Chemical characterization of environmental and industrial ... - CiteSeerX

Analyst, May 1998, Vol. 123 (833–842)

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Chemical characterization of environmental and industrial particulate samples† H. M. Ortner*a, P. Hoffmanna, F. J. Stadermannb, S. Weinbrucha and M. Wentzelc a Department of Chemical Analytics, Materials Science Division, Darmstadt Technical University, Petersenstr. 23, Darmstadt, D-64287, Germany b McDonnel Center for the Space Sciences, Washington University, Campus Box 1105, St. Louis, MO 63130-4899, USA c Max Planck Institute of Chemistry, Department of Biogeochemistry, Joh.-J.-Becker-Weg 27, Mainz, D-55020, Germany

The characterization of particles, especially aerosol particles, is of great importance to many scientific fields. A relevant brief overview is given. A rigorous scheme of sampling and in-depth characterization of particulate samples has been developed in the authors’ laboratories and by collaborative groups, including investigations by the following techniques: total reflection X-ray fluorescence spectrometry for quantitative bulk characterization; solid-state speciation by valence band X-ray spectrometry using electron microprobe or Mössbauer spectrometry (only bulk particle characterization possible); scanning electron microscopy and electron probe microanalysis for automatic semiquantitative single particle characterization of particles ≥ 0.5 mm; transmission electron microscopy for semiquantitative single particle characterization of particles with diameters down to 10 nm; high resolution-scanning electron microscopy, which is also capable of characterizing particles morphologically and qualitatively down to 10 nm in diameter; secondary ion mass spectrometry for the study of trace elemental distributions and isotopic ratios in particles with diameters above 1 mm. It is the aim of this paper to show the advantages and characteristics of this scheme of analysis to match today's requirements for topochemical methods of analysis. For this purpose a short overview of these methods for particle characterization is also presented. Keywords: Particle characterization; total reflection X-ray fluorescence spectrometry; electron probe microanalysis; transmission electron microscopy; secondary ion mass spectrometry; topochemical methods; element speciation; compound identification The characterization of particles, especially aerosol particles, is of relevance to a number of scientific and industrial fields, as outlined below. In atmospheric sciences, the characterization of individual aerosol particles, their size distribution and chemical composition is of great relevance to modelling atmospheric processes and for environmental control purposes.1,2 Compared with conventional bulk techniques, individual particle analysis provides additional and complementary information concerning origin, formation, transport and chemical reactions.3 Occupational health monitoring relies on particle collection and subsequent particle characterization to evaluate health hazards for workers exposed to dusts from foundries, calcination ovens, powder handling, milling, etc.4,5 †

Presented at The Third International Symposium on Speciation of Elements in Toxicology and in Environmental and Biological Sciences, Port Douglas, Australia, September 15–19, 1997.

Particle characterization is an important source of information for compound identification for cleanroom control:6,7 in microelectronic compound fabrication or in ultratrace analytical laboratories for quality control of ultrapure chemicals, it is essential not only to monitor particle concentrations in air, which is done routinely, but also to identify the particles for possible elimination of sources of particulate contamination. The identification of particles which caused malfunction of highly intergrated microelectronic devices is also very important.8 Powders are the basis of powder technologies. Usually, single particle characterization is not necessary in powder technologies and bulk characteristics are determined in routine quality control. However, single particle characterization is essential for the determination of so-called ‘heterogeneous impurities’ in raw and intermediate products in powder metallurgy.9–11 Heterogeneous impurities are particulate impurities in the raw and intermediate products of powder metallurgy with particle diameters above approximately 5 mm. They are introduced into the raw or intermediate powders at various stages of production as, e.g., ore or gangue particles, abraded particles from grinding, milling and mixing operations or by careless powder manipulation or storage (e.g., cigarette ash, textile fibers, hair, rubber particles, aerosol dust). Owing to the sintering process in powder metallurgy, such impurities are not homogenized as in melting operations. Hence the particles remain unaffected or react partially with the matrix material to form inclusions which usually act as centers for crack formation if the material is mechanically stressed. In fine wire drawing or foil production, wires break or the foils become perforated at the place of a heterogeneous inclusion. The analytical determination of heterogeneous impurities is therefore an important procedure in the quality control of raw and intermediate products of powder metallurgy.9–11 The characterization of wear particles, e.g., in polymer extrudates12 or in motor lubricants, is another important area of technological relevance. The quality control of composite particles designed for coating operations by plasma spraying13 or of particles used in powder technology and metallurgy is a further very important field. The characterization of complex particles is of relevance in some applications, e.g., for pigments, which will be discussed in some detail later. Heterogeneous catalysts are usually also applied in particulate form and their characterization is accomplished using diverse methods of solid-state analysis.14 Another important group of particles of relevance in analytical chemistry are chromatographic materials for column fillings for GC, HPLC, MSC (e.g., macroreticular or pellicular resins or materials for the preparation of TLC layers)15 (for acronyms, see the list at the end of the paper). A rather exotic but nevertheless important role is played by cosmic16–18 and terrestrial particles in the degradation of

