How to Choose an Acrodisc® Syringe Filter for Analytical Sample Preparation

Pall Life Sciences has specifically designed and certified our Acrodisc® PSF syringe filters to be fully compatible and reliable for use with automated equipment. The following special features make our syringe filters reliable for worry-free performance 24 hours a day:

• Smooth filter-to-filter release
• Consistent turret advancement
• Exceptional housing strength
• Strict "outside filter geometry"

Acrodisc PSF syringe filters from Pall are the only syringe filters to receive Caliper Life Sciences’ Automation Certified guarantee. This certification is granted to syringe filters that meet the stringent requirements for automated dispensing and robotic handling. Pall is manufacturing these products working in close partnership with Caliper Life Sciences Manufacturing and Engineering teams to ensure proper fit, function, and compatibility with Caliper workstations. Caliper Life Sciences is the manufacturer of the Caliper TPW* workstation, the APW* workstation, and the Multi-Dose* workstation. These robotic workstations are uniquely designed to work best with Caliper “Automation Certified” filters for proper operation. (*TPW, APW, and Multi-Dose are trademarks of Caliper Life Sciences.) 

Acrodisc PSF syringe filters are available with AutoPack™ packaging, convenient tubes that can be directly loaded onto automated testing systems. AutoPack packaging is also available for standard syringe filters with Nylon, PTFE, PVDF, and Glass Fiber membranes. 

Does the filter need to be resistant to bases, acids, or organic solvents? Chemical compatibility is a critical consideration when selecting the sample prep syringe filter or mobile phase disc filter for your application. The following broad guidelines can be used for basic information.

Aqueous Samples

Hydrophilic membranes, which have an affinity for water, are preferable when filtering aqueous samples. Use Pall filters with GHP, PES, Nylon, or PVDF membranes.

Gases and Aggressive Organic Solvents

Hydrophobic membranes repel water and are inert to aggressive organic solvents, making them ideal for gases and organic solvents. Choose Pall filters with PTFE membrane.

Aqueous and Organic-solvent Solutions

Different polymeric membranes have different chemical compatibilities. Based on the application and chemical compatibility, there may be one or several membrane and Acrodisc syringe filter possibilities. Generally, one filter type will not function for all applications due to limitations in hydrophobicity/hydrophilicity and chemical compatibility. However, Pall's patented hydrophilic polypropylene (GHP) membrane is a universal membrane for both aqueous and organic applications.

Exceptionally Low Extractable Levels

A filter is designed to increase accuracy by removing unwanted particles. However, the wrong filter can be a source of contaminants in the form of extractables that elute into the sample from the filter device. These undesired artifacts can jeopardize analytical results. Some extractable concerns include coelution, false quantitation, and extraneous peaks. Pall Life Sciences specifically selects the highest grade of materials and performs rigorous extraction methods on our membrane products to remove the occurrence of undesired artifacts.

HPLC Certified for Low Extractables

Pall Life Sciences HPLC certification ensures that analytical results will not be compromised by extractable filter materials. To verify low levels of UV-detectable extractables, samples of the HPLC Acrodisc syringe filter line are randomly taken and tested for compatibility with common HPLC olvents using standard HPLC conditions.

IC Certified for Low Levels of Inorganic Extractables

Pall certifies Ion Chromatography (IC) Acrodisc® syringe filters have been tested using a highly sensitive IC protocol to monitor inorganic extractables. For ion chromatography applications, only the IC Acrodisc (PES) syringe filter is certified for low levels of inorganic extractables. Actual background levels of filter extractables are typically less than 20 ppb for chloride, 6 ppb for nitrate, 1 ppb for phosphate, and 10 ppb for sulfate.

Low Sample Adsorption

Choose a low adsorbing filter such as the Acrodisc syringe filter with GHP membrane. GHP membrane is extremely low in biomolecule binding. Typical binding levels are far below 1%.

Color-coding for Easy Identification

All analytical sample prep Acrodisc syringe filters and their packaging have color-coded printing with membrane type and pore size on each filter:

 GHP
 Nylon
 PTFE
 Glass Fiber
 PVDF
 Polyethersulfone (PES)

The particulate contained within a fluid affects the life of a filter. As particles are removed from the fluid, they block pores and reduce the useable portion of the filter. Particulate-laden fluids generally plug a filter more quickly than "clean" fluids. Increasing the Effective Filtration Area (EFA) can lengthen the life of a filter.

