ELECTRON MICROPROBE ANALYSIS OF ATMOSPHERIC
AEROSOLS
D. K. Landstrom, et al
Battelle Memorial Institute
Columbis, Ohio
31 December 1969
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U. S. DEPARTMENT OF COMMERCE
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ANALYSIS
MICROPROBE
ELECTRON
AEROSOLS
ATMOSPHERIC
NATIONAL AIR POLLUTION CONTROL
ADMINISTRATION
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FINAL REPORT
on
ELECTRON MICROPROBE ANALYSIS
OF ATMOSPHERIC AEROSOLS
to
NATIONAL AIR POLLUTION CONTROL
ADMINISTRATION
Contract CPA 22-69-33
December 31, 1969
by
D. K. Landstrom
Doyle Kohler
BATTELLE MEMORIAL INSTITUTE
Columbus Laboratories
505 King Avenue
Columbus, Ohio 43201
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MANAGEMENT SUMMARY
Contract No. CPA 22-69-33
"Electron Microprobe Analysis of Atmospheric Aerosols"
Background
This research program is designed to investigate the use of the electron micro-
probe for the determination of the fine structure and composition of airborne particulates
of significance in air pollution studies. The results of this study should give NAPCA and
others interested in air pollution a good overall picture of the applicability of the electron
microprobe to small-particle analysis and characterization.
Significance of Research Results
The value of the electron microprobe for small-particle analysis has been demon-
strated, and it has been shown that sample-collection techniques are presently the limit-
ing factors in the amount of compositional information that can be obtained from small
particles. A relationship between lead, bromine, and chlorine in automobile engine
exhaust and atmospheric samples has been demonstrated and recommendations have been
made regarding the type of instrumentation best suited for small-particle analysis.
Applications of Research Results
The data generated in the program show the many aspects where the electron
microprobe can aid in small-particle analysis. The need for research in specimen
preparation is necessary so that a sample is suitable for the particular requirements
of the microprobe and other associated techniques. Further investigation of the chem-
ical relationships of elements in air pollution particles is necessary to determine their
effects on the environment.
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TABLE OF CONTENTS
Page
SUMMARY 1
INTRODUCTION 1
OBJECTIVES 2
EXPERIMENTAL WORK 2
Test Aerosols 2
Source Aerosols 8
Vehicle Exhaust 8
Automobile Exhaust, Stage 5, Beryllium Disk 8
Automobile Exhaust, Stage 4, Beryllium Disk 12
Automobile Exhaust, Stage 5, Electron Microscope Grid. . . 12
Atmospheric Samples 15
Atmospheric Samples, Stage 4, Beryllium Disk 15
Atmospheric Samples, Stage 5, Beryllium Disk 15
Atmospheric Samples, Stage 2, Beryllium Disk 23
Atmospheric Samples, Stage 3, Beryllium Disk 27
Evaluation of Experimental Data 28
Energy Dispersive Analysis 29
CONCLUSIONS 32
RECOMMENDATIONS FOR FUTURE WORK 34
APPENDIX A
GENERAL OUTLINE OF THE ELECTRON MICROPROBE POTENTIAL AND
APPLICATIONS TO AIR POLLUTION RESEARCH A-l
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ELECTRON MICRO PROBE ANALYSIS
OF ATMOSPHERIC AEROSOLS
by
D. K. Landstrom and Ooyle Kohler
SUMMARY
This report describes a research effort to determine the feasibility and capability
of electron microprobe analysis as a method for obtaining detailed compositional and
structural information about pollutant aerosol particles in the atmosphere. Extensive
data is presented from known test aerosols, automobile engine exhaust, and atmospheric
samples. It was determined that the sampling method is presently the limiting factor in
the detection and analysis of small particles, and, by preparing samples to minimize
X-ray background, particles as small as 0. 1 micron can be successfully analyzed. The
theoretical limitations of the microprobe for both wavelength-dispersive and energy-
dispersive spectrometers are discussed, and recommendations are made about analysis
systems for small particles.
INTRODUCTION
The problems associated with air pollution are well known and are becoming in-
creasingly more important as the population and industrial activities expand. The effects
of suspended particulates in the air can be observed every day and include soiling,
deterioration of materials, corrosion, smog, and physiological reactions. Other long-
term effects not so readily observable also are a direct result of particulate contamina-
tion. Actually, very little is known about the composition and fine structure of individual
airborne particles, especially those in the size range from about 1 micron in diameter
and below.
Some of the classical methods of analysis such as dustfall estimates, total sus-
pended matter, particle size, or gross chemical analysis often do not provide enough
information about individual particles. Even such valuable techniques as X-ray diffrac-
tion, X-ray fluorescence, electron microscopy, light microscopy, mass spectrometry,
and many others do not provide compositional data on very small individual particles
except in a very few special cases. It is the analysis of single selected particles that
presents both an opportunity and a challenge for expanding the information available
about the effects of air pollution. Very little is known about the effects of individual
particles on people, plants, animals, or objects. Often, particles of interest may be
present in specific particle size ranges, but very little is known about what particle size
range can be expected for a given composition. In many cases, a single particle of
interest may be lost among thousands of particles that may be harmless or of no particu-
lar interest. In atomic energy operations, the detection of just one particle of fissionable
material among many other particles could be of major importance, especially from the
health physics standpoint. The electron microprobe analyzer appears to have great
promise as a method for the analysis of individual particles in the micron range.
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A discussion of the problems and theoretical considerations of the electron m
probe as applied to small-particle analysis is important to the work described here.
Because this discussion is so extensive, it has been written separately as an Appendix.
OBJECTIVES
The research program described herein was planned to achieve the objectives set
forth by the National Center for Air Pollution Control to determine the capacity of elec-
tron microprobe analysis as a method for obtaining detailed compositional and structural
information about pollutant aerosol particles in the urban atmosphere and from specific
sources. More specifically, the major objectives of the program were to perform,
through electron microprobe analysis, detailed nondestructive chemical analysis of aero-
sol particles collected from the ambient atmosphere and from specific sources, and
thereby define the types of quantitative and qualitative information about aerosol fine
structure and composition obtainable with the present state of the art. The study should
demonstrate the sensitivity and the limitations of the method, provide information on the
importance of instrumental design parameters, and outline future capabilities as pro-
jected from the present state of the art.