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material surfaces exposed to space in the low earth orbit (LEO) range, i.e., at altitudes of 400–600 km above the Earth’s surface.19–22 The most alarming observation was that more than 80% of these particles are of anthropogenic origin, i.e., manmade debris (e.g., paint particles from space shuttles, particulate residues from solid-state rockets). The differentiation between cosmic and anthropogenic particles is only possible by isotope ratio measurements of particles or what remains of such particles on degraded material surfaces.20,23 Owing to speeds of typically several km s21 on impact, virtually no particles survive collisions with space exposed material surfaces and what remains is a thin layer of re-condensed matter after being vaporized during impact. This layer can only be qualitatively analyzed by SIMS. Material degradation by the combined effect of erosion by particles and oxidative processes on such eroded surfaces by the atomic oxygen which is still present in the LEO region is becoming a serious problem for satellite lifetimes in this region.20–22 A similar exotic field is the forensic characterization of the diverse particulate material from crime scenes which is used as physical evidence and is a major task in forensic science.24–26 Volcanic dust particles and interplanetary dust particles are studied intensely in geology: explosive volcanic eruptions can eject vast amounts of gaseous and particulate material into the atmosphere within a brief period. Much of this material remains in the troposphere for a considerable length of time.27 These particles can greatly reduce the radiation of the sun reaching the earth surface and thus decisively influence weather patterns. On the other hand, interplanetary dust particles are studied intensely since they often stem from the very beginning of the formation of our solar system 4.5 billion years ago and some of them might even stem from pre-solar system times.18,23,28–32 Taking into account this broad range of interest in particle characterization, a multi-method approach for the comprehensive characterization of particles was developed for a particle diameter range from 10 nm to 100 mm which is relevant in the above fields of study. A thorough evaluation of the particle size distribution and of the lifetimes of particles of varying size in different domains of the Earth’s atmosphere showed that particles smaller than 100 nm in diameter exhibit very short lifetimes, mainly due to agglomeration.33 Furthermore, most sources of particles introduced into the atmosphere emit particles of 100 nm and more in diameter, and many natural sources emit particles even in the 1 mm range.33 This is the reason why our main scheme of particle characterization is designed for particles > 100 nm. On the other hand, particles with diameters down to 10 nm are of relevance for health hazard studies to evaluate the toxicity of dust in many industries.5 In this case, the application of TEM is necessary for particle size distribution measurements in the nanometer range and for phase and/or compound identification of such particles. Alternatively, HR-SEM can also be applied and we have just started to use this method.34 Nanometer sized particles have also become technologically important in materials science for the production of materials with new and interesting properties. They have also revolutionized established technologies and led to the development of materials with greatly improved high-tech properties.35–37 The characterization of such particles is also accomplished by the combined use of TEM and HR-SEM. The large diameter end of aerosols is again limited by the increasingly short lifetimes of large particles in the range of 25–100 mm in diameter depending, as can be expected, on particle density and morphology. This upper limit is equally significant for health hazard investigations, since coarse particles are usually trapped in our respiratory system before the particles can reach the deeper bronchial system and the lungs. For various reasons, this is also the upper limit of particles of technological interest (heterogeneous particles in powder

metallurgy, wear particles, harmful particles in microelectronics technology).8–12 Experimental The following instrumentation was used and/or developed in the course of our work. All-PTFE five-stage cascade impactor for particle collection A five-stage cascade impactor constructed totally from PFTE was built for contamination-free and isokinetic particle collection with the following five stages at a flow rate of 2 l min21:38 0.1–0.4, 0.4–1.8, 1.8–6.8, 6.8–25 and > 25 mm particle diameter. The particles were collected on glassy carbon discs of high purity (Fa. Hochtemperaturwerkstoffe, Maikingen, Germany) of 3 cm diameter and 3 mm thickness, which were ground and highly polished. TXRF Two instruments were used for TXRF measurements: for elements with atomic number > 15, a Seifert (Ahrensburg, Germany) instrument with Mo target X-ray tube (50 kV, 30 mA) with an Si(Li) detector (Kevex, Mainz-Finten, Germany) (with Be window) and a Tracor multi-channel analyser (Bruchsal, Germany); and for elements with atomic numbers from 8 to 23, a laboratory-constructed TXRF spectrometer39 with Cr-target X-ray tube (30 kV, 25 mA) with an Si(Li) Quantum detector (with diamond window) from Kevex, a Spectrace 6100 multi-channel analyser and a vacuum sample chamber. Mössbauer spectrometry A laboratory-made constant-acceleration type Mössbauer spectrometer equipped with a 1024-channel analyser operating in the time-scale mode and a 925 MBq 57Co/Rh source was employed. For further details, see ref. 40. This work was undertaken in collaboration with J. Ensling and P. Gütlich, Institute of Inorganic and Analytical Chemistry, University of Mainz, Germany. EPMA A Cameca (Paris, France) CAMEBAX SX 50 electron microprobe was used with one energy dispersive Si(Li) detector (Princeton Gamma-Tech) and with four WDX spectrometers, three vertically and one horizontally mounted (for the evaluation of rough surfaces). Experimental details of EPMA can be found elsewhere.41 TEM A Philips CM 20 UT (Eindhoven, Netherlands) (maximum accelerating voltage 200 kV) with EDX system (Noran, Middleton, WI, USA) with a Ge detector (thin window, applicable to detection of elements with Z > 4) and Voyager (Noran) data evaluation system was used. Sample collection was performed on Formvar foils on Cu grids. Elimination of magnetic particles was achieved with a permanent magnet. This work was undertaken in collaboration with G. Miehe, Department for Structural Research, Material Science Division, Technical University of Darmstadt, Germany. HRSEM A Philips 30 XR-FEG with an EDX system (EDAX, Mahwah, NJ, USA) comprising an Si(Li) detector with an ultra-thin


