Sampling and characterization of individual particles in occupational health studies† Hugo M. Ortner Department of Chemical Analytics, Faculty of Materials Science, Technical University of Darmstadt, Petersenstr. 23, D-64287 Darmstadt, Germany Received 25th March 1999, Accepted 3rd June 1999 1 Aim of investigation In practice, this is much too time-consuming, especially in today’s world of overrationalization.This is the reason why, 2 Experimental procedures 2.1 Sampling in this review, it is intended to provide an overview on the 2.2 HR-SEM possibilities and limitations of the most essential topochemical 2.3 EPMA and topostructural methods for single particle characterization. 2.4 TEM An overview must embrace the most important information 2.5 TXRF about a specific method. To take an example of everyday life: 3 Results and discussion it is not necessary that everybody understands how a TV set 3.1 Sampling operates, but everyone should know what it can be used for 3.1.1 Sampling for TXRF, HR-SEM and EPMA and how it is operated.The latter responsibility is, of course, investigations the domain of the respective specialist for the complex instru- 3.1.2 Sampling for TEM work on polycarbonate filters mentation involved in topochemical and topostructural backed up by a metal grid analysis. 3.2 Characterization Results on investigations of soot particles should 3.2.1 HR-SEM demonstrate the relevant possibilities.18–21 Soot particles were 3.2.2 SEM studies of soot and carbonaceous particles chosen because they exhibit a great morphological variety in 3.2.3 Environmental SEM (ESEM) the micrometre as well as the nanometre domain.Special 3.2.4 Semiquantitative single particle analysis by EDX attention will be given to a critical comparison of the various and WDX methodological possibilities. This seems important because, in 3.2.5 Nanometre particle characterization by HR-SEM spite of a multitude of excellent monographs on the various and TEM individual methods, a critical comparison of these techniques, 3.2.6 EFTEM for element mapping and speciation with especially with respect to particle characterization, is not nanometre resolution readily available. A much broader overview on single particle 3.2.7 Some examples for HR-TEM of particles characterization with many topochemical methods was given 4 Conclusion recently.6 5 Acknowledgements The most important and widely used method for 6 List of acronyms used morphological and compositional particle characterization is 7 References scanning electron microscopy (SEM) in combination with X-ray spectrometry (XRS).11 Normally, XRS is performed in SEM by energy dispersive X-ray spectrometry (EDX).Due 1 Aim of investigation to some inherent disadvantages of the latter, wavelength The characterization of individual aerosol particles, their size dispersive X-ray spectrometry (WDX) using crystal specdistribution, chemical composition and inner structure is of trometers is applied in addition to EDX in the costly electron great relevance to atmospheric sciences as well as occupational microprobes (EMP).The method is then called electron probe health monitoring.1–6 Single particle characterization is only microanalysis (EPMA), although there is no basic diVerence feasible with modern topochemical methods of analysis.6–10 between SEM-EDX and EPMA.14 The latter instrumentation The latter, however, are not always available in laboratories is, however, optimized for quantitative microanalysis, whereas dedicated to occupational health monitoring.In materials SEM’s is usually optimized with respect to morphological science, on the other hand, the topochemical and topostructu- investigations. If the latter is extended into the single nanomeral characterization of solid materials is a key issue.9–12 Single tre range, high resolution (HR)-SEMs are used with so-called particle characterization, therefore, can be considered a typical field emission guns for bright and highly focused electron interdisciplinary area of current research.In such a wide area, beams ensuring the highest possible lateral resolution in imagan optimal cooperation of allrounders and specialists seems ing.14 The advantages and disadvantages of WDX and EDX especially important for the quality and success of the respect- analysis will be discussed with respect to particle characterizive research work.13 For the mutual understanding of the ation in some detail, since this information is rarely available various cooperating disciplines, a basic understanding of the elsewhere.Unfortunately, quantitative particle analysis by diverse scientific areas is of paramount importance.It is, of SEM-EDX or EPMA-WDX is not easy for various reasons.22 course, principally possible to study relevant voluminous However, with appropriate correction procedures, semiquanmonographs for the various methods to extract the necessary titative analysis can be performed semiautomatically for a information for a basic understanding of the various fields.14–17 multitude of particles collected preferably on glassy carbon discs.This is of paramount significance for the toxicity evaluation of particles, because it will be shown that particles †Presented at AIRMON ’99, Geilo, Norway, February 10–14, 1999. J. Environ. Monit., 1999, 1, 273–283 273collected at diVerent industrial workplaces exhibit a very 2.3 EPMA complex composition contrary to toxicity studies with single A Cameca (Paris, France) CAMEBAX SX 50 electron compounds.23,24 Since every analytical method has its limimicroprobe was used.The instrument was equipped with an tations, HR-SEM and transmission electron microscopy energy dispersive Si(Li) detector (Princeton Gamma-Tech, (TEM) will be comparatively discussed in order to demon- Princeton, NJ, USA) and with four WDX spectrometers, three strate the importance of TEM investigations, namely for the vertically and one horizontally mounted (for the evaluation of study of the crystal structure of nanometre domains of particles rough surfaces).Experimental details of our EPMA work can or of the elemental composition of nanodomains by energy be found elsewhere.22 filtering TEM (EFTEM), a method which has been introduced only recently in TEM work.25,26 By an appropriate combi- 2.4 TEM nation of techniques, it is possible today to study the structure as well as the composition of particles from the atomic level Two TEM instruments were used.A Philips CM 12 (120 kV) over nanometre domains to micrometre features. This will be equipped with an EDX system from EDAX with an ultrathin mainly demonstrated by investigations on soot particles. window for the detection of elements with Z>5 and with a Since sampling is an essential part of every analytical GATAN (Pleasanton, CA, USA) parallel electron energy loss procedure, relevant sampling techniques for the methods spectrometry (EELS) system.Liquid nitrogen cooling of the described here will be outlined.It is evident that sampling sample holder is possible. A Philips CM 20 UT (200 kV) with techniques have to be optimized for the subsequently applied an ultratwin lens and EDX system from Noran (Middleton, method of characterization.27 WI, USA), with a Ge detector and an ultrathin window for the detection of elements with Z5, was also used. 2 Experimental procedures 2.5 TXRF 2.1 Sampling Two