Instrumentation, data evaluation and quantification in on-line aerosol mass spectrometry

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1 JOURNAL OF MASS SPECTROMETRY J. Mass Spectrom. 2007; 42: Published online in Wiley InterScience ( SPECIAL FEATURE: TUTORIAL Instrumentation, data evaluation and quantification in on-line aerosol mass spectrometry Klaus-Peter Hinz and Bernhard Spengler Institute of Inorganic and Analytical Chemistry, University of Giessen, Schubertstrasse 60, D Giessen, Germany Received 21 February 2007; Accepted 31 May 2007 On-line micro- and nanoparticle mass spectrometry has evolved into a prominent analytical method for the characterization of airborne particles, particle populations and aerosols over the recent years, driven by essential developments in instrumentation, data evaluation and validation. In this tutorial, the fundamental aspects of the technology and methodology for qualitative and quantitative on-line aerosol particle analysis are discussed. Specific properties of the on-line mass spectrometric instrumentation for particle analysis are described, combined with a discussion of basic differences of the instruments and demands for future improvements of instruments and data analysis techniques. Optimized technology and methodology in particle analysis is expected to lead to essential growth of the knowledge and to quality improvement of the description of atmospheric processes and health effects in the future. Copyright 2007 John Wiley & Sons, Ltd. KEYWORDS: chemical composition; mass spectrometry; on-line particle analysis; atmospheric aerosols; clustering algorithms INTRODUCTION Mass spectrometric analysis techniques have been widely used for the characterization of single particles and particle populations over the last 20 years. Various instrumental designs have been developed for laboratory and field applications to improve knowledge regarding particle effects on atmospheric processes and on human health. These instrumental developments has also had impact on improved handling of particles in technical applications and influenced the development of new methods for particle emission reduction. Nanotechnological processing, bio-aerosol identification and climate modelling essentially depend on data from detailed and reliable particle analyses. Effects of particles on humans and the environment are controlled by many different properties such as particle size, shape, chemical composition, number concentration, surface consistency and others. Identification and characterization of aerosol particles, as well as knowledge of their behaviour in the various layers of the atmosphere, are essential for a better understanding of the earth s climate, cloud formation and visibility effects. 1 3 Among the human health effects directly correlated to the presence of particles are an Ł Correspondence to: Klaus-Peter Hinz, Institute of Inorganic and Analytical Chemistry, University of Giessen, Schubertstrasse 60, Bldg. 16, D Giessen, Germany. klaus-peter.hinz@anorg.chemie.uni-giessen.de increased mortality in regions with high outdoor particle concentrations, 4 6 indoor allergic reactions caused by house dust or passive smoking and an increased resistance of bacteria against antibiotics in hospitals Both natural and anthropogenic particle sources are known to influence the local, regional and global density and composition of particle populations. Volcanoes, biomass burning, particle generation from ocean waves (wind-blown sea spray and bubble bursting) and desert storms are some of the main natural particle sources. Industry, motor vehicles and emissions from private homes by coal-fired heating are the most important anthropogenic particle sources. Size and chemical composition are two prominent particle parameters. They influence directly, e.g. particle transportation in the atmosphere or into the lungs, and control reactions with surrounding gases, surfaces or other particles. The demand for an improved instrumentation for studying particle behaviour and physico-chemical characteristics was first mentioned by Friedlander in the 1970s. 12,13 Since then, various instrumental approaches have been described using different particle detection and analysis methods. Off-line versus on-line analysis Aerosol analysis methods can be subdivided into two principal groups of techniques, off-line methods and on-line methods. Copyright 2007 John Wiley & Sons, Ltd.

2 844 K.-P. Hinz and B. Spengler Off-line techniques are characterized by distinct collection and transportation procedures prior to laboratory analysis, resulting in a rather slow overall process. Sizeresolved analysis of particles is possible with off-line methods, employing either particle impactors with size-selective collection stages or other size-selective instruments such as differential mobility analysers (DMA). Chemical characterization of the collected particles can be performed either with bulk-analytical methods or with single-particle techniques. Bulk methods such as atomic absorption spectroscopy (AAS), 14 proton-induced X-ray emission (PIXE) 15,16 or ion chromatography 14,17 can provide quantitative information on specific inorganic particle constituents (e.g. soluble ion fraction). Electron microscopy techniques, such as scanning electron microscopy (SEM), transmission electron microscopy (TEM) 18,19 and electron probe X-ray microanalysis (EPXMA), 20 as well as laser microprobe mass analysis (LAMMA, LMMS) are important techniques for off-line single-particle analysis. A fundamental limitation of off-line techniques is that sampling and transportation procedures can cause artefact formation, chemical transformation of constituents or loss of volatile components. Nevertheless, information obtained from off-line techniques can be valuable since a well-defined sample preparation can allow obtaining reproducible information on quantity of particle components and particle morphology (e.g. from SEM data). In combination with online techniques, such data can be used for cross-correlation and inter-comparison experiments for a comprehensive characterization of aerosols. 