window, applicable to the detection of elements with Z > 4, was used. This work was undertaken in collaboration with A. Balogh, Department of Thin Films, Material Science Division, Technical University of Darmstadt, Germany. SIMS A Cameca IMS 5f instrument was used. Primary ion sources were a duoplasmatron for O2+, O2 and Ar+ and a source for Cs+. Primary accelerating voltages up to 17.5 kV, continuously adjustable, were applied. Primary beam diameters down to 0.5 mm were used. Mass resolution up to MRP 20 000 was achieved with a double focusing mass spectrometer. A secondary electron detector was applied for surface image formation in the scanning mode. Three detectors for secondary ions were used: an electron multiplier for low count rates up to 106 counts s21, a Faraday cup for count rates >105 counts s21 and a channel plate for image formation in the direct mode. An electron source for active charge compensation for nonconducting samples and an oxygen jet to the sample surface for improved secondary ion yields were also used. Results and discussion Bulk characterization of particulate samples for main, minor and trace components by TXRF It is beyond the scope of this paper to give an overview of the wide range of methods used today for the bulk characterization of particulate samples (e.g., AAS, XRS, ICP-OES, MS).42 Only the use of TXRF is outlined here since TXRF as a trace and microanalytical technique matches the specific requirements of impactor-collected particulate samples in an ideal way: it covers the small area where particles are deposited below the respective impactor jets and allows a safe multi-element quantification by internal standardization.43 TXRF has been used extensively for quantitative aerosol analysis44,45 and is one of the most important methods for this purpose. The following strategy of investigation for the quantitative characterization of aerosols was developed by us. Aerosol collection with a one-stage impactor was used for particles with a minimum diameter of 100 nm for relatively long periods (hours) in order to obtain the necessary sensitivity for element detection in the pg m23 range in clean-room atmospheres. Collection times, must of course, be shorter (minutes) for heavily contaminated atmospheres in industry in the range of mg m23 for health hazard evaluation. Quantitative determination of the elemental composition of the collected aerosol was carried out by the use of two TXRF instruments. Glassy carbon carriers of selected purity were chosen for particle collection. Quantitation was accomplished by use of Sc and Y as internal standards for low- and high-Z elements, respectively. Details of the procedure are described elsewhere.6,7 For a more detailed study of particles with diameters > 0.5 mm, of particle size distribution, particle morphology and of the chemical composition of individual particles, the procedure described later is used. The quantitative evaluation of the bulk composition of particles proved to be very useful for the identification of sources of contamination in clean rooms leading to appropriate actions of prevention.6,7 This identification is not feasible by the routinely carried out clean bench testing by use of particle counters. Such investigations are also of great value in the periodic evaluation of the atmosphere of clean room production sites in the microelectronics industry.

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but also in speciation for which some possibilities are outlined below. Only Mössbauer spectrometry is a mere bulk analytical method; all others also allow single particle characterization. Valence band X-ray spectrometry by EPMA–WDX (bulk or single particle speciation) This method is already well established for the identification of binding states in solids, especially for low- and medium-Z elements.46,47 The basis is the ‘chemical shift’ of X-ray lines if the onset of the electron jump to an inner orbital vacancy lies in the valence band of the respective atom. Generally the line shift is small (about 1–3 eV) in comparison with the peak width of Xray lines (FWHM ≈ 40 eV). In geology and mineralogy, a method was developed to determine the FeII/FeIII ratio in samples by measurement of the variation of the position and shape of the Fe La and Fe Lb lines.48 The La/Lb intensity ratio might also vary. Rohr49 studied various binding states of Pd by modifying the method of Höfer et al.48 These methods have the great advantage over other established methods for solid-state speciation (XPS, AES) that they exhibit much greater detection sensitivity and lateral resolution (compared with XPS only in this case). Bulk speciation by the method of Rohr can be carried out directly on samples collected on glassy carbon discs. For particles > 5 mm, single particle speciation is also possible. Mössbauer spectrometry (only bulk speciation) This method was specifically applied for iron speciation in large, integral aerosol samples.40 Iron is one of the most abundant elements in solid and aqueous atmospheric samples.50 It is usually introduced into the atmosphere as soil dust, fly ash from power plants and waste incineration facilities, from exhausts of combustion engines and generally from industrial operations.51 A thorough study of the significance of iron for atmospheric redox reactions and on the presence of iron in the above-mentioned sources was recently carried out by Weber.38 In the frame of this work, integral aerosol samples of several grams were collected from an inlet of an air-conditioning device in Darmstadt that had a large air throughput (17 000 m3 h21). Aliquots of these samples were studied by Mössbauer spectrometry.40 Unfortunately, this method is not capable of analyzing the usually collected microgram samples. It allows, however, the solid-state speciation of iron compounds present in aerosols. Magnetite, hematite, goethite and iron(ii) and iron(iii) silicates were found in the inspected samples.40 Of course, for a number of other elements speciation by Mössbauer spectrometry is also possible.52 Auger electron spectrometry (AES) (bulk or single particle speciation) AES can be used especially for the speciation of low-Z elements with excellent lateral and depth resolution because Auger signal shapes are frequently sensitive to relevant binding situations. We investigated the composition of dispersoid particles in oxide dispersion strengthened (ODS) steels by AES on in situ fractured surfaces.53 The relevant dispersoid sizes were in the submicrometer range. In combination with argon sputtering, indepth profiles of the TiO2–dispersoid–ferritic matrix interphase boundary were obtained with nanometer depth resolution for various elements of interest.

