Patent Application
20130321804
The present invention relates generally to methods and apparatus for the collection and analysis of aerosol particles.
An aerosol is a suspension of fine solid particles or liquid droplets in a gas. There is a need to analyze the particles of an aerosol especially where the particles of aerosol are unidentified or contain pollutants that must be monitored or limited. There are numerous approaches to provide instruments and methods for the collection and analysis of particles of an aerosol. However, each of these approaches has had certain drawbacks with respect to development of hand-portable instruments and sensors to allow real-time chemical analysis of aerosols with excellent limit of detection. As such, there is a need for improved real-time or semi-continuous methods and apparatus for the collection and analysis of particles of an aerosol. Most current technologies used for real-time chemical analysis of aerosols are not amenable to compact, hand-held instrumentation.
The present invention provides for collection of airborne particles on a tip of a electrode (approx. several micrometers to several millimeters in diameter), followed by ablation and atomic emission detection of the particulate matter on the tip by a microwave-induced plasma. The present invention provides an apparatus and a method for its use. The overall process of measurement of atomic spectra of aerosols can be divided into three steps: i) Particle collection and preconcentration; ii) Introduction of pulsed microwave-induced plasma; and iii) measuring the ensuing atomic emission and recording the signals.
In one embodiment, particles of a flow of aerosol are collected and analyzed by passing them through a housing having an inlet area, an outlet area, and a collection and analysis area. A collection electrode has a tip disposed in the flow path in the collection and analysis area. Particles are collected on the tip of the collection electrode. A microwave pulse is applied to the collection and analysis area such that a plasma is created. Atomic emissions produced during at least part of the microwave sustained plasma are collected for analysis of the ablated particles. A plurality of other embodiments are also provided.
This invention involves collection of airborne particles on a tip of an electrode (approx. several micrometers to several millimeters in diameter), followed by ablation and atomic emission detection of the particulate matter on the tip by a microwave-induced plasma. The present invention provides an apparatus and a method for its use. The overall process of measurement of atomic spectra of aerosols can be divided into three steps: i) Particle collection and preconcentration; ii) Introduction of pulsed microwave-induced plasma; and iii) measuring the ensuing atomic emission and recording the signals.
A collection electrode 20 is provided in the collection and analysis area 18. This electrode may also be called a microelectrode. Particles of the aerosol flowing along the flow path are collected on the tip of the electrode 20. This may be done in a number of ways. As shown, a high voltage power supply 22 is connected to the collection electrode 20. In one approach, the aerosol is passed through an optional aerosol charger 24 (to impart electrical charge to the aerosol particles) prior to being introduced into the collection and analysis area 18. The charger may be of any type known to those of skill in the art. The collection electrode 20 may then be held at a bias voltage relative to the charged particles of the aerosol such that charged particles collect on the tip of the electrode 20.
In another approach, a second electrode 26 is provided, which may be coaxially aligned with the collection electrode 20. The second electrode 26 may be connected to a power supply (to apply high potential) or to ground, and the collection electrode 20 may be connected to a power supply or ground. The second electrode 26 maybe provided with a corona current such that an excess of ions are produced around the second electrode. The particles of the aerosol are then charged by these ions. Again, the collection electrode 20 may be held at a bias voltage so as to attract the particles. Alternatively, the second electrode 26 may be held at a second voltage relative to the bias voltage so as to create an electrical field forcing the charged particles toward the collection electrode.
Particles may be collected on the collection electrode in any of a number of other ways (in addition to electrostatic method described above). For example, particle preconcentration on the electrode can be accomplished or assisted via an aerodynamic lens or a focused particle beam using orifices, nozzles, or capillaries. An additional particle-free sheath flow arrangement surrounding the aerosol sample flow can be used to improve particle preconcentration on the electrode 20. Any of these approaches, or combination thereof, may be used with or without an upstream charger 24.
After the particle collecting step, the present invention provides for creating a plasma and collecting the atomic emissions to analyze the particles. In accordance with the present invention, the plasma is created and/or sustained by a pulse of microwave from a microwave source.
