Environ. Sci. Techno/. 1983, 17, 457-462
Laser Microprobe Mass Analysis (LAMMA) as a Tool for Particle Characterization: A Study of Coal Fly Ash Eric Denoyer,t David F. S. Natusch,$ Patrick Surkyn, and Fred C. Adams" Department of Chemistry, University of Antwerp, B-2610 Wilrijk, Belgium
Laser microprobe mass analysis (LAMMA) is used for the characterization of a fly ash derived from a conventional coal-fired electrical generating plant. The mass spectra provide data on the chemical composition including speciation data on the matrix and minor constituents and organic constituents present at the surface. It appears that LAMMA constitutes a fast and probably cost-effective tool for qualitative screening and for establishment of some physical and chemical characteristics of fly ash. However, at present the quality of the information is insufficiently high to warrant its use as a screening tool for particle characterization.
Introduction Assessment of the potential environmental and toxicological effects of particulate material emitted to the atmosphere, together with design of emission-control strategies, required detailed physical and chemical characterization of the particles. A large number of such studies dealing with a variety of particle types has been reported in the literature (1-8). The information that is most frequently required includes particle morphology, particle size distributions of number, mass for elemental and organic species, identity of inorganic compounds and minerals, and surface-enrichment and solubility characteristics. Determination of such a body of information is time consuming and costly and requires deployment of instrumentation and methodology that is not usually present in a single laboratory. In fact, full and detailed physical and chemical characterization of particulate matter is not really required in most cases, and a more qualitative investigation, such as oultined in the level I Source Assessment Protocol developed by the United States Environmental Protection Agency (9),is often sufficient. Consequently,it is desirable to develop a methodology that is capable of providing a sufficient amount of information in a short time with minimal sample handling, preferably utilizing a single instrument. To this end we have investigated the utility of laser microprobe mass analysis (LAMMA) for studying environmental particles. The LAMMA technique, which is described in detail elsewhere (10-12), utilizes a focused laser beam to vaporize and partially ionize the sample material. The ions produced are then extracted into a time-of-fight mass spectrometer for mass analysis in either positive or negative ion modes. Sample observation is achieved with a high-resolution optical microscope, so in principle LAMMA is capable of providing information about the morphology, size distribution, chemical composition, and possibly even the surface composition of individual particles. Furthermore, the time required for a single analysis is short, and sample preparation is relatively simple. The use of LAMMA in aerosol research has been demonstrated previously (5, 13-15). Present address: American Cyanamide, Stamford, CT 06904. t Present address: Liquid Fuels Trust Board, Wellington 1, New Zealand. 0013-936X/83/0917-0457$01.50/0
In order to establish the degree to which LAMMA is useful for particle characterization, the technique has been applied to a sample of fly ash derived from a conventional coal-fired electrical generating plant. This fly ash has previously been extensively characterized, and a large body of information is available against which to judge that provided by LAMMA. It is the objective of this paper to report the results of these studies and to assess the extent to which either existing or new physical and chemical data are provided.
Experimental Section Materials. Coal fly ash was collected in bulk from the electrostatic precipitation of the Corrette plant in Billings, MT. Instrumentation. For a full description of the LAMMA-500 instrument (Leybold-Heraeus, GmbH, Koln, FRG) we refer to other publications (11,12). A Q-switched Nd:YAG laser is used to generate intense light pulses with a duration of 15 ns. The radiation is quadrupled in frequency to 265 nm. A He-Ne laser continuously emitting in the red is aligned collinearly to the invisible UV light of the Nd:YAG laser, The He-Ne laser beam eases the alignment of the UV laser beam with respect to the axis of the microscope and serves for aiming at a selected area on the specimen. For mass spectrometric analysis the sample needs to be placed into the vacuum. It is transferred onto a coated electron-microscopicgrid that fits into a x-y movable sample stage. A standard quartz cover slide serves as vacuum seal and optical window. The ions that are formed by laser irradiation of the selected sample area are accelerated into a time-of-flight mass spectrometer, which can be used for the analysis of positive or negative ions. An open secondary electron multiplier with 17 CuBe dynodes is used for ion detection and has a gain of lo6 or higher. The analog signal of the detector is digitized with %bit resolution and stored in a transient recorder of 2048 channels. The recorded spectrum is displayed on a CRT screen. It can be plotted by a strip-chart recorder or transferred to a data system. The most attractive features of the LAMMA instrument for particle analysis are spatial resolution down to 1 qm, high collection efficiencies (10-50%), mass resolution of about 700-800, and ease of operation, simplicity, and speed. The laser energy on the sample can be varied with a set of attenuating filters and is monitored by an energy meter. It is either adjusted to provide complete vaporization of a micrometer-size sample of the fiber or else adjusted to the lowest energy that provides a mass spectrum. Adsorbed impurities onto the fiber surface can be detected in this laser desorption (LD) operation mode (16). Procedures. Fly ash particles were mounted on a transmission electron microscopy grid coated with a thin Formvar film (thickness -0.1 pm). This was achieved simply by touching the coated grid to the bulk fly ash. The grid was then mounted directly in the sample chamber of the instrument. Microscopic examination of samples to identify morphological characteristics was made by using both 400X and 1250X magnification. For particle analyses however,
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Environ. Sci. Technol., Vol. 17, NO. 8, 1983