EXPERIMENTAL, WORK
This program on "Electron Microprobe Analysis of Atmosphere Aerosols" was
expected to begin on January 1, 1969, and continue for 6 months. Due to delays princi-
pally in receiving samples appropriate for the study, no technical work other than that
of a survey nature was performed on this contract until April, 1969, and the overall
time period of the program was extended to 12 months.
The first samples were delivered on March 27, 1969, and consisted of known test
aerosols as will be described below. Results were reported in a letter report dated
July 1, 1969, and are summarized below.
Test Aerosols
Two sets of samples collected from Stages 2, 3, 4, and 5 of a Battelle cascade
impactor were received and consisted of aerosols of NaCl and Zn(NH4)2(SO4)2'6H2O,
respectively. Two types of collections were received for each aerosol, one using
polished beryllium disks for a substrate and the other using Formvar-coated electron
microscope grids.
Results obtained on the NaCl beryllium-disk samples were generally good, and
pictures showing the location and elemental distributions were obtained for all four
cascade impactor stages. These samples also allowed a comparison of a new-type
combination scanning electron microscope/microprobe (Materials Analysis Company
Model 400S) with an older, more conventional microprobe (Materials Analysis Company
Model 400). The superior performance of the 400S is clearly indicated by a direct
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comparison of photographs obtained of identical sample areas in the two instruments.
Figure 1 is a back-scattered electron (BSE) image taken on the older microprobe show-
ing several particles from Stage 2 of the cascade impactor. Figures 2 and 3 are-X-ray
distribution photographs of the sodium and chlorine, respectively, in the same area as
shown in Figure 1. These figures should be compared with Figures 4 and 5, which show
the BSE and the chlorine X-ray images respectively, of the same sample. (Note the
slightly lower magnification of Figures 4 and 5.) Comparison of these figures clearly
show the superior resolution obtained in the new microprobe. Note also the better X-ray
resolution shown in the chlorine picture.
As is mentioned in the general discussion on microprobe analysis (Appendix)
instrumental conditions are important for maximum particle resolution. Figures 6 and
7 illustrate the effect of beam current on the resolution. (Higher beam currents cause
the beam to expand in size.) Figure 6 is a BSE image taken at a specimen current of
0.009 ua. Note that Figure 6 represents the same area as in Figures 1 and 4, except at
higher magnification. Figure 7 is a similar photograph taken at 0. 03 ^a (normal operat-
ing conditions for X-ray analysis). It is clear that there is a loss of resolution in
Figure 7 as well as a smearing effect that makes the particles appear larger than they
actually are. A rate-meter scan of this same area for chlorine using the same instru-
ment conditions as for Figure 7 is shown in Figure 8,
A useful limit of magnification for this type of scanning electron microprobe where
the beam diameter is on the order of 0. 1 to 0. 2 micron is approximately 1000-3000X.
Figure 9 shows three of the particles from Stage 2 of the cascade impactor at 3000X.
At this magnification, very little structure is observable, and the surface detail of this
type of specimen would have to be studied in a scanning electron microscope for proper
resolution. The X-ray sensitivity is still good at this magnification; Figures 10 and 11
show the distribution of Na and Cl, respectively. Note the higher noise level (back-
ground counts) in the Na distribution, due to a less favorable peak-to-background ratio
for this element in our microprobe. Figures 9, 10, and 11 represent a practical limit
for this type of specimen preparation and our particular commercial microprobe.
In general, the samples obtained on electron micro-scope grids were not as satis-
factory as those on beryllium disks. The instrument conditions necessary for X-ray
analysis usually caused the support film to buckle or melt. Figure 12 is a BSE image
obtained from the Stage 2 cascade impactor sample. It is interesting to note that the
NaCl cubic shape ia visible on these samples, whereas those on the beryllium-disk
samples are generally spherical or oval in shape. If the beryllium-disk had been
exposed to more atmospheric moisture than the grid samples, the shape differences
could be explained as due to the increase in moisture.
Particles were detected and analyzed from Stages 3, 4, and 5 on the beryllium
disks; but the picture quality of these was progressively worse from those presented
for Stage 2 and, because of the difficulty of reproduction, micrographs are not included
here. Sodium and chlorine could be detected in particles that were not visible optically
but which could be detected by the scanning electron beam system of the microprobe.
There was some agglomeration of particles on Stages 3, 4, and 5 that gave relatively
high Na and Cl counts, while the smallest particles (approximately 0. 1 to 0.2 ju in
diameter) gave only e. few counts above the background level.
Figure 13 illustrates one method of detecting particles that is often more sensitive
than the standard BSE image. This method of data presentation is called oblique pro-
jection (OP) and consists simply of modulating the y-deflection of the oscilliscope with
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43 OX
FIGURE 1. BACK-SCATTERED ELECTRON IMAGE OF SAMPLE FROM STAGE
NO. 2 OF CASCADE IMPACTOR (NaCl Be DISK) MODEL 400 MICROPROBE
43 OX
FIGURE 2. SODIUM DISTRIBUTION IN THE SAME AREA AS FIGURE 1
Rate Meter Mode, Model 400 Microprobe
43 OX
FIGURE 3. CHLORINE DISTRIBUTION IN THE SAME AREA AS FIGURE 1
AND 2
Pulse Mode, Model 400 Microprobe
300X
FIGURE 4. BACK-SCATTERED ELECTRON IMAGE OF SAN 1C FRO*
STAGE NO. 2 OF CASCADE IMPACTOR (NaCl Be DI? '<) N' >'. L 4u
SCANNING ELECTRON MICROPROBE
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300X
FIGURE 5. CHLORINE DISTRIBUTION IN THE SAME AREA AS FIGURE 4
Rate Meter Mode, Model 400S Scanning Electron Microprobe
700X
FIGURE 7. SAME AS FIGURE 6 EXCEPT TAKEN AT 0. 03 UAMP
SPECIMEN CURRENT
700X
FIGURE 6. BACK-SCATTERED ELECTRON IMAGE OF SAMPLE FROM
STAGE NO. 2 OF CASCADE IMPACTOR (NaCl Be DISK) MODEL 400S
Taken at 0.009 y AMP Specimen Current
700X
FIGURE 8. CHLORINE DISTRIBUTION IN THE SAME AREA AS
FIGURE 7
Rate Meter Mode, Model 400S
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3000X
FIGURE 9. BACK-SCATTERED ELECTRON IMAGE OF SAMPLE FROM STAGE
NO. 2 OF CASCADE IMPACTOR (NaCl Be DISK) MODEL 400S
3000X
FIGURE 11. CHLORINE DISTRIBUTION IN THE SAME AREA AS FIGURE 9
AND 10
.r.»^ -. .
