Elemental Analysis of Single Micrometer-Size Airborne Particulates by Ion Microprobe Mass Spectrometry J. A. McHugh and J. F. Stevens General Electric Company, Knolls Atomic Power Laboratory, Schenectady, N . Y. 12301 The analysis of single micrometer-size particulates has been accomplished using ion microprobe mass spectrometric techniques. The ion microprobe possesses a capability for rapid analysis and provides a high elemental detection efficiency for most elements, including the lightest. These factors make it a very attractive tool for the compositional analysis of small particles. The techniques of preparing, mounting, and analyzing particles as small as 1.5 pm are described. A sample of airborne particulates was examined by light microscopy and a group of 10 particles in the 2- to 8-pm size range with the characteristic oil-soot morphology was selected for ion microprobe analysis. The results showed that the average oil-soot particle i s characterized by high levels of 0 , V, C, Na, Ca, and K.
complished in the work of Liebl (Z2), and Castaing and Slodzian (13, Z4). The intent of the present paper is to describe the application of a new and highly sensitive analytical method-ion microprobe mass spectrometry-to the elemental analysis of single particles collected from the atmosphere. In order to demonstrate the capability of the ion microprobe in this area, we collected airborne particulate matter on filters and selected a class of particles in the 2- to 8-prn size range for analysis. The particle class selected was oil-soot and compositional results are presented for a number of these particles.
AIRBORNE PARTICULATES represent many particle classes of varying chemical composition. To extract maximum information from a particle collection, it is necessary to obtain elemental compositions of individual particles. With such data, one can more easily interpret, pinpoint, and eventually control sources of particulate pollution. To date, the majority of studies dealing with the elemental composition of airborne particulates have been done by elemental analyses of gross particle collections. Some of the conventional methods that have been used in air particulate analysis studies, such as emission spectrography (Z), atomic absorption ( 2 ) , spark source mass spectrometry (3), and neutron activation ( 4 ) , are not suitable in their present form for analyzing single micrometer-size particulates. One method in existence for a number of years, the electron microprobe, possesses the capability of elemental analysis of microvolume samples and has been used in the characterization of single micrometer-size particles (5-8). The high elemental sensitivity provided by ion sputtering and secondary ion emission in analytical solids mass spectrometry has been well documented (9-11) and the extension of these methods to micro-area analysis problems was ac-
The work presented in this paper was performed with a Liebl (Z2) design Applied Research Laboratories Ion Microprobe Mass Analyzer. A schematic drawing depicting the ion microprobe and its operation is given in Figure 1. The instrument operates in the following way: an ion beam is extracted from a high brightness arc discharge source, mass analyzed, and demagnified to a small spot ( 52 pm FWHMfull width at half maximum) at the sample. The secondary ions generated by bombarding the sample with an energetic ion beam are extracted into the secondary mass analyzer by a potential difference between the sample surface and the extractor electrode. The number of ions of a particular element emerging from the sample surface can be related to the concentration of the element at the sample surface. The secondary ions are mass analyzed by a double-focusing mass spectrometer and the resolved ion beam is detected by a detector sensitive to single ions (15). A visual image of a specific isotope distribution on the sample surface is generated through synchronizing the sweep of the primary ion beam and the electron beam in the cathode-ray tube, and then using the secondary ion signal to modulate the intensity of the electron beam in the oscilloscope. Examples of ion images produced by the ion microprobe are given in Figure 2 . The ion micrographs were taken of a 6-pm diameter dust particle that by chance settled out on a gold film sample. The exposure time required to generate an ion micrograph (Polaroid film picture of cathode-ray tube display) is from 1 sec to about 3 minutes depending on the concentration and the secondary ion emission characteristics of the element in the sample. A ,Millipore 0.45-pm membrane filter and air filtering apparatus were used to collect a sample of airborne particulates. The filter was dissolved in acetone, centrifuged, and the residue ua5ht.d with acetone. The particulate residue was dispersed in ethyl alcohol and a part of this dispersion was dried on a clean glass microscope slide. A microscopic examination of this particle collection showed many particle types and classifications. Oil-soot particles in this sample comprised about 1 of the particles above 2 pm in diameter. The morphological characteristics of a typical oil-soot particle can be summarized as follows: a foamy or lacy, vitreous, brown to black
