Radiant energy – Ion generation – Field ionization type
Reexamination Certificate
1997-09-23
2001-07-10
Berman, Jack (Department: 2881)
Radiant energy
Ion generation
Field ionization type
C250S288000, C250S287000, C250S42300F
Reexamination Certificate
active
06259101
ABSTRACT:
FIELD OF THE INVENTION
The present invention is a method for detecting, sizing or otherwise analyzing aerosol particles wherein a beam of radiation (e.g., electromagnetic radiation) interacts with one or more particles in a beam of particles that are traveling in a colinear fashion with the beam of radiation. When the beam of radiation interacts with a particle in the beam of particles, it causes that particle to emit either mass or energy. That mass or energy is then detected by one or more detectors or sensors which provide data that permit the particle to be detected, sized or otherwise analyzed (e.g., the chemical composition of the particle can be determined). The present invention also includes instruments that use the above-described method.
BACKGROUND OF THE INVENTION
The instruments that are currently available for detecting, sizing or otherwise analyzing aerosol particles with radiation (e.g., electromagnetic radiation) use a design whereby a beam of radiation interacts with particles traveling in a beam of particles so as to cause the particles to emit mass or energy which is then detected and analyzed. The beam of radiation and the beam of particles in these instruments are perpendicular to one another. As discussed hereinafter, this design has disadvantages that are solved by the method of the present invention and the instruments that use that method.
Particles less than 1 &mgr;m in diameter comprise more than 98% of the aerosol population in the atmosphere. Current on-line chemical characterization instruments that use light scattering to detect single particles in-flight are suitable for field studies but cannot detect ultrafine aerosols (i.e., particles with a diameter of less than about 200 nm). An example of this type of instrument is described in U.S. Pat. No. 4,383,171, which issued on May 10, 1983, to Sinha et al.
A solution to this problem was reported by W. D. Reents, Jr., et al. in an article entitled, “Single Particle Characterization by Time-of-Flight Mass Spectrometry”, Aerosol Science and Technology 23: 263-270 (1995). The entire disclosure of this article is expressly incorporated herein by reference. In the Reents device, a focused pulsed excimer laser is used to ablate and ionize individual particles without the use of light scattering to sense the presence of a particle, followed by time-of-flight mass spectrometric analysis. By removing the light scattering element of the instrument, Reents was able to detect single particles as small as 0.02 &mgr;m or 20 nm in diameter. However, the fraction of particles that were detected in the particle beam was unsatisfactory.
SUMMARY OF THE INVENTION
The present invention is a method for detecting, sizing or otherwise analyzing aerosol particles wherein a beam of radiation (e.g., electromagnetic radiation) interacts with one or more particles in a beam of particles that are traveling in a colinear fashion with the beam of radiation. When the beam of radiation interacts with a particle in the beam of particles, it causes that particle to emit either mass or energy. That mass or energy is then detected by one or more detectors or sensors which provide data that permit the particle to be detected, sized or otherwise analyzed (e.g., the chemical composition of the particle can be determined). The present invention also includes instruments that use the above-described method. For example, the present invention includes an instrument for the on-line chemical analysis of aerosol particles, especially submicron size aerosol particles. The instrument includes a time-of-flight mass spectrometer wherein an ionization laser beam, which is aligned in a colinear fashion with the beam of particles being analyzed, is free-fired at a high repetition rate to ablate some of the particles and form ions which then travel down a flight tube and contact a detector. The spectra that are produced when ions from the ablated particles contact the detector are recorded and analyzed to determine the identity of the particles that were ablated by the laser.
The instruments of the present invention are designed so that a beam of radiation (e.g., electromagnetic radiation such as a laser beam) interacts with one or more particles in a beam of particles that are traveling in a colinear fashion with the beam of radiation. This design increases the size of the source region, which is the region where the beam of electromagnetic radiation interacts with one or more particles in the beam of particles with sufficient energy to cause the one or more particles to emit mass or energy that can be detected by one or more sensors or detectors. By increasing the size of the source region, the instruments of the present invention are much more likely to have a successful interaction between the beam of radiation and one or more particles (i.e., an interaction that causes the one or more particles to emit mass or energy that can be detected). This in turn means that the instruments of the present invention are much more likely to be able to detect the presence of particles in a sample of gas that contains particles. Further, since the increase in the size of the source region means that the instruments of the present invention will have a greater number of successful interactions between the radiation beam and particles in the particle beam than an equivalent instrument where the radiation beam and particle beam are perpendicular to one another, the instruments of the present invention will obtain more data (i.e., from the detectors or sensors) in a given period of time from a sample of gas containing particles. This results in faster and better detection, sizing or chemical analysis of the particles.
The above-described advantages of using a design where the beam of radiation and the beam of particles are aligned in a colinear fashion is useful in most devices or instruments that use a beam of radiation which interacts with particles in a beam of particles to detect, size or otherwise analyze the particles in the beam. For example, instruments that use atomic emission (e.g., laser induced breakdown spectroscopy), phosphorescence or fluorescence to detect, size or analyze (chemically or otherwise) aerosol particles will be improved by using the method of the present invention wherein the beam of radiation is aligned in a colinear fashion with the beam of particles. In particular, mass spectrometers are improved by using the method of the present invention. In the following paragraphs, a brief summary of the preferred embodiment of the present invention (i.e., a laser mass spectrometer) is provided.
To overcome the limitations of the prior art devices, the time-of-flight mass spectrometer of the present invention was constructed for on-line analysis of aerosols including ultrafine aerosols down to 10 nm and possibly smaller. This instrument, which is field transportable, eliminates the collection and mounting steps normally associated with microprobe analytical methods and is not subject to the particle size limitation inherent with other on-line single particle techniques that use light scattering for particle detection. Instead, the ablation laser is free-fired at a high repetition rate and spectra are saved only when a particle is ablated and ions are formed. The main limitation to free running the laser is a low duty cycle in acquiring spectra. To increase the probability of ablating a particle, the ionization laser beam is aligned colinear with the particle beam giving a much larger ionization region than the currently available instruments which have the ionization laser oriented perpendicular to the particle beam. Modeling of ion trajectories in the mass spectrometer shows that ions produced across the ionization region are efficiently transmitted to a microchannel plate detector.
In the mass spectrometer of the present invention, particles enter the source region of the mass spectrometer through an inlet which confines the particles to a narrow beam. The narrow beam of particles then intersects an excimer laser beam. When a particle is ablate
Carson Peter
Johnston Murray V.
Mallina Ramakrishna
Wexler Anthony S.
Berman Jack
Connolly Bove Lodge and Hutz LLP
University of Delaware
Wells Nikita
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