Aerosols, PIXE analyses

One of the most successful applications of PIXE has been in the analysis of air pollution particulate matter. Atmospheric particulate matter is typically collected by impaction on a filter paper, which provides an ideal thin sample for PIXE analysis. Another aspect of PIXE that is very important for the analysis of aerosol samples is the ability to analyze a large number of samples in a short time. PIXE analyses typically take less than a minute, and the entire irradiation, counting, sample changing, and analysis procedure can be automated. 

Combustion of coal produces ash that can be transported through the air. Slagging and fouling problems can also be predicted from elemental analysis. Therefore, elemental analysis of both the coal as well as the ash are important. Procedures for dissolution and analysis of coal and combustion products of coal have been reported [334-336]. Laser ablation sampling has been successfully used for coal and combusted materials [337,338]. The direct introduction of slurries has also been used [339]. Comparison of ICP-MS and PIXE analysis of coal combustion aerosols showed that analysis errors can occur in ICP-MS if particle vaporization is incomplete in the ICP [340]. 

PIXE analysis of 89 samples of the fine component of the aerosol collected at Campo Icaro (Terra Nova Bay) was used to quantify Ca, Cl, Cu, Fe, K, Mn, Ni, S, Si, Ti and Zn (81). The authors pointed out that contamination from the nearby base cannot be excluded a priori this, however, can be easily estimated by means of elemental tracers such as V. The same research group extended the observations of this type in a later study on the aerosol coarse fraction covering also Al, Br, Co and Cr (82). Elemental concentrations were given, with Co being at the lowest and K at the highest level (0.053 0.030 and 90 100 ng m , respectively). 

Kasahara M, Takahashi K, Sakisaka M, Tomita M (1993) Standard samples and calibration of PIXE analysis. Nucl Instrum Methods Phys Res B 75 136-139 Kasahara M, Ogiwara H, Yamamoto K (1996a) Soluble and insoluble components of air pollutants scavenged by rain water. Nucl Instrum Methods Phys Res B 118 400-402 Kasahara M, Park JH, Chatani S (1996b) Size distribution and solubility of 15 elements in atmospheric aerosols. Int J PIXE 6 299-310... 

Analytical features. As a typical example of the form in which experimental data appear during PIXE analysis, Fig. 33.4 presents an X-ray spectrum obtained on a sample of coarse fraction atmospheric aerosol deposited on a polycarbonate membrane filter of 7 pm thickness. The spectrum was taken with protons of 2 MeV bombarding energy in a run of 1,200 s and beam intensity of 30 nA. The physical conditions in this case correspond to the approximation of the thin uniform homogeneous sample. Full and dashed lines show curves fitted by the spectrum evaluation code to the total spectrum and to its continuous background, respectively. The general features of the spectrum can be well explained on the basis of the simplified diagrams in Fig. 33.1. 

Minimum detection limits in PIXE analysis of atmospheric aerosol samples... 

Applications The main application fields of PIXE are earth science, air pollution studies (aerosol analysis), mineralogical studies, forensic science, arts and archaeology. In the external-beam PIXE technique, the proton beam is taken out to ambient air. This mode finds application in the analysis of art objects (paintings, books, etc.). 

The fine particle airstream from the cyclone was sampled by two total filters in parallel. A Millipore Fluoropore 47 mm diameter Teflon filter with a 1 pm pore size was used for the first seven samples. Subsequent samples were obtained with a 0.4 pm pore size 47 mm Nuclepore polycarbonate filter because particle absorption measurements and elemental analysis by particle induced X-ray emission (PIXE) were easier and more accurate using the Nuclepore filters. In parallel with the Nuclepore filter, a TWOMASS tape sampler collected aerosol using a Pallflex Tissuequartz tape. The aerosol deposit area was 9.62 cm on the Nuclepore and Millipore filters and 0.317 cm on the Tissuequartz tape. The flow rate was 16-20 1pm through the Nuclepore and Millipore filters and 10 1pm through the Tissuequartz tape. Each Millipore or Nuclepore filter was placed in a labeled plastic container immediately after collected, sealed with Parafilm, enclosed in a ziplock bag, and placed in a refrigerator in the trailer. The tape in the TWOMASS sampler was advanced between samples. The tape sample was removed about once every 8-10 weeks and stored similarly to the Nuclepore filters. The TWOMASS was cleaned at that time. All samples were stored in an ice chest during the return trip to Caltech. Field blanks were handled identically to the samples. Of approximately 100 filter samples collected in 1979, 61 were selected for analysis. The 61 were chosen to span the variation in bjp and to obtain representative seasonal and diurnal samples. Sample times varied from 6 to 72 hours, with an average of 20.1 hours. 

