New method for contamination testing

Tuesday, April 13, 2010 @ 03:04 PM gHale


When it comes to testing such things as food for contamination, the concept of lab-on-a-chip works in theory. In reality, it needs to provide quick and easy answers.

The goal is simple: Sample in, answer out. But for these miniaturized laboratory analysis systems analyzing complex solutions containing particulates or other contaminating materials the first part of the process isn’t so easy.

Sample preparation from these solutions can take time, be expensive and time-consuming, involving complicated laboratory methods performed by skilled technicians. This can significantly diminish the benefits. But that may soon change.

There is now a simple way to analyze samples that are complex mixtures, such as whole milk, blood serum and dirt in solution, said researchers at the National Institute of Standards and Technology (NIST).

The new technique, developed by NIST researchers Elizabeth Strychalski and David Ross, in collaboration with Alyssa Henry of Applied Research Associates Inc. of Alexandria, Va., is gradient elution moving boundary electrophoresis (GEMBE).

Because of its ability to easily and rapidly characterize complex mixtures with minimal preparation, the researchers said GEMBE shows promise for applications, such as monitoring contaminants in food or water supplies, determining nutrient levels in soil, detecting biochemical warfare agents, and diagnosing medical conditions. GEMBE uses a combination of electrophoresis and variable pressure-driven flow through a microchannel. Electrophoresis uses electricity to push a mixture in solution through a channel, forcing the individual components to separate as they move at specific rates based on their individual properties, such as size and electrical charge. Complex samples can be difficult to separate cleanly because components in these samples (for example, the fat globules in milk or proteins in blood) can “foul” microfluidic channels in a way that prevents reliable detection of the needed sample components.

The new technique solves this problem by pumping a buffer solution under controlled pressure in the opposite direction. This opposing pressure flow acts as a “fluid gate” between the sample reservoir and the microchannel. Gradually reducing the pressure of the counterflow opens the “gate” a little bit at a time. When the pressure flow becomes weak enough, the component’s electrophoretic motion pushes it against the pressure flow and into the channel where it undergoes detection. In this way, different components enter the channel at different times based on their particular electrophoretic motion. Most importantly, the channel does not back up because the unwanted material in the sample stays out because of the pressure flow.

Researchers validated their GEMBE analysis technique by testing it with solutions of whole milk, dirt, estuarine sediment, coal fly ash, pulverized leaves and blood serum. In all cases — and without the muss and fuss of pre-analysis sample preparation — the system was able to reproducibly separate and quantify specific components from the solutions, including potassium, calcium, sodium, magnesium, lithium and melamine.

“GEMBE is well-suited to the microfluidic analysis of ‘real-world’ samples,” Strychalski said. “We have shown that the method can handle solutions containing particulates, proteins and other materials that would confound the majority of other microfluidic techniques.”

The next steps are to miniaturize the desktop equipment now used in the system and integrate all of the parts to develop a true “lab-on-a-chip” field analyzer that can rival the effectiveness of a full-scale facility.



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