Solid-state speciation (bulk and topochemical)

X-ray induced photoelectron spectrometry (XPS) (usually bulk speciation; single particle speciation feasible with special instrumentation)

In many cases, there is considerable interest not only in an integral quantitative evaluation of impactor collected particles,

XPS is the most commonly used method of solid-state speciation with equally excellent depth resolution as AES in the

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single nanometer range.54,55 Lateral resolution, however, is usually limited to the nanometer range.55 Only the most modern XPS instruments can achieve a lateral resolution in the low micrometer range and could thus be used for single particle XPS characterization.55 The application of XPS to environmental particulate samples has been discussed extensively by Xhoffer et al.56 Micro-Raman spectrometry (bulk or single particle speciation) With the advent of well focused laser beams as sources for the excitation of Raman spectra and parallel developments in relevant instrumentation, the analysis of discrete particles has become possible by Raman spectrometry. Although additional vibrations might be caused by a well defined geometry of the inspected particles, this is usually of no concern for irregularly shaped particle assemblies which are generally present in aerosol samples. A more detailed discussion of micro-Raman spectrometry for particulate samples can be found elsewhere.56 Transmission electron microscopy with electron energy loss spectrometry (TEM/EELS) Speciation with a spatial resolution in the 10 nm range is possible by EELS with TEM since EELS peaks contain binding information in the near edge fine structures. This is the method for solid-state speciation with the best spatial resolution of all known respective methods. Relevant recent instrumental advances are discussed in a later section. The method has been used for aerosol characterization by Jambers et al.57

aeroplane23 and which also obviously solidified from the liquid state. EPMA A qualitative approach for the characterization of a great number of individual particles based on element distribution maps was developed recently.41 In this procedure, the size, shape and qualitative chemical composition of each particle can be deduced from a combination of secondary electron (SE) or backscattered electron (BSE) images and element distribution maps for those elements which have been chosen by the above SEM survey. The knowledge of the qualitative chemical composition of individual particles is sufficient for many applications. For example, we have recently investigated the significance of iron in atmospheric processes.38,40 The ironbearing particles were characterized by our analysis procedure and were classified into several categories (e.g., metal, oxide, silicate). We are also using this analysis procedure for source apportionment of aerosols, where characteristic elements for each source are known from bulk measurements by TXRF (see earlier). In addition to the qualitative determination, a semiquantitative estimate of the chemical composition of each particle can also be obtained.41 For this purpose, count rates for each particle are derived from the element distribution maps and are corrected for matrix and geometric effects using the particle

Topochemical characterization of solid aerosols SEM The first step used in our scheme for particle characterization is always an SEM survey, in which important information is collected on the following aspects: (i) particle abundance: important for optimizing the particle collection parameters for an appropriate number of particles per unit area; (ii) size distribution: important on its own; it also has to be decided whether EPMA can be applied as a next step or, in the case of very small particles, whether TEM or HR-SEM would have to be selected; (iii) particle morphology: in many cases, certain particle morphologies give important evidence, e.g., as to emission sources; (iv) particle homogeneity or heterogeneity: for larger particles, a heterogeneous structure of aggregates of smaller particles often becomes visible. Several particles (the number depends on the extent of morphological and constitutional variation of the inspected particles) are then qualitatively analyzed by EDX for the selection of elements which need to be mapped by EPMA. Usually, oxygen and carbon, which are not detectable by our EDX system, are included in our WDX element mapping. Whether or not element mapping for nitrogen should be included has to be checked by WDX analysis of a representative number of particles. Since B and F are usually not expected in particles, their presence is only checked in special cases. Fig. 1 shows an interesting example of two compositionally identical particles, which are also morphologically very similar but which were collected at very different places: Fig. 1(a) depicts a nickel particle which obviously had been introduced into the atmosphere in the liquid form from a smelting process in a nickel refinery.58 Fig. 1(b) shows a nickel particle which was collected in the outmost atmosphere by a high flying

Fig. 1 (a) Spherical nickel particle which was obviously emitted from a smelting process at the Monchegorsk nickel refinery. Courtesy of Y. Thomassen, Norwegian National Institute of Occupational Health, Oslo. (b) Nickel particle which was collected in the upper atmosphere by a high flying aeroplane.23