instruments were used for TXRF measurements: for elements with atomic number greater than 15, a Seifert A self-designed one-stage cascade impactor made totally of (Ahrensburg, Germany) instrument with Mo target X-ray tube PTFE is used in combination with an Edwards (Newcastle, (50 kV, 30 mA) with an Si(Li) detector (Kevex, Mainz-Finten, UK) high vacuum pump EDM 6 (700W). This impactor Germany) with Be window and a Tracor (Bruchsal, Germany) allows a high sampling rate of 1 mP h-1.Nevertheless, sam- multichannel analyser; for elements with atomic numbers from pling times of 10 h are usually necessary for clean room control 8 to 23, a laboratory-constructed TXRF spectrometer with Cr in order to ensure a high detection sensitivity (on the order of target X-ray tube (30 kV, 25 mA) and with an Si(Li) Quantum pg m-3) in bulk concentration measurements by total reflec- detector (with diamond window) from Kevex (Germany), a tion X-ray fluorescence (TXRF).A cut-oV size of 0.1 mm was Spectrace 6100 multichannel analyser (Mountain View, CA, chosen in order to obtain a wide particle size spectrum to be USA) and a vacuum sample chamber.deposited and to meet the lower dimensional limit of particle diameters in the microelectronics industry.28 3 Results and discussion A five-stage cascade impactor was also constructed totally from PTFE. Isokinetic particle collection at a flow rate of 3.1 Sampling 2 l min-1 is performed with the following five stages: >25, Sampling was optimized for the diVerent modes of particle 25–6.8, 6.8–1.8, 1.8–0.4 and 0.4–0.1 mm in equivalent projected characterization and will be discussed accordingly in two area diameter.20 sections.For both impactors, the particles are collected on highly polished glassy carbon discs of high purity (Fa. Hoch- 3.1.1 Sampling for TXRF, HR-SEM and EPMA investi- temperaturwerkstoVe, Maikingen, Germany) of 3 cm in diamgations.Sampling for these investigations is preferably carried eter and 3 mm in thickness. These discs can be reused after out by one-stage or multistage impaction using highly polished being washed with dilute, ultrapure nitric acid (1510) and glassy carbon discs of high purity as impaction plates. One- subsequently three times with high purity water.They can also stage impaction is used if extremely low particle densities have be repolished after frequent usage.21 to be monitored as is the case, for example, for clean room For particle collection on Nucleopore filters, personal inhalcontrol. 28 In most other cases with suYciently high particle able dust collectors with small personal air pumps were used densities a self-designed five-stage cascade impactor is used.20 by our partners.23,24 A pore size of 0.1 mm of the Nucleopore Both impactors are constructed totally from PTFE to ensure filters was preferred in order to collect the whole particle size contamination free and isokinetic particle collection.The range of interest of inhalable, thoracic and respirable particle glassy carbon exhibits suYcient electrical conductivity so that fractions.27 conductive particle coating is generally not necessary even for Particle collection for TEM investigations was performed insulating particles (e.g.SiO2 particles). Only for large particles on thin Formvar foils backed up by Cu grids.20 (generally greater than 5 mm in diameter) is carbon coating applied if massive charging is observed.The selected glassy 2.2 HR-SEM carbon material does not exhibit any interfering elemental background signals in HR-SEM and EPMA work. Only in A Philips (Eindhoven, Netherlands) XL 30-FEG scanning electron microscope equipped with a Schottky emitter gun was TXRF, due to its inherently much superior detection limits, do weak background signals for iron, zinc and calcium have used in combination with an EDX system (EDAXTM DX4 Mahwah, NJ, USA) comprising an intrinsic germanium detec- to be taken into account.21 Of course, pure glassy carbon does not generate any further interfering X-ray lines except C Ka.tor, resolution 137 eV at 5.89 keV, with an ultrathin window applicable to the detection of elements with Z5. Generally, Its oxygen surface content is minimal and does not interfere with the semiquantitative oxygen determination of particles.particles collected on glassy carbon discs do not have to be coated prior to SEM inspection. This is only necessary for the Of course, semiquantitative carbon analysis is only possible for rather large particles for which the signal contribution of >25 mm stage and infrequently for the 25–6.8 mm stage and for large insulating particles.In such cases, carbon coating is the substrate is negligible. A further advantage of glassy carbon discs is their easy cleaning. They can be used many preferred over gold coating. 274 J. Environ. Monit., 1999, 1, 273–283times and can be repolished to restore the extremely smooth described cascade impactors. For this purpose, a Formvar foil is placed on the glassy carbon disc.Usually, slight wetting of and flat surface which is especially important for TXRF work. For HR-SEM work, it is advantageous that this surface does the foil is suYcient to adhere it to the glassy carbon disc for particle collection. Very fine particles with diameters below not contribute any morphological details which could interfere with the nanostructural details of the collected particles.For 100 nm cannot be collected by impaction. In order to collect such particles, a porous Formvar foil is placed on a ceramic low particle densities, the rather limited area of particle deposition (#5 mm2) is advantageous over filter sampling. filter support and air is sucked through with a small pump. In some cases for industrial aerosols, mere exposure of the Especially in occupational health control, sampling with impactors is not feasible at present with suYciently small Formvar foil to the relevant atmosphere for a few minutes is suYcient.For EELS investigations, porous Formvar foils have devices for personal sample collection. Hence, particle collection without any size fractionation is standard on porous filter to be used in order to keep the sample thickness to a minimum.Small particles adhering to the fringes of the pores of the filter materials in combination with suction of air through the filter. According to our experience, only Nucleopore filters exhibit a can thus be inspected without an underlying substrate. smooth enough surface to be used for subsequent SEM work. 3.2 Characterization However, the filters must be coated with at least 30 nm of Pt or Au in order to ensure suYcient electrical conductivity and A broad discussion and scheme of our multimethod approach to suppress the massive background for carbon and oxygen. of single particle characterization have been given previously.6 The filter material is polycarbonate. A pore size of 0.1 mm is Here, some examples from recent work will be presented and generally preferred by us thus ensuring particle collection important relevant aspects will be discussed, related to the down to 100 nm in diameter. combined use of the electron probe methods HR-SEM, EPMA There are, nevertheless, decisive disadvantages as compared and TEM.to collection by impactors for SEM work.