«.-.* - *
xv-
3000X
FIGURE 10. SODIUM DISTRIBUTION IN THE SAME AREA AS FIGURE 9
Rate Meter Mode, Model 400S
a**
20 OCX
FIGURE 12. BACK-SCATTERED ELECTRON IMAGE OF SAMPLE
STAGE NO. 2 OF CASCADE IMPACTO*
Rate Meter Mode, Model 400S
uectron
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1000X
FIGURE 13.
OBLIQUE PROJECTION MADE OF
SAMPLE FROM STAGE NO. 2 OF
CASCADE IMPACTOR (Nad Be DISK)
200X
200X
FIGURE 14. BACK-SCATTERED ELECTRON IMAGE OF SAMPLE FROM
STAGE NO. 4 OF CASCADE IMPACTION (AUTO EX-
HAUST. Be DISK) SHOWING PARTICLES IN SCRATCH
FIGURE 15. LEAD DISTRIBUTION IN THE
SAME AREA AS FIGURE 14
200X
FIGURE 16.
CHLORINE DISTRIBUTION IN THE
SAME AREA AS FIGURE 14
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the BSE signal or the specimen current. The ordinary BSE image is produced by i.
lating the Z axis (spot intensity). Thus, in the oblique projection mode, very small
changes in the back scattered signal that might not be observable in the regular image can
be easily observed. Topographical detail is also enhanced, although interpretation the
image is often difficult.
The Zn(NH4)2(SO4)2-6H2O samples were all unsatisfactory for probe analysis. The
sample area could not be located in the light microscope or by a scanning electron image.
Some isolated clumps of material were found, but these appeared to be debris or contam-
ination on the beryllium surface. Consequently, no further work was carried out on
these samples.
Source Aerosols
Vehicle Exhaust
In April, 1969, some automobile engine exhaust samples were received, and early
rmcroprobe results on them were not encouraging. In these early samples, only gross
quantities of carbon could be detected, with traces of Si, Ca, Fe, and Cl. No lead was
detected, which was quite surprising to us since engine exhaust samples we had prepared
here under another program all showed easily detectable lead. At first, it was thought
that the particle collection time was not sufficient, and, indeed, this seemed to be part
of the problem; however, when later samples were received (June, 1969) the same lack
of definitive particles was noted. It was evident that there was a definite sampling prob-
lem; the beryllium disks simply were not retaining the impinging particles as they should,
and this was partially confirmed by one of the samples that was received at this time.
A beryllium disk from Stage 4 of the Battelle cascade impactor was examined and
very few particles were found, with the exception of one area where a large amount of
lead was detected. This area is shown in Figure 14, which is a BSE image of this area.
Close inspection of this region showed that this area was a surface scratch on the beryl-
lium plate and that this was the reason that the particles were trapped. Chart scans and
photographs were obtained of this area, with Figures 15 and 16 showing the distribution
of lead and chlorine, respectively, in the same area as of Figure 14. Photographs were
also obtained showing the Si, Fe, Cr, Mn, and Ni distribution in this area, but they are
not included in this report. Many Si particles were observed as well as many Fe
particles. One fairly large particle (= 20)u, near arrow in Figure 14) was positively
identified by quantitative analysis to be type 304 stainless steel.
In October, 1969, the first automobile exhaust samples were received that were
useful for extensive investigation. Acting on our suggestion, NAPCA had put a very thin
coating of silicone grease on the surface of the beryllium disks to aid in particle capture
and, in addition, had used a longer sampling time. These samples were very good for
microprobe analysis, and the results are reported below.
Automobile Exhaust, Stage 5, Beryllium Disk. The deposit from the cascade im-
pactor was contained in one small spot near the center of the beryllium disk. Unlike
previous samples, no difficulty was experienced in locating the depop.t area, which
appeared to be a thick agglomeration of particles that thinned out toward the edges of the
deposit. Near the edges and surrounding area, individual particles could be detected,
although many of the larger ones appeared to be made up of clumps of many smaller
particles.
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TABLE 1. RELATIVE CONCENTRATIONS OF ELEMENTS DETECTED IN AUTOMOBILE EXHAUST SAMPLES
H = high, M = medium, L = low, T = trace, ND = not detected.
Element
Sample Description
Stage 5, Be disk,
random area
Stage 4, Be disk,
random area
Stage 4, Be disk,
high iron
Stage 4, Be disk,
high zinc, type 1
Stage 4, Be disk,
high zinc, type 2
Stage 4, Be disk,
high lead
Stage 5, EM grid,
random area
Pb
H
H
L
M
L
H
H
Cl
H
M
L
M
L
H
L
Br
H
M
T
L
L
H
H
Al
M
M
ND
ND
ND
ND
ND
Zn
M
L
T
H
H
T
T
Fe
M
M
H
L
L
L
L
S
L
L
T
T
T
T
L
P
L
L
L
T
T
T
L
Si(a)
M
H
H
L
L
H
ND
Ca
M
L
L
L
L
L
L
Cu
ND
ND
ND
H
T
ND
H
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10
In order to obtain a general idea of the deposit composition, the electron mi<
probe beam was positioned in the densest portion of the deposit and a wavelength scan
for all detectable elements was run. * Table 1 gives a summary of the elements detected
in all of the automobile exhaust samples and presents a very rough estimation of th»
relative concentration of the various elements.
