Abstract
A prototype aerosol detection system is presented that is designed to accurately and quickly measure the concentration of selected inorganic ions in the atmosphere. The aerosol detection system combines digital microfluidics technology, aerosol impaction and chemical detection integrated on the same chip. Target compounds are the major inorganic aerosol constituents: sulfate, nitrate and ammonium. The digital microfluidic system consists of top and bottom plates that sandwich a fluid layer. Nozzles for an inertial impactor are built into the top plate according to known, scaling principles. The deposited air particles are densely concentrated in well-defined deposits on the bottom plate containing droplet actuation electrodes of the chip in fixed areas. The aerosol collection efficiency for particles larger than 100 nm in diameter was higher than 95%. After a collection phase, deposits are dissolved into a scanning droplet. Due to a sub-microliter droplet size, the obtained extract is highly concentrated. Droplets then pass through an air/oil interface on chip for colorimetric analysis by spectrophotometry using optical fibers placed between the two plates of the chip. To create a standard curve for each analyte, six different concentrations of liquid standards were chosen for each assay and dispensed from on-chip reservoirs. The droplet mixing was completed in a few seconds and the final droplet was transported to the detection position as soon as the mixing was finished. Limits of detection (LOD) in the final droplet were determined to be 11 ppm for sulfate and 0.26 ppm for ammonium. For nitrate, it was impossible to get stable measurements. The LOD of the on-chip measurements for sulfate was close to that obtained by an off-chip method using a Tecan spectrometer. LOD of the on-chip method for ammonium was about five times larger than what was obtained with the off-chip method. For the current impactor collection air flow (1 L/min) and 1 h collection time, the converted LODs in air were: 0.275 μg/m3 for sulfate, 6.5 ng/m3 for ammonium, sufficient for most ambient air monitoring applications.
Highlights
Analysis of atmospheric aerosol content relies heavily upon the sensitivity of sampling and chemical analysis
The droplet mixing between sample chosen for each assay and sequentially dispensed from on-chip reservoirs
The droplet mixing and reagent wasand completed a few secondsinand theseconds final droplet was transported to the detection between sample reagent in was completed a few and the final droplet was transported position as soon as the mixing was finished
Summary
Analysis of atmospheric aerosol content relies heavily upon the sensitivity of sampling and chemical analysis. The long sampling times precluded researchers from detecting concentration changes that occurred on short time scales Several on-line methods have been developed to avoid sampling artifacts and answer the need for automatic high time resolution measurements of aerosol chemical composition [5,6,7,8,9,10]. While these systems have been providing invaluable data in numerous laboratory and field studies, they have their disadvantages, such as fairly large form factors and high costs to purchase and operate
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