low nonspecific binding, compatible with organic and aqueous solvents, cannot handle the majority of strong acids and caustic solutions, dimethyl sulfoxide, dimethylformamide (DMF), acetone, ketones, esters, and ethers; This chart provides a quick reference to selecting syringe filters for aqueous solutions. If you’re looking for: Then use: Hydrophilic membrane PES, Nylon, CA, RC, Anopore, or Glass Fiber Hydrophobic membrane PTFE, PVDF or PP Compatible with aqueous samples only CA, Nylon, or PES Compatible with organic and aqueous samples Hydrophilic PTFE, Nylon, PP, Hydrophilic PVDF, RC, or Anopore Compatible with gaseous samples PVDF, or PTFE Can handle high temperature liquids PES, CA, or PTFE Low protein binding CA, PES, PP, RC, Anopore, or Hydrophilic PTFE Nonspecific binding Nylon or PVDF Excellent flow rates CA, Nylon, PES, PTFE, PVDF, RC, or Anopore Well-defined pore structure PVDF or Anopore Large particulate filtering Glass Fiber or Anopore Low extraction Glass Fiber or RC High throughput loading Nylon, PTFE, or Anopore Choosing a Pore size The pore size of your syringe filter is determined by the size of the particles you need to filter out of your aqueous solution. Syringe filters for aqueous solutions are available in these pore sizes: 0.05 µm, 0.10 µm, 0.22 µm, 0.45 µm, 0.70 µm, 0.80 µm, 1 µm, 3 µm, 5 µm, and 10 µm. As a guide, the finer the pore, the more pressure needed to pass your sample through the syringe filter. For instance, if you want to filter out particulate 0.2 microns in diameter, then choose a syringe filter with a 0.20 µm pore size. Choosing a Diameter Syringe filter size is determined by the size of your sample. In other words, the amount of volume you plan to push through the filter. As a general rule, use a syringe filter with a greater membrane area if you plan to push a high volume of aqueous solution through it. Syringe filters for aqueous solutions are available in diameters of: 4mm, 13mm, 17mm, 20mm, 25mm, 30mm, and 33mm. Here’s a guide to help you choose your syringe filter size: 4 mm filters – for small volume samples less than 1 ml 13 mm filters – for sample volumes between 1 and 10 ml 17 mm filters – for sample volumes greater than 10 ml 25 mm filters – for most applications, ideal for high throughput labs 30 mm filters – for large sample volumes greater than 100 ml Sterile or nonsterile Syringe filters for aqueous solutions are available in sterile or nonsterile options. Which option to choose is based on your sample’s processing application. If you want a sterilized aqueous solution, sterile syringe filters come individually wrapped to ensure optimal performance. If your aqueous solution is going to be filtered again through a sterilization process, then a nonsterile syringe filter should meet your needs. Still Have Questions? Get more information from our filtration guides and charts here. If you still aren’t sure what you need, contact us via email, phone, or support ticket here.