- Email: [email protected]
Characterization of individual airborne particles - methods used and their limitations
trends in analvtical
Y8
Yoneda, Nakazawa, Yamazaki, Hare, P. LePage, Karger,
separation can be optimized by modifying the mobile phase. What remains to be done is to elucidate the nature of the forces that control the separation. In any event, the use of metal cations greatly facilitates the altering of the chromatographic system to the benefit of the analyst.
185,
I!),
Gilon, 203,
8
Sousa,
I,.
Am.
Leshem,
R. and
Grushka,
E., Tishhee,
I\. and
Hare,
E. (3981)
J.
Chromatqqr.
( lY80)
P. E.
J. ~tm.
Chem.
Sot.
102, 51l5 Oelrich,
13-24
L. K.,
( 1Y74) ,j.
C., 365
Gil-Av,
E., Preusrh,
H. and WIhrlm.
E. (lY80)
HRC’+
Cc’.
3.
26Y
Davankov, V. A. (1980) in Giddings,.J. C., Grushka, E., Cazrs, .J. and Brown, P. R. (eds), AdLlances in Chromatqraph_v, Marcel Dekkrr, 18. Ch. 4, p. 139 6 1’oshikawa, Y. and Yamasaki, K. ( 1970) Inorg. ,Vucurl. Chem. Letl. 6 35 25. 7 Sousa,
C., Leshem, R., Tapuhi, Y. and Grushka, E. (1979) ./. Sac. 101, 7612 C., Leshem, R. and Grushka, E. (1980) Ilnal. Chem. 52,
1206
Buss, D. R. and Vermeulen, I‘. (1968) Ind. Eng. Chem. 60, 12 E., Feihush, B. and Charles-Sigher, R. (1966) Gil-A\,, Tetrahedron Lett. 10, 1OOY Audebert, K. (197’3) ./. Liq. Chromatqr. 2, 1063-1095 Blaschkr, G. (1’380) :In,ym. Chem.. 92, 14-25 (lY80); Int. Ed. Erg/.
H. and Yoshizawa, T. (1976) Chem. Lett 707. H. and Yoneda, H. (1978) J. Chromatogr. 160, 89 S. and Yoneda, H. (1979) J. Chromatogr. 177, 227 E. and Gil-A\,, E. (197Y), Science 204, 1226 J. X., Lindner, b’., Davies, G., Seitz, D. E. and B. L. (1979) Anal. Chem. 51, 433; (1979) j. Chromatqtr.
Chem.
Gilon,
References
uo; 1, no. #. I981
323
Gilon, Am.
chemisty.
Hoirinan, Chem.
Sot.
1). H., 96,
Kaplan,
L. and
Cram,
Sogah, G. D. Y., Holyman, .J. (1!)78) ./. Anr. Chem. Sot. 100, 4.569
Eli
Characterization methods
1). H. and
Cram,
S., Chow,
F. and
Karmen,
;I.
( 1980)
J.
Chromafqr.
1YY.
D. .J.
7 100
K.,
Lam, 2Y5
D.
Grxrhka
.Ina!~‘tical
and
R.
ChemiJtrv.
Leshenr The
zcwrk nt the Deportment Hehretce
1 .nil,er-\i!v
o/. It~o~~auic~
o/‘, jewsalem.
NM/
Jerrcwlem.
Iwael.
of individual airborne particles used and their limitations
-
The characterization of individual airborne particles plays an important role in assessing environmental contamination. The morphology, elemental composition and surface properties of these particles must be determined for complete characterization. Manfred Grasserbauer and Chariklia Malami Vienna, Austria Characterization of individual airborne particles through size, shape, morphology and chemical identity (by elemental or direct compound-specific in-situ microanalysis) 1.2~. is of basic importance in environmental aerosol studies, because these data may yield information about _ the origin especially anthropogenic of particles, contaminants, - chemical transformation processes in the atmosphere, and - physiological effects of air contamination (especially by absorption through respiratory organs), solubility of material and even toxicity.
Survey of methods The availability and choice of methods for the characterization of individual airborne particles is determined primarily by the range of particle sizes to be examined - that is those respiratory particles between, approximately, 100 nm-5 nm in (aerodynamic) diameter. Therefore, only physical techniques 0 16.~-9936/~I/10-0OoRU2.75
having a spatial resolution of the order of the particlc size, which will permit selective analysis, can bc applied. The most important methods, the analytical information they yield and the lower limit particle size detectable by them (lateral resolution of analysis) are listed in Table I.
of
Sampling
and sample preparationl.2
For the characterization of individual airbornr particles sampling procedures and conditions have to be chosen which satisfy the following requirement; the particles under investigation have to he spatially separated on a suitable substrate to enable analysis of individuals. The substrate can be either a metallographically polished disk of a material which does not interfere with the analysis, or a thin film or plate transparent to electrons or photons. If it is important to avoid the, often difllcult, processing of the collected sample, rapid deposition methods (which take from a few seconds to a few minutes) with Nuclepore@-filters, impactors and ccntrifuges should be applied. The collection of airborne particles by cascade impactors appears to be especially useful, since size fractionation is achieved in the Q I%iIElswirr Scwnlilir Pul,li,bil,R(:,lmpan~
trends in nnn!yticnl
chemistry.
