Final Report: Microchip Capillary Electrophoresis for Online Measurement of Inorganic AerosolsEPA Contract Number: EPD04012
Title: Microchip Capillary Electrophoresis for Online Measurement of Inorganic Aerosols
Investigators: Hering, Susanne
Small Business: Aerosol Dynamics Inc.
EPA Contact: Manager, SBIR Program
Project Period: March 1, 2004 through August 31, 2004
RFA: Small Business Innovation Research (SBIR) - Phase I (2004) RFA Text | Recipients Lists
Research Category: Air Quality and Air Toxics , Particulate Matter , SBIR - Air Pollution , Small Business Innovation Research (SBIR)
Knowledge of the chemical composition of airborne particles is essential to understanding their sources, atmospheric processing, and effects on human health and welfare. Uninterrupted data sets with wide spatial resolutions are especially important for epidemiological studies examining health impacts of airborne fine particles. Such monitoring requires a robust and affordable instrument that could be widely deployed for the routine identification of the chemical constituents of atmospheric fine particles.
In a special call for air monitoring methods, the U.S. Environmental Protection Agency (EPA) asked for “continuous chemical speciation monitors for PM 2.5 … that provide concentration data in real time or at least hourly.” EPA specified that “The monitor should be comparable (or better) than filter-based chemical speciation methods, while reducing impacts from positive and negative interferences, attaining high levels of precision for a variety of seasonal and aerosol conditions, and exhibiting excellent time resolution.”
Phase I addressed the need for an affordable and reliable measurement of the composition of PM 2.5. Aerosol Dynamics, Inc., focused on the proof-of-concept for a compact and inexpensive system for in situ monitoring of one important set of the constituents of PM 2.5, namely the inorganic ions. The target analytes are sulfate and nitrate. Sulfates have long been identified as an important constituent of airborne particles, and have been implicated in human health effects. Nitrates are expected to increasingly contribute to fine particle concentrations as sulfur dioxide emissions are controlled, thereby freeing a larger fraction of the gaseous ammonia to combine with gas phase nitric acid to form particulate nitrate.
Aerosol Dynamics, Inc., explored the feasibility of an entirely new particle collection and analysis approach that utilizes emerging laboratory on a chip (LOC) techniques. The atmospheric aerosol collection interface is accomplished with a novel “growth tube” technique recently developed by Aerosol Dynamics, Inc., for water-based particle condensation counting. The growth tube encapsulates airborne particles in water droplets that are large enough to be readily deposited by impaction directly into a sample reservoir on a microchip. Once deposited, the sample is analyzed by capillary electrophoresis (CE) coupled with conductivity detection. Aerosol Dynamics, Inc.’s aim is the online, time-resolved quantification of inorganic ions in ambient aerosol. A portable instrument is envisioned that will provide a 5-minute time resolution for a suite of PM 2.5 ions with detection limits of 100 ng/m 3 or better.
The overall goal of this research project is the development of an inexpensive monitor, which could be fabricated at a cost of about $3,000, for the measurement of anion (sulfate and nitrate) particulate concentrations in urban areas throughout the country. The overall focus is a proof-of-concept of the LOC-CE technique for measurement of sulfate and nitrate in airborne particles, and its interface with a particle collector based on a water-based particle condensation instrument for the collection of particles onto the microchip.
The specific goals of this research project are to: (1) develop a particle collection interface to allow direct deposition of particles onto the LOC-CE microchip with an efficiency of 98 percent or greater for particles above 0.1 μm, and 90 percent for particles from 0.02 to 0.1 μm; (2) build and test an LOC-CE that provides quantification of sulfates and nitrates for solution concentrations of 50 μM; (3) demonstrate the capability of the method using laboratory-generated sulfate aerosols; and (4) assess the feasibility of the approach based on the Phase I results.
Particle collection and analysis systems were designed and constructed separately to permit optimization and testing in parallel at Aerosol Dynamics, Inc., (particle growth and collection) and at Colorado State University (microchip ion analysis).
Particle Growth and Collection
The particle collector system consists of Aerosol Dynamics, Inc.’s water condensation particle size amplifier followed by a single-jet impactor. The active component is a cylindrical growth tube with inner walls made of a porous material, which is saturated with liquid water. The first half of the tube is cooled and acts as the saturator. The second half is heated and acts as the condenser. Because water vapor diffuses more quickly than the nitrogen molecules that comprise the major fraction of the surrounding air, the mass diffusion of water vapor from the walls is faster than the thermal diffusion. This produces supersaturation of water vapor in the central core of the flow, and hence, particle growth through condensation.
For Aerosol Dynamics, Inc.’s inorganic ion measurement system, most of the aerosol mass is in the size range from 0.1 to 1 μm. Thus, the focus is to efficiently collect those particles with diameters above 0.08 μm. A theoretical analysis suggests that a temperature elevation of 10ºC above the temperature of the inlet sample flow will enable the activation and growth of particles larger than 0.04 μm.
The size-dependent particle collection efficiency of the device was measured using size-classified ambient and laboratory generated sulfate particles. For ambient aerosol, the 50 percent collection cutpoint is at 20 nm, with 90 percent collection at 30 nm, and greater than 99 percent for particles above 60 nm in diameter. For sulfate, the cutpoint is lowered to 7 nm, with 90 percent collection at 13 nm. This is achieved at a pressure drop of 0.5 kPa (2 in water) and without exceeding temperature of 24ºC anywhere in the system. Data for ambient aerosol are somewhat less efficient than predicted by theory, yet meet the goals for particle collection efficiency.