ZAF procedures of Armstrong.59,60 In contrast to the qualitative procedure, spectrometer defocusing cannot be neglected in semiquantitative analysis, even at high magnifications. The correction is performed using algorithms which were also developed in our group.61 This semiquantitative approach turned out to be necessary, e.g., for particle characterization in occupational health monitoring. In the course of the evaluation of samples collected at the largest nickel refinery in the world at Monchegorsk on the Kola peninsula (Russia), it became apparent that the differentiation of particles containing various amounts of nickel oxides and nickel sulfides in contrast to nickel sulfate particles was only feasible by semiquantitative characterization.58 In contrast to a previously developed similar procedure,56 we use WDX in order to include the important elements C, O and N. Unfortunately, this method is not capable of analyzing particles smaller than 0.5 mm in diameter for physical reasons. Therefore, other methods and instrumentation have to be applied for smaller particles. TEM of particles smaller than 0.5 mm in diameter Samples for TEM investigations are usually obtained on the last (fifth) stage of the cascade impactor with a Formvar foil placed on the glassy carbon disk. The Formvar foil is a polycarbonate filter reinforced by a copper grid. Very fine particles which cannot be collected by impaction are deposited on a Formvar foil which is placed on a ceramic filter support by suction with a small pump. In some cases, for ‘concentrated’ industrial aerosols, mere exposure of the Formvar foil to the contaminated atmosphere for a few minutes is sufficient. The Formvar foil is carbon coated prior to TEM inspection. Particles can be characterized in only a limited number since automation comparable to that outlined above for the EPMA procedure is not yet available. Particles thicker than 50 nm cannot be penetrated by the primary electron beam. However, thin areas of such particles can be inspected at their edges. The following information can be obtained (i) Particle size distribution and partly morphology can be studied by imaging with the transmitted electron beam. (ii) Phase identification is possible by selected area electron diffraction (SAED). If point patterns were obtained for single crystals, the observed reflections can be converted into the respective lattice constants. The latter are used for compound identification by search in the ICSD data bank using the procedure ‘PIEP’ which was recently developed by Miehe.62 (iii) In the case of very small crystallites and polycrystalline particles, diffraction rings are obtained. Since the atomic number dependences of the scattering power for electrons and X-rays are similar, the relative intensities in the corresponding electron and X-ray powder diffractograms are also similar. It is therefore possible in such cases to carry out a compound identification with the JCPDS data bank for X-ray powder diffraction data. (iv) Compound identification for amorphous particles is possible in simple cases (no multi-phase particles) by the determination of the elemental composition either by EDX or by EELS or, preferably, by a combination of the two methods. In favorable cases, speciation is possible because EELS peaks often contain binding information in the respective near edge fine structures.63 EDX and EELS are complementary since EELS is most sensitive for low-Z elements whereas EDX is more powerful for medium- and highZ elements by the nature of the underlying physical processes. Xhoffer et al.56 gave a more detailed account of EELS for single particle analysis. Space resolved speciation on a nanometer scale of 3d transition metals in mineralogical and geochemical samples by EELS has been extensively practised for several years.64–67 There are some very promising developments in TEM–EELS which will have a substantial influence for single particle

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characterization: up to now, EEL spectra were usually acquired from a small selected area (point analysis). Now, the twodimensional acquisition of any spectral feature of the EEL spectrum has become possible with what is called energy filtering transmission electron microscopy (EFTEM) or electron spectroscopic imaging (ESI).68,69 Energy filtering devices for the TEM have become commercially available only recently and the Gatan imaging filter (GIF) can be attached to almost any 100–400 kV TEM.70 With ESI, energy filtered images are acquired which can then be combined to show the distribution of elements in the specimen with nanometer resolution.68 This procedure has advantages over the STEM–parallel EELS combination since acquisition times for high resolution measurements are longer than those for ESI.68,71 Aerosol particles sometimes exhibit surprising nanometerrange fine structures. Fig. 2 shows a typical example of particles which were collected from the exhaust of an Otto-type engine.38 Most of the detected particles exhibited this special morphology of elongated crystals. Although diffraction reflections proved the crystallinity of these particles, the observed reflections could not be attributed to any tabulated compound. EDX analysis indicated the presence of carbon, nitrogen, oxygen and chlorine in a mass ratio of 98:77:1:6. Fig. 3 shows a typical particle from the waste incineration plant at Darmstadt. Sampling was carried out in the purified flue-gas of the plant.38 Graphitic carbon is the main component. Other constituents also present could not be identified.

Fig. 2 Transmission electron micrograph of a particle collected from the exhaust of an Otto motor. Magnification: 324 400; 0.5 mm = 1.22 cm.

Fig. 3 Transmission electron micrograph of a compound particle from the waste incineration plant in Darmstadt. Magnification: 3171 920; 50 nm = 0.86 cm.

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Elements present were O, Cl, K, Fe, Co, Ni and Zn. In both Figs. 2 and 3 the nano structure of the inspected particles is evident. HR-SEM investigation HR-SEM, usually using a cold field emission cathode to achieve the highest brightness of the primary electron beam at very low beam diameter (down to 2 nm), is also capable of characterizing particles down to 10 nm in diameter or elucidating details of larger particles.34 It has the great advantage over TEM in that it shows the morphology of particles irrespective of their size, whereas in TEM analysis particles which are not penetrated by the primary electron beam only appear black, without further morphological details. In addition, software for morphological automatic particle characterization is usually available so that a much greater number of particles can be morphologically characterized. Owing to the much smaller volume of particles in the nanometer range as compared with the micrometer range, only EDX analysis is possible and usually long counting times are necessary. In this case, problems might be encountered with the measurement of light elements (C, N, O) due to both unresolvable line overlaps with lines of transition metals such as Ti and a lack of sensitivity. Trace characterization of single particles by SIMS and other mass spectrometric methods Not many methods are available for the determination of trace elements in single particles.56 SIMS is known to be the most sensitive topochemical method,54,55 and it has been successfully used for the trace characterization of single particles.23 Since it uses minimal sample amounts during analysis, it can be considered to be a non-destructive technique. A further unique characteristic of SIMS is its ability to determine isotopic abundances and isotopic ratios in single particles, which is important for differentiating between cosmic and earth debris particles in erosion studies of space exposed materials.23,32 Owing to its good lateral resolution (better than 0.5 mm), in the beam scanning mode using both oxygen and cesium primary ions, particles down to 0.5 mm in diameter can be characterized. From numerous trace element measurements of Stadermann23 on cosmic particles, it can be deduced that at least the order of magnitude of trace constituents can be reliably determined by SIMS in single particles. It is interesting that isotopic ratio measurements of single grains of cosmic dust by SIMS have yielded important information on pre-solar matter and on nucleosynthesis as the theory of element formation in stars.18 LAMMS has been used extensively for single particle characterization and is a powerful technique especially for the characterization of poorly vacuum resistant organic particles.56,72 Equivalent in performance to SIMS, LAMMS is an off-line method because particles must be collected and mounted on a substrate. Detection of trace metals is feasible at the mg g21 level and speciation of inorganic compounds (especially those containing nitrogen and sulfur) is possible. Distinction between surface and volume compositions of a particle is also possible, in addition to the detection of trace organics (especially aromatics). However, large pulse-to-pulse and also particle-to-particle variations of the ion signal intensity inhibit quantification.73 For inorganic trace analysis in single particles, LA–ICP-MS is probably a more powerful technique, owing to the much more efficient ionization of material in the inductively coupled plasma. Laser ablation of the sample, on the other hand, should only produce a very fine aerosol which is quantitatively transported into the inductively coupled plasma by an argon carrier gas. Laser desorption/ionization (LDI) coupled with time-offlight MS (TOF-MS) was used by Johnston and Wexler73 for the

