Ask TLD: Filtration FAQs

A hydrophilic filter membrane is readily wet using water and is most commonly used for various water filtration protocols, while a hydrophobic filter membrane does not wet with water. Instead, hydrophobic filter membranes are used with low-polarity organic solvents or oils.

Polyethersulfone (PES) are hydrophilic membranes with low protein binding that aids in preventing the target-analyte from binding. PES filter membranes also possess high flow rates and are most commonly used for filtering liquids where the target analyte is present in the filtrate of a solution.

In contrast, a glass filter membrane is commonly composed of borosilicate and has a high throughput. Glass fiber filter membranes are often used for gas filtration when the collection of air particulate is desired, to remove larger course particulate, or to analyze the cleanliness of water. Glass fiber filters also offer high-temperature resistance in open-air environments and high particle retention.

Filter paper grade is based on particle retention size, thickness, and weight, which provides insight for determining what filter paper is needed for a specific process.
Filter paper grade indicates the degree to which a filter paper can capture and retain particles. The finer the filter paper grade, the smaller the particles it can capture. The thicker the filter paper, the higher the grade its grade.

Additionally, the heavier the filter paper, the higher its grade. Different grades are used in a variety of processes such as water filtration, air filtration, and gas filtration. In all cases, the filter paper grade should be selected based on the size and type of particles that need to be filtered.

The desired retention can be determined by assessing the liquid or gas that will be filtered. If the particulate is fine, a filter with a small particle retention size will be needed to remove all particulates efficiently. However, if the particulate is large, a filter with a larger particle retention size and faster flow rate can be used.

A general-purpose qualitative filter paper can be used for basic vacuum filtration and must be size matched to the Buchner funnel. Filter paper grade 1 is one of the most commonly used general-purpose filter papers since it offers a reasonable flow rate and medium-size particle retention.

While a coffee filter does work well as a filter, its intended use is not in a lab or industry setting. A coffee filter can be used in a qualitative environment; however, it should never be used for analytical analysis because impurities present in the coffee filter are unknown.

To determine the filter paper size needed for a Buchner funnel, measure the inner diameter of the funnel in millimeters (mm).

It depends; filters made for higher pressure need Luer Lok Syringes that use a screw-on connection to secure the filter. However, most low-pressure applications use a Luer Slip Syringe, where the filter is secured by pushing it onto the tip of the syringe. Syringe filters will universally fit most syringe tips, but checking the filter connection's inner diameter and the syringe’s outer diameter is essential to ensure proper fitment.

Ashless filter paper is commonly used in analytical chemistry for gravimetric analysis, which quantitatively uses the mass of the collected residue to determine an analyte.

Ashless filter paper can also be used during filtration, as its material composition is designed to reduce or eliminate the presence of ash in the filtrate. It is ideal for trace analysis, as it is free from chemical and mechanical impurities that could otherwise affect the accuracy of the results. Additionally, it is resistant to the effects of many solvents, meaning it can be used to filter a variety of solutions.

Removing all super-fine particulates, such as ash, using a filter is difficult due to how small the ash particles are. Often celite is packed on top of the filter as a caked filter to aid in filtering out ash or other super-fine particulates.