A BSE image of the deposit area was obtained (Figure 17) and individual X-ray line
scans for Pb, Si, S, Fe, Ca, Al, Zn, P, Br, and Cl were made along the line visible in
Figure 17. These profiles were taken directly from the oscilliscope screen of the micro-
probe at various scale factors, so the intensities (peak heights) of the scans are not
directly comparable. The scans do provide a general idea of the elemental distributions,
and it is easy to see, for example, the obvious segregation of individual particles con-
taining iron. The lead, bromine, and chlorine are all fairly evenly distributed in the
deposit; the Ca, Si, Al, S and, to a lesser extent, the Zn and P show some evidence of
segregation into individual particles.
Chart scans for each of these elements along the scan line of Figure 17 were also
obtained which provide greater details than it is possible to obtain with an oscilliscope
profile. The chart scans confirm the indications shown on the X-ray line scans and also
show the deposit to be less dense at the center; i.e., the deposit is somewhat ring-
shaped. Due to the difficulty of reproduction, the chart scans are not included in this
report but are available for study if desired. **
An area near the edge of the deposit was located and BSE pictures at 1000X of
some scattered individual particles were obtained. X-ray distribution photographs were
obtained for Pb, Cl, Br, Zn, Fe, P, and Si. No photographs for S and Ca were obtained,
since these elements were found only in heavier regions of the deposit. These photo-
graphs show that areas high in lead are also high in chlorine and bromine. Phosphorus,
zinc, and aluminum also appear to be associated with these areas. The calcium, iron,
and silicon seemed much more segregated and not necessarily associated with high-lead
areas.
An attempt was made to obtain more nearly quantitative data from this sample;
Table 2 shows the results for two of the stages. Obviously, these values will not total
100 percent, since there are large quantities of hydrogen, nitrogen, and oxygen associ-
ated with these deposits and these were not determined. The carbon value is probably
low due to large absorption effects from the lead in the deposit.
TABLE 2. SEMIQUANTITATIVE ANALYSIS OF AUTOMOBILE
EXHAUST SAMPLES ON BERYLLIUM DISKS
Sample
Description
Stage 5,
Stage 4,
Be disk
Be disk
Pb
17.6
14.3
Cl
2.7
1.0
Br
4.3
1.7
Element (weight percent)
Al
1.0
1.2
Zn
3.5
Trace
Fe^a) S P
0.2 0.3 0.5
0.8 0.7 0.3
SiO>)
4.1
8.1
Ca
0.2
1.0
C
11.
10.
5
0
(a) Highly segregated -higher values obtainable on individual particles.
(b) Background from sllicone grease subtracted.
Due to the difficulty of reproducing wavelength chart scans (which may be 4 to 6 feet long), none of the charts are included
in this report. Results will be summarized, and reference to the original charts can be made if questions arise.
Chart scans were obtained for all of the samples documented in this report None are included for the reasons mentioned above
but are on hand for stuoy if desired. In general, the chart scans provide greater resolution and greater accuracy in locating
specific panicles, hence were mainly used to pick specific particles for analysis
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11
tsox
B.S.E. Image Showing Scan Line
FIGURE 17. X-RAY SCANS ALONG LINE SHOWN IN PHOTOGRAPH, STAGE 5,
Be DISK, AUTOMOBILE EXHAUST
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12
Comparisons between the two stages are quite interesting, and it is easy to see
that the larger, more segregated particles (Fe, Si, and Ca) are higher on Stage 4 than
on Stage 5; the Pb, Cl, Br, and Zn, which appears to be related to smaller particle size,
are higher on the Stage 5 sample. It would be interesting to see what type of elemental
separation (by particle size) would be possible using the other stages of the cascade
impactor. Unfortunately, no other stages were supplied, so complete comparisons are
impossible. It could be conjectured that the cascade impact or might be useful for sepa-
ration purposes, not only on a particle-size basis, but on an elemental basis if specific
elements are present in certain size ranges. This could be quite important in some
analytical techniques where, for maximum sensitivity, elements of little or no interest
could be eliminated or greatly reduced in the analyzed sample.
Automobile Exhaust, Stage 4, Beryllium Disk. The deposit area on this sample
appeared slightly less dense than that obtained on Stage 5. A greater number of individ-
ual particles were also visible in this sample. As in the case of Stage 5, a wavelength
scan for all detectable elements was run on a representative area, and the results are
similar. Relative concentrations for Stage 4 can be found in Table 1.
A BSE image of the deposit was obtained (Figure 18) and individual X-ray line
scans for Pb, Fe, Al, Br, Zn, S, Si, Cl, P, and Ca were made along the line visible
in the photograph. Again, the heights of the traces are not directly comparable, but
provide only a relative elemental distribution along the scan line. Elemental segregation
is easier to see in this sample than in the Stage 5 samples.
Chart scans along the scan line of Figure 18 were also obtained but are not included
in this report. Segregation of Fe, Si, Ca, Zn, and, to a less extent, Pb, Cl, Br, and S
can be clearly shown in these charts.
An area near the edge of the deposit was located and BSE and elemental distribu-
tion pictures were obtained for Pb, Br, Si, Fe, Cl, Zn, and Cr. The S, P, and Al were
not intense enough to provide a good picture. These pictures are not included in this
report.
Since individual particles could be found easily in this sample, particles with high
values of various elements were located and individually analyzed with a wavelength scan.
The results are summarized in Table 1 and include particles having high iron, high zinc,
and high lead content.
Several particles containing high sulfur content were found in this sample and a
sulfur wavelength-shift measurement was made. Figure 19 shows the sulfur spectrum
from the sample compared with the spectrum from PbS and CaSO4. In general, Fig-
ure 19 shows that most of the sulfur present in this sample is in the sulfate form. There
is a reproducible double peak in the sample spectrum that might indicate a small per-
centage of elemental sulfur; however, this could not be confirmed without extensive
additional work with better standards and spectrometer alignment.