vol. I. no. I, 1981
99
sampling procedure and the sample can then be concentrated in a small spot of only several mm in diameter on almost any substrate (e.g. Au-foils, carbon grids). If sampling with Hi-Vol-Filters is applied, or if it is performed for a longer time to obtain time-averaged values, a further sample preparation step has to be taken. There are many possible ways ofdoing this. One of the most important procedures, especially for scanning transmission electron microscopy (STEM) investigations, is suspension in collodion-amyl-acetate followed by deposition on a suitable substrate. For enrichment of certain materials it may be useful to ash the ma.jor matrix before analysis (e.g. microwave ashing of organic debris before analysis of asbestos fibers). For analvsis with electron and ion beams, coating of the part’icles with carbon or gold is necessary (layer thickness approximately 20 nm). In general, sample preparation has to be chosen and adapted carefully for a specific analvtical goal. However, a lot of research is still necessary ‘in order to minimize losses and obtain meanin,qful information.
Fi,q.
I. Secondclg
(dinmeter
electmn
micqrnph
qf‘,ilicnte
pnrtirle
Jiom con1,J(r nsh
10 nm).
Elemental analysis and morphology ‘l’he most \vidc$. used analytical technique for characterization of Individual airborne particles is based on the combination of morphological imaging and N-ray microanal~.sis’.’ (applying electron probe microanalysis (EP;\I.\), scanning electron microscop) (SELI) and S’l‘E,\l). Qualitati\.r and scmiquantitati\.e N-ray analysis is often sufficient for identification ifthe particle has a characteristic morpholog).. Secondar) electron micro,qraphs. \vhich sho~v the particle shape
Llrthod
‘I‘)-l)ic;il inliwmation
.\ppl-osimatr IOWCI.particlr sizr limit (di:tm~tw. urn)
and surface morphology are most useful for particle sizes above 0.1 nm. Fig. 1 contains a silicate particle which had been molten (droplike shape). indicating that it is a typical airborne particle originating from coal firing. Particles emitted from oil firing or generated by other processes ha\.e a completely different morphology and can be difrerentiated clearly using this analytical technique’. Transmitted electron images (obtained \\-ith TEA1 or S’l’E;\l) provide information on the particle shape. enabling the morphological characterization of submicrometer particles \vith a spatial resolution of bettel than 10 nm. For more complete identification of these particles. imaging has to be combined \vith highl! sensiti1.e S-ray analysis. best achieved lvith STE;\l (due to its specific construction features). \Yith this technique the characteristic emission particles from leaded gasoline can be identified (Fig. 2). The particles originating from the lead additi1.e are typically round and about 0.1 pm or less in diameter. Ho\\-ei-er. there are situations kvhere the combination of morpholoLg! \vith qualitative or semiquantitati\.r S-ra!. microanalysis is not silficient: in these cases g~7r2fifa/ii*~ _Y-M~I~ nnal~sis has to be applied for particle characterization.
Analysis for asbestos ‘l’raw clcwirnts. isotope ratios, surfiw conipositiou Lasrr mass spxtromctric analysis (I,.\hlhl:\)
Elrmt.utar)~ composition ratios
isotopt~
C:ompoulid Surhw
ccmiposition
I 0.5
Quantitative S-ray analysis of individual particles is especially important in the study of asbestos pollution. Asbestos fibers are present in the respirablc fraction of urban aerosols in concentrations I\-hich are usually above se\.eral rig/m,, (origin: abrasion of. brake linings, plasters and roofs), They exist in much higher concentrations in factories \\hich manufacture asbessites: in some cases tos products 01 near mining asbestos fibers are e\.en present in drinking Icater (e.g. due to ‘l‘aconitc mining)‘. Duluth, Lake Superior.
. 100
trends in ana!,ltiral
Although there are contradictory evaluations of the health risk due to asbestos pollution, it seems that long time exposure to asbestos fibers causes pleuromesothelioma and even lung tumors. It is therefore generally agreed that for the reliable assessment of asbestos pollution analytical methods have to be developed which provide an accurate identification of fibrous materials (especially for the different varieties of asbestos), and a determination of the fiber concentration. This analytical goal is extremely difficult to achieve since the average concentration ofasbestos fibers in an aerosol is very low and the fibers are very small (diameter from about 20 nm to several hundred nm, length from about 100 nm to several pm). For these reasons the methods used for compound-specific
chemistrv,
uol. 1. no. I, 1981
average analysis (like X-ray diffraction techniques) are not suitable. The only successful approach consists of single particle quantitative X-ray analysis, in combination with morphological characterization. For assessment of the submicrometer fraction of asbestos fibers only the STEM is suitable; this also provides electron diffraction patterns which yield additional information. Quantitative X-ray analysis ofsuch fibers (Fig. 3) is based on the ratio-method, which evaluates the intensity ratios of suitable element pairs characteristic of a specific form of asbestos. These intensity ratios are then converted into concentration ratios, with relative sensitivity factors obtained from homogeneous reference materials. Careful optimization of the measurement and spectral evaluation techniques and detailed assessment of the statistical
_--.-.LEAD BROMIDE (Cl-ILORIDE)
STEil EO = 40
KEV
T = 200
SEC
BR
PB
i!