Filters come in a variety of sizes ranging from the area within a single well of a 96-well plate, to spin filters and syringe tip filters. Acrodisc syringe filters are available in 25 mm Acrodisc PSF syringe filters as well as in 13 and 4 mm diameters for smaller sample volumes. All are available in a variety of membrane and pore size choices.

Hold-up Volume

Another aspect of choosing the right filter size is the hold-up volume. This is the volume of liquid remaining in the filter after use. A filter with a low hold-up volume is recommended for use with expensive fluids or those with limited availability. What device size will assure complete sample filtration with minimal hold-up volumes? Pall offers a broad range of device sizes. The minispike outlet, available on the 13 mm device, allows for minimal sample hold up and easy dispensing into autosampler vials. Additional options include the 4 mm Acrodisc syringe filter, the Nanosep® MF centrifugal device, or AcroPrep™ filter plates.

Syringe Filter and Spin Device Selection

The table below outlines general guidelines to the appropriate filter size for different volumes of fluid.

Volume to be Filtered	Filter Type	Typical Hold-up Volume
< 500 µL	Nanosep MF Device	< 2 µL
< 900 µL	AcroPrep 96 1 mL Filter Plate	< 18 µL/well
< 2 mL	4 mm Acrodisc Syringe Filter	< 10 µL
< 2 mL	AcroPrep 24 Filter Plate	< 50 µL/well
< 10 mL	13 mm Acrodisc Syringe Filter (Minispike Outlet)	< 14 µL
< 10 mL	13 mm Acrodisc Syringe Filter	< 30 µL
< 125 mL	25 mm Acrodisc PSF Syringe Filter	< 200 µL

Consider Prefiltration

For difficult-to-filter samples, it is best to use a syringe filter with a glass fiber prefilter over the membrane. The Acrodisc PSF syringe filter with GxF multi-layered glass fiber prefilter is the best option for extremely particulate-laden samples. Our traditional Acrodisc syringe filters with GHP and Nylon membranes are also available with a single-layer glass fiber prefilter.

The Acrodisc PSF syringe filter has a serial glass fiber (GxF) prefilter to allow for maximum throughput and faster flow rates than standard glass fiber prefilter devices. The multi-layered prefilter, rated from > 40 to 1 µm, traps particulate, thereby extending filter life.

Assumption: it is desirable to extend the life of your column and reduce maintenance due to particulate in the pumping system thereby giving more analyses per dollar spent. Given this assumption, the filter pore size should be determined based on the column packing size. As you can see below, the column packing particles touch each other. Ideally, you would not want contamination to fit into the space between the particles of packing. This space (labeled Flow Path) is identified below. The idea is to find out how large that space is and remove particles that size.

For example, start with a 3 µm packing size and draw a few well-placed equilateral triangles as suggested in the figure below. Determine the largest particle capable of fitting through the column by circumscribing an equilateral triangle with a side length of 0.75 µm.

Now enlarge the diagram and look further. Visualize a series of equilateral triangles whose side length gets down to 0.75 µm. The figure below illustrates a right triangle whose short side describes the radius of the sphere. The angle is one half of 60º, or 30º. The horizontal side of this new right triangle has a length of 0.375 µm (half of 0.75 µm). Calculating the tangent of 30º gives the ratio of the length of the opposite side over the adjacent side, in this case 0.58. This means that the short side of the triangle is equal to 0.58 x 0.375 or 0.217 µm. Coincidentally, this is also the radius of the particle. So, if the column packing is 3 µm in diameter, the flow path is 0.43 µm.

When an HPLC column has a packing size of 3 µm or smaller, you should use a 0.2 µm filter because a 0.45 µm filter may let particles through that will plug the column.

Are You Concerned About Accurate Retention of Particulate?

For liquid chromatography systems using columns with larger than 3 µm packings, the filtration industry standard is 0.45 µm for syringe filters and mobile phase membranes. For columns with 3 µm or smaller packings, microbore columns, or when concerned about microbial growth, a 0.2 µm filter is recommended. Once the best pore size rating is chosen for the application, you must rely on the filter manufacturer to provide an accurate pore size rating. As described in, “Prolonging HPLC Column Life,” accurate pore size rating is important to the economics of the appropriate filter choice.