(i) The background morphology of the filter pores. This 3.2.1 HR-SEM. The start of our investigation is always a morphology is taken care of with special software for particle sample survey by SEM in combination with EDX analysis, identification by WDX analysis.22 including light element analysis down to boron. (ii) The filter material exhibits only a limited resistance Morphologically interesting particles may be studied in detail against electron bombardment which eventually leads to cracks and under magnifications of up to ×150 000.Micrographs in the filter which raise the carbon and oxygen background. taken at magnifications of ×100 000 or more are considered (iii) Frequently, an uneven coating thickness of the sputtered to be high resolution scanning electron micrographs because metal layer leads to further background variations for C and they embrace an area of only 1 mm×1 mm.Qualitative X-ray O in WDX and EDX analysis. Consequently, semiquantitative analysis can be carried out for particles with diameters down XRS on filters is inferior to that on glassy carbon. to 50 nm. In principle, it should also be possible to carry out (iv) Problems arise in the evaluation of particle size a semiquantitative analysis of small particles in the range from distribution measurements irrespective of the applied method. 100 nm in diameter upwards at least for elements above Even in visual inspection (i.e. adding human intelligence to sodium. The problems connected with the evaluation of light otherwise automatic procedures), it is not possible to dis- elements (second period elements of the Periodic Table) by tinguish airborne particle agglomerates from those forming in EDX will be discussed later.EDX analysis in the HR-SEM and around the pores. essentially extends the range of particles to be analysed quali- Fig. 1 shows a good example of the aggregation of various tatively from 0.2 mm in diameter, which is the limit of WDX particles in a large pore of an 8 mm Nucleopore filter, which analysis in the electron microprobe, down to 50 nm in diameter.clearly demonstrates the problem with particle collection by filter methods. Still more problems in single particle charac- 3.2.2 SEM studies of soot and carbonaceous particles. Soot terization are encountered by the use of filter materials with a particles are amongst the most common particles of open air rough surface, such as glass fibre or other fibrous filter as well as industrial indoor aerosols. Soot is often also called materials.black carbon (BC), which is the most polymerized and refractory fraction of combustion aerosols, as opposed to organic 3.1.2 Sampling for TEM work on polycarbonate filters carbon (OC) or carbonaceous particles.29 Soot and carbonbacked up by a metal grid.Samples for TEM investigations aceous particles exhibit a great morphological variety and are usually collected on the last (fifth) stage of the above often a very intricate micro- and nanostructure,30 which is best studied by HR-SEM. The formation of the respective aerosols may be explained:30,31 (i) by the dismutation of carbon monoxide through the reaction: 2COPC+CO2 (ii) by polymerization (dehydrogenation of the fuel, denoted as RiH) under the oxidative action of the hydroxyl radical or any oxidant: RiH+OH*ARi*+H2O 2Ri*+OH*ARiO*+Rh* CO+OH*PH*+CO2 H2+OH*AH*+H2O Rh+RiHARh+i (polymerization) There are other routes of formation of carbonaceous aerosols which are, however, not considered here.The first Fig. 1 Scanning electron micrograph of an 8 mm pore of a Nucleopore formulated reaction is a typical vapour phase condensation filter which is completely clogged with particles, collected in the Soweto area (South Africa).19 reaction which occurs with many technological processes and J. Environ. Monit., 1999, 1, 273–283 275seemingly produces similar morphologies of the condensation product as shown below.Fresh combustion-derived particles consist of a carbonaceous matrix with adsorbed or included trace elements such as S, N, K, Fe, etc. Only fossil fuel derived aerosols do not contain any detectable amounts of K, because this type of fuel is very impoverished in K by comparison with vegetation.29 In a morphological study of highly structured particles, it might be helpful to remember that, in a solid state object, several structural categories are superimposed one over the other.11 (i) The first structural level is always the atomic structure in the subnanometre range which ideally would be the crystal structure of graphite for soot particles.It has been shown that this structure indeed makes up the backbone of a soot particle, but the crystal lattice exhibits numerous imperfections,29,32 Fig. 3 Scanning electron micrograph of a large dendritic soot particle such as: oxygen and hydrogen containing functional groups collected on the ‘Kleiner Feldberg’ near Frankfurt/Main, Germany, especially on the inner and outer surfaces; relatively active on a glassy carbon disc.21 carbon sites where the aromatic ring structure is disturbed; and wavy lattice planes with varying crystal lattice parameters.city of Frankfurt, Germany. Its large particle diameter again It should be emphasized that there is no distinct boundary ranges well above 10 mm.21 between the structural levels and that the graphitic backbone Fig. 4 presents another such particle collected on the island of a soot particle dimensionally may extend into the 10 nm of Helgoland during the NORDEX ‘96 campaign,34 under range.Of course, many organic components, such as hydrohigh magnification: the grainy structure of the branches is carbons (C14–C35), PAHs (>4 rings) and nitro-PAHs (>3 clearly seen, but the grains also form compact branches which rings), may be adsorbed or chemisorbed on the graphitic core ensure good rigidity.This is in contrast to soot particles which of soot particles together with sulfates and other trace were inspected earlier in TEM and which exhibited a pro- elements.33 In addition, completely amorphous carbon makes nounced spherical nanostructure without such bridges.20 up an essential part of soot particles.Industrially produced carbon black also exhibits such an (ii) The next dimensional category is the nanostructure of unlinked structure of large aggregates of spherical particles such a particle. This is the first structural category which can with diameters from 1 nm upwards.35 be studied by HR-SEM. It has been shown by Cachier29 that The bridged, dendritic structure shown in Figs. 2–4 seems young combustion particles exhibit an extended dendritic to be typical for certain gas condensation reactions. Fig. 5 structure composed of spheres or links with diameters of shows a strikingly similar morphology for the deposition of 10–30 nm. Fig. 2 shows an extraordinarily large soot particle molybdenum metal on a tungsten foil substrate. The molyb- with a very intricate dendritic structure.Such particles were denum was also deposited from the vapour phase during the collected in the Soweto area in South Africa during the winter course of a reduction process in a wet hydrogen atmosphere.36 time.18 Soweto is a predominantly residential city, variously It is understandable that such intricate structures as shown estimated at 1.5 to 2 million people, situated 20 km southwest in Figs. 2–5 are not stable for a long time. Especially under of Johannesburg. Coal combustion is used predominantly for atmospheric conditions, soot