17,27 33 In contrast to off-line methods, on-line techniques employ a direct aerosol inlet to transfer the particles within milliseconds from their native (airborne) environment into the analytical high-vacuum system using differentially pumped transfer units. On-line mass spectrometers are therefore characterized by a minimized chemical modification of particles and a high temporal resolution (due to very fast and immediate analysis) combined with analytical specificity on the single-particle level. These features also lead to the demand of instrumental mobility for on-line particle analysis at the locations of interest (e.g. ships on oceans, higher altitudes on mountains or in airplanes, in rural or urban areas as well as in certain indoor environments). Such instruments have to have superb long-term stability of all operating parameters after transportation and should be able to analyse particle populations instantaneously and with high frequency in single-particle mode. Analytical methods for on-line particle characterization differ considerably in instrumental parameters (regarding, e.g. particle inlet system, detection and sizing system, particle ionization method, mass spectrometric analyser) and in the data evaluation techniques employed (e.g. hierarchical classification, fuzzy c-means or k-means clustering, artificial neural networks). Off-line particle analysis is still a valuable technology, especially for detailed, specific investigations and for investigations of aerosol locations where on-line instruments cannot be used because of size or operating conditions. Over the last 15 years, on-line mass spectrometric systems, however, have increased in number dramatically, owing to their clear advantages. Currently, at least 20 research groups use laserbased on-line particle mass spectrometers and about 50 mass spectrometers with on-line thermal particle vapourization coupled with electron-impact ionization are under operation worldwide. In the current situation of available technology and application of particle mass spectrometry to, for example, the identification of natural and anthropogenic sources, bio-aerosol detection, atmospheric research or to other fields of high complexity, it appears that instrumental techniques such as on-line and off-line methods still have to be employed in combination to give a comprehensive and true picture of the investigated aerosol. Single-particle versus multi-particle detection There is another clear methodological distinction besides that of on-line and off-line particle analysis, which is single-particle versus multiple-particle detection. While laserbased time-of-flight mass spectrometers typically analyse particles individually, other mass spectrometric techniques such as thermal ionization systems average data from several (mostly an unknown number of) particles to form one data set. To distinguish between the two principal techniques, we will call the former systems single-particle mass spectrometers (SPMS) and the latter multiple-particlemass spectrometers (MPMS). The aim of the present paper is to highlight the specific properties of on-line instrumentation for aerosol particle analysis (both SPMS and MPMS), to discuss fundamental differences of the instruments and the necessary future developments for an improved quality of analytical results concerning particle composition by mass spectrometric investigation. HISTORY A short historical overview will reflect the main directions of instrumental developments, evaluate the achieved results and describe the new designs for particle analysis. Around 1980, several research groups were performing experiments for on-line particle analysis using nozzles and pulsed valves as inlet systems and electron impact (EI) and/or heated surfaces for particle ionization The first approach employing a continuous laser beam for singleparticle detection by light scattering and a pulsed UV laser for particle ionization was described in 1984 by Sinha. 38 These early instruments, however, used quadrupole or magnetic sector field mass spectrometers for mass analysis. Acquisition of a complete mass spectrum from a single particle or an ensemble of particles was not possible with such scanning mass analysers. Most of the current state-of-the-art instruments for single-particle detection and ionization are based on the 1988 proposal of Marijnissen 39 employing time-of-flight mass spectrometers (TOF-MS) with laser-based ion sources. The first instrument using a TOF-MS for single-particle analysis (SPMS) was realized by McKeown et al., 40 quickly followed by several other groups having developed individual instrumental solutions. Instrumental improvements have been made since this time concerning the inlet system (see Ref. 47), ionization method (see Refs 48,49)

3 On-line aerosol mass spectrometry 845 and mass analysers (e.g. bipolar TOF mass analysis or ion trap analysis. 54 ) Another major group of on-line instruments is aerosol mass spectrometers, employing thermal vapourization of particles and EI ionization combined with quadrupole mass analysers These MPMS systems are also realized as orthogonal-acceleration reflectron TOF instruments with improved mass resolution. They now provide the singleparticle mode as well. 58,59 The development of on-line instruments for particle analysis was influenced to a large extent by new technologies in the fields of lasers, vacuum devices, data acquisition devices and computers. As a result, new instruments could be developed as transportable devices for sampling and analysing particles during field measurements in desired locations 51,54,60 or on mobile platforms such as airplanes 59,61 63 or trucks. 53,64,65 The interest in on-line technology for particle analysis also pushed the commercialization of this technology, leading to an SPMS system (Model 3800, TSI Inc., St Paul, USA) and an MPMS system ( Aerodyne Aerosol mass spectrometer (AMS), Aerodyne Research Inc., Billerica, USA). During the past 10 years, several review and tutorial papers have been published reflecting the current developments and state of the art of particle analysis methods ,66 69 These reviews cover instrumentation for physical and chemical particle characterization, 68 fundamentals of aerosols associated with mass spectrometric analysis methods, 49 instrumentation for the chemical bulk composition analysis and/or single-particle composition analysis of aerosols, 20,48,70,71 detailed descriptions of particle mass spectrometer designs 47,49,69 and historical overviews. 71 The scope of this paper consists in the discussion of fundamental aspects of instrumentation, data evaluation and quantification in on-line aerosol particle analysis. Necessary developments for an enhanced application of on-line particle analysis techniques and for a reliable characterization of particle populations are discussed. INSTRUMENTATION STATE OF THE ART This section summarizes and discusses selected features of on-line particle mass spectrometers and their influence on the analytical results. Details of specific instruments can be found in the cited literature. 