I
I
0 I
PB
Fig.
2. STLV
micrograph
and .Y-rev-spectrum
qfautomohile
exhaust
particle
in urban
aerosol.
~~‘ollrctiotr
mode;
caccade
im/lactor.
STEM E, = 40
CHRYSOTILE
KEV
T = 200SEC
1 UM
Fig.
3. STb7‘;M
micrograph
and .Y-_mv-spectrum
qfasbestos
fiber.
trends in nn&tic.ai
chemist~r.
vol. 1. no. 4. 1981
101
and systematic errors of X-ray analysis result in an accuracy of k 25% for (major) elemental concentrations in fibers of 200 nm diameter. To appreciate this accuracy, it has to be recognized that the amount of fiber analyzed in such a measurement is only about 10-r” g. Quantitative STEM-analysis therefore, yields sufficiently accurate concentration ratios for characteristic elements to enable one to distinguish between the different chemical forms of asbestos.
An identification scheme for asbestos fibers can then be established (Fig. 4). In the first step of this identification process the particles are characterized according to their shape and fibers are identified as such if their length-to-width-ratio is larger than 3 (L:D>3). An electron diffraction pattern or an X-ray spectrum is then recorded for each fiber. In many cases the SAED-pattern is sufficient for the identification of asbestos and classification of its type (particularly for i
1 STEM-IMAGE 1
FIBRES I
PARTICLES
r
. _,
I
5. Po.\itiw
.recondnrr
.._
,_
.
moss/charge
I I:ig.
_j
ion mn.~
.spurtrum
ql.l)nrtirlP,/i-o~,r
_,
..-
1 urbnn
newsol
(.\ir~~bu~F),
Pnrtirle
dinmcter
10 nm.
trends in ana
I(12
chrysotile, which has a distinctive diffraction pattern). In many other cases, however, the electron diffraction patterns are not clear enough for identification (particularly if the fiber has endured structural changes due to thermal exposure in the abrasion process, as in Then X-ray analysis must be used. brake linings). This characterizes the fibers on the basis of concentration ratios which are unique for each variety of asbestos. Determination of the asbestos concentration is achieved by counting the number of positively identified asbestos fibers and relating this to sampling parameters. Although the above technique for the assessment ofasbestos pollution is very tedious, it is the one which yields the most reliable results.
Trace element analysis The electron probe techniques described so far have one limitation in common: X-ray analysis can only be present in concentrations performed on elements Therefore, these methods are not above 0.1-l%. adequately sensitive for the identification of trace elements in single particles. For trace analysis in individual airborne particles which could yield important information about their origin - ion probe microanalysis (IPMA)‘, and laser mass spectrometric analysis (LAMMA)” can be used. The detection power of IPMA lies in its signal generation and detection characteristics, in the rig/g to pg/g range. However, practical detection limits depend mainly on the elimination of interferences in These are complex, due to the the mass spectra. occurrence of molecular, cluster and multiply charged ions. Fig. 5 shows that in spite of these basic problems several trace elements, such as Li, B, Na, Mg, P, S, Ti, Cr, Ni, Sr and Pb could be detected in an oxidic
chemistry,
vol.‘1
1no.4.1981
airborne particle in an urban aerosol’. An improvement in the detection capabilities for trace elements can be expected by the use of ion probes, which offer high mass resolution. A further severe limitation of IPMA for characterization of individual airborne particles is its rather poor lateral resolution, which results in the exclusion of the major fraction of respirable particles for investigation. LAMMA is useful for the detection of trace elements of low and high atomic numbers but for those of medium atomic number, which include many of the anthropogenic components (e.g. most metals), it is of little use - heavy interferences occur which cannot be resolved due to the low mass resolution of a time-offlight-spectrometer. Fig. 6 shows the LAMMAspectra of three aerosol particles of different origin. The characteristic elements can be identified, but trace element identification is limited. Additional information can be obtained from the molecular fragmentation peaks. The C&-peaks in spectrum 0 indicate that the particle consists of highly condensed hydrocarbons, i.e. soot. Although the lateral resolution of LAMMA is much better than that of IPMA it is still insufficient for a large fraction of the respirable particles. LAMMA’s greatest advantage is its potential for use in the direct analysis of particles in biological tissues. For this purpose histological investigations by highresolution light microscopy are combined with microanalysis of areas of as little as 1 pm in diameter. For medical purposes this approach could be complemented by (S)TEM investigation of the tissues, which is more difficult to perform, but has a significantly higher spatial resolution. It should be noted that both IPMA and LXM,ClA are only semiquantitative techniques for particle
Fi
(Id) Ic
2
5
x
0
a
particles
(Kat
finann.
