Measurement methods particle composition

Instruments and methods that can measure single-particle composition in real time down to sizes of fresh nuclei ( 1 nm). 

Among the multivariate statistical techniques that have been used as source-receptor models, factor analysis is the most widely employed. The basic objective of factor analysis is to allow the variation within a set of data to determine the number of independent causalities, i.e. sources of particles. It also permits the combination of the measured variables into new axes for the system that can be related to specific particle sources. The principles of factor analysis are reviewed and the principal components method is illustrated by the reanalysis of aerosol composition results from Charleston, West Virginia. An alternative approach to factor analysis. Target Transformation Factor Analysis, is introduced and its application to a subset of particle composition data from the Regional Air Pollution Study (RAPS) of St. Louis, Missouri is presented. 

Because of the uncertainties In the use of source-emissions Inventories to estimate contributions from various sources to ambient levels of suspended particles, many workers have been developing and testing aerosol receptor models (1 ). The basic Idea of receptor models Is that chemical compositions of particles from various types of sources are sufficiently different that one can determine contributions from the sources by making detailed measurements of the compositions of ambient aerosols and of particles from the sources. Several computational methods have been used... 

Emission of electrons from the particle surface has also been used in laboratory studies to probe surface composition. Electron emission has been induced by UV irradiation, for example, by Burtscher and Schmidt-Ott (1986) to probe perylene on the surface of carbon particles. In a series of laboratory studies, Zie-mann et al. (1995, 1997, 1998) have demonstrated the potential utility of secondary electron yield measurements as a technique for probing particle surface composition. In this method, particles are bombarded with... 

Just as DENs particle sizes have some distribution (albeit relatively narrow), there is surely some distribution in particle compositions for bimetallic DENs. This is a fundamentally important aspect of DENs, particularly with regard to their catalytic properties however, there are presently no reliable characterization methods for evaluating particle composition distributions. One method that has been applied to PdAu [21] and PtPd [19] DENs, as well as dendrimer-templated PtAu [24] is to collect single particle EDS spectra from several (15-20) nanoparticles. These experiments indicate that individual particle composition distributions may vary widely, but the difficulty in obtaining data from the smallest particles may skew the results somewhat. EDS spectra collected over large areas, which sample tens or hundreds of particles, generally agree well with the bulk composition measurements [24] and with stoichiometries set in nanoparticle synthesis [19,21,24]. 

The method selected depends upon the kind of material to be measured. If particles are confined to narrow limits of size, screens or microscopic methods of direct measurement may be used. When particles are distributed over a wide range of sizes we must choose indirect methods such as sedimentation or centrifuging. There is no simple method of measurement in either case, and the results are not always susceptible of interpretation unless the composition of the material is known. This will be even more evident when we consider sedimentation methods applied to particles varying widely not only in size but also in density. 

Conventionally, particles are collected on a filter substrate, where they can be probed by a variety of methods. Table 5 lists the analytical measurement capabilities of the four most commonly used techniques for the analysis of particle composition, and Fig. 2 shows an idealized approach... 

Like cements, the elemental composition is determined by XRF or AAS techniques. The XRF bead is made using lithium tetraborate at 1050°C. Sulfide content cannot be determined by XRF. Sulfite, SO3 , and sulfate, S04 , are safely analyzed by XRF. Na2C03 -I- K2CO3 fusion is carried out for Ca, Mg, Fe, and A1 analysis by AAS. Lanthanum chloride is used as a sulfate interference suppressant. Gravimetric sulfate determinations are also carried out by precipitation as barium sulfate. The Leco Carbon-Sulfur Analyzer can also be used for quality control purposes. The fluoride is determined by XRF or a pyrohydrolysis method. The measurement of particle size distribution is carried out in a manner similar to that for cements and clays. 

The XRF method is widely used to measure the elemental composition of materials. Since this method is fast and non-destructive to the sample, it is the method of choice for field applications and industrial production for control of materials. Depending on the applieation, XRF ean be produced by using not only x-rays but also other primary excitation somces like alpha particles, protons or high energy electron beams. 

Therefore, several active methods were developed for locating uranium-containing particles. Scanning electron microscopy (SEM) is one of the most powerful techniques used for locating uranium-containing particles. The sample is placed on a stub that is inserted into the vacuum chamber of the SEM and is scanned by an electron beam. Typical peaks are obtained whenever uranium-bearing particles are encountered so that they can be mapped and their coordinates recorded. Characterization of elemental composition can also be carried out by SEM, but for isotopic measurements the particles must be transferred to a suitable device (usually TIMS or ICPMS). 

The great diversity of application, the size range of atmospheric aerosol particles, the physical and chemical concentration variations, and the variety of measurement principles available imply many different combinations of application and measurement methods and procedures. Therefore, this chapter is focused on the most important methods in use. The methods applied for atmospheric aerosol sampling include filters and cascade impactors which collect the aerosol particles onto a surface. The collected sample must therefore be evaluated for size and composition. Because accumulation mode aerosols (fine aerosol particles) contain a substantial fraction of liquid material at normal temperatures and humidities, these fine aerosol particles must be sized in situ without precipitation. In some extreme cases, such as in Los Angeles smog, the liquid content may be as high as 75% or 80% of the total mass (Ho et al., 1974). 

Bertram and Thornton recently introduced a method for performing direct aerosol flow tube uptake studies on ambient aerosol particles [148, 149]. They are able to correlate observed N2O5 reactive uptake coefficients with simultaneous measurements of aerosol composition using other techniques. When the flow tube technique of Bertram and Thornton is coupled with a technique capable of detecting the presence of surface-active organics in the aerosol, potential exists for the influence of these organics on N2O5 uptake by ambient aerosol to be inferred. 

Source sampling of particulates requites isokinetic removal of a composite sample from the stack or vent effluent to determine representative emission rates. Samples are coUected either extractively or using an in-stack filter EPA Method 5 is representative of extractive sampling, EPA Method 17 of in-stack filtration. Other means of source sampling have been used, but they have been largely supplanted by EPA methods. Continuous in-stack monitors of opacity utilize attenuation of radiation across the effluent. Opacity measurements are affected by the particle size, shape, size distribution, refractive index, and the wavelength of the radiation (25,26). 

True Density or Specific Gravity. The average mass per unit volume of the individual particles is called the tme density or specific gravity. This property is most important when volume or mass of the filled composition is a key performance variable. The tme density of fillers composed of relatively large, nonporous, spherical particles is usually determined by a simple Hquid displacement method. Finely divided, porous, or irregular fillers should be measured using a gas pycnometer to assure that all pores, cracks, and crevices are penetrated. 

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