If you are using a filter in automated workstations, it is important to choose filters that are reliable and will move consistently and smoothly within the automated sample handling apparatus.

• Acrodisc PSF syringe filters from Pall Life Sciences 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, robotic handling, and fit.

• Acrodisc PSF syringe filters are designed specifically to meet  the exacting requirements of automation systems. They demonstrate smooth filter-to-filter release, consistent turret advancement, exceptional housing strength, and a strict outside filter geometry to ensure proper alignment and consistent operation.

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 broad guidelines below can be used for basic information.

Aqueous Samples
Hydrophilic membranes, which have an affinity for water, are preferable when filtering aqueous samples. Use Pall Life Sciences 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 Life Sciences 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 Life Sciences’ 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.

A filter is designed to increase accuracy by removing unwanted particles.

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. Our membranes have been tested for compatibility with common HPLC solvents (water, acetonitrile and methanol) using established HPLC procedures. In addition, to verify low levels of UV-detectable extractables, samples of the entire HPLC Acrodisc syringe filter line are evaluated prior to release.

IC Certified for Low Levels of Inorganic Extractables
Pall Life Sciences 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.

Sample Adsorption
Unwanted drug binding as well as the presence of possible extractables eluted from the filter during routine pharmaceutical sample analysis can be a serious problem and cause out-ofspecification results. No single analytical method can provide reliable information on comparative filter properties and the full range of extractables for all filters. Therefore, choose a low adsorption 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%.

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.

25 mm Acrodisc PSF syringe filters, as well as 13 and 4 mm diameters for smaller sample volumes, are available in a variety of membrane and pore size choices.

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 Life Sciences 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.

Table 2

Volume to be Filtered	Filter Type	Typical Hold-up Volume
< 500 µL	Nanosep MF Device	< 2 µL
< 2 mL	4 mm Acrodisc Syringe Filter	< 10 µL
< 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

For difficult-to-filter samples, the Acrodisc PSF syringe filter with GxF multi-layered glass fiber prefilter is the best option.

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.

Figure 3

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.

Easy Identification
13 and 25 mm 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)

To extend the life of your column and reduce maintenance due to particulate in the pumping system, thereby giving more analyses per dollar spent, a filter’s pore size should be determined based on the column packing size. As you can see in Figure 4, 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 in Figure 4 below. The idea is to find out how large that space is and remove particles that size.

Figure 4

For example: start with a 3 µm packing size and draw a few well-placed equilateral triangles as suggested in Figure 5. Determine the largest particle capable of fitting through the column by circumscribing an equilateral triangle with a side length of 0.75 µm.

Figure 5

Figure 6

Now enlarge the diagram and look further. Visualize a series of equilateral triangles whose side length gets down to 0.75 µm. Figure 6 illustrates a right triangle whose short side describes the radius of the sphere. The angle 0 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. Accurate pore size rating is important to the economics of the appropriate filter choice.