(n)
anthrotqenic
(b)
mineral
(c)
soot particle
(Inflow)
partirle
particle
(h&q Li. :II.
(.Il.
(lge. Zn) K.
(.‘a/
(CJ
Fi,c. 7. Laser
(.VH#):S04-FarMe u?th impactor
ofairborne
II kte+)
raman (diameter
.rpecVrum o/ Y nm) collec~ted
on South Pole (Et;.
page)
Fig.
8. Depth
HI&i’).
di.r/ribution
Fe, Ba and Pb in aerosol particle
nm diameter
(Nexbu+)
SOUTH POLE
ofH. oJ IO
trends in ana~tical chemistry, vol. 1,
103
no. 4,1981
analysis because of the matrix effects which occur in signal generation. However, an advantage which both techniques have over other methods and which has not yet been exploited sufficiently, is that isotopes are actually measured. Detection of unnatural isotopic ratios is therefore possible. This is a feature that might be used for tracer studies in emission and transportation processes.
Direct compound-specific analysis For the direct compound-specific identification of individual airborne particles, Laser-Raman Microanalysis (LRMA) can be applied’J. Fig. 7 shows the vibrational spectrum of an ammonium sulphate particle collected at the South Pole with an impactor. Study of the vibrational frequencies can provide not only the identification of functional groups but also, in many cases, the definition of the molecular formula (in this case: ( NH4) $O.+) . LRMA has enormous potential as a tool in aerosol research, since the identification ofcompounds present in the aerosol is most important and LRMA is the only technique which can provide this information directlv. All the other techniques of individual particle analysis identify compounds indirectly (by elemental analysis and calculation of stoichiometry). However, the technique still has to undergo further development before it can be used for aerosol analysis. Its ma.jor limitations
1
ri
x* 28S,+
Surface analysis The surface composition of aerosol particles is an especially important factor in determining their physiological effects. Their surfaces interact with body fluids and there is often a significant enrichment of anthropogenic elements (e.g. metals) on particle surfaces as a result of condensation and adsorption processes. For the determination of the surface composition of individual particles auger electron spectroscopy (AES) and IPMA can be used. AES offers more analytical capabilities, because a high lateral (- 100 nm) and depth resolution (- l-5 nm) can be achieved. Furthermore, selection of individual particles for analysis is easily performed with the aid of secondary electron imaging. However, problems arise due to electron induced heating of the particles. IPLIA can be used to measure depth distribution bv continuous registration of the secondary ion intensities during the sputter process thus allowing a direct comparison between the surface and bulk composition. Fig. 8 shows the depth profiles of six elements in a single dust particle. A significant enrichment of the anthropogenic element Pb could be registered.
Conclusion and prospects Urbcn
ld
at the moment are a lateral resolution of about 1 nm and the low intensity of the Raman radiation which results in long measurement times for the spectra (up to several hours).
Dust
Parttcle
w 56Fe/
j’
\
-(
28S1
The combination of different techniques of in-situ microanalysis enables the characterization of individual airborne particles with respect to their morphology, elemental composition, compound and surface enrichments. Such information is vital for the successful solution of a large number of problems in aerosol research, but it is not yet used as widely as it could be. The major reason for this is almost certainly because only a few laboratories in the world are able to apply several of the techniques, described above in combination. However, this \vill change as more and more aerosol research centers are equipped vvith such instrumentation. As far as future research is concerned. it has to be emphasized that much work has yet to be done on the following: development of sampling and sample preparation methods; optimization and application of the techniques for direct compound-specific and surface analysis; especially important, the combination of histological tissue characterization with in-situ particle analysis.
References
1
Depth,
pm
i
Grasserbauer, AI. ( 1978) in :Inn!,ris oJ_iirborne Portirles 6~ P~nicol Methods (hlalissa. hl. ccl.) CRC-Press Inc.. \Vest Palm Beach. Florida hl&rone. I\‘. C.. Delly, J. G. (1973) The Particle :ll/ns. vol. 1-l. :\nn ;\rbor Scimc~. ;\m Arbor. hlichigan Heinrich. Ii. F. J. (ccl.) (1980) C/ mrorteri:ntion qf’Porticles. SBS SP 533. \Vashington
trends in analytical chemistry, vol. I, no. 4,198I
104
Grasserbauer, M. (1978) Mikrochim. Acta 1, 320 Gravatt, C. C.? LaFleur, Heinrich, K. F. J. (eds) (1978) Proceedings of the Workshop on Asbestos: Definitions and Measurement Methods, NBS SP 506, Washington Malami, Ch., Grasserbauer, M. Z. Anal. Chem. in Press Newbury, D. E. (1980) in Characterization of Particles (Heinrich, K. F. J. ed.) NBS SP 553, Washington Kaufmann, R., Wieser, P. (1980) in Characterization of Particles (Heinrich K. F. J. ed.) NBS SP 553, Washington Etz, E. S., Blaha, J. J. (1980) in Characterization of Particles (Heinrich, K. F. J. ed.) NBS SP 553, Washington
Circle no. 60”
advertising enquiry form
M. Grasserbauer is Professor of Analytical Chemistry at the Technical University of Vienna, Institute for Analytical Chemistry, Getreidemarkt-9, A-1060 Vienna, Austria. He is currently head of the Department of Physical Analysis. His current research interests are in the field of physical microanalysis of solids - especially characterization of airborne particles and quantitative distribution analysis of trace elements in semiconductors, metals, ceramics and geological materials (using different electron and ion probe techniques). Ch. Malami, a graduate of the Technical University of Athens, Greece, was a doctoral student of Prof Grasserbauer and obtained her Ph.D. in 1981.