(1) N. L. Morrow and R. S . Brief, Eiicrru/r. Sci. Tcdr/iu/., 5, 786
(1971). (2) T. J. Kneip, M. Eisenbud, C . D. Shehlow. and P. C. Freudenthal, J . Air Pollrrt. Courr. Ass., 20, I44 (1970). (3) E. R. Blosser and It. J. Thompson, Nineteenth Annual Con-
ference on Mass Spectrometry and Allied Topics, Atlanta, Cia., May 2-7, 1971. (4) S.S. Brar, D. M. Nelson, E. L. Kanabrocki, C . E. Moore, C. D. Burnharn, and D. Hattori, E/rt-iro/i.Sci. T d r i i c ; / . ,4, 13 (1970). (5) E. W. White, P. F. Denny, and S. M. lrviny, in “The E\ectron Microprobe,” T. D. McKinley, K. F. J. Heinrich, and D. B. Wittry, Ed., Wiley, New York, N.Y., 1966, pp 791-804. (6) D. K. Landstrom and D. Kohler, Battelle Mem. Inst., Columbus, Ohio, for National Air Pollution Control Administration, NTIS Repi., No. PB 189-282, December 1969. (7) G. L. Ter Haar and M. A. Bayard, Nrrrurc., 232, 553 (1971). (8) A. M. Langer, A. D. Mackler, I. Rubin, E. C . Harnmond, and I. J. Selikoff, Scimca. 174, 585 (1971). (9) T. L. Collins, Jr., and J. A. McHugh, Adccrir. M c ~ s sSpectrum., 3, 169 (1965). (IO) J. A. McHugh and J. C. Sheffield, ANAL.CHFM..37, 1099 (1965). (1 1) Zbid., 39, 377 (1967).
EXPERIMENTAL METHODS AND TECHNIQUES
(12) H. Liebl, J . Appl. Phys., 38, 5277 (1967). (13) R. Castaing and G. Slodzian, J . hficrosc., 1, 395 (1962). (14) G. Slodzian, / l i r / i . Phys. (Ptrris),9, 591 (1964). (15) L. A. Dietz, Rec. Sci. hsrrtrm., 36, 1763 (1965).
ANALYTICAL CHEMISTRY, VOL. 44, NO. 13, NOVEMBER 1972
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PRIMARY I O N
CMVDENSERLENS
ECTIVE LENS
SAMPLE
Figure 1. Schematic drawing of the Applied Research Laboratories ion microprobe mass analyzer
spherical glass structure (16). Particles that fit the oil-soot morphology were selected for ion microprobe analysis. A particle analysis with the ion microprobe requires that a particle of interest, free of extraneous material, be placed on a flat, electrically conducting surface. The following is a brief description of the methods and techniques developed for handling and transferring “bare” particles as small as 1.5 pm. The particles are picked up for transfer to the ion probe mount with a glass microneedle (drawn to a 2-pm tip) attached to a Beaudouin micromanipulator. This pickup is accomplished simply by bringing the particle and tool into contact. The manipulator is small and fits onto the stage of a Leitz Ortholux microscope. Vibration problems encountered with a separately supported manipulator are reduced considerably with this arrangement. The manipulator is pneumatically controlled from a unit placed on the bench top and a single lever provides the X-Y motion with a continuously variable magnification control. The particle adhering to the needle is brought in contact with the surface of the ion microprobe particle mount and the particle is transferred. Often it is necessary to introduce a second needle on a second micromanipulator to aid in the removal of the particle from the first tool. All operations are accomplished under 200X magnification with a microscope equipped with both transmitted and reflected (vertical illumination) light. The work area used for these operations must be clean and free of severe vibrations. The mount on which the particles are placed must have certain special characteristics that satisfy the analysis needs. It must have low secondary ion background in the mass regions of interest, and have a smooth, flat light-reflecting surface with index marks. It must also have good vacuum properties, be an electrical conductor, and be reuseable to some extent. We have found two materials, high purity tantalum and vitreous carbon, that satisfy our requirements. These materials are electrical conductors and can be polished (with standard metallographic polishing methods) to give a surface finish suitable for locating I - p n particles by light
microscopy. Since the tantalum is very pure, interferences caused by impurities present in the Ta mount are virtually non-existent. On the other hand, the vitreous carbon we employed has traces of calcium, titanium, and alkali impurities which are detectable under small particle operating conditions. The levels (