In one approach, the microwave pulse is applied to the collection and analysis area and the presence of the electrode in the microwave cavity concentrates the electromagnetic field on its tip, subsequently leading to formation of plasma on the tip upon introduction of pulsed microwaves (as one example, f=2.45 GHz, peak power=3 kW may be used). The plasma leads to ablation, atomization, and atomic emission of the particle-bound analyte. The lifetime of the plasma can be changed by changing the duration of the microwave pulses. A plasma lifetime of the order of 5-40 ms can be obtained in this system. The longer lifetime of microwave plasma, compared to laser and spark plasmas, leads to larger atomic emission signals and better sensitivity.
The presence of the electrode in the cavity itself is sufficient to create the plasma on its tip. To further improve the reproducibility of the microwave plasma characteristics, a number of other techniques may be employed, which enhance the concentrations of electrons and ions in the vicinity of the collection electrode to allow efficient reproducible coupling of microwaves.
In a first enhancement approach, a stable, continuous ac or dc corona is introduced on the tip of the collection electrode 20 (on which the particles are collected), which provides additional seed electrons and ions for effective coupling of microwaves. This significantly improves the repeatability and stability of the pulsed microwave plasma. In one configuration, the corona is produced on the second electrode 26 (held at high potential) during the particle collecting step, but during microwave introduction, the corona is created on the tip of the collection electrode 20 by reversing the polarity. Other configurations using different combinations of polarities of high voltage and collection or other electrodes are possible.
In a second enhancement approach, a corona electrode 40 is provided for production of a corona. This corona electrode (or electrodes if multiple are provided) are located very close to the collection electrode 20 (so that a high concentration of electrons is obtained in the vicinity of the collection electrode).
In a third enhancement approach, a bias voltage (high enough to create a microwave discharge but not a corona) is applied to either the collection electrode 20 or the second electrode 26 to create a microwave discharge.
A fourth enhancement approach involves introduction of a pulsed spark or laser plasma, which leads to ablation and atomization of the particulate sample on the collection electrode, followed by immediate introduction of the pulsed microwaves, providing a tandem scheme. This scheme uses the pulsed high-voltage spark or laser plasma to ablate the particulate sample preconcentrated on the electrode, whereas the microwave plasma energy promotes and sustains atomic emissions over an extended period of time.
In a fifth enhancement approach, an alternate source of seed electrons is provided, such as UV or soft X-ray photoionization, to induce ionization of air in the vicinity of the collection electrode 20. Source 42 may represent such a source.
In a sixth enhancement approach, an additional source of ions and electrons, produced in a gas phase outside the cavity, may be brought in into the microwave cavity using a carrier gas flow to allow effective coupling of microwaves, or using small radioactive alpha and beta sources to provide ionized particles.
The next step in practicing the present invention is that the resulting atomic emissions are detected using an optical spectrometer.
Referring now to
The collection electrode is preferably formed of an electrically conducting material, such as metal, with the material chosen so as not to interfere with the spectrographic analysis of the analytes of interest and does not enhance ‘matrix effects’. Exemplary materials are tungsten or platinum. The second electrode may be similarly constructed. The bullet and the entire cavity may be metal or other conducting material, and may be grounded.
In another approach, where preconcentration on the electrodes is not needed, aerosol particles could be analyzed on-the-fly, in a microwave plasma produced between two opposing electrodes using any combination of the techniques described above. In this case, the electrodes serve only to create repeatable and robust microwave plasma between the electrodes. The other above-discussed enhancement approaches may additionally or alternatively used with this on-the-fly analysis.
The embodiments disclosed herein may be used in combination with the teachings of co-pending patent applications Ser. Nos. 13/315,344 and 13/315,372, the entire contents of which are incorporated herein by reference.
The herein disclosed embodiments may be altered in various ways without departing from the scope or teaching of the present invention. It is the following claims, including all equivalents, which define the scope of the invention.
This application claim priority to U.S. provisional patent application Ser. No. 61/652,593, filed May 29, 2012, the entire content of which is incorporated herein in its entirety.
| Number | Date | Country | |
|---|---|---|---|
| 61652593 | May 2012 | US |
