Filtration makes up a key procedure in a wide range of laboratories, from basic biological and chemical research to industrial applications, including food processing, pharmaceuticals, and more. Many biologists, for example, filter smaller and smaller volumes of sample. The amount that qualifies as truly “small volume” sample filtration depends on whom you ask. For example, Vivek Joshi, principal scientist at EMD Millipore, a division of Merck KGaA (Darmstadt, Germany), says, “In my perspective, small volume is about a couple [of] hundred microliters of sample or less.” Navin Pathirana, global products manager - lab devices at GE Healthcare (Waukesha, WI), draws the line at up to one milliliter. Some researchers take the line even higher.
“The reason people are going down to these small volumes is because of the research they are conducting,” Joshi says. For small-volume sample filtration, the research often involves analysis of tissue-level analytes or even components at the single-cell level.
For example, when a drug researcher uses small animals, such as a mouse or rat, for pharmacokinetic or pharmacodynamic studies, only small amounts of plasma can be withdrawn. To prepare such a small sample for analysis, it must often be filtered. When performing high-performance liquid chromatography (HPLC), for instance, Joshi says, “you need to at least filter the sample because particles and impurities affect the HPLC performance.”
While the sample volume decreases, researchers want to run more samples. “Even smaller labs are trying to process more and more samples,” Joshi explains. “So scientists look for samplefiltration solutions for small volumes that make the process simpler and faster.” In fact, small academic laboratories to larger biotechs and big pharma keep needing to process more samples. Consequently, these samples must be processed as efficiently and consistently as possible.
Saving the sample
As researchers filter small volumes of sample, they can afford to lose less and less. “When you do filtration on a membrane,” Joshi says, “some amount of sample is held by the membrane.” That loss can be fluid or even the analyte of interest.
When customers do fewer than ten samples a day, they can use syringe filters. “At that number of samples, it’s not that big of a deal,” Joshi says. “If you process hundreds of samples a day, you might go to a 96- or 384-well plate to process multiple samples in one go.”
For scientists who filter ten to 100 samples a day, they needed a better solution. “They could not go to a plate-based solution because they didn’t process enough samples, but the syringe format is not convenient when you have to process so many samples.” He adds that about 60–70 percent of researchers filter ten to 100 samples a day.
So researchers want to use filtration systems that work with small volumes, preserve sample, and fit their sample-throughput needs. Because of so many needs, says Joshi, “we introduced our Samplicity, which is a vacuumbased filtration system using membranes that allows eight sample filtrations side by side.” This system filters a sample as small as 100 microliters and preserves 80 percent of the sample.
The accuracy of HPLC depends on sample preparation, including filtration. In this case, researchers don’t want to lose sample to the filter, and they also do not want anything from the filter getting in the sample. “So researchers should use filtration systems that are extremely clean and certified for HPLC applications,” Joshi says.
The challenges in small volumes get even more complicated with ultra-HPLC (UHPLC). “A consequence of switching from HPLC to UHPLC is that the sample-filtration step becomes the bottleneck,” says Pathirana. “Often the time it takes to filter and get the sample ready to load onto the autosampler is at least equal to the run time of the sample.” In addition, says Pathirana, “filtration of the sample is critical for UHPLC because of the particle size of the media in UHPLC columns.”
Despite those challenges, researchers will continue to make the transition to uHPLC. “The benefit to the customer from switching to UHPLC from HPLC is improved resolution, sensitivity, and speed.” Pathirana says. “However, for the customer to see the full benefit of speed, the sample filtration speed also needs to be improved. This is where we see opportunities to improve the UHPLC workflow.”
For one thing, researchers can use a combined filter and autosampler vial, which GE Healthcare makes for its Mini-UniPrep syringeless filter. So in one step the sample gets filtered and put into an integrated autosampler vial. This system comes in plastic or glass versions (new Mini- UniPrep G2).
The technology depends on the technique
What a researcher needs when filtering small-volume samples really depends on just what is being done. It depends on the kind of sample and its source; it depends on the question at hand and how it will be analyzed.
Oliver Hyman, a doctoral student at Arizona State University, once filtered samples of pond water—ranging from 60 to 1,000 milliliters—in search of DNA from a pathogen. Hyman says, “I used a small Lure Lok syringe to push the water through a 0.22 micron filter.” This technique provided some clear benefits to Hyman. As he says, “I liked it because it was much simpler than sampling animals.” Still, some technological improvements would have made this work even more effective. As Hyman explains, “It would be nice to have tools—pumps, prefilters, and so on—to help filter more water.”
So even as sample volumes plummet in some research areas, the volume of samples to filter expands. Consequently, the approaches to filtration grow in step. Today, researchers can choose from many approaches, even ones specialized for processes such as UHPLC and high-throughput techniques. Sometimes, the choice depends on the particular scientist doing the work.
Mike May is a freelance writer and editor living in Texas. You may reach him at firstname.lastname@example.org.