on-line characterization of single particles from a sampled air stream. Several particles can be sampled and analysed per second, allowing the compilation of large data sets in a very short time. This allows an improvement in the precision of analysis of particles having similar size and composition by averaging the recorded spectra. The average composition of each group is then quantitatively determined by comparison with spectra of relevant standard particles having known size and composition. PIXE is another method with a trace characterization capability for particles with detection limits of the order of 1–10 mg g21.56,74–77 Other variants of nuclear microprobe techniques are still restricted to a few laboratories worldwide but can provide a wealth of information for individual particles.56 mPIXE uses a well focused proton beam and has been used for the single particle characterization of giant North Sea aerosols and other samples for major, minor and trace elements.74–77 Three-dimensional SIMS characterization of large particles In recent years it has been shown that 3D imaging is a powerful new application of secondary ion mass spectrometry.78,79 This method combines the surface imaging capabilities of SIMS with depth profiling, creating layer-by-layer images of elemental distributions as the primary ion beam sputters deeper and deeper into the sample. The amount of data produced during a typical measurement of this type can easily reach several hundred megabytes, but with suitable imaging software it is possible to convert this information into easy to understand three-dimensional visualizations of elemental distributions within a given sample volume near the original surface. When the images are created by rastering the primary beam, the lateral resolution achievable is only limited by the beam diameter, which can be significantly smaller than 1 mm. The depth resolution is only limited by the thickness of the ion beam mixing layer (at best several nanometers). In general, particles represent one of the least suited categories of samples for analysis by 3D SIMS. Not only do particles often have a heterogeneous composition, leading to a series of artefacts, but also their morphologies often make meaningful 3D SIMS impossible without special sample preparation.80 One important requirement for SIMS measurements, a flat sample surface at the beginning of analysis, is generally not complied with. 280 0

x-axis z-axis 2

190

130

y-axis 0.0

370

Count rate Oxygen-18

100.0

Fig. 4 3D view of the oxygen distribution in an oxide coated mica particle. The full block represents a volume of 2003200310 mm3


However, in a few cases the 3D SIMS technique can successfully be applied to particle analysis. One example is shown in Fig. 4. Here we analyzed a mica platelet with a diameter of approximately 150 mm that is covered with Ti and Cr oxide layers on both sides and is used as a coloring pigment. Such coloring pigments are today produced in considerable amounts and their application is widespread, from cosmetics and children's toys to car lacquers.81 These pigments consist basically of mica platelets of a certain diameter which are coated with different oxides (e.g., TiO2, Fe2O3, Cr2O3). The nature of these oxides and the thickness of the coating determine the color which is achieved mainly by interference of the reflected light and this causes a pearly lustre effect. Double oxide layers are also sometimes used such as mica–TiO2–Fe2O3

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or mica–TiO2–Cr2O3. A cut-out 3D view shows the O distribution inside the particle in Fig. 4. Clearly visible are the two disk-like areas where O is enriched; the oxide coating of the particle. The mica platelet itself is between the top and bottom oxide areas. Since the 3D measurement was done for a total of 10 elements, it was possible to study the particle's internal structure in detail, to verify the lateral homogeneity of the layer's composition and to check the uniformity of the coating thickness. The oxide layers in this 3D image appear thicker than the mica platelet owing to their lower sputter rate. Data processing can correct for this effect as described elsewhere.80 If particles can be embedded in a conductive resin and the sample prepared in this way can be ground and polished, the

