Quantitative analysis of cascade impactor samples – revisited

Concentrations of aerosols collected in Singapore during the three months long haze period that affected the whole South-East Asian region in 1997 are reported. Aerosol samples were continuously collected by using a fine aerosol sampler (PM2.5) and occasionally with a single orifice cascade impactor (CI) sampler. Our results show that in the fine fraction (<2.5 μm) the concentrations of two well-known biomass burning products, i.e. K and S were generally increased by a factor 2–3 compared to the non-hazy periods. However, a discrepancy was noticed, at least for elements with lower atomic number (Ti and below) between the results obtained by the fine aerosol sampler and the cascade impactor. Careful analysis by means of Nuclear Microscopy, in particular by the Scanning Transmission Ion Microscopy (STIM) technique, revealed that thicknesses of the lower CI stages exceeded thick target limits for 2 MeV protons. Detailed depth profiles of all CI stages were therefore measured using the STIM technique and concentrations corrected for absorption and proton energy loss. After correcting results for the actual sample thickness, concentrations of all major elements (S, Cl, K, Ca) agreed much better with the PM2.5 results. The importance of implementing thick target corrections in analysis of CI samples, especially those collected in the urban environments, is emphasized. Broad beam PIXE analysis approach is certainly not adequate in these cases.     

Aerosol composition and source apportionment in Santiago de Chile

Santiago de Chile, São Paulo and Mexico City are Latin American urban areas that suffer from heavy air pollution. In order to study air pollution in Santiago area, an aerosol source apportionment study was designed to measure ambient aerosol composition and size distribution for two downtown sampling sites in Santiago. The aerosol monitoring stations were operated in Gotuzo and Las Condes during July and August 1996. The study employed stacked filter units (SFU) for aerosol sampling, collecting fine mode aerosol (dp<2 μm) and coarse mode aerosol (210 mass of particles smaller than 10 μm) and black carbon concentration were also measured. Particle-Induced X-ray Emission (PIXE) was used to measure the concentration of 22 trace elements at levels below 0.5 ng m⁻³. Quantitative aerosol source apportionment was performed using Absolute Principal Factor Analysis (APFA). Very high aerosol concentrations were observed (up to 400 μg/m³ PM₁₀). The main aerosol particle sources in Santiago are resuspended soil dust and traffic emissions. Coarse particles account for 63% of PM₁₀ aerosol in Gotuzo and 53% in Las Condes. A major part of this component is resuspended soil dust. In the fine fraction, resuspended soil dust accounts for 15% of fine mass, and the aerosols associated with transportation activities account for a high 64% of the fine particle mass. Sulfate particle is an important component of the aerosol in Santiago, mainly originating from gas-to-particle conversion from SO₂. In the Gotuzo site, sulfates are the highest aerosol component, accounting for 64.5% of fine mass. Direct traffic emissions are generally mixed with resuspended soil dust. It is difficult to separate the two components, because the soil dust in downtown Santiago is contaminated with Pb, Br, Cl, and other heavy metals that are also tracers for traffic emissions. Residual oil combustion is observed, with the presence of V, S and Ni. An aerosol components from industrial emissions is also present, with the presence of several heavy metals such as Zn, Cu and others. A factor with molybdenum, arsenic, copper and sulfur was observed frequently, and it results from emissions of copper smelters.     

PIXE investigation of aerosol composition over the Zambian Copperbelt

Atmospheric sulphate aerosol concentrations are of interest in climate change studies because of their negative climate forcing potential. Quantification of their forcing strength requires the compilation of global sulphur emission inventories to determine the magnitude of regional sources. We report on measurements of the ambient aerosol concentrations in proximity to a copper refinery in the central African Copperbelt, along the border of Zambia and the Democratic Republic of the Congo. This region is historically regarded as one of the largest African sources of sulphate aerosols. Sulphate is produced by oxidation in the atmosphere of SO₂ emitted during the pyrometallurgical processing of Cu–Co sulphide ores. Since the last quantification of sulphur emissions (late 1960s), there has been large-scale reduction in copper production and more frequent use of the leaching technique with negligible sulphur emissions.