Y8
Yoneda, Nakazawa, Yamazaki, Hare, P. LePage, Karger,
separation can be optimized by modifying the mobile phase. What remains to be done is to elucidate the nature of the forces that control the separation. In any event, the use of metal cations greatly facilitates the altering of the chromatographic system to the benefit of the analyst.
185,
I!),
Gilon, 203,
8
Sousa,
I,.
Am.
Leshem,
R. and
Grushka,
E., Tishhee,
I\. and
Hare,
E. (3981)
J.
Chromatqqr.
( lY80)
P. E.
J. ~tm.
Chem.
Sot.
102, 51l5 Oelrich,
13-24
L. K.,
( 1Y74) ,j.
C., 365
Gil-Av,
E., Preusrh,
H. and WIhrlm.
E. (lY80)
HRC’+
Cc’.
3.
26Y
Davankov, V. A. (1980) in Giddings,.J. C., Grushka, E., Cazrs, .J. and Brown, P. R. (eds), AdLlances in Chromatqraph_v, Marcel Dekkrr, 18. Ch. 4, p. 139 6 1’oshikawa, Y. and Yamasaki, K. ( 1970) Inorg. ,Vucurl. Chem. Letl. 6 35 25. 7 Sousa,
C., Leshem, R., Tapuhi, Y. and Grushka, E. (1979) ./. Sac. 101, 7612 C., Leshem, R. and Grushka, E. (1980) Ilnal. Chem. 52,
1206
Buss, D. R. and Vermeulen, I‘. (1968) Ind. Eng. Chem. 60, 12 E., Feihush, B. and Charles-Sigher, R. (1966) Gil-A\,, Tetrahedron Lett. 10, 1OOY Audebert, K. (197’3) ./. Liq. Chromatqr. 2, 1063-1095 Blaschkr, G. (1’380) :In,ym. Chem.. 92, 14-25 (lY80); Int. Ed. Erg/.
H. and Yoshizawa, T. (1976) Chem. Lett 707. H. and Yoneda, H. (1978) J. Chromatogr. 160, 89 S. and Yoneda, H. (1979) J. Chromatogr. 177, 227 E. and Gil-A\,, E. (197Y), Science 204, 1226 J. X., Lindner, b’., Davies, G., Seitz, D. E. and B. L. (1979) Anal. Chem. 51, 433; (1979) j. Chromatqtr.
Chem.
Gilon,
References
uo; 1, no. #. I981
323
Gilon, Am.
chemisty.
Hoirinan, Chem.
Sot.
1). H., 96,
Kaplan,
L. and
Cram,
Sogah, G. D. Y., Holyman, .J. (1!)78) ./. Anr. Chem. Sot. 100, 4.569
Eli
Characterization methods
1). H. and
Cram,
S., Chow,
F. and
Karmen,
;I.
( 1980)
J.
Chromafqr.
1YY.
D. .J.
7 100
K.,
Lam, 2Y5
D.
Grxrhka
.Ina!~‘tical
and
R.
ChemiJtrv.
Leshenr The
zcwrk nt the Deportment Hehretce
1 .nil,er-\i!v
o/. It~o~~auic~
o/‘, jewsalem.
NM/
Jerrcwlem.
Iwael.
of individual airborne particles used and their limitations
-
The characterization of individual airborne particles plays an important role in assessing environmental contamination. The morphology, elemental composition and surface properties of these particles must be determined for complete characterization. Manfred Grasserbauer and Chariklia Malami Vienna, Austria Characterization of individual airborne particles through size, shape, morphology and chemical identity (by elemental or direct compound-specific in-situ microanalysis) 1.2~. is of basic importance in environmental aerosol studies, because these data may yield information about _ the origin especially anthropogenic of particles, contaminants, - chemical transformation processes in the atmosphere, and - physiological effects of air contamination (especially by absorption through respiratory organs), solubility of material and even toxicity.