particles coagulate and are fre- domestic heating and cooking during winter resulting in dense quently covered with adsorbed layers of material present smoke pollution. simultaneously in the atmosphere.29,33 The particle in Fig. 6 is (iii) This specific nanostructure makes up a particle with a an obviously aged but still dendritic soot particle with partly diameter in the 1–10 mm range and a microstructure is again swollen links. No other elements were detected by EDX superimposed over the atomic structure and the nanostructure. analysis. Fig. 3 shows another soot particle with a very open dendritic Fig. 7 shows a condensed soot particle with a still grainy structure very similar to that shown in Fig. 2. It was collected in a very diVerent part of the world, on the ‘Kleiner Feldberg’, a meteorological station in the Taunus mountains near the Fig. 4 High resolution scanning electron micrograph of a small Fig. 2 Scanning electron micrograph of a highly dendritic young and large soot particle collected on a glassy carbon disc at Soweto dendritic soot particle collected on the island of Helgoland, Germany, on a glassy carbon disc.34 (South Africa).19 276 J.Environ. Monit., 1999, 1, 273–283Fig. 5 Scanning electron micrograph of the typical branched Fig. 8 Scanning electron micrograph of three spherical soot particles morphology of the deposition of molybdenum metal on a tungsten with a diVerent nano- and microstructure collected at the ‘Kleiner foil substrate.Deposition took place by a chemical vapour transport Feldberg’ near Frankfurt/Main, Germany.21 (CVT) mechanism by decomposition of a gaseous compound.36 surface with some still grainy areas along what looks like grain boundaries of the shell areas. Fig. 9 shows another typical morphology of a carbonaceous particle: this particle was again collected at the ‘Kleiner Feldberg’ near Frankfurt.21 Such particles are frequently observed together with spherical fly ash particles composed of, for example, Si, Al and O.Such a small spherical particle is also visible in Fig. 9. It is interesting that such ‘sponge-like’ carbonaceous particles are even found in the upper stratosphere: Fig. 10 Fig. 6 High resolution scanning electron micrograph of an aged soot particle. The links have either grown together or are covered with a coating. However, no other element but carbon was detected by EDX.19 Fig. 9 Scanning electron micrograph of a spherical carbonaceous particle with a spongy structure collected at the ‘Kleiner Feldberg’ near Frankfurt/Main, Germany.21 Fig. 7 High resolution scanning electron micrograph of an aged soot particle with a grainy nanostructure, but with little open porosity. The particle was collected at the ‘Kleiner Feldberg’ near Frankfurt/Main, Germany. No other element was detected but carbon by EDX.21 nanostructure, but with little porosity. It is assumed that this is a still later stage of the particle aging process.Fig. 8 presents further morphological variations in the nanoand microstructure of carbonaceous particles: the largest particle still exhibits a rather grainy and porous nanostructure, but also a partial shell on its lower right side. There was only carbon detectable in the shell by EDX analysis. The smallest, sphere-like particle below the other two exhibits an intermedi- Fig. 10 Scanning electron micrograph of a carbonaceous particle with ate but compact structure (between grainy and shell structure). minor sulfur and nitrogen contents of strikingly similar morphology It also consists totally of carbon, as well as the particle on the to that shown in Fig. 9. This particle was collected on a high flying aeroplane at an altitude of about 20 km.37 right-hand side, which exhibits a rather solid shell on its J.Environ. Monit., 1999, 1, 273–283 277shows a mainly carbonaceous particle with minor sulfur and can be introduced as a gas into the specimen chamber so that saturated chamber gas pressures can be reached and main- nitrogen concentrations [secondary ion mass spectrometry (SIMS) analysis] with a very similar morphology to that in tained.Water can even be condensed onto as well as removed from the sample in a controlled manner. This allows the Fig. 9. It was collected by NASA scientists on a high flying aeroplane at an altitude of about 20 km.37 morphological and analytical investigation of samples under moist conditions (not freeze dried), which would otherwise The hypothesis of the possible formation of carbonaceous particles mainly by incomplete combustion of long chain develop morphological artifacts when put into vacuum.In order to create a water saturated atmosphere at low pressure, hydrocarbon fuels, e.g. in Diesel motors or aerojet turbines, is in agreement with the fact that incomplete combustion is a a Peltier cooling stage is available which can, of course, also be used for the removal of water from samples by freeze drying.problem in such engines and the backbone for further polymerization already exists in the form of the long carbon chains Furthermore, a specimen chamber gas handling system allows the introduction of various gases at low pressure. Air which undergo particle dehydrogenation in the combustion process. Hence, the particle collected at high altitude most can be introduced for oxidation studies of low or high temperature materials (in dry or moist air), while a hot stage is also likely stems from the exhaust of such a high flying aeroplane.Its minor nitrogen concentration is also indicative of a high available for temperatures up to 1500 °C. The evolved gases of a hot stage experiment (for which temperature–time pro- temperature combustion process.Previous TEM investigations of particles collected from the exhaust of an Otto motor in gramming is possible) can be introduced into a quadrupole mass spectrometer via a small capillary and analysed. Gas combination with EDX analysis indicated the presence of carbon, nitrogen, oxygen and chlorine in a mass ratio of concentrations of less than 1 ppm to 100% can be determined.41 The successful SEM work at low pressure is only possible 985775156.20 Nitrogen uptake of pyrolytic carbon layers on polycrystalline electrographite has been observed for graphite with some constructional developments.41 (i) The beam gas path length, i.e.the distance which the tubes used in atomic absorption spectrophotometry upon cooling under nitrogen of the tubes after pyrolytic coating.38 primary electron beam has to overcome from the high vacuum system of the electron optics to the sample, is kept to a Hence, soot particles stemming from high temperature combustion in Otto or Diesel motors or from aerojet turbines are minimum.This is also important for the lateral resolution of X-ray detection which is hampered by primary electrons being liable to contain considerable amounts of nitrogen.Fig. 11 exhibits carbonaceous and mineral particles which scattered by gas molecules. (ii) Secondary electron (SE) detectors have to be modified were collected at Soweto.18,19 The very small, nanometre sized soot particles are again partly composed of dendritic structures. to so-called ‘gas amplification detectors’ whose operation is analogous to the flow proportional gas detector used in However, the spherical particles in the 100 nm to several mm range are also purely carbonaceous particles.WDX.39,41 