Introduction
With nearly 200,00 HPLC detectors in operations worldwide, these units are laboratory essentials. Whether analyzing samples or preparatory scale-up, filtration begins at the foundation of your experiment. Filtration is often perceived as a technique to only remove contaminants from the sample, but this is one factor of many facets that can improve your analysis and knowing more about this process may even help troubleshoot issues that may arise throughout your method. Adhering to proper sample filtration practices may avoid less downtime from equipment failure by particulates entering the HPLC system. This may circumvent future house calls to instrument repair technicians and metrologist, saving time and money.
Mobile phase
By using a bottle top dual unit, like the SolvFil™, (pictured below) one can remove dust and other particulates from the mobile phase and degas in one simple step. This is especially important for mobile phases containing buffers, as any undissolved salt can create blockages in the system. This filter degasses mobile phases directly into reservoirs, making it incredibly easy to degas and load onto an HPLC system. This avoids the double transfer from using typical older glass housing and also circumvents the precarious situations of tipping expensive breakables.
Sample Membrane Filtration: Extractables
“Extractables” are compounds which can come from your HPLC syringe filter membrane and transpire in the filtrate (i.e. the sample). These extractables can present problems in HPLC analyses where they may co-elute and interfere with compounds of interest. The concentration of extractables can be deduced by two strategies. One is a common practice in the laboratory by discarding the first milliliter or so of filtrate, which often contains the highest level of extractables. Another option is to select a high-quality filter that will further reduce this effect.
The MicroSolv AQ™ Brand NDX™ - Depth 0.45μm pharma-grade nylon filters were compared to those from a different market leading manufacturer. A solvent of 70:30 acetonitrile: DI water was passed through each filter. In one instance, the filtrate was sent directly into an autosampler vial (Figure 1, “No Wash”). In another, the first 1mL was sent to waste and then the rest was collected in an autosampler vial (Figure 1, “Wash”). All four vials were used in HPLC studies to determine whether extractable peaks were present. The peak areas of the observed extractables are shown in Figure 1. The AQ™ filters yield substantially lower extractables than competitive filters, hence minimizing potential interferences in your HPLC analyses. Further studies demonstrate additional advantages of the MicroSolv AQ Brand NDX - depth filters.
Keep Retention Volume to a minimum
Retention Volume or hold-up volume is the amount of liquid remaining in the filter after passing all the liquid through. This can be particularly important when handling expensive or limited samples. As shown in the figure below, the AQ™ filters have much less liquid remaining in the filter compared to an ordinary filter.
Lastly, the filters have a significant lifetime. To test this, a “dirty” solution was filtered until the pressure became too high. The volume at which this occurs was recorded for each filter type, shown in the figure below.
Filtration is a very cost-effective insurance to avoid the introduction of particulates to your analytical instrumentation and HPLC column. The objectives of filtration are not only to retain undissolved particulates to the sample but also to not add extractables to the sample. The advantages of these filters are apparent in terms of lowest extractable content, retention volume, and filter lifetime. By employing these filters with premium quality, avoid the likelihood of introducing extractables to samples. These AQ™ Filters have proven to be a valuable benefit to any analytical laboratory with high throughput or research and development areas that handle low abundance samples.