47,59,67 The principal set-up of on-line instrumentation for particle analysis is sketched in Fig. 1. aerosol inlet system mass spectrometer data evaluation knowledge Figure 1. Scheme of a mass spectrometric system for aerosol analysis including instrumentation and data evaluation. The overall systems consist of a particle inlet system, a particle sizing system, a particle vapourization and ionization system, a mass analyser and a data evaluation system. Inlet systems Particle inlet systems are used to transfer particles from their natural environment into the vacuum chamber for physical and chemical mass spectrometric analysis, with a minimum of chemical modification. The aim of inlet systems is a high transfer rate for all particle sizes and particle shapes. Typical nozzle/skimmer systems (Fig. 2(A)) have particle-size-dependent transmission efficiencies between less than 1% to almost 100% for particle diameters of 200 nm to 2 µm. 72 The low transmission efficiency for small particles is a result of the divergence of the particle beam. The divergence angle downstream of the inlet nozzle increases with decreasing size of the particles owing to a smaller inertia. Airborne particles in this size range are accelerated in nozzle/skimmer systems to velocities of m/s because of the formation of a supersonic jet of air resulting from the pressure gradient. Distances between nozzle entrance and particle ionization region of a few centimetres and residence times of particles in the inlet system prior to analysis of several milliseconds can be realized with ionization lasers having short trigger delay times (a few hundred nanoseconds) such as excimer or nitrogen lasers. 32,42,60,73 The application of Nd : YAG lasers having longer trigger delay times ( µs) 51,53,74 or instruments using two lasers for separate particle vapourization and ionization require larger distances between particle detection and ionization 75,76 in order to form longer transient times allowing for longer delays. Such instruments suffer from lower particle detection efficiency (due to particle beam divergence), but on the other hand, take advantage of an improved particle size determination quality (see section Particle Sizing ). As a function of the distance between the nozzle and the ionization region, the particle beam can have diameters up to some millimetres in the plane of particle ionization. This influences the total detection and analysis rate because laser focus diameters in the range of only a few tens to a few hundred micrometres are typically used. A stronger dispersion of particles of different velocities during the longer transient path, however, provide a better resolution of velocity determination (and thus size determination). Another important group of particle inlet systems are aerodynamic lenses 55,77 80 (Fig. 2(B)). A critical orifice upstream of the aerodynamic lens with diameters of µm reduces the pressure, while an expansion volume in front of the lens leads to a relaxation of the gas flow. Lower pumping capacities compared to nozzle/skimmer systems are necessary. Different pressure ratios between the entrance and exit of the lens can be chosen to select a particle size range of interest by using controlled valves 53,81 or a series of critical orifice diameters. 82 The flow through the lens must be subsonic and laminar to achieve a well-focussed particle beam of a few hundred micrometres to a few millimetres in diameter at a distance of cm behind the exit of the lens for particle diameters of 100 nm to 3 µm. 53,55,80 The

4 846 K.-P. Hinz and B. Spengler A nozzle skimmer aperture aerosol pump mass spectrometer pump particle detection/ ionization vacuum chamber B aerosol pump mass spectrometer orifice aerodynamic lens pump nozzle aperture Figure 2. Scheme of the principal set-up of a nozzle/skimmer inlet system (A) and a particle inlet system using an aerodynamic lens (B). Both systems are differentially pumped. optimum size range typically covers a decade in diameters with transmission efficiencies of nearly 100%. 55,83 Two important parameters of the inlet systems are currently under critical investigation: (1) a particle-shape dependence of particle transmission and (2) evaporation and condensation of material during particle flight through the inlet systems. Irregularly shaped particles experience additional forces in a gas stream compared to spherical particles, resulting in larger divergence angles when leaving the inlet system. Experiments with aerodynamic lenses as inlet systems showed an increase of the detection efficiencies by a factor of 5 10 for spherical compared to non-spherical particles. 75,80,84 Furthermore, alignment of fibrous particles in laminar flows have been reported. 85 The shape of a particle can also depend on its composition and water content. 84,86 Variations in transmission efficiency due to particle shape have to be taken into account when interpreting quantitative information of specific particle groups. Inlet systems with nozzle/skimmer arrangements have not been investigated systematically regarding a possible shape dependence of the particle beam divergence. The higher acceleration of the particles in these inlet systems compared to aerodynamic lens systems, as well as the shorter distances between inlet system and particle ionization region, may lead to a reduced shape (and composition) dependence of the particle detection efficiency. Ambient particles are typically mixtures of various components and contain water depending on the relative humidity. Adsorption of water to such particles can lead to almost spherical particles under high humidity conditions and therefore the influence of particle shape on the particle detection efficiency is reduced. 87 Evaporation or condensation of volatile components can change the portions of specific components in the particles during their flight through the inlet system to the ionization region. In nozzle/skimmer systems, the adiabatic expansion leads to a considerable particle cooling with the possibility to conserve volatile components and water. On such cold particles, condensable substances (e.g. water) can be deposited during the flight through the nozzle system depending on particle surface consistency and relative humidity of the air. Condensation values of a few percent in volume have been reported. 88 In nozzle/skimmer systems, wet particles can lose a significant portion of water (up to 20%) during a 5-ms flight in a vacuum chamber by evaporation. 47 Aerodynamic lens systems are longer than nozzle/skimmer systems and particle velocities are lower ( m/s for particle sizes of 100 nm to 3 µm). 53,55,63 Residence times in vacuum prior to ionization for aerodynamic lens systems are therefore between a few ten milliseconds and a few hundred milliseconds and can lead to significant evaporation. 47,56 In thermal desorption instruments (MPMS), a strong evaporation effect and a low relative humidity condition can also influence particle shape and detection efficiency owing to particle bounce on the vapourizer surface. 63 Particle sizing Particle size is an important parameter for aerosol characterization. In this section, only a short discussion of the basic configurations for particle sizing is presented. Reviews of sizing systems and influences on particle detection can be found in the literature. 