Backscattered electrons suVer negligible energy loss in the gas phase and retain suYcient energy to activate large From these examples, it should become clear how important HR-SEM studies together with EDX analysis are for the area scintillators without post-specimen acceleration.39,41 (iii) An advantage of elevated pressure operation is the elucidation of the genealogy of many particle types, such as soot and carbonaceous particles.automatic discharge of the negatively charged surface of insulators due to gas ions which are attracted by this charge and ‘neutralize’ it.The gas ions above the sample are generated 3.2.3 Environmental SEM (ESEM). There is a problem with quite a number of aerosol particles which are either easily by the primary electron beam and by SE detector ionization processes.39,41 decomposed or dehydrated in the vacuum of a SEM. Other particles are sensitive to electron bombardment, especially in the more energetic electron beam of the TEM.For example, 3.2.4 Semiquantitative single particle analysis by EDX and WDX. As has been shown elsewhere,23,24 semiquantitative organic particles are not stable in the TEM and change their morphology dramatically when hit by the electron beam. Most single particle analysis is necessary especially in occupational health monitoring, since generally particles collected at indus- ammonium salts are decomposed in the SEM.Since this is a general problem, especially of biological trial sites are not composed of single compounds, for which medical investigations, e.g. of their carcinogenic eVect, have samples, low pressure SEMs have been under development for quite some years.39,40 There now seems to be a breakthrough been performed.42,43 This has been verified for Ni containing particles collected at the largest nickel refinery in the world at with what is called the ESEM series of SEMs.41 Water vapour Monchegorsk on the Kola peninsula (Russia),24 and for Mn containing particles collected in the manganese alloy producing plant of Elkem manganese US PEA in Norway.23 In both cases, particles are not just composed of Ni and S or Mn and S, but also contain varying amounts of O together with minor amounts of Fe, Ca, Na and other ubiquitous elements.Very often, they probably exhibit an oxide layer on the surface which makes further investigations by methods with high depth resolution, such as X-ray photoelectron spectroscopy (XPS), Auger electron spectrometry (AES) or SIMS, important for further characterization.11 However, TEM studies are probably the best choice, because only TEM with appropriate additional instrumentation, such as EDX and EELS, can yield nanostructural and nanocompositional information.The feasibility of low Z element determination in an individual particle is absolutely essential to identify the chemical compound(s) which make up the particle. However, a few general remarks on light element X-ray analysis seem appro- Fig. 11 Scanning electron micrograph of carbonaceous and mineral priate.14,44 H, He and Li do not yield X-ray peaks, but Be, B, particles on a Nucleopore filter collected at Soweto (South Africa).19 C, N, O and F are detectable. The long wavelengths (low The sphere-like particles and the very small particles are soot.The larger irregularly formed particles are mineral particles. photon energies) of the K lines of these elements make the use 278 J. Environ. Monit., 1999, 1, 273–283Table 1 Micro- and nanometre particle characterization by HR-SEM, of special experimental techniques necessary. The long- EPMA and TEM wavelength X-rays are easily absorbed. Hence, the conductive coating applied to insulating specimens must be of a strictly Method Information content controlled thickness in order to apply respective corrections.A further problem is the deposition of carbon from vacuum HR-SEM Scanning electron Evaluation of micro- and nanomorphology contaminants under electron bombardment. It is therefore not micrographs with a resolution of 1 nm surprising that semiquantitative X-ray analysis, especially for Automatic particle size distribution analysis carbon and also for nitrogen, is still very problematic.Nitrogen +EDX Qualitative automatic analysis of single par- Ka exhibits a very large mass absorption coeYcient for carbon ticles with diameters 50 nm and is therefore quite sensitive to varying carbon contents (or Semiquantitative automatic analysis of single thin carbon layers).The usual assumption that X-ray spectra particles with diameters 100 nm, but problems with spectral resolution are independent of chemical bonding is also invalid for light elements. Line positions and shapes vary as a function of EPMA Morphological evaluation as for HR-SEM, elemental composition and chemical state.This has to be but generally with inferior resolution Automatic particle size distribution analysis taken into account if sample and standard diVer in this respect. and systematic shape characterization On the other hand, speciation is easily possible for light +WDX Qualitative automatic analysis of single par- elements by precision measurement of line position and ticles with diameters 100 nm shape.45 Semiquantitative automatic analysis of single Since there are two principal possibilities for semiquantit- particles with diameters 500 nm by WDX ative XRS in SEM or EPMA, these should be critically with much better spectral resolution than for EDX compared as to their respective advantages and drawbacks, Element speciation by precision measurement especially with respect to the determination of low Z elements. of line shifts and line shape analysis of bond- WDX is advantageous over EDX for the following reasons.sensitive lines of particles 500 nm in (i) The peak-to-background ratio is essentially better for diameter light elements when using multilayer materials (e.g. W–Si, TEM Mo–B4C, V–C). Peak intensities can be up to 20 times higher CTEM than those obtainable with conventional crystals.14 This is DiVraction contrast Morphology with about 1.5 nm resolution very important due to high soft X-ray absorption in the Absorption contrast Morphology of amorphous objects (replica sample.Although EDX analysis of light elements using Si(Li) surfaces or objects stained with heavy metal salts) or Ge detectors with ultrathin Be windows is now standard, Phase contrast Crystalline structure analysis with atomic peak-to-background ratios are worse than for WDX. The use resolution of very thin specimens of windowless EDX detectors,46,47 which were employed prior SAED Topostructural analysis, spot size of about to the introduction of EDX detectors with ultrathin windows, 500 nm in diameter by electron diVraction bears the danger of contamination of the detector surface, e.g.pattern in back focal plane by volatile particles or particle components, and is therefore SA-EDX Compositional analysis with same lateral resolution of 500 nm less practical than the application of the latter. STEM Nanomorphological investigations as for (ii) In EDX, there is frequently troublesome line interference CTEM of the light element Ka lines with the L and M lines of heavier Same contrast modes as above elements, especially of the third period transition metals, due CBED Topostructural analysis with a lateral resoto the significantly worse energy resolution in