Survey of methods The availability and choice of methods for the characterization of individual airborne particles is determined primarily by the range of particle sizes to be examined - that is those respiratory particles between, approximately, 100 nm-5 nm in (aerodynamic) diameter. Therefore, only physical techniques 0 16.~-9936/~I/10-0OoRU2.75
having a spatial resolution of the order of the particlc size, which will permit selective analysis, can bc applied. The most important methods, the analytical information they yield and the lower limit particle size detectable by them (lateral resolution of analysis) are listed in Table I.
of
Sampling
and sample preparationl.2
For the characterization of individual airbornr particles sampling procedures and conditions have to be chosen which satisfy the following requirement; the particles under investigation have to he spatially separated on a suitable substrate to enable analysis of individuals. The substrate can be either a metallographically polished disk of a material which does not interfere with the analysis, or a thin film or plate transparent to electrons or photons. If it is important to avoid the, often difllcult, processing of the collected sample, rapid deposition methods (which take from a few seconds to a few minutes) with Nuclepore@-filters, impactors and ccntrifuges should be applied. The collection of airborne particles by cascade impactors appears to be especially useful, since size fractionation is achieved in the Q I%iIElswirr Scwnlilir Pul,li,bil,R(:,lmpan~
trends in nnn!yticnl
chemistry.
vol. I. no. I, 1981
99
sampling procedure and the sample can then be concentrated in a small spot of only several mm in diameter on almost any substrate (e.g. Au-foils, carbon grids). If sampling with Hi-Vol-Filters is applied, or if it is performed for a longer time to obtain time-averaged values, a further sample preparation step has to be taken. There are many possible ways ofdoing this. One of the most important procedures, especially for scanning transmission electron microscopy (STEM) investigations, is suspension in collodion-amyl-acetate followed by deposition on a suitable substrate. For enrichment of certain materials it may be useful to ash the ma.jor matrix before analysis (e.g. microwave ashing of organic debris before analysis of asbestos fibers). For analvsis with electron and ion beams, coating of the part’icles with carbon or gold is necessary (layer thickness approximately 20 nm). In general, sample preparation has to be chosen and adapted carefully for a specific analvtical goal. However, a lot of research is still necessary ‘in order to minimize losses and obtain meanin,qful information.
Fi,q.
I. Secondclg
(dinmeter
electmn
micqrnph
qf‘,ilicnte
pnrtirle
Jiom con1,J(r nsh
10 nm).
Elemental analysis and morphology ‘l’he most \vidc$. used analytical technique for characterization of Individual airborne particles is based on the combination of morphological imaging and N-ray microanal~.sis’.’ (applying electron probe microanalysis (EP;\I.\), scanning electron microscop) (SELI) and S’l‘E,\l). Qualitati\.r and scmiquantitati\.e N-ray analysis is often sufficient for identification ifthe particle has a characteristic morpholog).. Secondar) electron micro,qraphs. \vhich sho~v the particle shape
Llrthod
‘I‘)-l)ic;il inliwmation
.\ppl-osimatr IOWCI.particlr sizr limit (di:tm~tw. urn)
and surface morphology are most useful for particle sizes above 0.1 nm. Fig. 1 contains a silicate particle which had been molten (droplike shape). indicating that it is a typical airborne particle originating from coal firing. Particles emitted from oil firing or generated by other processes ha\.e a completely different morphology and can be difrerentiated clearly using this analytical technique’. Transmitted electron images (obtained \\-ith TEA1 or S’l’E;\l) provide information on the particle shape. enabling the morphological characterization of submicrometer particles \vith a spatial resolution of bettel than 10 nm. For more complete identification of these particles. imaging has to be combined \vith highl! sensiti1.e S-ray analysis. best achieved lvith STE;\l (due to its specific construction features). \Yith this technique the characteristic emission particles from leaded gasoline can be identified (Fig. 2). The particles originating from the lead additi1.e are typically round and about 0.1 pm or less in diameter. Ho\\-ei-er. there are situations kvhere the combination of morpholoLg! \vith qualitative or semiquantitati\.r S-ra!. microanalysis is not silficient: in these cases g~7r2fifa/ii*~ _Y-M~I~ nnal~sis has to be applied for particle characterization.
Analysis for asbestos ‘l’raw clcwirnts. isotope ratios, surfiw conipositiou Lasrr mass spxtromctric analysis (I,.\hlhl:\)
Elrmt.utar)~ composition ratios
isotopt~
C:ompoulid Surhw
ccmiposition
I 0.5
Quantitative S-ray analysis of individual particles is especially important in the study of asbestos pollution. Asbestos fibers are present in the respirablc fraction of urban aerosols in concentrations I\-hich are usually above se\.eral rig/m,, (origin: abrasion of. brake linings, plasters and roofs), They exist in much higher concentrations in factories \\hich manufacture asbessites: in some cases tos products 01 near mining asbestos fibers are e\.en present in drinking Icater (e.g. due to ‘l‘aconitc mining)‘. Duluth, Lake Superior.