2,47,49 Two common methods of particle size determination are pre-selection of a certain size range on one hand, and velocimeter analysis with in-flight particle detection and sizing on the other. The latter method uses a distinct flight path for start stop measurement, somewhere downstream of the inlet system inside the vacuum chamber to determine the particle velocity. The majority of single-particle mass spectrometers use two separate continuous-wave laser beams to measure the transit time of individual particles as they pass through the two beams. The scattered light is detected and the particle velocity is calculated on the basis of the known distance between the two beams. The scattered light intensity is heavily dependent on the particle diameter and therefore application of this method is limited to a minimum particle size of about 100 nm in diameter. With a determined particle velocity the ionization laser can be actively triggered to strike the particle at the laser focus at the right time. There is a limitation to this method due to the fact that faster (smaller) particles might overtake slower (larger) particles during a velocimetric measurement if particle densities are high. This problematic effect cannot be avoided completely but has much greater relevance at longer flight path lengths between the two detection laser beams. The result of the particle overtake effect is wrong size determination and failure in particle ionization. 89 In most thermal desorption instruments (all versions of the Aerodyne AMS instrument) the particle velocity is determined on the basis of the time between the opening of a chopper wheel (duty cycle of 1.8%) and the arrival of the particles in the ionization region. 55 Extending the lower size limit to about 20 nm can be achieved by using an internally triggered (fixed repetition frequency) ionization laser without single-particle detection. 45,53,82 This blind particle analysis method has a rather low detection efficiency with no particle sizing capability. It can, however, be combined with pre-filtering of a certain particle size range by tuning the inlet system parameters. 82 Application of single-particle detection techniques is useful only for rather low particle fluxes because of the limited repetition rate of the ionization laser (e.g.

5 On-line aerosol mass spectrometry 847 maximum of about 200 Hz with excimer lasers) and the necessary data storage and handling times after mass spectral acquisition. 90 For high particle concentrations, gas dilution methods preceding the aerosol introduction can be employed. Analysis of high-particle-concentration aerosols can be favourably performed with MPMS such as the AMS system (Aerodyne Research Inc., Billerica, USA), characterizing ensembles of particles that reach the ionization region at the same time, with a lack of single-particle information. High particle densities are especially common in the size range below 100 nm. Particle vapourization and ionization Once the particles are detected and/or their size determined, they are vapourized and ionized. At present most of the instruments use either heated surfaces combined with EI or pulsed lasers for ionizing the particles. Various lasers or combinations of lasers have been used for desorption/ionization of particles. 47,49 The choice of a laser has a direct impact on the set-up for (light scattering) particle detection, on the electronic units for laser triggering and on the data collection rate. Nd : YAG lasers with internal trigger delays between 100 and 200 µs require distances between the particle detection zone and the ionization region of about 10 cm. The use of excimer or nitrogen lasers with internal trigger delay times of only a few hundred nanoseconds, instead, allow positioning the ionization laser beam much closer to the particle detection region with a minimum distance of about 1 mm. 32 As a result, measurements at higher particle densities are possible with increased particle ionization rates. The ion formation in single-particle analysis (SPMS) by intense laser pulses is a complex process and the mechanisms of ion formation are still a topic of current research. 47 The ion yield is a function of many instrumental parameters (e.g. laser wavelength, laser pulse energy, irradiance) and particle properties (e.g. morphology, light absorption efficiency, size and composition). For particles larger than about 800 nm, only incomplete evaporation and ionization can be achieved with typical laser irradiances as high as 10 9 W/cm 2. As long as the components are homogeneously distributed within the particle, the determined particle composition is still representative for the complete particle even with incomplete vapourization. Spectra of inhomogeneously mixed particles, however, will show a greater variability under such conditions. To achieve high irradiances, the ionization laser beam has to be focused to a certain spot size, depending on the laser pulse energy and the pulse duration. A large laser focus diameter of, e.g., 200 µm can easily be directed onto the particle beam with high spatial overlap. A small laser focus of only 30 µm indiameter,for example, has to be aligned with much higher accuracy, but with the advantage of a much higher irradiance and a better selectivity for single-particle detection. The number of ions produced from a certain particle is certainly a function of the laser wavelength used. Laser irradiance and laser fluence, however, are also very important parameters (see section Quantification ), as shown in Fig. 3. Incineration particles containing polycyclic aromatic hydrocarbon (PAH) compounds were measured with the LAMPAS 2 instrument at a laser wavelength of 337 nm (nitrogen laser). For this measurement, a reduced laser irradiance of about 10 7 W/cm 2 was used. A reduced number of fragment ions were observed, and organic molecules with higher masses could be detected with much higher probability under these conditions. Similar results were found with a laser wavelength of 266 nm and irradiances of 5 ð 10 7 W/cm 2, reflecting the importance of a controlled laser irradiance. 91 After in-flight (partial) disintegration of the particle by an intense laser pulse, a strong interaction of the produced ions, electrons and neutrals takes place in the dense plume of the ion source region, and ion formation, recombination or electron attachment can occur, e.g., by electron or proton transfer reactions. 92 The particle matrix (i.e. main particle components) often essentially influences the detection of minor particle components in analogy with the ion formation in matrix-assisted laser desorption/ionization (MALDI). 92,96 A reduction of such matrix effects can be achieved by separating the desorption and ionization process using a combination of a desorption (often infrared) laser and a UV laser. 75,97 99 Application of high irradiances of W/cm 2 to W/cm 2 completely vapourizes the particles and atoms are mostly converted to positively 100 NO K + 39 K 2 Cl intensity / a.u Cl - 35 NO 2-46 NaCl 2-93 KCl2-109 intensity / a.u Na + 23 PAH CN m/z Figure 3. Spectra of a single particle from waste incineration containing PAH. The sample was nebulized and introduced into the LAMPAS 2 instrument for analysis m/z 400