EDX.lution of 50 nm and better (function of sample EDX is advantageous over WDX for the characterization thickness) by electron diVraction.However, evaluation more diYcult than in CTEM of particles with diameters smaller than 0.5 mm. On the other EDX, EELS Elemental analysis with a resolution#sample hand, WDX is superior to EDX with respect to relative thickness (10 nm at best). EELS: element element sensitivities (or concentrations).11 Hence, for particles speciation by shape of ionization edges down to 0.5 mm in diameter, relative detection limits obtainable EFTEM Elemental mapping with single nm resolution by WDX are better than with EDX.However, EDX is superior (with stigmatic (and better; limitation: sample thickness and to WDX in terms of absolute detection limits due to a more electron spectrometer) quality of electron optics) Binding selective element mapping with favourable geometry for accepting X-ray pulses.This is the single nm resolution: NANOSPECIATION reason why only EDX is used in the TEM, since the volumes excited by the primary electron beam are on the order of 10-5–10-8 mm3 in the TEM as compared to several mm3 in the SEM.11 Of course, detection limits also depend on the primary beam intensity, which is usually higher in TEMs than without three-dimensional plasticity which is very valuable and typical for scanning electron micrographs.Due to the in SEMs due to the more frequent use of LaB6 cathodes or field emission guns as electron generating sources. EDX conse- transmission mode in the TEM, darker areas indicate extension of the morphologies in the Z direction if the density of the quently extends the range of particles to be analysed semiquantitatively in the SEM or EPMA at least down to 50 nm in particle is assumed to be homogeneous which is valid for soot particles.Qualitative EDX analysis for the Soweto particle in diameter. Still smaller particles can be morphologically and compositionally studied in the TEM as will be shown below. Fig. 12 indicates the presence of minor concentrations of O, Si, S and Cu.The latter, however, stems from scattered X-rays from the copper grid which mechanically stabilizes the 3.2.5 Nanometre particle characterization by HR-SEM and TEM. Table 1 gives an overview of the possibilities of particle polycarbonate filter. As was discussed elsewhere in more detail,11 there are several characterization by HR-SEM, EPMA and TEM.The latter allows the study of the nanomorphologies of particles in more instrumental possibilities in TEM for morphological, structural and elemental analysis. detail as long as the studied objects are still transparent for the primary electron beam (i.e. generally below several (i) Morphological contrast in TEM is usually a diVraction contrast based on coherent elastic scattering, leading to varying hundred nm in thickness). This is demonstrated in Fig. 12 for a soot particle collected at Soweto which shows the nanomor- fractions of primary electrons which remain in the transmitted beam in bright field images. This allows the inner nanomor- phology of the dendritic soot particle in great detail, but J. Environ. Monit., 1999, 1, 273–283 279mapping.25 Deposits of dusts in lung tissue have been investigated recently by EFTEM.48 Numerous anthracotic areas with plentiful inhaled soot particles were found in the lung of the 5300-year-old Tyrolean Iceman, together with mineral crystals (mainly muscovite) and organic threshing residues.49 The prevailing presence of soot most likely stems from open fires in late Neolithic houses.Such open fires were customary in farmhouses of Alpine regions even until the beginning of the 20th century in remote areas. Soot particles were probably the initial particles of relevance to the occupational health of mankind. 3.2.7 Some examples for HR-TEM of particles. Figs. 13–15 demonstrate the possibilities of CTEM bright field imaging for nanometre sized particle morphologies as well as HR-TEM imaging of crystalline particles or particle domains in the phase contrast mode.Fig. 13 shows particles which were recently collected within the framework of the LACE ’98 0.90 1.80 2.70 3.60 4.50 5.40 Cu Si O C S 200 nm campaign. The ‘Lindenberger Aerosol Charakterisierungs Fig. 12 TEM image of a soot particle collected at Soweto. Experiment’ (LACE) is part of a German aerosol research activity sponsored by the BMBF.The dark cloudy areas phology of objects to be made visible with a lateral resolution of about 1.5 nm. (ii) Absorption or mass thickness contrast is used to investigate the nanomorphology of amorphous objects which are often stained with heavy metal salts or of which replicas have been made. (iii) Phase contrast is produced from crystalline domains in the sample by coherent electron scattering.Image formation is usually accomplished by combination of the transmitted electron beam and some diVracted beam. This allows the image formation of crystalline domains with atomic resolution ( lattice imaging). Conventional transmission electron microscopy (CTEM) works with a strictly parallel primary electron beam. In order to select certain areas of the inspected sample, small apertures were originally used leading to structural analysis by selected area electron diVraction (SAED) or to compositional EDX analysis with a minimal spot size of about 500 nm ( limited by diVraction eVects at the aperture).For EDX analysis, spot measurements with a rigorously focused primary electron beam are also possible with a lateral resolution of about 30 nm (essentially being a function of the sample thickness due to a broadening of the spot by electron scattering in the sample). 200 nm soot biological particle O, S, C iron oxide (magnetite or maghemite) Fig. 13 TEM bright field image of nanometer sized particles together Electron scattering also limits the lateral resolution of EDX with electron diVraction patterns of the various particles.and EELS elemental analysis in scanning transmission electron microscopy (STEM), which usually uses a convergent electron beam focused at the sample plane. This improves lateral resolution in EDX and EELS analysis, but leads to problems in the evaluation of electron diVraction patterns since ellipsoidal areas instead of sharp spots are obtained in the back focal plane of the TEM.In modern instruments, all these possibilities can be used easily by switching from one mode to the other. 3.2.6 EFTEM for element mapping and speciation with nanometre resolution. A decisive improvement for the lateral resolution of EELS analysis was the introduction of stigmatic electron spectrometers which led to the method of EFTEM.25,26 This, on the other hand, necessitates very thin samples or sample areas for inspection (upper limit, 50 nm).Multiple electron scattering should not occur in the sample for the following reasons: it will hamper lateral resolution; it will lead to a loss of intensity of the discrete energy loss peaks; it will further raise the background of the EELS spectrum.26 Under optimal conditions, elemental mapping and even binding specific mapping (e.g.of amorphous carbon and diamond25) are feasible with single nanometre resolution. Even Fig. 14 HR-TEM image of the long, needle-like crystalline particle of Fig. 13 on an amorphous carbon foil. subnanometre resolution has been achieved for elemental 280 J. Environ. Monit., 1999, 1, 273–283reference oxides, it can be concluded that the particle is magnetite.50 This demonstrates well the possibility to perform speciation by EELS.The outer morphology of such iron oxide particles cannot be determined by TEM. Therefore, the typical globular morphology of the particles is shown in Fig. 16 as viewed in HR-SEM. SEM, on the other hand, cannot determine which kind of iron oxide is present, since they all exhibit similar globular forms and a precise oxygen quantification is not feasible by EDX analysis. This again shows the complementary nature of the two methods. 