Why You Should Filter Mobile Phase HPLC

Mobile phase filtration is essential in HPLC. It not only extends the lifespan of your HPLC system but also helps ensure consistent, accurate, and repeatable results. Here are some of the key reasons why filtering the mobile phase in HPLC is important.

Protecting the HPLC Pump 

The HPLC pump is highly prone to premature failure, especially when poor-quality solvents are used without filtration. Unfiltered solvents can introduce particles that accelerate wear on the pistons and seals, leading to irregular baselines and fluctuations in pressure and flow rate. To prevent this, it is recommended to filter solvents through a 0.45 µm membrane filter before use.

Pre-mixed Solvents, the filtering is already done for you. 

To avoid having to filter solvents in the lab, many opt to purchase pre-mixed solvents from reputable manufacturers. These pre-mixed solvents are filtered prior to packaging and give the lab the confidence to put the solvent directly on their instrumentation for HPLC analysis.

Filtering LCMS grade or LC grade solvents (or premixed solvents) in the lab can introduce additional variables and it is typically not done in everyday lab practice. For example, washing of glassware between filtering (if not done appropriately) can add to contamination. Also, the human element of setting up the filtration apparatus and selecting the compatible filter membrane adds additional variables and can lead to contamination.

Mobile Phase Additive and Microbial Growth 

Many LC methods today involve some mobile-phase additive, including buffering agents and ion-pairing reagents. It is strongly recommended to filter when buffering by adding a solid additive (salt) to the mobile phase. The increased risk of particulates from the solid outweighs the risks, such as human error and improper glass washing.

In the case of a liquid additive, it is best to filter after adding the additive when the concentration is high.  

Other HPLC Mobile Phase Considerations

Microbial growth can contaminate the pump both physically and chemically. A few of the best practices to avoid microbial contamination include: 

• Resist the temptation to top off solvent containers
• For buffered aqueous solutions, especially those near-neutral pH, prepare concentrated stock buffer solutions, refrigerate them upon storage, and dilute small quantities for use when needed. Prepare only enough of the working buffer to last one or two days to limit the time available for microbes to grow at room temperature. 
• Add an antimicrobial agent to the buffer

In summary, many people don’t filter in the lab because they buy high purity solvents. Although, when making your own buffered solutions in the lab (adding solid particulates), it is recommended to filter in the lab.