Syringe Filters - Are you paying too much?
Table of Contents
According to Wikipedia, a syringe filter is a single-use filter cartridge. It is attached to the end of a syringe for use. Syringe filters may have Luer lock fittings, though not universally so. The use of a needle is optional; where desired it may be fitted to the end of the syringe filter.
A syringe filter generally consists of a plastic housing with a membrane which serves as a filter. The fluid to be purified may be cleaned by drawing it up the syringe through the filter, or by forcing the unfiltered fluid out, through the filter.
In scientific applications, the most common sizes available are 0.2 or 0.22 µm and 0.45 µm pores. These sizes are sufficient for HPLC use. The smallest known sterile syringe micro filter have pore sizes of 0.02 µm. Membrane diameters of 13 mm, 25 mm and 30/33 mm are common as well.
The syringe filter body may be made of such materials as polypropylene and nylon. The filter membrane may be of PTFE, nylon, or other treated products for specific purposes. Most manufacturers publish compatibility wall charts advising users of compatibility between their products and organic solvents or corrosive liquids (e.g. trifluoroacetic acid). I have attached the National Scientific chart for your future reference.
Syringe filters may be used to remove particles from a sample, prior to analysis by HPLC. Particles easily damage an HPLC due to the narrow bore and high pressures within. Syringe filters are quite suitable for Schlenk line work, which makes extensive use of needles and syringes. Being relatively affordable, they may be used for general purpose filtration, especially of smaller volumes where losses by soaking up filter paper are significant.
Syringe filters are also available for the filtration of gases, and for the removal of bacteria from a sample.
Syringe Filter Diameter
Syringe filters are made in diameters from 4 to 50mm. The larger the diameter, the more particulates can be captured on the surface of the filter before it starts to clog resulting in backpressure and the amount of force that must be applied to the syringe plunger to extract the purified material. For labs with many samples, this is a huge issue as it can result in hand injuries.
Most labs obtain samples that they know from experience have “X” amount of particulates. And if a 25mm diameter filter works well on these samples, going to a larger diameter syringe filter doesn’t gain anything. They just take up more storage room and cost more. If samples are especially “dirty” a far better solution is to use a 2-stage syringe filter which has a pre-filter to capture the really big particles before the sample reaches the micron filter.
The 25mm diameter filter has been an industry standard for years. 13mm filters are available but in my experience, very few people buy these and while there is a small cost savings, it’s normally not worth the trouble.
Several years ago, Millipore introduced a Millex® 33mm filter which some people started using because of supposedly faster flow rates and operating pressures. However Millipore clearly was trying to come up with a bid “lock-out” specification that would ensure their products would win competitive bids. Since then, a number of competitors have gone to a 30mm filter that, while not meeting the lock-out spec, performs just as well for 99.9% of samples.
Filters can be broadly categorized into two types – Hydrophobic and Hyrdophilic. Many companies color code their filter housings so that the filter material can be easily identified which is a nice feature that avoids lab errors.
For general filtration, Nylon membranes are the least expensive and most resistant to harsh chemicals. The second most popular is PTFE which is available in both hydrophilic and hydrophobic versions. Here is a summary of the more common filter material types:
- Polytetrafluoroethylene (PTFE) Hydrophobic membrane. Resistant to organic solvents as well as strong acids and bases. Low protein binding. Low in extractables. Main applications are the filtration of non-aqueous samples. Prior to filtering of aqueous samples the membrane must be pre-wetted with a water-miscible organic solvent.
- Hydrophilic Polytetrafluoroethylene (PTFE L) Hydrophilic PTFE membranes are unsupported membrane that ideal for HPLC and mixtures of aqueous and organic solvents. Use with both aqueous and organic solvents, along with their mixtures.
- Polytetrafluoroethylene (PES) Hydrophilic membrane. Broad solvent compatibility. Suitable for filtration of aqueous and compatible organic solvents. Higher liquid flow than either PTFE or PVDF. Low in extractables. Low protein binding.
- Cellulose acetate (CA) Hydrophilic membrane. Limited solvent resistance. Very low protein binding capacity, which makes it an excellent choice for protein recovery applications.
- Polyvinylidene difluoride (PVDF) Hydrophobic membrane. Fast flow rate and very low protein binding. Generally compatible with most common solvents. Ideal for protein recovery applications.
- Mixed Cellulose Esters (MCE) Hydrophilic MCE membranes. Biologically inert mixture of Cellulose Acetate and Cellulose Nitrate membranes which have higher protein binding than CA for most proteins. High porosity provides high flow rate. Good use for aqueous based samples.
- Glass microfiber/glass fiber (GMF/GF) Hydrophilic material. Excellent compatibility with organic solvents and strong acids (apart from hydrofluoric acid) and bases. Either used as a pre-filter or as a final filter.