Samples were collected over four weeks, November–December 1996, at Kitwe, Zambia. A low volume two-stage time-resolving aerosol sampler (streaker) was used. Coarse and fine mode aerosols were separated at >2.5 and >10 μmad. Hourly elemental concentrations were determined by 3.2 MeV PIXE, and routinely yielded Si, S, K, Ca, Ti, Mn, Fe, Cu and Zn, above detection limits. Si, K, Ca and Fe (major crustal components) dominated the coarse elemental mass. In the fine stage, S and Si accounted for up to 80% of the measured mass, and S alone up to 60%. Time series analysis allowed the division of sulphur and crustal elements (Si, K, Ca, Fe) between (i) background concentrations representative of synoptic scale air masses; and (ii) contributions from local sources, i.e., copper smelter and re-suspended soil dust. Short duration episodes of S concentrations, up to 26 μg/m³, were found simultaneously with enhanced Cu, Fe and Zn. Contributions from individual pyrometallurgic processes and the cobalt slag dump could be distinguished from the elemental signatures. Periods of diminished sulphur concentrations were also identified, indicating a well-mixed regional air mass. These results will contribute towards validating global climate model predictions of aerosol forcing over central Africa.     

Hourly measurement of particulate concentrations with streaker samplers and optical methods

We have studied the possibility of continuous particulate monitoring, by an optical system, on the filters of a two-stage streaker sampler. The streaker deposits, collected for PIXE analysis, have been examined off-line by measuring the attenuation of a light beam. The optical analysis provides, in a few minutes, an estimate of hourly total mass concentrations, over a one week period, for both stages, very useful as a selection for further PIXE irradiation. We describe the set-up and the calibration procedures, where PIXE plays a fundamental role in resolving different aerosol compositions and in validating the prototype performance. The results obtained over 2 weeks of operation and the comparison with the aerosol component determined via PIXE analysis, are also discussed.     

Industrial aerosol characterisation at a remote site in South Africa

South Africa is the most industrialised country in southern Africa with approximately 1.1 Tg of sulphur emitted from anthropogenic activities per annum. Complex circulation patterns and highly stable vertical atmospheric conditions promote the accumulation of pollutants below 700 hPa or 3000 m asl. A remote site in the Eastern Cape of South Africa, Ben Macdhui (30.5°S 27.9°E, 3001 m) was selected for testing the hypothesis that industrial emissions, specifically sulphate, are transported over thousands of kilometres in anticyclonic type patterns of flow and exported from the subcontinent towards the Indian Ocean at about 30°S. Time resolved particulate sampling (streaker) was conducted between June 1995 and January 1997. To characterise the industrial aerosol signal, two intensive sampling campaigns (summer and winter) were undertaken in 1996. Aerosol samples were collected by a streaker sampler and an open-faced stacked filter unit (SFU). Samples were PIXE analysed to obtain elemental concentrations. The industrial signature detected at Ben Macdhui was characterised by elevated concentration of sulphur and iron in the fine fraction and fine sulphur detected in the coarse mode. Other sources identified from the elemental data were soil (Al, Si, Ca, Mg, K, S, Mn), biomass burning (fine K) and marine (Cl, Ca, Mg, S and coarse K). These four sources accounted for approximately 70% of the total detected elemental mass. Major individual contributions came from the crustal (53%) and industrial components (21%). Air parcel trajectory analyses confirmed that peak episodes of enhanced aerosol sulphur were related to transport from the industrial Highveld region of South Africa and conversely that clean air masses originated over the southern oceans.     