. 100
trends in ana!,ltiral
Although there are contradictory evaluations of the health risk due to asbestos pollution, it seems that long time exposure to asbestos fibers causes pleuromesothelioma and even lung tumors. It is therefore generally agreed that for the reliable assessment of asbestos pollution analytical methods have to be developed which provide an accurate identification of fibrous materials (especially for the different varieties of asbestos), and a determination of the fiber concentration. This analytical goal is extremely difficult to achieve since the average concentration ofasbestos fibers in an aerosol is very low and the fibers are very small (diameter from about 20 nm to several hundred nm, length from about 100 nm to several pm). For these reasons the methods used for compound-specific
chemistrv,
uol. 1. no. I, 1981
average analysis (like X-ray diffraction techniques) are not suitable. The only successful approach consists of single particle quantitative X-ray analysis, in combination with morphological characterization. For assessment of the submicrometer fraction of asbestos fibers only the STEM is suitable; this also provides electron diffraction patterns which yield additional information. Quantitative X-ray analysis ofsuch fibers (Fig. 3) is based on the ratio-method, which evaluates the intensity ratios of suitable element pairs characteristic of a specific form of asbestos. These intensity ratios are then converted into concentration ratios, with relative sensitivity factors obtained from homogeneous reference materials. Careful optimization of the measurement and spectral evaluation techniques and detailed assessment of the statistical
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trends in nn&tic.ai
chemist~r.
vol. 1. no. 4. 1981
101
and systematic errors of X-ray analysis result in an accuracy of k 25% for (major) elemental concentrations in fibers of 200 nm diameter. To appreciate this accuracy, it has to be recognized that the amount of fiber analyzed in such a measurement is only about 10-r” g. Quantitative STEM-analysis therefore, yields sufficiently accurate concentration ratios for characteristic elements to enable one to distinguish between the different chemical forms of asbestos.
An identification scheme for asbestos fibers can then be established (Fig. 4). In the first step of this identification process the particles are characterized according to their shape and fibers are identified as such if their length-to-width-ratio is larger than 3 (L:D>3). An electron diffraction pattern or an X-ray spectrum is then recorded for each fiber. In many cases the SAED-pattern is sufficient for the identification of asbestos and classification of its type (particularly for i
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chrysotile, which has a distinctive diffraction pattern). In many other cases, however, the electron diffraction patterns are not clear enough for identification (particularly if the fiber has endured structural changes due to thermal exposure in the abrasion process, as in Then X-ray analysis must be used. brake linings). This characterizes the fibers on the basis of concentration ratios which are unique for each variety of asbestos. Determination of the asbestos concentration is achieved by counting the number of positively identified asbestos fibers and relating this to sampling parameters. Although the above technique for the assessment ofasbestos pollution is very tedious, it is the one which yields the most reliable results.
Trace element analysis The electron probe techniques described so far have one limitation in common: X-ray analysis can only be present in concentrations performed on elements Therefore, these methods are not above 0.1-l%. adequately sensitive for the identification of trace elements in single particles. For trace analysis in individual airborne particles which could yield important information about their origin - ion probe microanalysis (IPMA)‘, and laser mass spectrometric analysis (LAMMA)” can be used. The detection power of IPMA lies in its signal generation and detection characteristics, in the rig/g to pg/g range. However, practical detection limits depend mainly on the elimination of interferences in These are complex, due to the the mass spectra. occurrence of molecular, cluster and multiply charged ions. Fig. 5 shows that in spite of these basic problems several trace elements, such as Li, B, Na, Mg, P, S, Ti, Cr, Ni, Sr and Pb could be detected in an oxidic
chemistry,
vol.‘1
1no.4.1981
airborne particle in an urban aerosol’. An improvement in the detection capabilities for trace elements can be expected by the use of ion probes, which offer high mass resolution. A further severe limitation of IPMA for characterization of individual airborne particles is its rather poor lateral resolution, which results in the exclusion of the major fraction of respirable particles for investigation. LAMMA is useful for the detection of trace elements of low and high atomic numbers but for those of medium atomic number, which include many of the anthropogenic components (e.g. most metals), it is of little use - heavy interferences occur which cannot be resolved due to the low mass resolution of a time-offlight-spectrometer. Fig. 6 shows the LAMMAspectra of three aerosol particles of different origin. The characteristic elements can be identified, but trace element identification is limited. Additional information can be obtained from the molecular fragmentation peaks. The C&-peaks in spectrum 0 indicate that the particle consists of highly condensed hydrocarbons, i.e. soot. Although the lateral resolution of LAMMA is much better than that of IPMA it is still insufficient for a large fraction of the respirable particles. LAMMA’s greatest advantage is its potential for use in the direct analysis of particles in biological tissues. For this purpose histological investigations by highresolution light microscopy are combined with microanalysis of areas of as little as 1 pm in diameter. For medical purposes this approach could be complemented by (S)TEM investigation of the tissues, which is more difficult to perform, but has a significantly higher spatial resolution. It should be noted that both IPMA and LXM,ClA are only semiquantitative techniques for particle
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trends in ana~tical chemistry, vol. 1,
103
no. 4,1981
analysis because of the matrix effects which occur in signal generation. However, an advantage which both techniques have over other methods and which has not yet been exploited sufficiently, is that isotopes are actually measured. Detection of unnatural isotopic ratios is therefore possible. This is a feature that might be used for tracer studies in emission and transportation processes.
