Aerosol mass spectrometers allow particles to be counted on the basis of size and chemical composition. In most instruments, individual particles are ablated with a pulsed laser to obtain a mass spectrum. Using this method to characterize ambient aerosols requires an understanding of biases induced by the measurement process. For particles less than 200 nm diameter, the efficiency of detection is shown to be dependent on both size and composition. These dependencies arise from the transmission characteristics of the particle inlet and the intrinsic ability of a particle to be vaporized and ionized. The relative contributions of each are determined for a series of atmospherically relevant test aerosols. Small particles are generally more difficult to detect and analyze than large particles because they are more difficult to focus through the inlet into a tight beam and because they are more difficult to ablate. Particles composed of polycyclic aromatic hydrocarbons, ammonium nitrate, and alkali metal ions are efficiently ablated by laser ablation. Aliphatic organics are less efficiently ablated, and ammonium sulfate is very difficult to detect in a positive ion spectrum. The mass spectra of ultrafine particles show extensive fragmentation, making it difficult to distinguish aliphatic and aromatic components. However, organic particles may be classified on the basis of inorganic impurities. Initial field measurements of ambient ultrafine particles are consistent with these conclusions.

Single-particle mass spectrometers are now commonly used to analyze atmospheric particles and generate tens of thousands of spectra from typical measurement campaigns. The ART-2a spectrum algorithm has been used to classify these spectra. In this work, we generate a range of particles that are models of those that are common in the atmosphere. A single-particle mass spectrometer is used to analyze these known particles, and the spectra are classified using ART-2a. The optimum vigilance parameter is approximately 0.5 while the optimum learning rate is approximately 0.05. The classifications elucidate limitations in generation of test particles, their analysis by single-particle techniques, and their classification by ART-2a.

) states that the film pinholes in the evaporated device give rise to the growth of dark spots. However, physical lamination by soft, conformable electrodes has an advantage in that film pinholes will not make contact with the conformable electrodes. Although we did not observe the symptoms of pinholes from the current density at low voltages of our evaporated devices, pinholes could be a source for growth of dark spots in evaporated devices. The ultimate goal of carbon nanotube research is the fabrication of functional macroscopic structures that can fully utilize the individual nanotube properties. Although ordered nanotube assemblies [1,2] are required for making integrated devices, simpler macroscopic constructs that reflect the intrinsic random fiber morphology of the nanotubes offer potential applications such as media for aerosol filtration. Although gas-phase filtration properties of nanotube macrostructures have never been studied, there have been reports on nanotube pore structure [3,4] and gas permeability of nanotube membranes. [5] In this communication, we present the first investigation of the use of multiwalled carbon nanotubes (MWNTs) as highly efficient, airborne particulate filter media. Filter efficiencies in excess of 99 % were achieved from films of MWNTs deposited onto cellulose fiber filters. The MWNT-coated filters exhibited low pressure drops and better filter quality than cellulose filters even for very low MWNT coverages (0.07 mg cm ±2 ). The MWNT filter performance was comparable to the highest efficiency HEPA (high efficiency particulate air) filter standards. Filtration of airborne particulate matter is essential in many instances, including air purifiers, respiratory protection equipment, and clean rooms. Fibrous filters are the most common type of filter media used for such applications, and are usually made from cellulose, glass, or polymer fibers. Fibrous aerosol filters do not work like sieves (that allow only particles smaller than the holes to pass through), rather, the mechanisms of interception of particles by the fiber surface, inertial impaction of a particle on a fiber, and Brownian diffusion of particles in the filter pores are mainly responsible for particle retention in the filter. These three mechanisms each dominate for different particle sizes. Fiber diameter is an important parameter that affects filter performance. Typically filters have fiber diameters on the order of 10 lm, therefore filters made from nanotubes with diameters in the range of 20±50 nm should display unique properties due to their small dimensions. Figure 1 shows scanning electron microscopy (SEM) images of the MWNT-coated filter morphology. We also thank George Karles, Vicki Baliga, Randall Baren, and Prof. P. M. Ajayan (RPI) for valuable assistance and fruitful discussions relating to this work. We are grateful to NanoTechLabs Inc., for the MWNT samples.

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