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^OFlV Using drops to collect samples When rain falls, our thoughts range from rainbows to water for the garden to alternative plans for canceled outdoor activities. But raindrops are also efficient collectors of gases. Purnendu Dasgupta and colleagues at Texas Tech University have devised a droplet technique that mimics Mother Nature's efficiency at collecting atmospheric gases. Their unusual approach has resulted in a 1-ppb detection limit for many gases, without the need to perform the labor-intensive steps of taking gas and rainwater samples. Dasgupta became interested in trace environmental analysis in graduate school. The public had become concerned about the effects of S02 in the environment. Although emission control systems were removing the majority of sulfur gases from car exhaust and industrial smokestacks, the scientific and regulatory communities still needed to know how much was in the atmosphere and reacting to become H2S04 aerosols. "The analytical challenge shifted to measuring smaller quantities [of pollutants] that get into our lungs," says Dasgupta. "The driving force behind environmental research is human health, what happens to people when they are exposed [to pollution]. Today, we have the ability to make the measurements; it's the sampling that's the problem."
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Dasgupta with a monitor showing a liquid film on the tip of a capillary.
The droplet-based sampler consists of a capillary tube inserted into a larger tube. An appropriate solution is pumped through the capillary and a droplet or wire-supported film is formed at the tip of the capillary to collect the gas that flows through the larger tube. As the sample gas flows around the drop, gas molecules diffuse into it. The drop is aspirated back to an in-line analysis system, such as an LC or CE system, and a new drop is formed and another sample collected. "Just as the raindrop samples the environment, the drop at the tip of the capillary samples the immediate environment surrounding it. The drop is the interface between the outside world and the analytical system." The advantages of this type of integrated sampling and analysis system are that there is less chance of contamination, and it's not necessary to send samplers out into the field to collect large quantities of samples. Forming drops is also easy to do—their size and shape are very reproducible. The drops can also be probed optically or electrochemically with current technology, rather than being aspirated back into a system.
Analytical Chemistry, August 1, 1995
A bonus is that the technique discriminates between gases and particles. As the surface of the drop evaporates, the vapor sets up a barrier that prevents particles from entering the drop. However, the apparatus can be modified to collect particles by attaching a dry wire at the capillary tip and applying a voltage to it. The particles are thenrinsedfrom the wire (the rinsate forming a drop) and the solution is aspirated back into the system. Eventually, nonaqueous solutions will be used to dissolve particles that don't dissolve in water, and it should also be possible to directly insert the particle-coated wire into a GC or GC/MS system. Dasgupta also envisions drop sampling being used like a membrane. "For example, a water drop in an oil sample. Analytes can diffuse from the oil into the water, just like they would across a membrane. Membranes can be used only so many times. You can keep making drops forever." He says that "what is appealing about this technology is its small size and the renewability of the drops. This is what will spur researchers in other areas to apply the droplet sampler to their analytical problems."