4 Conclusion The possibilities of single particle characterization are demonstrated with some of the most important topochemical methods based on primary electron beams. As unfortunately typical for any profound analytical problem, a thorough approach to single particle characterization is only possible by a combined use of diVerent methods: in the presented combination the methods used are highly complementary.As typical for any type of analysis, sampling has to be optimized separately for the various methods. Fig. 15 HR-TEM image of one of the iron oxide particles composed By showing some of the most striking morphologies of soot of magnetite (Fe3O4) or maghemite (c-Fe2O3).The crystal pattern exhibits twin formation, which is only observed for very small particles and carbonaceous particles, the significance of HR-SEM for (10 nm in diameter). the study of the morphological nanostructure of particles was emphasized. Soot and carbonaceous particles were selected because they are probably the particles with the highest and mainly on the right side of the needle-like particle in Fig. 13 oldest relevance to occupational health, as demonstrated by are soot agglomerates. Two of the three particles on the left the plentiful inhaled soot particles in lung samples of the side of the needle-like crystal are iron oxide particles. Their 5300-year-old Tyrolean Iceman.From the various morpho- electron diVraction pattern is shown to the lower right of logies found for soot, the dendritic morphologies with the Fig. 13. The larger particle between the two iron oxide particles highest specific surface area are most likely those with the is of biological origin. The long, needle-like particle consists most pronounced tendency to interact with the nasal, thoracic of carbon, oxygen and sulfur (EDX analysis).Its crystallinity and bronchial system of humans. is demonstrated by the HR-TEM image shown in Fig. 14. The next question of single particle characterization is the Unfortunately, it is not yet possible to assign the proper chemical composition. This is best answered by automated compound to the observed lattice data and its elemental semiquantitative EPMA analysis applying WDX for particles composition, since organic compounds are not contained in down to 0.5 mm in diameter.The advantages and disadvantages the JCPDS or ICSD data banks which were available. Fig. 15 of WDX and EDX analysis have been discussed, especially shows one of the smaller iron oxide particles of which the with respect to light element analysis (C, O, N), which is of upper iron oxide agglomerate of Fig. 13 is composed, again in paramount importance for particle characterization. A further the phase contrast mode at high resolution. The streaky advantage of WDX is the possibility of element speciation by pattern running through the centre of the globular particle precision measurement of bonding sensitive X-ray lines.The indicates twin formation. The crystal structure derived from known composition of a representative multitude of particles the single crystal diVraction pattern in Fig. 13 indicates magis the basis for an initial evaluation of the possible health netite or maghemite. These two iron oxide compounds exhibit hazards for workers in specific industrial environments. From similar lattice constants so that a decision on one of these the initial results of particle characterization in the manganese compounds from the electron diVraction data is not feasible.and nickel industry it appears that the composition of most However, EELS analysis of this particle can answer this particles is complex. Generally, such particles seem to be question unambiguously.From the evaluation of the energy composed of several phases, which consequently calls for position of the iron peak and by comparison with relevant particle investigations with methods capable of elucidating the inner nanostructure and nanocomposition. The most important instrumentation for this purpose is the transmission electron microscope with its modern auxiliary instrumentation for nanoanalysis, i.e.the combination of EDX and EELS. The most advanced modification of EELS has been named energy filtering TEM, and is based on the recent development of stigmatic electron spectrometers capable of yielding element maps of suYciently thin specimens with single nanometre resolution. In addition, element speciation is also frequently possible by evaluation of the shape of element ionization edges.25,26 Since the crystallinity of certain phases is of importance for health hazard evaluations, the possibility to study crystallinity by the application of phase contrast in TEM is also an important feature of single particle characterization in TEM.Consequently, the combined use of HR-SEM, EPMA-WDX Fig. 16 HR-SEM image of very small iron oxide particles. and TEM with EDX and EFTEM can be considered a very J. Environ. Monit., 1999, 1, 273–283 281powerful approach to single particle characterization. For ZAF Matrix correction procedure for XRS in SEM special cases, the use of ESEM can also be important if and EPMA for atomic number eVects (Z), mass relevant particles are either not stable in vacuum and/or are absorption eVects (A) and fluorescence eVects (F) decomposed by electron bombardment. 7 References 5 Acknowledgements 1 K. R. Spurney, ed., The Physical and Chemical Characterization of The author thanks the following persons for their contribution Individual Particles, Wiley, New York, 1986. to the presented micro- and nanographs: Michael Wentzel and 2 J.BuZe and H.P. Van Leeuwen, eds., Environmental Particles, Martin Ebert for the HR-SEM figures; Dr. Gabriele Gorzawski Environmental Analytical and Physical Chemistry Series, Lewis, Chelsea, MI, 1992, vol. 1. (Department of Applied Mineralogy, Faculty of Geo-Science 3 R. M. Harrison and R. Van Grieken, eds., Atmospheric Particles, and Geography of the Technical University of Darmstadt) for IUPAC Series on Analytical and Physical Chemistry of the TEM figures; Dr.F. J. Stadermann (McDonnel Center Environmental Systems, Wiley, Chichester, 1998, vol. 5. for the Space Sciences, Washington University, St. Louis, MO, 4 K. Willeke and P. A. Baron, eds., Aerosol Measurement, USA) for the micrograph of the carbonaceous particle shown Principles, Techniques, and Applications, Van Nostrand Reinhold, in Fig. 10. New York, 1993. The author would also like to thank the following partners 5 J. H. Vincent, Aerosol Science for Industrial Hygienists, Elsevier Science, Oxford, 1995. for their excellent cooperation without which this work would 6 H. M. Ortner, P. HoVmann, F. J. Stadermann, S. Weinbruch and not have been possible: Prof. Dr. Stephan Weinbruch M.Wentzel, Analyst, 1998, 123, 833.