Semi quantitative data for this sample are given in Table 2.
Automobile Exhaust, Stage 5, Electron Microscope Grid. The deposit on the
microscope grid was quite thick and had the same ring structure as was visible on the
Stage 5 beryllium-disk sample. A wavelength scan on the thick deposit area was obtained,
with the relative concentrations for this sample summarized in Table 1.
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13
I50X
B.S.E. Image Showing Scan Lines
FIGURE 18. X-RAY SCANS ALONG LINE SHOWN IN PHOTOGRAPH, STAGE 4,
Be DISK, AUTOMOBILE EXHAUST
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(A
CaS04 standard
Sample auto
exhaust stage
no. 4
PbS standard
5.3736
5.3728
\
5.3720 A decreasing
FIGURE 19. ELECTRON MICROPROBE ANALYSIS OF AUTOMOBILE EXHAUST - SULFUR WAVELENGTH SHIFT
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15
A BSE image of the deposit was obtained (Figure 20} and individual X-ray line
scans for Cl, Pb, Br, Al, S, Fe, Zn, Si, Ca, and P were made along a line which is
visible in the photograph. Segregation of some of the elements is clearly visible.
Areas near the edge of the deposit areas were examined for individual particles,
but there was no area defined clearly enough to obtain suitable photographs.
Atmospheric Samples
Atmospheric Samples, Stage 4, Beryllium Disk. This sample deposit was much
lighter than those obtained for the automobile exhaust samples, but it still was quite
adequate for analysis. Most of the samples was concentrated in a half circle and con-
tained many individual particles scattered over the beryllium surface.
Initially, the beam was positioned in the densest portion of the deposit and a wave-
length scan for all detectable elements was run. Table 3 is a summary of the elements
detected in this sample and all of the other atmospheric samples. No attempt was
made to obtain quantitative data in other than relative terms.
A BSE image of the deposit area was obtained (Figure 21) along with individual
X-ray pulse mode pictures for Pb, Si, Br, Zn, Fe, Mn, Ca, S, Cl, K, and Al (Fig-
ure 22 through 32, respectively). Since these pictures are typical of those obtained for
all the other samples, only this set is reproduced; however, Simular photographs were
obtained on all of the atmospheric samples. Segregation of some of the elements is quite
evident.
Individual particles with high iron, high zinc, high lead, and high calcium contents,
respectively, were easily located in this sample, and wavelength scans were obtained.
The data are summarized in Table 3. BSE images and X-ray distribution photos were
also obtained for each of these particles but are not included in this report. The high
calcium particle (Stage 4, Table 3) is interesting to examine since the wavelength scan
of this particle shows trace amounts of tin, titanium, and chromium. It is important
to realize that, unless specific elements are sought or particle analysis is performed
on an individual and systematic basis, elements which are present in only a few particles
can easily be missed in an analysis. This aspect is covered more fully in the Appendix.
This atmospheric sample was extensively examined for lead particles that had no
bromine or chlorine associated with them, but none could be found; i.e., all particles
containing lead also contained bromine and chlorine, as was also true of the automobile
exhaust samples.
Many particles were found in which no elements could be detected, and these were
probably hydrocarbon deposits.
Atmospheric Samples, Stage 5, Beryllium Disk. The deposit area of this sample
appeared to be a uniform circle slightly less dense in the center. The beam was focused
on a dense, representative area of the deposit and a wavelength scan was made. The
results are show in Table 3. It is interesting that this random area contained fewer
detectable elements than did the sample from Stage 4 (random area, Table 3).
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16
I50X
B.S.E. Image Showing Scan Line
FIGURE 20. X-RAY SCANS ALONG LINE SHOWN IN PHOTOGRAPH, STAGE 5,
E.M. GRID, AUTOMOBILE EXHAUST
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17
TABLE 3. RELATIVE CONCENTRATIONS OF ELEMENTS DETECTED IN ATMOSPHERIC SAMPLES
H = high, M = medium, L = low, T = trace, ND = not detected.
Element
Sample Descpntion
Pb Cl Br Al Zn Fe Mn
Si*
Ca Cu K Ng Mg
(b)
(d)
Stage 4, Be disk,
random area
Stage 4, Be disk,
high iron
Stage 4, Be disk,
high zinc
Stage 4, Be disk,
high lead
Stage 4, Be disk,'
high calcium
Stage 5, Be disk,
random area
Stage 5, Be disk,
high sulfur
Stage 5, Be disk,
high iron
Stage 5, Be disk,
high lead
Stage 2, Be disk,'
high copper
Stage 2, Be disk,""
high titanium
Stage 2, Be disk, 'f>
high lead
Stage 2, Be disk,
high sodium
Stage 2, Be disk,
high iron
Stage 2, Be disk, (g>
high calcium
Stage 3, Be disk,
high iron
Stage 3, Be disk,
high calcium
Stage 3, Be disk,
high zinc
Stage 3, Be disk
high lead
Stage 3, Be disk,
high sulfur
Stage 3, Be disk,
high aluminum
L TT T LH MLNDH
T ND ND ND T H M M ND L
ND L ND ND H M L L ND L
H HH LNDMNDHTH
ND T ND L L M T H ND H
L ND ND T ND T T M ND H
L ND ND ND ND T ND H ND M
ND ND ND M ND H ND H T H
H MHNDTTNDMLH
ND M ND H ND T ND M ND H
ND ND ND ND ND T ND ND ND H
H LLHNDHLLTH
ND ND ND ND ND ND ND L ND L
ND ND ND ND L H ND M ND H
ND T ND T ND T ND L ND H
ND ND ND ND ND H L
-------
150X
FIGURE 21. B.S.E. IMAGE STAGE 4, Be DISK,
ATMOSPHERIC SAMPLE
150X
FIGURE 23. Si DISTRIBUTION IN AREA SHOWN IN FIGURE 20
150X
FIGURE 22. Pb DISTRIBUTION IN AREA SHOWN IN FIGURE 20
oo
150X
FIGURE 24. Br DISTRIBUTION IN AREA SHOWN IN FIGURE 2'
-------
150X
FIGURE 25. Zn DISTRIBUTION IN AREA SHOWN IN FIGURE 20
150X
FIGURE 21. Mn DISTRIBUTION IN AREA SHOWN IN FIGURE 20
150X
FIGURE 26. Fe DISTRIBUTION IN AREA SHOWN IN FIGURE 20
150X
FIGURE 28. Ca DISTRIBUTION IN AREA SHOWN IN FIGURE 20
-------
150X
FIGURE 29. S DISTRIBUTION IN AREA SHOWN IN FIGURE 20
150X
FIGURE 31. K DISTRIBUTION IN AREA SHOWN IN FIGURE 20
150X
FIGURE 30. O DISTRIBUTION IN AREA SHOWN IN FIGURE 20
150X
FIGURE 32. Al DISTRIBUTION IN AREA SHOWN IN FIGURf 20
-------
21
I SOX
B.S.E. Image Showing Scan Line
Pb
Br
Al
FIGURE 33. X-RAY SCANS ALONG LINE SHOWN IN PHTOGRAPH, STAGE 5,
Be DISK, ATMOSPHERIC SAMPLE
-------
to
o
i_
o
v>
c
0>
PbS standard
Sample atmospheric
stage no. 5 Be disc
5.3736
5.3728
5.3720 X decreasing
FIGURE 34. ELECTRON MICROPROBE ANALYSIS OF ATMOSPHERIC SAMPLE - SULFUR WAVELENGTH SHIFT
-------
Z3
A BSE image of the deposit was obtained (Figure 33) and individual line scans
for Pb, K, Si, Si, Br, and Al were made along the line visible in the photograph.