- Polypropylene (PP) Slightly hydrophobic membrane. Resistant to a wide range of organic solvents. Also good choice used as a prefilter or as a final filter.
Selection of the filter material should be made on the following criteria:
- Compatibility with the sample you are filtering.
- Compatibility with the test you will be performing on the sample.
- Conformity with the S.O.P. for the test you will be performing.
- Ability to filter the majority of samples with minimal back pressure.
I can think of no other lab item that more vendors sell! (If you can think of one, I would like to hear from you.) As an example, Graphic Controls – the company whose name says that they make chart paper – has a line of syringe filters! Walking around the exhibit hall at Pittcon, you see syringe filters in nearly every booth! I am bombarded weekly with emails from Chinese manufacturers who offer syringe filters such as this ad from Microlab Scientific Co., Ltd based in Hong Kong, China:
We advise our clients against purchasing Chinese manufactured syringe filters directly from China regardless of the price as there is no responsible agent in the U.S. if you run into problems.
In my experience, the market leaders in the U.S. for syringe filters are:
- EMD Millipore
- Pall Life Sciences
- GE Whatman
- Thermo / National Scientific / Nalge
- Corning Life Sciences
- Fisherbrand Private Label
- VWR Private Label
Beyond this listing there are literally dozens of suppliers who have a small piece of the market.
As a general rule, the more recognizable the brand name, the higher the price. For example, if you compare a PES 0.45micron 25mm bulk, non-sterile syringe filter, you will find that the most expensive filter is a GE Whatman filter at well over $3.00 each. In the middle of the pack are the Fisher and VWR private label filters which come in around $1.25 each. And at the low end of the scale a the U.S. companies who purchase from Asia under their brand at about $0.80 each. And if you would purchase direct from China, you could purchase this filter for around $0.40 each, not including the shipping.
If you use 10,000 filters per year and you buy a $3.00 filter you will pay $30,000. At the other end of the scale you will pay $4,000. So the question is, is the difference worth $26,000 to you? In some cases, the answer may be yes! Especially if you are required to do extensive parallel testing to re-validate an assay.
But in many cases, the only reason for paying this huge difference is because lab people just aren’t willing to risk adding a new variable to a test that works. So the question is, what are the risks? Would the testing you are doing be subject to the variables? Let’s look at what the risks are:
- Sample contamination. This comes from two sources, the cleanliness of the syringe filter and whether or not the chemicals you are filtering will leach materials from the filter or the housing. Another potential source is the rupturing of the filter due to over-pressuring which would allow non-filtered materials to pass into your sample. (This is usually easy to detect because you the syringe plunger suddenly goes from really hard to push to really easy.)
- Sample retention. Is the filter material or housing binding something in your sample that it shouldn’t?
- Filter clogging / backpressure. Overloading the filter with particulates or high viscosity materials can cause filter rupture at worst or some serious cramps in your hands and wrists. If this is happening, you will hear from your lab workers one way or the other. The solution to this problem may be a pre-filter or better yet – having a pre-filter available in addition to your routine filters if you visually see that a sample is especially “dirty.”
- DNA / RNA / Ulra-Low Particulates – If you are concerned about human contamination, and not just sterilization, then buying syringe filters that are entirely machine manufactured in a clean room environment would be prudent. And these will cost more.
For large customers who purchase lots of syringe filters…
Negotiate a contract directly with the manufacturer. Work with your contracted distributor to negotiate prices directly with the manufacturer. You should expect prices to be AT LEAST in the $1.00 - $1.25 range for non-pre-filter syringe filters regardless of the brand. DO NOT BE SATISFIED WITH YOUR DISTRIBUTORS CONTRACT PRICE!
Buy in bulk packs of 1,000 rather than packs of 50 / 100. Many vendors offer big discounts if you purchase in a bulk case rather than the standard packs. These part numbers may not even be advertised.
Investigate your distributor’s Private Label brand. For example, VWR’s private label syringe filter is supplied by Pall Life Sciences. Your distributor rep may not tell you (if the rep isn’t seasoned, he/she may not know.) Fisher’s private label probably comes from some Thermo company. If these are the brands you buy anyway, you just might save a lot by leveraging their buying power.
For the mid-sized customer who tests samples with a known particle content…
If you are experiencing problems. Identify the problem. Is it filter loading / backpressure? If so, before going to a pre-filter, look at alternate brands or sizes and save the pre-filter option as a last resort. If you are experiencing rupturing of the filter, find a new brand! If you have contamination, look at a PTFE membrane or at other brands.
If you are not experiencing problems. Then there is no better time to look for less expensive alternatives! You know the filter membrane, you know the micron size, now you are just looking for a supplier who will sell their products for less. Look to the lesser known, U.S. based manufacturers or distributors who provide great products. We recommend:
- Foxx Life Sciences (a new entry for 2016)
If you use syringe filters, why not call us for a quote? We may be able to save you a lot!