(Department of Applied Mineralogy, Faculty of Geo-Science 7 J. Injuk, L. De Bock and R. Van Grieken, in Atmospheric and Geography of the Technical University of Darmstadt); Particles, IUPAC Series on Analytical and Physical Chemistry of Prof. Dr. Harold J. Annegarn (Schonland Research Centre Environmental Systems, ed. R. M. Harrison and R.Van Grieken, for Nuclear Sciences, University of the Witwatersrand, Wiley, Chichester, 1998, vol. 5, p. 173. 8 C. XhoVer, L. Wonters, P. Artaxo, A. Van Put and R. Van Johannesburg, South Africa); Dr. Gu� nter Helas (Bio- Grieken, in Environmental Particles, Environmental Analytical and geochemistry Department, Max Planck Institute for Physical Chemistry Series, ed. J. BuZe and H. P.Van Leeuwen, Chemistry, Mainz, Germany); Dr. Yngvar Thomassen Lewis, Chelsea, MI, 1992, vol. 1, pp. 207–245. (Department of Occupational Hygiene, National Institute of 9 H. M. Ortner, Quim. Anal., 1997, 16 (Suppl. 1), 15. Occupational Health, Oslo, Norway). 10 S. Weinbruch, M. Wentzel, M. Kluckner, P. HoVmann and H. M. Ortner, Mikrochim. Acta, 1997, 125, 137. 11 H. M. Ortner, Ortsaufgelo� ste oder topochemische Analytik—ein 6 List of acronyms used U� berblick (Space resolved or topochemical analysis—an overview), in Analytiker Taschenbuch, ed.H. Gu� nzler, A. M. Bahadir, AES Auger electron spectrometry K. Danzer, W. Engewald, W. Fresenius, R. Galensa, W. Huber, BC Black carbon (soot) M. Linscheid, G. Schwedt and G. To� lg, Springer, Berlin, 1998, BMBF Bundesministerium fu� r Bildung und Forschung vol. 19, pp. 217–261. (Federal Ministry for Education and Research in 12 H. M. Ortner, GIT-Fachz. Labor, 1991, 35, 891. Germany) 13 H. M. Ortner and P. Wilhartitz, Fresenius’ J. Anal. Chem., 1990, CBED Convergent beam electron diVraction 337, 686. CTEM Conventional transmission electron microscopy 14 J. I. Goldstein, D. E. Newbury, P. Echlin, D.C. Joy, A. D. Romig, C. E. Lyman, C. Fiori and E. Lifshin, Scanning Electron CVT Chemical vapour transport Microscopy and X-ray Microanalysis, Plenum, New York, 2nd EDX Energy dispersive X-ray spectrometry edn., 1992. EELS Electron energy loss spectrometry 15 K. F. J. Heinrich, Electron Beam X-ray Microanalysis, Van EFTEM Energy filtering transmission electron microscopy Nostrand Rheinhold, New York, 1981.EMP Electron microprobe 16 K. F. J. Heinrich and D. E. Newbury, eds., Electron Probe EPMA Electron probe microanalysis Quantitation, Plenum, New York, 1991. 17 L. Reimer, Transmission Electron Microscopy: Physics of Image ESEM Environmental scanning electron microscopy Formation and Microanalysis, Springer, Berlin, 3rd edn., 1993. FEG Field emission gun 18 M.Wentzel, H. J. Annegarn, G. Helas, S. Weinbruch, HR-SEM High resolution scanning electron microscopy A. G. Balogh and J. S. Sithole, South Afr. J. Science, in the press. ICSD Inorganic crystal structure data base 19 M. Wentzel, PhD Thesis, Department of Chemistry, Technical JCPDS Joint Committee of Powder DiVraction Standards University of Darmstadt, in preparation.LACE Lindenberger Aerosol Charakterisierungs 20 S. Weber, PhD Thesis, Department of Chemistry, Technical University of Darmstadt, 1997. Experiment 21 M. Ebert, PhD Thesis, Department of Chemistry, Technical NASA National Aeronautics and Space Administration University of Darmstadt, in preparation. (USA) 22 S. Weinbruch, M. Wentzel, M. Kluckner, P. HoVmann and NORDEX Measuring campaign ‘NORDEX ‘96’ on the H.M. Ortner, Mikrochim. Acta, 1997, 125, 137. island of Helgoland. For details, see ref. 27. 23 S. Gunst, S. Hetland, H. M. Ortner, A. Skogstad, Y. Thomassen, OC Organic carbon (particles) S. Weinbruch and M. Wentzel, Chemical composition of individ- PAH Polyaromatic hydrocarbon ual aerosol particles from working places in the production of manganese alloys, in preparation.PTFE Poly(tetrafluoroethylene) 24 B. L. W. Ho� flich, H. M. Ortner, A. Skogstad, Y. Thomassen, SAED Selected area electron diVraction S. Weinbruch and M. Wentzel, Chemical composition and size of SA-EDX Selected area energy dispersive X-ray individual aerosol particles from working places in a nickel spectrometry producing factory, in preparation. SE Secondary electrons 25 F.Hofer, P. Warbichler and W. Grogger, Spektrum der SEM Scanning electron microscopy Wissenschaft, 1998, 10, 48. 26 R. F. Egerton, Electron Energy Loss Spectroscopy in the Electron SIMS Secondary ion mass spectrometry Microscope, Plenum, New York, 1996. STEM Scanning transmission electron microscopy 27 D. Mark, in Atmospheric Particles, IUPAC Series on Analytical TEM Transmission electron microscopy Physical Chemistry of Environmental Systems, ed. R. M. Harrison TXRF Total reflection X-ray fluorescence (spectrometry) and R. Van Grieken, Wiley, Chichester, 1998, vol. 5, p. 29. WDX Wavelength dispersive X-ray spectrometry 28 M. Ebert, J. Dahmen, P. HoVmann and H. M. Ortner, XPS X-ray photoelectron spectrometry Spectrochim. Acta, Part B, 1997, 52, 967. 29 H. Cachier, in Atmospheric Particles, IUPAC Series on Analytical XRS X-ray spectrometry 282 J. Environ. Monit., 1999, 1, 273–283and Physical Chemistry of Environmental Systems, ed. R. M. 38 U. Rohr, PhD Thesis, Department of Chemistry, Technical University of Darmstadt, 1996. Harrison and R. Van Grieken, Wiley, Chichester, 1998, vol. 5, 39 S. Weinbruch, M. Wentzel, M. Kluckner, P. HoVman and p. 295. H. M. Ortner, Mikrochim. Acta, 1997, 125, 255. 30 K. Katrinak, P. Rez and P. R. Buseck, Environ. Sci. Technol., 40 D. G. Danilatos, Microchim. Acta, 1994, 114/115, 143. 1992, 26, 1967. 41 The X company brochure of Philips. 31 E. D. Goldberg, Black Carbon in the Environment, Wiley, New 42 E. Nieboer and G. G. Fletcher, Determinants of reactivity in metal York, 1985. toxicology, in Particles in Our Air: Concentrations and Health 32 W. Huettner and C. Busche, Fresenius’ Z. Anal. Chem., 1986, EVects, ed. R. Wilson and J. D. Spengler, Harvard University 323, 674. Press, Boston, 1996, pp. 111–130. 33 V. Perret, C. K. Hungh, P. O. Droz, T. Vu Duc and M. Guillemin, 43 M. Costa, Fresenius’ J. Anal. Chem., 1998, 361, 381. Poster within the AIRMON ‘99 Conference, Geilo, Norway, 44 G. F. Bastin and H. J. Heijligers, GIT-Fachz. Labor, 1991, 35, 145. February 10–14, 1999. 45 A. Meisel, G. Leonhardt and R. Szargan, X-ray Spectra and 34 M. Ebert, P. HoVmann, H. M. Ortner and S. Weinbruch, Chemical Binding, Springer Series in Chemical Physics, Springer, Chemische Charakterisierung atmospha�rischer Partikel im Berlin, 1989, vol. 37. Rahmen der Meßkampagne NORDEX ‘96, in NORDEX ‘96 46 P. Fruhstorfer and R. Niessner, Mikrochim. Acta, 1994, 113, 239. Workshop, November 1997, ed. N. Beltz, ZVV-Verlag, 47 R. S. Hamilton, P. R. Kershaw, F. Segarra, C. J. Spears and Frankfurt/M, 1998, pp. 60–64. J. M. Watt, Sci. Total Environ., 1994, 146/147, 303. 35 J. C. Bokros, Chemistry and Physics of Carbon, Marcel Dekker, 48 F. Hofer and M. A. Pabst, Micron, 1998, 29, 7. New York, 1972, vol. 9. 49 M. A. Pabst and F. Hofer, Am. J. Phys. Anthropol., 1998, 107, 1. 50 G. Gorzawski and P. Van Aken, personal communication, 1999. 36 W. Schulmeyer, PhD Thesis, Material Science Department, Technical University of Darmstadt, 1999. 37 F. J. Stadermann, PhD Thesis, Heidelberg University, 1990. Paper 9/02398D J. Environ. Monit., 1999, 1, 273–283 2