Vespel, Tefzel or PEEK Valve Rotor Seals ?

One of the most common HPLC preventative maintenance parts is the injector valve rotor seal. Worldwide, the majority of these chromatography parts are produced by Rheodyne (IDEX) or Valco Instruments (VICI) and used by the major instrument manufacturers in their products. These valve seals are critical to maintaining a leak free, high pressure seal inside the injector. They are subjected to a lot of wear and chemical exposure with use. They have a finite lifetime which may be very short, in some applications (a few months) or last for many years in others. *The most common reason for rotor seal damage is a lack of flushing when buffers are used. Regular flushing down of the flow path (without the buffer) is required to maintain a clean flow path. Buffer deposits and crystals scratch and damage the rotor surfaces. Depending on the specific use and application, rotor seals are often replaced as a preventative measure once every 6 or 12 months. They should also be replaced whenever they are scratched, heavily worn, no longer sealing well, leak or become contaminated. Failure to maintain your injector's parts may lead to HPLC carry-over contamination problems.

The choice of rotor seal material should be based on: (1) the types of chemicals it will come into contact with; (2) the working pH range; (3) the temperature range.

• Note: When choosing a valve rotor seal material, please refer to the valve manufacturer's information, compatibility and advice. Blends and properties may vary between vendors so always verify compatibility with them before use.

 
Common HPLC Rotor Seal Material Types:

 Vespel ^®: Chemically, this is a polyimide blend (DuPont). One of the most widely used materials for HPLC valve rotor seals. It has excellent chemical compatibility with most HPLC mobile phases, excellent temperature stability and a pH limit of 10. An excellent choice for most applications.

Tefzel ^®: Chemical name, ethylene-tetrafluoroethylene (aka, "ETFE"). It has excellent chemical compatibility with most HPLC mobile phases and a higher pH limit of 14. Tefzel's preferred applications are where very high (>9) or very low (<3) pH solutions or mobile phases are used. Not compatible with some chlorinated solvents and in most forms it has a temperature limit of 50°C.

PEEK: Chemical name, polyether-ether ketone. Known for applications where biocompatibility and / or high temperatures are of concern. Like Tefzel, it has excellent chemical compatibility at room temperature with most HPLC mobile phases. Most vendors report a working pH range between 1 and 14. Unlike Tefzel, it has a much higher temperature limit (i.e. 100°C or higher), but its resistance to some chemicals appear to degrade with increasing temperature. Contraindicated where it will come into contact with solutions of THF, methylene chloride, DMSO or concentrated acids (i.e. nitric or sulfuric). Some sources have observed problems with chloroform as well, especially with PEEK tubing, so it may not be recommended for those applications.

 

For more information on troubleshooting HPLC injector valves, please refer to this linked article, "Troubleshooting HPLC Injectors (Manual and Automated)". 

Syringe Filter Selection for HPLC or LC/MS samples

This article will address the use of disposable female, Luer-compatible, syringe filters without built-in pre-filters for the filtration of individual samples into vials for HPLC or LC/MS analysis. - Note: 96 or 384 multi-well filtering plates provide for a better solution when large quantities of samples need to be filtered. Note: The presented filter membrane material selection criteria also applies to mobile phase filtration too.

The choice of syringe filter depends on the: filter size (volume) of your sample, the chemical compatibility of the housing and membrane and desired pore size. Selection of the wrong filter size can result in too much sample holdup volume (loss of sample on filter) or overloading of the filter (allowing unfiltered material to pass through). If a membrane or housing is chosen which is not chemically compatible with your solution, then contamination of the sample or rupture of the assembly can result. Choosing a filter with too large a pore size can result in material passing through it which could clog or contaminate the solution (i.e. plug an HPLC system or result in a loss of sterility of a solution). Protein binding affinity is another characteristic of filter membranes and if you are filtering samples of biological interest, then you will also want to consider this specification in your selection criteria too (though it will not be discussed in this article).