6 848 K.-P. Hinz and B. Spengler charged ions and electrons. This method improves atomic quantification at the cost of information of molecular particle components. Thermodynamic calculations of the ion production rate are difficult because of many (mostly unknown) multifactorial parameters. 47,102 Reliable quantitative predictions for individual components in single ambient particles are therefore not possible. 75 Laboratory measurements with model particles are nevertheless meaningful and necessary to determine the response of different instruments or instrument types regarding certain particle components. Relative ion yields of species determined for certain particle types (minerals, salts, carbonaceous material, etc. 32,93,95,103 ) can be used to compare quantitative data derived from an ensemble of particles of a certain type with chemical bulk data. A direct application of such determined ion yields to single ambient aerosol particles is of limited value because of diversity and heterogeneity of such particles. Therefore, inter-comparison experiments with different on-line particle instruments have been performed as a first approach for obtaining a more comprehensive picture of the aerosol of interest. Dedicated methods of particle desorption/ionization have been applied to the detection of specific particle components. Various organic compounds, for example, were analysed by resonance-enhanced multi-photon ionization (REMPI), 76 by a combination of infrared laser evaporation and UV laser ionization, 97,107 by laser vapourization combined with chemical ionization 108 or by soft ionization techniques such as vacuum ultraviolet (VUV) photoionization and lithium attachment. 69,109,110 After particle vapourization, EI with standard electron energies of 70 ev, as used in the AMS instrument, 55,57 and electron attachment 111,112 can also be employed for ionization of the evaporated material. The use of thermal desorption provides for evaporation of non-refractory aerosol compounds. Quantitative analysis of such components is possible (e.g. ammonium, nitrate, sulfate and organics) when coupled to EI ionization. Quantitative analysis of the complete particle population with this method alone is, however, not possible since several refractory components such as elemental carbon, mineral dust and sea salt cannot be detected with this technique. Mass spectrometer Several types of mass spectrometers have been used for on-line particle analysis and identification of particle compounds. Currently the most commonly used mass analysers are time-of-flight instruments, quadrupole mass filters and quadrupole ion traps. The generation of ions from particles requires their destruction, and therefore only one single measurement per particle is possible. Both positive and negative ions as well as electrons are produced by the ionization process. An ideal mass analyser should therefore be able to detect these ion species within the same measurement. Time-of-flight instruments are often used in single-particle (SPMS) and multiple-particle ( ensemble, MPMS) instruments (e.g. thermal desorption instruments) because the registration of a complete mass spectrum per measurement is possible. 74,82,109,113 Furthermore, simultaneous detection of positive and negative ions is possible in dual time-offlight instruments, as is desired for single-measurement analyses Bipolar ion detection has been shown to provide complementary information on a single particle while retaining the correlation between both types of ions. In-flight single-particle ionization generates an ion plume that expands rapidly into the vacuum chamber. The extent of particle vapourization (complete or incomplete) influences the preferred direction of the plume expansion, and the related variations of ion flight times have been observed. 47,114 With static electrical fields for ion acceleration in TOF instruments, only a small part of the ions can be focussed in time onto the ion detectors. Delayed ion extraction (DE) can be employed to improve mass resolution and mass accuracy, resulting from a dilution in space of the initially dense ion plume and a kinetic compensation of spatial ion distributions in the axial direction. 53,114 High mass resolution and high mass accuracy are essential prerequisites for high-quality evaluation of SPMS data employing statistical methods. Application of ion reflectors further improves mass resolution and mass accuracy in TOF mass spectrometers for particle analysis. 51,53,58,59,113 In single-particle instruments (SPMS) typical values of mass resolving power are in the range of for reflectron arrangements depending on the experimental set-up (e.g. flight tube lengths, acceleration voltages, design of ion source). A mass resolving power of more than 4000 at m/z D 200 u has been reported using a double reflector arrangement at the expense of sensitivity in an AMS instrument. 59 Quadrupole mass filters were often used in the early days of on-line particle analysis, and currently several thermal desorption MPMS instruments employ such analysers. 38,55 57,63 The two operational features, mass range scan and unipolar ion detection, prevent measurements with high temporal resolution and high specificity. Quadrupole ion trap mass spectrometers provide nearreal-time single-particle analysis and additionally allow MS-MS experiments. 54,115 Data evaluation Besides a reliable operation of the instrument, a fast and validated data handling and data analysis system is a major prerequisite for a successful application of on-line mass spectrometric particles analysis. Data evaluation covers statistical procedures for data reduction and extraction of desired information (e.g. by classification algorithms), chemical and physical interpretation of data and comparison and combination of all data obtained from the employed instruments and other (e.g. meteorological) data sources during a measurement period. Various statistical, computer-based algorithms have been developed in aerosol research to sort the particle data and extract the relevant information covered in the data sets. Essential requirements for the application of these algorithms are, e.g., mass determination as exact as possible, a minimum of one mass peak per spectrum, an automatic spectra storage and at least 8-bit A/D conversion during spectra recording. In SPMS, typically a number of 1000 detected spectra can be acquired and archived per hour. During long-term