A BSE image and X-ray distribution photos were obtained for this sample; however,
since they are basically similar to those already presented for the Stage 4 sample, they
are not included in this report.
Individual particles with high specific-element content were located and wavelength
scans for several of these particles are summarized in Table 3. Particles run include
high sulfur, high iron, and high lead. BSE image and X-ray distribution photos were
obtained for each of the particles examined. As in the previous samples, particles having
a high lead content also contain bromine and chlorine.
A sulfur-wavelength-shift measurement was made (Figure 33), similar to that run
for the automobile exhaust sample (Figure 19). Figure 34 shows the sulfur spectrum
from the sample compared with the spectrum from PbS. As in the case of the automobile
exhaust, the majority of the sulfur present in the sample is in the sulfate form. The
double peak again might indicate the presence of elemental sulfur, but this would have to
be confirmed by additional measurements.
Atmospheric Samples, Stage 2, Beryllium Disk. The sample density was lower on
this sample than any previous samples, but many large individual particles were found
scattered over a large area. Since the deposit was so widely and thinly dispersed on
the beryllium disk, a random-area wavelength scan was not made. A BSE image
(Figure 35) was obtained as well as X-ray distribution photographs for Ca, Ti, Fe, Ca,
Zn, Pb, Br, Al, Si, S, Cl, K, and Mg. The X-ray distribution photos are not included
in this report, since they are similar to those already shown for the Stage 4 sample.
The chart scans obtained for this sample confirmed the presence of many individual
large particles.
150X
FIGURE 35. BSE IMAGE SHOWING SAMPLE DEPOSIT,
STAGE 2, Be DISK, ATMOSPHERIC SAMPLE
Wavelength scans for particles high in specific elements were obtained, with the
results summarized in Table 3. Particles run include high copper, high titanium, high
lead, high sodium, high iron, and high calcium.
-------
iii
40
30
20
in
at
u
I
<«-
o
w
f
10
12 II 10 9876 54 32 10
Co/Si Ratio
FIGURE 36. FREQUENCY OF INTENSITY RATIOS FOR CALCIUM-SILICON (100 PARTICLES MEASURED)
-------
i i i i i i i i
n r
30
M
w
20o
4-
k-
s.
V
of
30 28 26 24 22 20 18 16 14 12 10 8 6 4 2 O
Ca/AI Ratio
m
FIGURE 37. FREQUENCY OF INTENSITY RATIOS FOR CALCIUM-ALUMINUM (100 PARTICLES MEASURED)
-------
3O
M
01
201
0»
101
20 19 18 17 16 15 14 13 12 II 10 9 6 7 6 5 4 3 2 10
Si/AI Ratio
Cv)
FIGURE 38. FREQUENCY OF INTENSITY RATIOS FOR SILICON-ALUMINUM (100 PARTICLES MEASURED)
-------
27
For this sample, it was decided to see if there were many particles with similar
elemental compositions or if the compositions varied greatly from particle to particle.
Consequently, the method of White, Denny, and Irving(1) was used to analyze 100 of the
calcium-containing particles. This method recognizes that for small particles, the
particle-to-particle variation in absolute X-ray intensities for a given element may vary
by over an order of magnitude, depending on the fraction of the incident electron beam
absorbed by the particle. They found that the intensity ratio for two elements is
essentially independent of particle size and shape in a mixture of particles of the same
composition. Thus, by measuring intensity ratios and plotting against known composi-
tion, one can determine quite accurately the composition of unknown particles.
The procedure followed in making the ratio measurement was to select a particle
from the X-ray image; then the electron beam was positioned directly on the particle
and both elements were counted simultaneously for 10 seconds. Duplicate intensity
readings were made for each particle. There is a very definite advantage in the sim-
ultaneous measurement of the two elements, since an error in beam positioning is
cancelled out as are errors in resetting the spectrometers. Of course, there is a cor-
responding savings in time for each analysis.
Figure 36 is a graph showing the relative frequency of intensity ratios for Ca/Si
on 100 measured calcium-containing particles in this sample. Figure 37 is a similar
graph showing the Ca/Al intensity ratio, while Figure 38 shows the Si/Al ratio. The
wide variations in intensity ratios is apparent from these figures, and we can thus con-
clude that there is a wide variation in the composition of individual particles. Extensive
statistical analysis involving several thousand particles would be necessary to make
more definite statements. For this type of analysis, a probe with automated readout is
nearly a necessity, as operator time becomes excessive.