Direct compound-specific analysis For the direct compound-specific identification of individual airborne particles, Laser-Raman Microanalysis (LRMA) can be applied’J. Fig. 7 shows the vibrational spectrum of an ammonium sulphate particle collected at the South Pole with an impactor. Study of the vibrational frequencies can provide not only the identification of functional groups but also, in many cases, the definition of the molecular formula (in this case: ( NH4) $O.+) . LRMA has enormous potential as a tool in aerosol research, since the identification ofcompounds present in the aerosol is most important and LRMA is the only technique which can provide this information directlv. All the other techniques of individual particle analysis identify compounds indirectly (by elemental analysis and calculation of stoichiometry). However, the technique still has to undergo further development before it can be used for aerosol analysis. Its ma.jor limitations
1
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Surface analysis The surface composition of aerosol particles is an especially important factor in determining their physiological effects. Their surfaces interact with body fluids and there is often a significant enrichment of anthropogenic elements (e.g. metals) on particle surfaces as a result of condensation and adsorption processes. For the determination of the surface composition of individual particles auger electron spectroscopy (AES) and IPMA can be used. AES offers more analytical capabilities, because a high lateral (- 100 nm) and depth resolution (- l-5 nm) can be achieved. Furthermore, selection of individual particles for analysis is easily performed with the aid of secondary electron imaging. However, problems arise due to electron induced heating of the particles. IPLIA can be used to measure depth distribution bv continuous registration of the secondary ion intensities during the sputter process thus allowing a direct comparison between the surface and bulk composition. Fig. 8 shows the depth profiles of six elements in a single dust particle. A significant enrichment of the anthropogenic element Pb could be registered.
Conclusion and prospects Urbcn
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at the moment are a lateral resolution of about 1 nm and the low intensity of the Raman radiation which results in long measurement times for the spectra (up to several hours).
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The combination of different techniques of in-situ microanalysis enables the characterization of individual airborne particles with respect to their morphology, elemental composition, compound and surface enrichments. Such information is vital for the successful solution of a large number of problems in aerosol research, but it is not yet used as widely as it could be. The major reason for this is almost certainly because only a few laboratories in the world are able to apply several of the techniques, described above in combination. However, this \vill change as more and more aerosol research centers are equipped vvith such instrumentation. As far as future research is concerned. it has to be emphasized that much work has yet to be done on the following: development of sampling and sample preparation methods; optimization and application of the techniques for direct compound-specific and surface analysis; especially important, the combination of histological tissue characterization with in-situ particle analysis.
References
1
Depth,
pm
i
Grasserbauer, AI. ( 1978) in :Inn!,ris oJ_iirborne Portirles 6~ P~nicol Methods (hlalissa. hl. ccl.) CRC-Press Inc.. \Vest Palm Beach. Florida hl&rone. I\‘. C.. Delly, J. G. (1973) The Particle :ll/ns. vol. 1-l. :\nn ;\rbor Scimc~. ;\m Arbor. hlichigan Heinrich. Ii. F. J. (ccl.) (1980) C/ mrorteri:ntion qf’Porticles. SBS SP 533. \Vashington
trends in analytical chemistry, vol. I, no. 4,198I
104
Grasserbauer, M. (1978) Mikrochim. Acta 1, 320 Gravatt, C. C.? LaFleur, Heinrich, K. F. J. (eds) (1978) Proceedings of the Workshop on Asbestos: Definitions and Measurement Methods, NBS SP 506, Washington Malami, Ch., Grasserbauer, M. Z. Anal. Chem. in Press Newbury, D. E. (1980) in Characterization of Particles (Heinrich, K. F. J. ed.) NBS SP 553, Washington Kaufmann, R., Wieser, P. (1980) in Characterization of Particles (Heinrich K. F. J. ed.) NBS SP 553, Washington Etz, E. S., Blaha, J. J. (1980) in Characterization of Particles (Heinrich, K. F. J. ed.) NBS SP 553, Washington
Circle no. 60”
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M. Grasserbauer is Professor of Analytical Chemistry at the Technical University of Vienna, Institute for Analytical Chemistry, Getreidemarkt-9, A-1060 Vienna, Austria. He is currently head of the Department of Physical Analysis. His current research interests are in the field of physical microanalysis of solids - especially characterization of airborne particles and quantitative distribution analysis of trace elements in semiconductors, metals, ceramics and geological materials (using different electron and ion probe techniques). Ch. Malami, a graduate of the Technical University of Athens, Greece, was a doctoral student of Prof Grasserbauer and obtained her Ph.D. in 1981.