7 On-line aerosol mass spectrometry 849 measurements, several thousand to millions of spectra are obtained and have to be analysed. Reduction of data and extraction of desired information is an essential task of the method. Different pre-processing algorithms are used to prepare the spectra for data evaluation and classification, such as spectra normalization and determination of centroid mass values, peak intensities and peak areas. Pre-scaling with the square root of peak areas can reduce the influence of the strongest ion signal in the spectrum. 116 Such preprocessing algorithms, however, have to be employed carefully to avoid incorrect classification. Data are translated into multi-dimensional vectors for each single particle and then combined in a data matrix for a complete particle population. In another pre-processing approach, the timeresolved incidence abundance of specific ion signals of all spectra of a population is evaluated with 1-h temporal resolution. Such time-resolved chemical histograms show temporal variations of the relative abundances of specific components and can be used to select periods of interest. 53,106 Several classification procedures are used to determine main classes within the available data set, such as principal components analysis, 50 k-means clustering, 53,117 fuzzy c- means clustering, 118 artificial neural networks 82,119,120 and hierarchical cluster analysis. 117,118,121 The latter method combines similar spectra or spectra patterns in a joining tree (dendrogram), whereas the other procedures calculate the basic particle classes displayed as spectra patterns. These patterns are representative for a certain type of aerosol particles and can be chemically interpreted. Most of these algorithms also determine the portion of the classes in the classified population as quantitative information (see section Quantification ). Sorting the spectra into sub-populations with respect to particle size or periods of spectra detection, followed by individual classification steps, results in a more detailed description of particle populations during long-term measurements. 106 After classification of the sub-populations, the individual patterns were compared using hierarchical cluster analysis (HCA) to find similarities between the sub-populations and to identify major particle classes. The pre-determination of hundreds of classes within ten thousands or hundreds of thousands of spectra followed by a hierarchical cluster analysis to a few main classes is a similar approach for data reduction employing visualization as circular dendrograms. 117 This procedure can lead to a stronger averaging of spectra. Particle classes of low abundance can easily get lost with this approach, especially if low-abundant classes are similar in pattern to dominant classes. Currently, only a few studies describe inter-comparison of different data evaluation procedures, such as artificial neural networks, fuzzy c-means clustering, k-means clustering and hierarchical clustering. 106,116,121 Artificial neural networks (e.g. ART-2a 119 ) group particles as a function of their similarities. Two parameters, the vigilance factor and the learning rate, influence the number of determined particle classes and the convergence of the calculation. The ART-2a algorithm was successfully applied to laboratory and ambient particle populations. 82, A large number of determined classes or non-converging classifications, however, sometimes complicate a reasonable data evaluation. 121,125 The fuzzy c-means (fuzzy clustering) algorithm performs a soft attribution of individual particles to classes (groups of chemically similar particles) on the basis of membership degrees of similarity. 118 The k-means algorithm, instead, is a hard clustering method assigning the particles to classes by yes/no decisions. 53 Application of these two classification algorithms, fuzzy c-means clustering and k-means clustering, to a data set of single-particle spectra showed minor differences in spectra patterns and class abundances. 106 Several variants of hierarchical clustering, k-means clustering algorithms and of the artificial neural network ART-2a have been applied to discriminate data sets of single-particle spectra obtained from several ambient sources. 116 These algorithms showed variations in the assignment of the particles to a certain class as a function of the number of clusters and showed differences of processing time for classification depending on the number of spectra. The processing time was also influenced by the number of identified peaks per spectrum, the chosen mass range, quality of software programming of the classification algorithms and computer performance. Therefore pre-sorting of spectra (see above) is essential for future extended application of clustering methods to large numbers of particle spectra (e.g. several million spectra). k-means clustering and specific hierarchical clustering algorithms showed the best classification results for most particle populations under investigation. 116 The optimal classification parameters mentioned in this paper will have to be confirmed for data from different SPMS systems for future generic applications. Furthermore, fuzzy c-means and other clustering algorithms have not been included in this study. Another example of a comparison of different clustering methods applied to a population of 299 single-particle spectra (aerodynamic particle diameter 0.5 µm) will be described in the following. The particle population was classified with three different algorithms: principal components analysis (PCA), fuzzy c-means clustering (FCA) and an artificial neural network (Kohonen). The three methods are described only briefly here. Details can be found elsewhere. 50,118,119,126 PCA constructs new variables (called principal components ) by combination of highly correlated original variables. The principal components are uncorrelated and orthogonal to each other. This often leads to problems in generating representative spectra patterns of the determined classes. FCA is an iteration process looking for local minima of distances between similar vectors (objects) and the corresponding class centers. The determined class centers of the population can be displayed as vectors or spectra. The smaller the distance of a single object (single-particle spectrum) to a class center, the higher is the membership degree of this particle to this class. After application of artificial neural Kohonen networks, spectra patterns can be generated by averaging all single spectra belonging to a certain class. The ART-2a algorithm is based on a similar concept. 82