Trace elements in sera from patients with renal disease

In hemodialysis (HD) patients, an accumulation of trace elements such as aluminum, copper, silicon and vanadium has been reported. Aluminum-caused encephalopathy and aluminum-related bone diseases are important trace element-related complications. Using particle induced X-ray emission (PIXE) we determined concentrations of aluminum, silicon, copper, zinc, selenium and bromine in sera of 29 patients with HD, 14 nondialysis patients with renal disease (RD) and 27 normal controls. The concentration of serum silicon of the patients with HD was 107.4 ± 61.3 μmol/l, which is markedly higher than that of normal controls (48.3 ± 25.8 μmol/l, p < 0.0001). The serum concentrations of zinc and bromine in patients with HD were 11.9 ± 1.7 and 21.3 ± 3.0 μmol/l, respectively. Both were markedly lower than those of normal controls (15.6 ± 2.6, 69.2 ± 8.3 μmol/l, p < 0.0001). The concentrations of aluminium and bromine in the serum of patients with RD were 171.9 ± 64.3 and 81.9 ± 11.6 μmol/l, which were markedly higher than those of normal controls (p < 0.0001, p < 0.001). No significant differences were observed in the concentration of copper and selenium among three groups.     

Quadrupole-doublet with permanent magnets for proton beam focussing

For PIXE studies on inhomogeneous samples with medium spatial resolution of about 50 μm two quadrupole doublets with Co₅Sm pole pieces for focussing 2.0 and 2.5 MeV proton beams, respectively, have been designed and tested. Technical data and first results are presented.     

PESA as a complementary tool to PIXE at CTU Prague

Proton Elastic Scattering Analysis (PESA) is a simple convenient method for hydrogen analysis in thin samples. A Proton Induced X-ray Emission (PIXE) target chamber was equipped with a PIPS detector for detection of forward-scattered protons. One of the objectives was to perform PIXE and PESA analyses of air particulate targets simultaneously. Tests and calibrations were fulfilled mainly with Mylar foils 1.5–6.5 μm thick in the proton energy region between 1.35 and 2.3 MeV. The energy dependence of scattering cross section is different from the Rutherford formula. Comparison of PIXE/PESA analysis with the Guelph Scanning Proton Microprobe Laboratory at the University of Guelph, Canada on seven aerosol samples was carried out. The intercomparison results validated our PESA/PIXE quality assurance protocol. In addition, repeated measurements of Gelman Teflo^TM filters indicated a gradual increase of hydrogen content by 1 ng/cm² per 1 μC/cm² proton dose.     

Aerosol characterisation in Italian towns by IBA techniques

The composition of particulate matter in the atmosphere of four major Italian towns (Florence, Genoa, Milan and Naples) has been studied with the extensive application of IBA techniques. The aerosol has been collected simultaneously in the four towns during the first weeks of year 2001, by two-stage continuous streaker samplers, which provide the separation of the particulate matter in two fractions. The concentrations in air of about 20 elements, and the total particulate mass, have been extracted in the PM2.5 and PM10 fractions with hourly resolution by PIXE, PIGE and optical analyses of about 2700 samples. IBA analyses have been performed at the 3 MeV external proton beam of the INFN accelerator facility at the University of Florence.     

Use of the nuclear microprobe at the University of Arizona for the study of heavy metal deposition in rabbit renal tissue

Industrial wastes consigned to disposal sites frequently contain substantial amounts of heavy metals. We have successfully applied proton induced X-ray emission analysis (PIXE) in the conduct of heavy metal (Hg, Cd, Cr, As) toxicity studies using precision cut rabbit renal cortical slices. The large beam diameter (4000 μm) of the proton macroprobe at The University of Arizona Ion Beam Analysis facility allowed an overall concentration of the metal(s) of interest in the samples to be determined, but lacked the ability to resolve point concentrations in the tissue. The ability to locate these areas has now been made available to us with the addition of a rastering microprobe (μ-PIXE) to the facility. Studies now being conducted in our laboratory using this micro-technique include analysis of renal tissue taken from rabbits injected intraperitoneally with HgCl₂, K₂Cr₂O₇, and NaAsO₂. The small beam size (3 μm) and the ability to raster this beam over areas of up to 125 μm × 125 μm has allowed regional mapping of endogenous and non-endogenous metal concentrations and revealed trends in heavy metal deposition in in vivo treated renal tissue, significantly increasing the amount of information obtained from these animal studies using PIXE alone. The combination of small beam size, high resolution, and multi-element detection makes μ-PIXE a powerful tool for investigating the impact of non-endogenous metals on the kidney.     

