Radiant energy – Ionic separation or analysis – With sample supply means
Reexamination Certificate
2001-08-21
2004-06-15
Lee, John R. (Department: 2881)
Radiant energy
Ionic separation or analysis
With sample supply means
C250S281000, C250S287000
Reexamination Certificate
active
06750449
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to apparatus and methods for analyzing and characterizing airborne particulate matter and more particularly to such apparatus and methods operating in real time.
This invention was developed with the use of funding provided in part by the combination of funding from Westinghouse Savannah River Technology Center pursuant to contract no. 3-30-1905-xxxx-32-4716 and the National Science Foundation pursuant to grant DMR-9727667.
The development of analytical methods for the characterization of airborne particulate matter has become an area of increasing activity over the last 15 years. The driving forces for these investigations lie across many fields of application, including environmental health and safety, atmospheric sciences, clean room quality control, and battlefield
on-proliferation monitoring.
Perhaps the greatest impetus for the development of new apparatus and methods of particle characterization has been the evolution of new air quality standards currently underway in the United States. Specifically, the U.S. Environmental Protection Agency (EPA) has proposed the PM2.5 standard for airborne particulate matter (hereafter “the PM2.5 standard”). The PM2.5 standard limits the density of particles of less than 2.5 micrometers in diameter to a value of 15 &mgr;g/m
3
on an annual basis. The PM2.5 standard does not include any parameters that refer to the chemical composition of airborne particles, only their size distribution/density (2).
1
The majority of the fields of application mentioned above rely on both particle classification based on size distribution as well as chemical composition (i.e., higher levels of information are required). As the pathogenic effects of particle composition become more apparent (which may in fact be driven through the development of improved analytical methods), it is easy to envision that more comprehensive regulations will evolve.
The numbers within the parentheses refer to the numbered endnotes at the end of the application.
The characterization of airborne particulate matter is usually classified according to the sample system from which the chemical information is desired; batch or single particle. Given the wide range of possible sample types and information requirements, it is clear that no single method will be applicable in all cases. In fact, the classification of batch or single particle fields of application are an effective way to address capabilities.
In a batch sample system, particle size distributions and gross composition information are usually the intended goals. This type of analysis is most often applied in process monitoring situations where particle size distribution is often the most relevant piece of information.
In the case of single particle analysis, particle size and composition (elemental or molecular) are determined in order to assess the “chemistry” of a given system. These evaluations seek to define size/composition relationships, determine the distribution of species within/on a single or individual particle, and study chemical reactions at a particle's surface. Beyond providing basic size/composition information, the requirements placed upon analytical instruments used in particle characterization include aspects of sample size, sample preparation/processing, analytical time frame, portability/remote monitoring capabilities, and instrument cost and complexity.
The most significant advances over the last decade in the analysis of airborne particulate matter have occurred in the area of single particle analysis (3-16). Some of these analytical methods involve collection of particles on inert filter supports for subsequent analysis by microbeam techniques. Others involve direct, real time sampling/analysis of individual particles by instruments which may be taken out into the field.
Van Grieken et al. Analyst, volume 120, pages 681-692 (1995) have reviewed the application of charged particle microbeam methods (e.g., EPMA, PIXE, SIMS) for the characterization of individual, collected particles. While these methods are quite powerful, particularly when used in tandem, the acts of particle collection, transport, and analyses in high vacuum environments present a number of drawbacks including poor temporal resolution, questions of representation, and possible loss of volatile analytes.
Real time single particle analysis methods often involve laser-induced vaporization/excitation/ionization and atomic emission or mass spectrometric detection. In the realm of elemental analysis, laser induced breakdown spectroscopy (LIBS) (4, 5) and laser excited atomic fluorescence (LEAF) (6) following thermal dissociation provide individual particle information. Hahn (5) used the 1064 nm fundamental output of a Nd:YAG laser to vaporize desolvated particles produced with a conventional solution nebulizer. Use of 1 &mgr;m diameter Fe-doped polymer beads permitted the establishment of emission intensity/particle size relationships. Implicit in any quantification scheme of this sort is the summation of the responses of all species present in each particle. This of course requires a priori knowledge of the sample composition.
Panne and co-workers (6) diverted one-half of the flow from a nebulizer/desolvation system through a differential mobility particle sizer (DMPS) and the other half through the path of a vaporization/excitation laser. Temporal resolution of Pb atomic fluorescence from the background plasma emission permitted very sensitive detection (Pb
LOD
=47 ng/m
3
). Low analytical duty cycles (limited by laser repetition rate) and single-element operation of the system were acknowledged as limitations, though the high level of selectivity and possibility for miniaturization were seen as positive features.
Direct (vacuum) inlets are versatile means of introducing ambient or collected particles. In the majority of such systems, differentially pumped momentum separators (often called particle beam interfaces) provide the means for performing analysis by methods requiring vacuum environments (i.e., mass spectrometry) and optionally identification/analysis of single particles in real-time (7). The research groups of Prather (8-11), Johnston (12-14), and Ramsey (15-16) have each made unique contributions to the field.
Prather and co-workers (9-11) have described the use of aerodynamic particle sizing using a dual-laser triggering system followed by laser vaporization/ionization and time-of-flight mass spectrometry (TOF-MS). One possible implementation of this time-of-flight mass spectrometry device is described in U.S. Pat. No. 5,681,752 to Prather. The production of both positive and negative ion species has been used to advantage in gaining comprehensive information from single aerosol particles.
Johnston (12-14) has used the intensity of scattered laser radiation as a measure of particle size and as a trigger for subsequent laser vaporization/ionization and TOF-MS analysis. One possible implementation of this time-of-flight mass spectrometry device is described in U.S. Pat. No. 5,565,677 to Wexler et al.
Ramsey and co-workers (15, 16) have exploited the ability of quadrupole ion traps to operate in modes which either trap charged particles or perform mass analysis of laser-produced ions. In the former mode of operation, charged particles can be effectively levitated within the three dimensional trap. Use of the ion trap as a mass analyzer provides higher levels of chemical information than TOF-MS as collision-induced dissociation (MS/MS) of isolated ions can be performed. Similarly, Davis and co-workers have used electrodynamic traps as a means of isolating charged particles at atmospheric pressure for interrogation by Raman spectroscopy (17).
Chemical analysis of batch-type (not single particle) particulate samples most often involves collection of samples via directed flow through a quartz fiber filter having pore sizes on the order of 1 &mgr;m (18-23). Optical scattering or differential mobility particle sizing can be accomplished prior to deposition on
Clemson University
Dority & Manning P.A.
Lee John R.
Smith, II Johnnie L.
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