Elemental mass distribution - The aerosol sampled by the LPI for elemental analysis was impacted on coated mylar films affixed to 25 mm glass discs. The mylar had been coated with Apiezon L vacuum grease to prevent particle bound. The LPI samples were sent to Crocker Nuclear Laboratory for elemental analysis by PIXE using a focused alpha particle beam of 3 to 4 mm diameter. Nanogram sensitivities for most elements were achieved with the focused beam. A detailed description of the PIXE focused beam technique applied to LPI samples can be found in Ouimette (13). Based upon repeated measurements of field samples, the estimated measurement error was about 15-20% or twice the minimum detection limit, whichever was larger. 

The aerosol samples collected by the SFU were analyzed both gravimetrlcally for total suspended particulate mass less than 15pm, and by particle induced x-ray emission (PIXE) for elemental content. The filters were weighed before and after sampling using a Cahn 25 electrobalance sensitive to Ipg. Typical precision of TSP determined by this analytical method is 0.5pg/m for samples collected under conditions of low aerosol concentrations ( ). After weighing, the filters were analyzed for elemental content (elements heavier than Na) using the UC Davis PIXE system. This analysis technique is described in Cahill al ( ). 

Traxel, K., and U. Watjen, Particle-Induced X-Ray Emission Analysis (PIXE) of Aerosols, in Physical and Chemical Characterization of Individual Airborne Particles (K. R. Spumy, Ed.), Chap. 16, pp. 298-330, Ellis Horwood, Chichester, 1986. 

Away from cities the natural background level of chemical species in Amazon Basin atmosphere is extremely low. Thus, Artaxo Netto et al. (1982a,b) used proton induced X-ray fluorescence analysis (PIXE) to determine that the total concentration of the natural Amazon aerosol was under 10 Pg/m , one of the lowest natural backgrounds ever recorded. This measurement was taken about 60 km north of the busy city of Manaus, confirming incidentally the prevailing east-west direction of the wind. 

A reasonably complete analysis of the inorganic chemical composition of the aerosol requires much effort and involves, in addition to wet chemical methods, instrumental techniques such as neutron activation analysis, atomic absorption spectroscopy, or proton-induced X-ray emission (PIXE). These latter techniques yield the elemental composition. They furnish no direct information on the chemical compounds involved, although auxiliary data from mineralogy, chemical equilibria, etc. usually leave little doubt about the chemical form in which the elements occur. Thus, sulfur is present predominantly as sulfate, and chlorine and bromine as Cl- and Br-, respectively, whereas sodium potassium, magnesium, and calcium show up as... 

Summarizing these examples on XRF analysis of environmental matrices the methods may be arranged in the following order WDXRF < EDXRF (radioisotope) < EDXRF < PIXE < TXRF in respect to overall possibilities. This order may be directly derived from theoretical considerations however there are some differences in the predicted application of different XRF methods to the environmental matrices. In the case of aerosols and fly ash samples all methods permit the reliable determination of about 12 elements. PIXE has the advantage of reported 20 elements. The only positive impressive exception is the determination of 34 elements in Market (1996). 

From the earliest days of PIXE, biological applications have attracted much attention. The role of essential trace elements and correlations between the concentrations of these elements and different types of diseases have been widely studied. The more recent interest in environmental problems led to an increase in the use of PIXE. Since PIXE is an almost ideal technique for the multielement analysis of aerosols filtered out of the atmosphere, many... 

Aerosol work has also profited from micro-PIXE, as single particle analysis has become feasible. Accurate correlation between the chemical composition of such particles and their size and shape provides information about the sources of the particles and about the transformations that take place during atmospheric transport. In other environmental studies, micro-PIXE has been used to measure the uptake of trace elements in plants, roots, and trees. In parti cular, tree rings, which also record time information, have been analyzed in several laboratories. 

The multielement analysis of airborne particulate material (atmospheric aerosols) has been a very popular and highly successful application of PIXE since the early days of the technique. Considering that atmospheric aerosols are often collected as a thin sample layer on some thin filter or substrate film, that... 

In the past, a combination of several analysis techniques has often been applied to environmental aerosols because one technique can normally not give all requested answers. E.g. bulk elemental analysis by XRF or particle-induced X-ray emission (PIXE) or inductively-coupled mass or emission spectrometry (ICP-MS/ICP-OES) is often combined with ion chromatography (IC) to assess the soluble ions, gas chromatography mass spectrometry (GC-MS) for some organics, aethalometry for ambient soot determinations and gravimetry for aerosol mass per m. ... 

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