8 850 K.-P. Hinz and B. Spengler A relative intensity / a.u. relative intensity / a.u. B C 62 relative intensity / a.u m/z 161 Figure 4. Example of spectra patterns from an indoor particle population determined by fuzzy clustering (A), principal component analysis (B) and neural network analysis (C) for a common particle class (aerodynamic particle diameter: 0.5 µm). As an example of the results of the three different clustering procedures, Fig. 4 shows the patterns of a common particle class. It is obvious that the spectra patterns determined with the three algorithms show significant differences. A postprocessing quality evaluation of the clustering result can be performed with the FCA algorithm. The algorithm then calculates similarities between single particles and the determined class centers for each of the classification procedures (PCA, FCA, Kohonen). Membership degrees (similarities) can vary between 0 and 100%, while the sum of all class memberships of a single particle is always 100%. A validity parameter of a class center is the number of particles having a high membership degree to that class. 118 In Fig. 5, the number of particles are shown having a maximum membership to one of the classes of a certain percentage. About 48% of the particles, for example, have a membership of more than 90% to one of the classes for the fuzzy c-means classification, while less than 5% of the particles have a maximum membership below 40% to one of the determined classes. The latter (mixed or exceptional) particles cannot be attributed to one of the classes and have to be investigated separately. # particles / % FCA PCA neural network 0.2 to to to to to to to to 1.0 maximum membership Figure 5. Number of particles with a maximum membership to one of the determined particle classes in steps of 10% for three classification procedures (fuzzy clustering FCA, principal components analysis PCA and artificial neural networks). For the particle population described above, the patterns of FCA show the best agreement with the original single-particle spectra. Table 1 summarizes the results for

9 On-line aerosol mass spectrometry 851 two limits of membership degrees of 60 and 80%, respectively, for the three classification procedures. The number of particles that have a membership coefficient above these two limits is listed. For the FCA, for example, 60.9% of the particles belonged to one of the classes to a degree of more than 80%, indicating an external mixing of these particles. The corresponding values for PCA (28.8%) and the neural network (31.4%) show a reduced agreement of original single-particle spectra with calculated spectra patterns probably as a result of the nonfuzzy classification procedures and the process of generating the spectra patterns. The described behaviour is of course just a singular example that does not necessarily represent the situation for any other particle population. Classification of particle populations prior to particle evaluation is not always necessary for every particle population investigated. On-line evaluation by direct assignment of single-particle spectra to pre-selected particle classes can be performed in real time (e.g. using the FCA 118 ) and can, for example, be used to monitor the temporal evolution of specific particle types (classes) in long-term experiments. Monitoring clean room air quality or hospital air control are possible applications of this method. Analysis of mass spectrometric data acquired by the AMS thermal desorption instruments is primarily based on evaluation of fragmentation patterns, taking into account the natural isotopic ratios of the chemical elements. Patterns are recognized after laboratory measurements of particular standard species. On the basis of the known peak intensity ratios of the standards, the detected mass spectrum can be divided into partial mass spectra corresponding to particular chemical species (or groups of species). Model aerosols are further used to calibrate the instrument for quantification studies and for determination of mass concentrations of selected species (e.g. air, nitrate, sulfate, chloride, ammonium). 127,128 Variations of these mass concentrations can then be monitored in real time. 58,129 Specific data analysis techniques have been reported for determination of mass concentrations of hydrocarbon-like and of oxygenated organic aerosols (HOA and OOA) from thermal desorption data using selected ion signals. 130 APPLICATIONS Laboratory and field measurements Mass spectrometric systems have been applied for on-line aerosol characterization in a number of laboratory and Table 1. Number of particles (%) with a maximum membership to one of the determined particle classes above a threshold between 60 and 80% Membership to one class (%) Fuzzy clustering (%) Number of particles PCA (%) Neural network (%) > > field measurements. This section will briefly describe a few selected examples of current investigations to reflect the broad applicability of the method. Atmospheric research Atmospheric phenomena and processes such as visibility change, cloud and ice formation or global climate change are known to be strongly influenced by aerosol particles. The comprehensive characterization of aerosol particles and particle populations is a substantial challenge because of the small size of the particles, their low concentration in air and a large variability of their physical and chemical properties. Many field campaigns have been carried out to 27,53,59,74,82, identify rural and urban particle emissions, to determine particle sources and to monitor temporal changes of particle number concentrations in correlation to determined chemical particle classes and their abundances. Measurements with ground-based aerosol mass spectrometers in the upper troposphere 17,32 and aircraftbased instruments 59,61 63 resulted in the identification of characteristic particles affected by long-range transportation (e.g. mineral dust), of a significant amount of organic compounds in particles and of anthropogenic particles. Results from on-line particle analysis have substantially contributed to a better understanding of particle formation, transformation and transport in the atmosphere, to improved source apportionments and to a better characterization of the mixing states of aerosols. Future combined measurements of on-line instrumentation (gas phase and particulate phase), off-line instruments (e.g. bulk) and other data (e.g. meteorological) will further improve the description of atmospheric processes and climate changes on a regional and global scale, definitely with effects on environmental policy regulations. Traffic and industry emissions Identification and characterization of exhaust is of great importance because of the considerable effects of such emissions on global climate and human health. 137,138 Investigations have been performed to characterize emission of gasoline and diesel vehicles 29,64,65,75,76,124,139 and of aircraft exhaust during flight. 63 The results have shown a variety of particle compositions including carbonaceous particles, mixed particles containing soot and secondary components (ammonium, nitrate, sulfate), particles with inorganic components and particles with PAH compounds. Laboratory experiments with aerosol chambers and chassis dynamometer tests have been performed to investigate particle nucleation in the vehicle exhaust and coating or mixing of soot with organic or secondary compounds for a better understanding of atmospheric processes caused by exhaust particles. Bioaerosols and aerosol MALDI Detection and identification of bio-aerosol particles is another important field because of their possible health effects. Several research groups are currently working in this field to improve mass spectrometric instrumentation for fast, sensitive and selective identification of bacteria, spores and viruses. Detection of biological particles can be performed by light scattering as for inorganic particles 115,120,140 or by