Atmospheric Samples, Stage 3, Beryllium Disk. The particle density of this
sample was greater than that of the previous sample, and individual particle could easily
be seen. Figure 39 is a BSE image of this sample showing the particle distribution.
150X
FIGURE 39. BSE IMAGE SHOWING SAMPLE DEPOSIT,
STAGE 3, Be DISK, ATMOSPHERIC
SAMPLE
-------
28
X-ray distribution pictures were also taken for Ca, Si, Al, S, Cl, Fe, K, Ph, "r
and Mg, but these again are similar to those previously presented and are not included
in this report.
Wavelength scans for particles high in specific elements were obtained, with
results shown in Table 3. Particles run include high iron, high calcium, high zinc,
high lead, high sulfur, and high aluminum.
Evaluation of Experimental Data
When the rough data regarding particles containing high lead in the automobile-
exhaust samples and in the atmospheric samples are tabulated, it is easy to see the
correlation of the lead with chlorine and bromine (see Table 4). Study of this table
points the way for an interesting set of experiments on the lead-chlorine-bromine re-
lationship in atmospheric samples. Intensity ratios for these elements should be mea-
sured in many particles; the results should prove a definite correlation. It is unfortu-
nate that under the scope of this program this could not be done; however, since the
qualitative relationship has been demonstrated, the quantitative measurement should be
a routine matter. If the chlorine and bromine are examined in Table 4, it can be seen
that there is an increase in the relative amounts from Stage 2 to Stage 5 (Stage 2 trapping
the larger particles and Stage 5 the smallest). The lead concentrations for these stages
are simply listed as "high", since at the scale factors used to detect; the other elements,
the lead was off scale in all cases. Closer investigation has shown that the concentra-
tion of lead in the individual particles increases as the particle size decreases and that
there is a definite corresponding correlation for the chlorine and bromine values. Dis-
tributions on the cascade impactor stages for several other elements seem highly de-
pendent on the particle size, specifically iron, aluminum, and manganese, which de-
crease with particle size. Specifically, it seems that the high-lead-content particles
located on Stage 5 of the cascade impactor have higher lead, chlorine, and bromine than
those from the other stages (larger particle size). The larger particles contain higher
quantities of Al, Fe, and Mn. Thus, there seems to be a definite separation of the ele-
ments by particle size.
TABLE 4. RELATIVE CONCENTRATIONS OF ELEMENTS DETECTED
IN PARTICLES OF HIGH LEAD CONTENT FOR AUTOMO-
BILE EXHAUST AND ATMOSPHERlCiSAMPLES
Data Condensed From Tables 1 and 3.
Element
Sample Description
Stage 4,
Stage 2,
Stage 3,
Stage 4,
Stage 5,
Be disk,
Be disk,
Be disk,
Be disk,
Be disk,
auto exhaust
atmospheric
atmospheric
atmospheric
atmospheric
Pb
H
H
H
H
H
Cl
H
L
T
H
M
Br
H
L
M
H
H
Al
ND
H
.L
L
ND
Zn
-T
ND
ND
ND
T
Fe
L
H
L
M
T
Mn
ND
L
T
ND
ND
S
T
L
T
H
M
P
T
T
ND
T
L
Si
H
H
L
H
H
Ca
L
T
L
T
T
K
ND
L
L
H
L
H = high, M = medium. L = low, T = trace. ND = not detected.
-------
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29
Energy Dispersive Analysis
In order to test the relative sensitivity of energy-dispersive and crystal-dispersive
systems, a Nuclear Diodes lithium-drifted silicon semiconductor detector was fitted to
a Cambridge Scanning Electron Microscope.
Since a scanning electron microscope (SEM) is quite similar in principle to the
microprobe, it is quite easily adapted for energy-dispersive analysis.
The normal-wave length dispersive spectrometer such as is used in the micro-
probe analyzes X-rays by diffracting them off a crystal and observing the angle (wave-
length) where the Bragg equation is satisfied (NX = 2d sin 9). This type of detection is
capable of X-ray resolution of a few electron volts, hence it is no problem to separate
the wavelengths of adjacent elements. The wavelength dispersive spectrometer is,
however, a very low-efficiency device, and it has the disadvantage of being a "single
channel" device in that it can examine only one wavelength at a given time. Also, at the
very low beam currents used in a SEM, the intensity of X-rays produced is very much
less than is that produced in a microprobe, and, also, very long analysis times are
necessary.
An energy-dispersive spectrometer has the advantage of recording, essentially
simultaneously, the full spectrum of the X-rays present. This detector gives a signal
that is proportional to the energy of the X-ray photons and, thus, the X-ray spectrum
is dispersed according to its energy. This type of detector is very efficient, since it
can be placed close to the sample and can view a significant solid angle. Furthermore,
since the entire spectrum can be viewed and recorded in a very few seconds, the total
analysis time is generally short. The major fault associated with energy-dispersive
devices is their lack of resolution (about 200 ev), which is considerably poorer than the
crystal spectrometer. In many cases, however, this resolution is sufficient for prac-
tical detection of elements from about oxygen and higher in the periodic table.
The Stage 5 automobile exhaust sample collected on an electron microscope grid
was mounted and placed in the SEM, Since the electron microprobe had previously de-
tected particles with high concentrations of calcium in this sample, the pulse height
analyzers of the Nuclear Diode analyzer was set to accept only the calcium peak, and
a scan at 1000X was made over several areas of the sample. The SEM can be modified
to produce X-ray pulse pictures similar to those obtained on the microprobe, and by
this method an area was located that seemed to have a particle with calcium concen-
trated in it. Figure 40 is a high-resolution, back scattered electron image of the se-
lected area, while Figure 41 is the X-ray distribution for calcium obtained on the SEM.