LOC Design and Testing
Polymer-based microchips were designed and tested for the analysis of inorganic ions focusing on nitrate and sulfate. The chips, fabricated by poly(dimethylsiloxane), contain a sample deposit reservoir (where sample is introduced), a separation channel filled with buffer solution (where various ions are separated according to their electrical mobility), and a detector region (where passing ions are detected by changes in electrical conductivity). The microchip channels were treated to result in positively charged capillary walls so that the resulting bulk solution flow is in the same direction as the mobility of the anions being measured. Sample injections were performed hydrodynamically with a 10 second injection time. Separation was achieved by applying a potential of -400 V across the length of the separation channel. Conductivity detection was carried out using either gold or palladium microwire electrodes.
Several issues were evaluated with regard to the LOC-CE separation and detection of sulfate and nitrate ions, including: (1) design and optimization of an ion separation scheme; (2) performance testing of multiple conductivity detection schemes; (3) evaluation of system response versus concentration; (4) evaluation of measurement precision between analyses and between multiple microchips; (5) evaluation of measurement detection limits; (6) testing of ambient aerosol filter extracts, including comparison between LOC-CE and ion chromatography (IC) ion concentration measurements; and (7) extraction and analysis of aerosol deposits provided by the WCPC system.
The LOC-CE system that uses gold electrodes was found to exhibit the best performance. Sulfate and nitrate were easily separated, with a total time of analysis under 2 minutes. This is much faster than typically achieved in IC, and serves to illustrate the high chromatographic efficiency characteristic of CE separations.
The limit of detection was determined as 1 μM, comparable to that reported with the use of a conventional CE conductivity system. The range of sulfate and nitrate measurement using a linear calibration function was found to be from 1 to 250 μM, with an R2 value of 0.998 for sulfate, and from 5 to 100 μM, with an R2 value of 0.989 for nitrate.
One concern with new aerosol particle analysis methods is reproducibility. Run-to-run, chip-to-chip, and day-to-day reproducibility were investigated for LOC-CE analyses of sulfate and nitrate in laboratory standard solutions. Run-to-run reproducibility was 7.0 percent (n = 14) for sulfate and 3.7 percent (n = 14) for nitrate. Day-to-day and chip-to-chip variability were evaluated for a period of 7 days using seven different microchips. Sulfate exhibited a relative standard deviation of 7.1 percent (n = 21) and nitrate had a relative standard deviation of 7.9 percent (n = 21).
The optimized LOC-CE system was utilized in the latter stages of Phase I to measure concentrations of sulfate and nitrate in aerosol samples. Analyzed samples included an extract of ambient aerosol particles collected on a Teflon filter during a winter 2003 field study in Bondville, IL, and various laboratory and ambient aerosols deposited by the WCPC in the Aerosol Dynamics, Inc., laboratory. Comparisons between LOC-CE and IC measurements of the sulfate concentrations in three Bondville ambient aerosol extracts showed excellent results, with no significant differences between concentrations measured by the two techniques. Analyses of ambient and sulfate aerosols deposited using the particle growth and collection system developed as part of this research project also demonstrated the ability of the microchip analyzer to analyze realistic aerosol samples.
During Phase I, a “growth tube impactor” was developed to enlarge particles through water condensation in a thermally diffusive, laminar flow, with subsequent collection by impaction. The ability of this approach to deposit ambient aerosols as small as 0.02 μm into a 1 mm diameter reservoir at an air sampling rate of 1 L/minute, with a pressure drop of only 0.5 kPa (2 inches of water, or 0.5 percent of an atmosphere) was demonstrated. This approach to deposit laboratory and ambient aerosols onto the CE microchip for analysis was used successfully.
Furthermore, the ability of microchip CE to analyze aerosol particle extracts for inorganic ions was demonstrated. Aerosol Dynamics, Inc., has a clear capability to quantitatively measure sulfate and nitrate over a wide range of concentrations with a detection limit of 1 μM. A factor of 10 improvement is anticipated in this detection limit with further Phase II development efforts. Method precision also is good and likely to improve with further system optimization and quantification by peak area rather than peak height in Phase II.
Table 1 compares Phase I accomplishments with Phase I goals. As shown, the goals with respect to particle collection have been met, and the goals for microchip CE analyses have been exceeded. Based on particle collection rate, analytical detection limits, a 3-sigma signal-to-noise level, and an effective 10 μL extraction volume, the ambient detection limits for a collection period of 12 minutes are 80 ng/m3 for sulfate and 50 ng/m3 for nitrate. These limits meet the final product goal of 100 ng/m3 for a 15-minute time resolution. The current analytical precision is 7 percent. This is expected to improve with the introduction of internal standards, as outlined in proposed Phase II work. Even without significant improvement, an overall precision goal of 5 percent is reasonable.
Table 1. Comparison of Phase I objectives and achievements.
The combined technology of the growth-tube impactor with microchip CE provides major advantages over automated aerosol IC in terms of being able to analyze much smaller sample sizes, featuring lower reagent consumption, and being conducted by a smaller and much less complex/expensive instrument. Furthermore, microchip CE provides faster analysis than either IC or conventional CE. Extension to other inorganic and organic analytes is possible. The successful outcomes Phase I demonstrate Aerosol Dynamics, Inc.’s ability to develop a fully portable instrument that interfaces directly with the particle collection system during Phase II.