10 852 K.-P. Hinz and B. Spengler using fluorescence of specific chromophores. 141 Particles are partially vapourized and ionized in flight by a UV laser pulse, and ions are analysed using bipolar TOF or ion trap mass spectrometers. Extension of the mass range of analysed components of bio-aerosol particles by the so-called aerosol MALDI mass spectrometry is possible by on-line coating particles with a suitable matrix during flight and subsequent 115, analysis. This method can help identify bacteria or other biological microparticles by their fingerprints. The aerosol MALDI technique has also been used as a laboratory analytical method for bio-organic molecules. For that the analyte solutions were mixed with matrix solutions. The liquid was nebulized and introduced into the mass spectrometer through a nozzle/skimmer aerosol inlet. 145,146 The method can also be used for basic investigations of the MALDI process. Organic aerosol compounds Organic compounds of aerosol particles are known to have great impact on any kind of particle interaction with its environment. 147,148 Emissions of primary organic aerosols (POA) and formation of secondary organic aerosols (SOA) are currently under investigation regarding their effects on atmospheric processes. Hydrophilic and hydrophobic properties of organic constituents, for example, influence cloud formation and condensation processes on existing particles. Changes in optical properties are prominent effects in this respect. The great variety of organic compounds emitted from natural and anthropogenic sources is a great challenge for analytical methods (such as on-line particle mass spectrometry) and for concepts of data acquisition and evaluation. A few examples of organic particle analyses are mentioned in the following. Organic compounds in mixed particles have been identified using SPMS instruments. 61,64,91,106,149 Thermal desorption (MPMS) instruments have been operated in field campaigns for identification, separation and quantification of various groups of organic compounds (e.g. HOA and OOA). 59,130 Time-resolved observation of SOA formation has been performed using a SPMS in laboratory experiments. 150 The characterization of organic aerosols with on-line mass spectrometric techniques will be a major field of atmospheric research in the coming years, aiming at identification of particle sources and understanding chemical reactions in the atmosphere. Nanoparticles Ultrafine particles (d < 0.1 µm) have high number concentrations in the atmosphere. They considerably influence human health because of their deep deposition in the airways. 151 Knowledge of nanoparticle compositions in aerosols is important for understanding particle formation and growth in the atmosphere (e.g. by nucleation), for characterizing the impact of anthropogenic nanoparticles on human health or for controlling nanotechnological processes. Detection of single particles by light scattering is difficult for particle diameters below 100 nm because of the low intensity of scattered light. Therefore instruments have been developed for the analysis of such particles, based on internally triggered UV laser pulses with high repetition rate. 45,52,82,152 The probability of hitting a particle by an internally triggered laser pulse is rather small and therefore the detection of a mass spectrum is related to an individual particle. 152 A direct correlation of the number of atoms in the particle to the total signal area of a mass peak in the spectrum has been found for high laser irradiances to prove single nanometre-sized particle analysis. 153 Thermal desorption instruments have been used to vapourize nanoparticles, combined with EI ionization. 55,56 Alternatively, nanoparticles have been collected on a surface after direct particle inlet, desorbed by a pulsed infrared laser followed by soft photoionization using VUV radiation. 110 These two methods of nanoparticle analysis have in common the fact that typically more than one particle is analysed with each measurement. These instruments are therefore not SPMS but MPMS instruments for this mode of operation. This is also the case for instruments with internally triggered ionization lasers under conditions of high particle number concentrations (N > particles/cm 3 ). Regardless of single-particle capability, most instruments require (collective) multi-particle analysis in the nanoparticle mode, since separate single-nanoparticle detection is mostly below the limit of detection. With high laser irradiances of more than W/cm 2, quantitative information of nanoparticle elemental composition can be obtained. 100 Soft ionization methods are preferred to obtain molecular information on particle composition. On the other hand, the efficiency of ion generation decreases with decreasing particle diameter and depends on particle composition, 84 requiring optimization of experimental conditions specifically for each investigated particle population. The lower instrumental size cut-off is in the range of a few ten nanometres in diameter currently. Recently, first laboratory experiments have been reported using a combination of an ion trap and a reflectron time-of-flight mass analyser to investigate particles with diameters of 7 25 nm by photoionization. 101 The lower size cut-off has to be further reduced in order to improve knowledge of nanoparticle effects on environmental processes and human health. Inter-comparison of instruments Simultaneous operation of several different aerosol mass spectrometers and other instrumentation (bulk or other offline instruments 17,27,29,31 ) are important to study the response of these instruments and for instrumental calibration. Inter-comparison experiments show both similarities and differences of the detected spectra of the investigated particles. Common laboratory and field measurements of different instruments are necessary to determine their fundamental characteristics and to use the complementary information from each instrument for a comprehensive, validated and exact characterization of the investigated aerosol population. Quantification An essential challenge of on-line particle analysis is the quantification of specific single-particle components or particle ensemble components and the (number) quantification