Syringe Filter Size:

Filters are available in a variety of sizes which are generally in a disc shape and described by their diameter. Common sizes available for chromatography samples include: 3 mm, 4 mm, 13 mm and 25 mm (~25 - 30 mm) diameter discs. The larger the diameter of the disc, the larger the sample capacity, cross sectional surface area and potential hold-up volume of the sample on the filter. 

Hold-up volume is important because some of the sample will be retained inside the membrane and/or filter housing. If too large a filter is selected, samples with small volumes could be lost entirely in the hold-up volume on the membrane. Smaller filters have lower hold-up volumes. To extract as much sample as possible, be sure and use a post-filtration air purge to reduce the total hold-up volume.

If the volume of the sample you wish to filter is under 1 ml, then a 3 mm filter may provide the lowest hold-up volume and require the smallest amount of solution. To filter samples between 1 ml and 10 ml, the 13 mm diameter filter provides a good balance between hold-up volume and large filter surface area. Larger sample volumes from 5 ml to 50 ml are often filtered through the more common 25 mm diameter filters (~4 times the filtration area as a 13 mm disc).

Chemical Compatibility:

Membrane Material: This is where you really must consult the manufacturer’s own documentation for the most compatible filter membrane for both your sample and the solution that will flow through the filter. To simplify the selection criteria, we can make some generalizations about some of the different types available:

Cellulose Acetate (CA): Use with aqueous solutions and a few hydrocarbons only. Low protein binding so good for many biological samples. Not compatible with ACN or DMSO. Can be autoclaved.

Nylon: Great general purpose material and compatible with many HPLC solvents (including THF, alcohols, ACN), but not strong acids. Nylon has a high affinity to bind proteins. Can be autoclaved.

Polysulfone / Polyethersulfone Variants (PS / PES): Commonly used with tissue culture and ion chromatography samples. Stable with many strong bases and alcohols, but few HPLC solvents (as it is hydrophilic). Low backpressure and low protein binding. Not compatible with ACN. Can be autoclaved.

Polypropylene (PP): General purpose hydrophilic material with resistance to most acids, bases, DMF, DMSO and alcohols. Not recommended for use with hydrocarbons, esters or solvents such as ACN. Can be autoclaved.

Polyvinylidene difluoride (PVDF): Hydrophilic material with broad compatibility. Often a good choice for use with alcohols, hydrocarbons, biomolecules, ether and ACN. Low protein binding. Can be autoclaved.

Polytetrafluoroethylene (PTFE): Reported in most brochures to be chemically resistant to almost all solvents, strong acids and bases. Hydrophobic membrane should be pre-wetted when used with aqueous solutions. Low protein binding and very strong. Can be autoclaved.

Most chromatography grade syringe filters are constructed of either HDPE or PP. These materials are compatible with a wide range of HPLC solvents and both offer low levels of extractables. HDPE has been reported to be more chemically compatible with aqueous basic solutions of NH4OH than PP.

Pore Size:

This will depend on your application and a number of different pore sizes are commonly available from vendors (1 micron, 0.8, 0.45 and 0.22 micron are the most common): 

For example, is sterilization of the fluid the goal? If so, a 0.22 micron filter is generally accepted as the best choice.  


For most chromatography or LC-MS applications either a 0.45 or 0.22 micron filters are preferred.

Summary:

• Please refer to the various manufacturers data sheets to select an appropriate syringe filter with: (1) a low hold-up volume; (2) large enough size for the volume of sample; (3) which is chemically compatible with the solution and material you are going to inject through it and (4) lowest protein binding affinity (if applicable).
   
• To reduce the hold-up volume, use a post-filtration air purge to empty the filter.
   
• Minimize contamination from extractables (in the plastic) by pre-rinsing the filter membrane with some of the clean solution. This can reduce the amount of detectable extractables in your sample. PTFE based membranes have some of the lowest extractable levels so consider their use if this is an issue.
   
• If analyte binding is a concern, select one of the membranes which has the lowest binding affinity such as PVDF or PTFE.