Detection of non-sea salt sulphate aerosol at a remote coastal site in South Africa: A PIXE study

An aerosol sampling site based on a single-stage streaker sampler with four-hour time resolution was established in 1993 at Brandt-se-Baai, on the north-western coast of South Africa (31.5°S 18°E, 50 m asl). The site was deployed to monitor the generation of dust from the local mining operations and in part to test the hypothesis of large-scale air mass recirculation over Southern Africa. Streaker samples were analysed by PIXE for the usual crustal derived elements and sulphur. Sulphate aerosols over southern Africa are produced in the atmosphere by oxidation of SO₂, from industry, or DMS, from the ocean. Industrial related sulphate plays a key role in tracing long-range transport of anthropogenic pollutants.

We present results from selected monthly samples collected during 1996. Measured particulate sulphur has been apportioned between contributions from sea spray and non-sea salt sulphate (NSS) aerosols. Marine particulate sulphur, present in the regional background, was traced by its correlation with Na and Cl, which were routinely detected. The NSS content of these air masses was enriched with respect to the expected sea salt sulphur by two orders of magnitude. Al, Si, K, Ca, Ti and Fe, the major crustal elements, were measured only episodically with the occurrence of strong south-easterly winds. These winds are associated with wind blown dust from a mine located 4 km from the site.

These results are the first direct aerosol characterisation of the remote region of the western seaboard of southern Africa. Specifically the sulphur measurements at this site provide insights into relative contributions of natural DMS, industrial and sea salt contributions of particulate sulphate.     

Analysis of lichen thin sections by PIXE and STIM using a proton microprobe

In order to better understand the distribution pattern of mineral elements in lichen tissues, thin sections (15 μm) of the foliose, vagrant soil lichen Xanthoparmelia chlorochroa were examined using proton microprobe Particle induced X-ray emission (PIXE). This technique was used to make two-dimensional scans, with 5 μm resolution, across tissue cross sections of the test species. Element maps for Si, P, S, Cl, K, Ca, Ti, Mn, Fe, Cu, Zn, and As have been prepared. Several elements are strongly localized in the element maps. PIXE data are complimented with STIM, light micrographs, and SEM images. Preliminary data suggest that nuclear microprobe techniques may be useful in elucidating element absorption and transport mechanisms in lichens.     

Elemental composition of size-fractionated urban aerosol collected in Florence, Italy; preliminary results

An extensive investigation is in progress aiming at the characterisation of the air particulate composition in Florence. We present here the preliminary results concerning the analysis of size-fractionated aerosol samples taken by two-stage streaker samplers in two sites characterised by different urban settings (one in a heavy traffic area, one in a green area). The sampling period (21 January–22 February) includes two days during which the Municipality of Florence has banned the circulation of non-catalytic cars, due to the increase of NO₂ above the “recommended safety values”. Hourly concentrations of 20 elements from Na to Pb were determined using the external beam PIXE facility of the I.N.F.N. Van de Graaff accelerator at the Physics Department of the Florence University. Factor analysis on the data set confirms that traffic is the main source of atmospheric pollution in Florence. The ban of non-catalytic cars seems to have produced no effect on the concentration of detected elements (in particular Pb and Br).     