Table 1 Overview of multi-method particle characterization of solid aerosols Bulk characterization: TXRF quantification for main, minor and trace components Sample preparation: particle collection down to 100 nm particle diameter by one-stage or multiple-stage impactor on high purity glassy carbon discs. Quantitative analysis down to pg m23 by TXRF with internal standardization. Analysis for oxygen up to vanadium with a laboratory constructed TXRF instrument for low-Z elements. Analysis for vanadium to uranium with a TXRF instrument optimized for medium-Z elements. Solid-state speciation of bulk samples and/or single particles —By valence band X-ray spectrometry with EPMA–WDX (bulk and single particle) —By Mössbauer spectrometry for magnetic compounds (bulk only) Further possibilities not pursued by us up to now —By Auger electron spectrometry for light elements (bulk or single particle) —By X-ray induced photoelectron spectrometry (bulk or single particle) —By micro-Raman spectrometry (bulk or single particle) —By TEM–EELS (single very small particles) Topochemical characterization Sample preparation: (a) Particle collection down to 100 nm particle diameter by one-stage or multi-stage impactor on high purity glassy carbon discs or particle collection on polycarbonate or PTFE filters (b) Carbon coating by evaporation (if necessary) SEM survey —Selection of samples with the appropriate particle density ( if sampling with different sampling times was performed) —First survey of particle size distribution —First survey of particle morphologies —Check on particle homogeneity or heterogeneity and on particle aggregates —Qualitative analysis of individual particles by EDX for selection of elements to be studied by EPMA EPMA investigation (down to 0.5 mm diameter) —Elemental mapping (WDX or EDX) for elements selected by SEM —Qualitative particle evaluation —Semi-quantitative particle evaluation according to the procedure of Weinbruch et al.41 for unequivocal characterization giving: 4 elemental composition of particles (no trace contents) 4 number and size distribution of particles of specific composition 4 systematic assignment of size distribution and morphology to identified classes of particles 4 assessment of elemental homogeneity or heterogeneity of individual particles > 5 mm in diameter Investigation of particles smaller than 0.5 mm in diameter Sample preparation: particle collection with multi-stage impactor and TEM sample carrier (polycarbonate filter on copper grid) on proper stage of impactor TEM investigation —Particle size distribution and morphology —Phase identification by SAED for crystalline materials —Compound identification by determination of elemental composition by EDX–EELS for amorphous particles (or in addition to SAED for minor constituents) —In favorable cases: speciation by EELS (e.g., Fe2+/Fe3+) —New: element distribution maps with EFTEM with single nm resolution68,69 HR-SEM investigation Faster than TEM characterization especially for size distribution measurements. Particle composition only determinable by EDX. No speciation possible. Weak for light elements. Investigation of large particles ( > 5 mm diameter) 3D SIMS characterization Sample preparation: single particle transfer to SIMS sample holder by micro-manipulation or collection on glassy carbon discs as above on proper stage of impactor In special cases particle embedding in conductive resin and metallographic preparation Special procedures, e.g., for rigid particles between gold foils Study of 3D elemental distributions for major, minor and trace elements due to the high sensitivity of SIMS for most elements

840


internal structure of such a particle can then also be studied by SIMS with respect to elemental and isotopic distributions. Such efforts are under way in our laboratories.

Overview and scheme of multi-method particle characterization The complete scheme of particle characterization which is used by us is summarized in Table 1. It should be pointed out that a proper combination of methods will result in valuable information both for bulk characterization and for single particle evaluation. As an example, the combination of TXRF as a bulk method with SEM–EDX for cleanroom control should be mentioned.6,7 In this context, it is important to collect particles in a large volume of the relevant atmosphere (of the order of 10 m3) because of the extremely low particle concentrations to be measured (of the order of pg m23). Bulk particle concentrations are therefore measured after one-stage impactor sampling by

TXRF. A parallel morphological SEM inspection will reveal the particle size distribution of the collected particles. If particles with diameters > 0.5 mm are present they can be identified by SEM–EDX or WDX. Thereby, source apportionment is usually possible and the sources contaminating the cleanroom atmosphere can be identified and eliminated.6,7 On the other hand, particle characterization for occupational health monitoring requires a more precise compositional particle analysis, which can be performed for larger particles ( > 0.5 mm in diameter) by automatic WDX element mapping as outlined in Table 1 and in an earlier section. This has been carried out recently for particles collected at the Monchegorsk nickel refinery in Russia58 and at the Elkem Mangan in Norway.82,83 An important result of these investigations is that most of these particles exhibit a very complex composition which cannot be attributed to simple mineral phases such as NiS, NiO, MnO2 or MnS. This calls for a consecutive TEM characterization to study the phases present in such particles as is discussed earlier and outlined in Table 1. A further evaluation of such samples for the composition of very fine particles with

Table 2 Information content and particle size limitations of particle characterization methods Method Particle size limitation Morphology and size distribution SEM, EPMA ~ 100 nm diameter HR-SEM, TEM ~ 10 nm diameter Elemental and isotopic composition TXRF < 20 mm, no single particle identification EPMA–WDX

~ 0.5 mm diameter

SEM–EDX

~ 0.5 mm diameter

HR-SEM/EDX ~ 20 nm diameter STEM–EDX–EELS ~ 10 nm diameter CTEM–SAED

~ 300 nm diameter

EFTEM SIMS

Single nm ~ 0.5 mm diameter

SIMS

~ 0.5 mm in diameter

LAMMS

0.5 mm in diameter

LA–ICP-MS LDI–TOF-MS

0.5 mm in diameter

AES

~ 50 nm in diameter

PIXE

~ 5 mm in diameter

Information content ©Surface morphology, automatic particle size distribution measurement ®TEM: internal particle structure Measurement of integral elemental concentrations down to pg/m23 (lower concentrations may be reached by use of rotating X-ray anodes). Quantitative elemental composition for main, minor and trace elements down to oxygen with our equipment Semiquantitative single particle composition for major and minor elements down to boron. Automatic analysis, but extensive data evaluation necessary Same as for EPMA-WDX for automatic measurements. However, lower spectral resolution and inferior detection limits compared with WDX Semiquantitative or qualitative single particle composition for major and minor elements Semiquantitative or qualitative single particle composition of amorphous and crystalline particles for major and minor elements down to boron (EELS) or carbon (EDX) Compound identification by crystallographic data for well crystallized species, usually applied in combination with EDX–EELS measurements Element distribution maps for particles essentially thinner than 100 nm Qualitative single particle composition for major, minor and trace elements for all elements, in favourable cases down to the ppb level. Isotopic composition also determinable Method is practically non-destructive compared with LAMMS; many repetitive analyses are possible on one particle Qualitative single particle composition for major, minor and trace elements for all elements. Applicable also to vacuum sensitive particles Isotopic composition also determinable. The methods are destructive Same as for LAMMS, but very fast analysis feasible, making the system applicable to on-line measurements of air sampled particle streams Qualitative or semiquantitative single particle composition for major and minor elements down to lithium Quantitative single particle composition for major, minor and trace elements, usually down to sodium (Z = 11)