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observation at 400X was used. The laser power, which varies by *15% between pulses, was monitored for each pulse, and the power delivered to the sample was varied by means of transmission filters. For elemental analyses, the laser power was maintained at a high level to completely vaporize a particle and thereby maximize sensitivity. For analyses designed to identify molecular species, however, low laser powers were employed so as to promote desorption and minimize molecular fragmentation. Because of poor spectral reproducibility, which was also noted in other work (ZO), spectral information was obtained for a number of visually identical particles of similar size which were averaged. The protocol developed for investigation of a bulk particulate sample and employed in this study consists of several steps: (1) Between 100 and loo0 particles are examined microscopically and classified in terms of morphology and size by using conventional optical microscopic procedures (3). If appropriate, photomicrographs can be taken to provide a visual record. (2) Individual particles, representative of chosen size and morphological classifications, are analyzed under high laser power conditions in both positive and negative ion modes, to obtain data on elemental composition and some inorganic species. (3) The above procedure is repeated under low laser power conditions to obtain data on organic compounds that may be present (4) Specifically designed studies are conducted to confirm previous findings. These could include analyses of authentic compounds for spectra calibration and, possibly, analyses of selected areas of apparently heterogeneous large particles. It has been reported that surface analytical information may be obtainable by deploying the laser beam a t grazing incidence to a particle (17). Solubility behavior may be obtained by analyzing particles before and after leaching with water or a suitable organic solvent. Results obtained till now are not encouraging (see Results and Discussion). One of the calibration experiments performed as part of this study involved analyses of particles that had been doped with polycyclic aromatic compounds. T w o doping procedures were employed. In the first activated carbon (Norit, Jackson, FL) and AI,O, were placed in individual chloroform solutions containing several different concentrations of coronene and 9,10-diphenylanthracene,respectively, in the range 100-1oo0 N/mL. The solvent was removed by evaporation under a stream of nitrogen gas. The second procedure involved passing phenanthrene vapor through an expanded bed of fly ash. This method, which has been previously reported (18,Z9), achieved a level of 225 pg/g on the bulk fly ash.
Results and Discussion Microscopic Examination. Use of the optical microscopic capability of the LAMMA instrument established conclusively that, with the aid of a calibrated graticle, the particle number distribution can be determined almost as effectively as with a free-standing microscope. It should be recorded, however, that the optical qualities of the LAMMA microscope have been degraded somewhat due to the need to remove the plane of focus beyond its optimum position in order to accommodate the sample in the vacuum system of the instrument. Some difficulty was encountered initially in achieving even distribution of particles on the Formvar mounting film and avoiding clumping; however, with practice samples suitable for counting can be prepared. Examination of several samples indicated that the sample mounting procedure was uniformly selective for all particle types and sizes present in the well-mixed bulk fly ash. 458
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C d Flpwr 1. Secondary e k b o n m)aogaphs of the Wee morpholoaical classes In the Correne fly ash: (a) overall plcture: (b) clear glassy particles. type A ( c ) solid spheres. type B; (d) cahonacearr material. type C.
AU of the morphological formsthat have previously been reported for coal fly ash (20,21) could be observed and identified microscopically. For the purpose of the present investigation, however, only three classifications were employed. These consisted of the following: (1) Clear or white glassy particles whose morphology varies from a regular sphere to irregular rounded lumps. These particles, which previous work (22) has shown to have little or no ferromagnetic character, are classified as type A. (2) Solid black spheres that could be separated from the bulk fly ash magnetically (22,23). These are classified as type B. (3) Irregular black particles of apparently carbonaceous material, which were classified as type C. Examples of each of these particle types are depicted in the electron microphotographs in Figure 1. Elemental Composition. Analyses of individual coal fly ash particles under high laser irradiance conditions (-5 times the ionization threshold for the sample) enabled qualitative identification of more than 20 elements in the types A and B particles. Of these only six elements were systematically detected in all particles investigated (Figure 2). In general, the electronegative elements C, S,and CI and clusters of Si, Fe, and AI were observed in the negative spectra. Very few elemental constituents except Na and K were detectable in the type C particles. In addition, H, Li, Be, B, and 0 atomic ions can be detected in spectra that include the mass range 0-23 amu. These analyses showed that the heterogeneity between individual particles was significantly greater than the imprecision of the LAMMA method. Also, not all the elements listed for a particle type were observed in each particle. Such an observation, which supports the mechanisms of coal fly ash formation recently published by Natusch et al. (21).is a useful contribution of the LAMMA technique since electron microprobe analyses do not detect enough trace elements in coal fly ash to illustrate high
Abundance 100%
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Figure 2. Frequency of detection of elemental constituents In type A particles.
Figure 4. Posltlve LAMMA mass spectrum of NBS-309 glass research material (top), type A particles (middle), and type B particles (bottom).
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Flgure 3. Cumulative probability as a function of intensity ratio of 54Fe/44Cafor type A particles (0)and type B particles (A).
degree of heterogeneity that exists. Unfortunately, LAMMA does not enable definition of concentration dependences on particle size unless a sufficiently large number of particles are analyzed to account for both interparticle heterogeneity and the poor precision of the technique. It is considered that this is beyond the type of preliminary survey analysis being proposed. Consideration of the mass spectra obtained from each morphological classification indicated that the carbonaceous type C particles are quite uniform in composition and consist primarily of elemental carbon. However, in a statistical evaluation of the "Fe/%a peak height ratios for particle types A and B a discontinuity in the frequency distribution of the measured ratio for type B particles is observed at a value of 1.4, dividing the sampled population into two distinct subclasses. Thus, 76% of the type B particles analyzed have a composition in which the "Fe/Wa intensity ratio is >1.4% and 24% have this ratio