Suitable test structures for submicron ion beam analysis

For the precise determination of the sizes of submicron beam spots test structures with an excellent edge definition are required. For this purpose a semiconductor heterostructure consisting of an 1.62 μm GaInP epi-layer grown on (0 0 1) GaAs has been made, which provides atomically sharp edges for beam spot size measurements. Since the sample has been thinned down by standard transmission electron microscope (TEM) preparation techniques, it can be used for both PIXE and STIM. The sample has been investigated with a TEM and the ion nanoprobe LIPSION. A one-dimensional beam profile in the low current mode was determined by a STIM measurement using 2 MeV protons and yielded a FWHM of (41±4) nm, which is the smallest value reported so far for high energy nuclear micro- and nanoprobes. Furthermore we present nickel nanowhiskers produced at the GSI Darmstadt by electrochemical preparation of etched ion track membranes that have been used to obtain two-dimensional images of the shapes of submicron beam spots. For these measurements a scan over a single nickel nanowhisker having a diameter of 220 nm and a height of about 6 μm was performed.     

Size-fractionated aerosol composition during an intensive 1997 summer field campaign in northern Finland

A 12-stage small deposit area low pressure impactor (SDI) was used to collect size-fractionated aerosol samples during an intensive 1997 summer field campaign in northern Finland. The samples were analyzed for over 20 elements by PIXE, and some “difficult” elements such as As and Se could be quantified. The sea-salt and crustal elements had essentially a unimodal coarse size distribution with geometric mean aerodynamic diameter (GMAD) of about 4–5 μm. In one third of the samples, S showed only one mode in the fine size range, with GMAD of 0.4–0.5 μm. In the other samples, this fine S mode broke up into two modes, with GMADs of 0.3 and 0.6 μm, respectively. V, Zn, As, Se and Pb were mainly present in a single submicrometer mode, with GMAD of 0.4–0.5 μm for V and Se, and slightly larger (i.e., 0.6 μm) for the other three elements. The highest concentrations of S, V, Ni, Cu, Zn, As, Se and Pb were encountered in the SDI sample which had been collected in the period 14–16 July. During this sampling, the air masses came in from the west, but had recirculated over northern Scandinavia and passed over the Kola peninsula.     

The external beam microprobe facility in Florence: Set-up and performance

An external beam microprobe facility, based on a quadrupole doublet supplied by Oxford Microbeam Ltd, has been installed on a new beamline at the 3 MV single-ended Van de Graaff accelerator in Florence. The goal was to obtain a beam with a spot size on target of 10–20 μm and a current in the order of at least 1 nA, in order to allow PIXE, PIGE and RBS elemental analysis in air or in a helium atmosphere. The beam was extracted from the vacuum lines through a 0.1 μm thick Si₃N₄ window to minimise lateral straggling. The design goals have been successfully achieved; the measurements of the beam spot characteristics in vacuum as well as in air and in helium atmosphere, are here reported.     

Analysis of atmospheric aerosols by PIXE: the importance of real time and complementary measurements

Particle-Induced X-ray Emission (PIXE) has been used for more than 30 yr in many urban and background air pollution studies. The technique has certainly contributed to the understanding of source-receptor relationship for aerosol particles as well as to aerosol physics and chemistry. In the last few years, where aerosol issues were strongly linked to global climate change through the relationship between aerosol and atmospheric radiation points to new challenges in atmospheric sciences, where PIXE could play an important role. Also the recognition for the inter-relationship between aerosol and liquid and gas phases in the atmosphere makes important to integrate PIXE aerosol analysis with other complementary measurements. The use of Nephelometers and Aethalometers to measure scattering and absorption of radiation by aerosol particles can be done in parallel with particle filter collection for PIXE analysis. Parallel measurements of trace gases using traditional monitors as well as with new techniques such as Differential Optical Absorption Spectroscopy (DOAS) that can provide concentration of O₃, SO₂, NO₃, NO₂, HCHO, HNO₃, Benzene, Toluene, and Xylene, is also important for both urban and remote aerosol studies. They provide information that allows a much richer interpretation of PIXE data. Recently developed instruments that provide real time aerosol data such as the Tapered Element Oscillating Microbalance (TEOM) PM₁₀ monitor and automatic real time organic and elemental carbon analyzers provide extremely useful data to complement PIXE aerosol analysis. The concentrations of trace elements measured by PIXE comprise only 10–30% of the aerosol mass, leaving the organic aerosol characterization and measurement with an important role. The aerosol source apportionment provided by PIXE analysis can be extended with other aerosol measurements such as scattering and absorption, estimating for example, the radiative impact of each discriminated aerosol source. The aerosol bulk PIXE measurements can be complemented with soluble concentrations provided by Ion Chromatography (IC) and Inductively Coupled Plasma-Mass Spectrometry (ICP-MS). Recent developments in remote sensing techniques and products also enhance significantly regional aerosol studies. Three-dimensional air mass trajectories should be integrated in aerosol studies for urban and remote areas. The applications of these techniques to study urban aerosols from São Paulo and Santiago de Chile have broadened extensively the scientific scope of these studies.     