Particle speciation EPMA–WDX

~ 0.5 mm

STEM–EELS

~ 20 nm

EFTEM TEM–EDX, EELS EFTEM AES

Single nm ~ 30 nm Single nm ~ 30 nm

XPS

mm range or above

Mössbauer Spectometry Micro-Raman

Integral method for mg samples

Single particle speciation by AES, since AES signal shapes contain bonding information, especially for low atomic number elements Single particle speciation by XPS, since the energetic fine structure of photoelectron spectra is bonding sensitive Identification of compounds of elements exhibiting Mössbauer effect

~ 1 mm diameter

Functional group analysis of single particles

(a) Single particle speciation by semiquantitative determination of elemental composition (b) Single particle speciation by valence band X-ray spectrometry and comparison of data with such of reference compounds (tedious but possible even for fifth period elements of the PSE) Single particle speciation by EELS since EELS peak shapes contain bonding information with stigmatic electron spectrometers Single particle speciation by semiquantitative determination of elemental composition


diameters < 0.5 mm by TEM and/or HR-SEM is also an important goal of our future activities. Table 2 gives an overview on the information content of the methods addressed in this paper and on particle size limitations. It tries to give an updated brief overview with emphasis on topochemical methods of analysis as compared with earlier and partly more extensive overviews of this field of particle characterization.56,57 Conclusion A main incentive to this paper was to show the interdisciplinarity of particle characterization and its importance to many and diverse fields of science and technology. We have also tried to show that besides the conventional bulk methods of particle characterization used extensively and routinely such as AAS, XRS, ICP-OES and MS there is a wealth of topochemical methods which can contribute especially to single particle characterization. It is the multi-method approach which produces valuable synergistic effects in an in-depth interpretation of relevant results. Of course, it is neither possible nor meaningful to use all the described methods together to provide solutions to questions related to particle characterization. However, certain combinations have proved to be very successful in our experience, e.g., the combination of the bulk method TXRF with the topochemical methods of SEM and EPMA, e.g., in the field of particle characterization for cleanroom control of ultratrace analytical laboratories dedicated to quality control of reagents for the microelectronics industry. The procedure developed by us for semiquantitative particle characterization by EPMA has revealed a complex composition of particles collected in the metallurgical industries. This now calls for further characterization of the phase composition of such particles by TEM–SAED–EDX–EELS. SIMS, on the other hand, can reveal the three-dimensional compositional structure of large particles with certain geometries or the isotopic pattern of trace elements of particles, e.g., to distinguish between terrestrial and cosmic particles in nearEarth space. It is to be expected, that certain combinations of the methods of solid-state characterization addressed here will lead to many new insights into the very diverse fields of science and technology for which particle characterization is of relevance.

HR ICP-OES, MS ICSD JCPDS LA–ICP-MS LAMMS LC LDI–TOF-MS MRP MSC PIXE PSE PTFE SAED SE SEM SIMS STEM TEM TLC TXRF WDX XPS XRS ZAF

1 2 3

5

List of acronyms AAS AES

BSE 3D EELS EDX EFTEM EPMA ESI FEG FWHM GC HPLC

Atomic absorption spectrometry Auger electron spectrometry (this is the common acronym in topochemical analysis. Unfortunately it is identical with the acronym for atomic emission spectrometry in bulk analysis. In this paper, AES is used exclusively for Auger electron spectrometry) Backscattered electron Three-dimensional Electron energy loss spectrometry Energy disperse X-ray (fluorescence spectrometry) Energy filtering transmission electron microscopy Electron probe microanalysis Electron spectroscopic imaging Field emission gun Full width at half maximum Gas chromatography High-performance liquid chromatography

High resolution Inductivity coupled plasma optical emission spectrometry, mass spectrometry Inorganic Crystal Structure Data Base Joint Committee of Powder Diffraction Standards Laser ablation–inductively coupled plasma mass spectrometry Laser micro(probe) mass spectrometry Liquid chromatography Laser desorption/ionization–time of flight mass spectrometry Mass resolution power Molecular sieve chromatography Proton induced X-ray emission (spectrometry); m-PIXE in frequently used for PIXE with focused proton beams Periodic system of the elements Polytetrafluoroethylene Selected area electron diffraction Secondary electron Scanning electron microscopy Secondary ion mass spectrometry Scanning transmission electron microscopy Transmission electron microscopy Thin-layer chromatography Total reflection X-ray fluorescence (spectrometry) Wavelength-dispersive X-ray (fluorescence spectrometry) X-ray induced photoelectron spectrometry X-ray spectrometry Atomic number–absorption–fluorescence (correction procedure)

References

4

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6 7 8 9 10 11 12 13

14 15 16

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Paper 7/07457C Received October 16, 1997 Accepted January 22, 1998