Determination of boron in Zr–Nb alloys by glow discharge quadrupole mass spectrometry

Direct determination of boron in Zr–2.5%Nb, Zr–1%Nb alloys and zirconium metals which are extensively used as structural materials in nuclear reactors has been carried out by glow discharge quadrupole mass spectrometer (GD-QMS). Relative sensitive factor (RSF) values for boron were determined using different solid standard reference materials (Zircaloy and steel). A comparison of the GD-QMS results obtained using these RSF values, with DC–Arc-AES (direct current arc atomic emission spectrometry)/certified values showed reasonably good agreement in all the Zr-based materials analysed for boron in the range of 0.1–7 mg kg⁻¹. Quantitation of boron in Zr matrix is possible even with a steel standard when certified for Zr and B. Internal precision (intra-sample precision) was found to be typically ±4% RSD (relative standard deviation) and the inter-sample precision was ±10% RSD for boron at 0.1 mg kg⁻¹ levels. The overall accuracy of the procedure was found to be ±8% at 0.5 mg kg⁻¹ levels of boron using Zircaloy and steel standards. Under optimised experimental conditions the detection limit for boron was found to be ±13 μg kg⁻¹.     

Magnetic force microscopy studies on the magnetic ordering in organic materials induced by high-energy proton irradiation

In this study, ferromagnetic microstructures in highly oriented pyrolytic graphite and superparamagnetic spots in polyimide foils were created by 2.25 MeV proton microbeam irradiation and characterized using atomic and magnetic force microscopy. For this purpose, graphite samples were irradiated with cross-like patterns of 15 μm × 15 μm size using ion fluences in the range of (0.003–2.5) × 10¹⁸ cm⁻². The irradiated crosses showed strong magnetic signals and a complex domain structure in the magnetic images depending on the geometrical dimensions of the crosses. Furthermore, polyimide foils were irradiated with microspots and fluences in the range of (0.016–3.1) × 10¹⁹ cm⁻². Magnetic force microscopy shows very strong phase shifts in these irradiated areas.     

On the accuracy of element concentrations and masses of micron sized samples determined with the Heidelberg proton microprobe

We have already tested the reliability of element-normalized PIXE (Proton Induced X-ray Emission) data on small (10–100 μm) particles that we routinely obtain with the Heidelberg proton microprobe. Thus, we here discuss the accuracy of quantitative results, i.e., absolute concentrations inferred from PIXE analyses of such particles. We investigated and reduced the effects of mechanical vibrations and of the instability of the electronic devices on the achievable minimum beam spot. We implemented into our computer code a mapping software for qualitative element distributions (PIXE) and quantitative mass images using STIM (Scanning Transmission Ion Microscopy). The STIM images determine the area density required to calculate from PIXE spectra the absolute element concentrations in thin samples. The accuracy of absolute PIXE concentrations is tested by measurements on 15 μm soda lime glass microspheres. Finally, the complete results of PIXE and STIM analyses of an interplanetary dust particle (IDP) are described.