Figure 42 shows a BSE image of the same area taken on the electron microprobe; the
superior resolution of the SEM photograph (Figure 40) is evident. Figure 43 is the
X-ray distribution for calcium obtained in the microprobe, and, in this case, it is easy
to see the superior X-ray peak-to-background resolution of the microprobe when com-
pared with the SEM X-ray distribution (Figure 41).
The electron beam of the SEM was positioned on the particle showing the high
calcium, and the spectrum was recorded by the energy-dispersive spectrometer
(Figure 44). Elements that can positively be identified from this spectrum are Cu
(from the electron microscope grid), Al, Cl, Ca, Pb, and Br.
-------
1000X
FIGURE 40. SCANNING ELECTRON MICROGRAPH OF AREA
FROM STAGE 5, AUTO EXHAUST. E. M. GRID
1000X
FIGURE 42. B. S.E. IMAGE OF SAME AREA AS FIGURE 39
TAKEN ON ELECTRON M1CROPROBE
1000X
FIGURE 41. X-RAY DISTRIBUTION FOR Ca IN SAME AREA AS FIGURE 39
Energy Dispersive Spectrometer Mounted on S. E. M.
1000X
FIGURE 43. X-RAY DISTRIBUTION FOR Ca IN SAME AR A
AS FIGURES 39 AND 41
Electron Microprove Wavelength Dispersive Spectrometer
-------
31
10
8
2 6
x
in
I 5
o a
CuL£
CuKa
6 10 12
Channel Number
14 16 18 20
FIGURE 44. SPECTRUM OBTAINED BY ENERGY-DISPERSIVE SPECTROMETER,
STAGE 5, AUTOMOBILE EXHAUST, E. M. GRID, HIGH CALCIUM
PARTICLE
-------
32
This same particle was run in the electron microprobe, and the same elements
were found (considerably higher peak-to-background ratio), as well as Si, S, Zn,
and Fe.
In this case, the energy dispersion gave a spectrum of the major elements in about
36 seconds, compared with about 55 minutes for the wavelength scan on the micropiobe.
It did not detect a few elements that were found in trace amounts by the microprobe.
The question must be asked, "Just what information is desired in particle
analysis? " If one were seeking only the major components of many particles, then the
energy-dispersive system is clearly superior. On the other hand, if one is interested
in trace components in particles containing a specific element, then the wavelength
spectrometer is superior because the wavelength spectrometer can be set to a specific
element and can find a particle containing that element just as fast and with greater
sensitivity than the energy-dispersive spectrometer.
In this particular example, if sulfur had been present in more than trace amounts,
it would have interfered with the chlorine peak, and the energy-dispersive spectrome-
ter could not have separated the true peaks. The same is true of the silicon-aluminum
pair. Perhaps a computer could smooth out the curves, but the time element for analy-
sis then becomes important. The wavelength dispersive spectrometer has no difficulty
in separating these adjacent-element peaks and also has the advantage of providing
quantitative data if desired. The energy-dispersive system is semiquantitative at best.
CONCLUSIONS
The efforts on this program demonstrate clearly that the electron microprobe
analyzer is a useful and proper instrument for the examination of air pollution particu-
late matter. The present limitations are not so much that of the instrument but rather
the type and method of specimen collection. During this project, it became apparent
that, given a suitable sample, a great deal of information can be obtained on particles
down to about 0. 1 \JL in diameter. At the present state of the art, particles smaller than
this cannot be examined because the X-ray count ratios are too low, and the particles
are almost invisible on the electron-scanning image and are not visible at all on the
light optical system. It seems clear that the minimum size of particle permitting ac-
curate quantitative analysis lies between 1 and 5 microns. It is possible to obtain in-
formation on the composition of smaller particles, but such information is generally
qualitative at best, especially in the situation where X-rays may be excited simultan-
eously from the particle and a substrate material; once the characteristic radiation
from the particle is overwhelmed by the background radiation from the substrate, even
qualitative analysis of particles much smaller than 0. 5 ju is impossible.
If the specimen collecting conditions are directed to producing dispersed parti-
cles on a substrate producing no X-ray background (such as beryllium or carbon films),
interference from the substrate is avoided and particles much smaller than 1 micron
can be analyzed qualitatively with a fair degree of certainty. If two or more elements
are present, some degree of quantification is possible using the ratio method described
previously.
-------
33
The importance of using the electron microprobe in conjunction with other analy-
sis techniques cannot be overemphasized. Use of electron microprobe analysis, com-
plemented by scanning electron microscopy, X-ray diffraction, electron microscopy,
optical microscopy, mass spectrometry, and chemical analysis becomes a powerful
system for characterizing particles. The practical usefulness of combining these tech-
niques is particularly evident in a laboratory where all of the instruments are available
and there is constant and close cooperation among those responsible for using the vari-
ous methods.
The question as to which instrument is best suited for particle analysis has no
real answer; it all depends on what information is desired. The scanning electron mi-
croscope is indispensable if surface-morphology information is desired; no present
commercial microprobe can supply the necessary resolution to provide surface detail
on 0. \-fji particles. The compromises necessary to provide this resolution detract
from the X-ray and/or light optical performance of the basic microprobe instrument.
If the problem is to detect one particle in several thousand with a specific elemental
composition, an automated microprobe may best fit the requirements. If a fast general
analysis of the major elements in many particles is desired, energy-dispersive analy-
sis on a scanning electron microscope or conventional electron microprobe is probably
more suitable.
Ideally, one should have at least three separate instruments - the electron mi-
croprobe, the scanning electron microscope, and the transmission electron microscope,
with techniques developed so that the specimens can be interchanged easily between
these instruments and the same area and/or particle easily located. This is not as dif-
ficult as it may sound, and only by a combination of these techniques can the maximum
amount of information be obtained about a particle or particles.
In commercial instrumentation, the electron microprobes currently available do
provide much better scanning facilities than those available just a few years ago, but
this scanning facility is approaching the limit that can be expected without degrading
X-ray performance. Instruments incorporating microanalysis and transmission elec-
tron microscopy have been built, but at this time are not commercially available in a
useful form. Energy-dispersive systems should be watched closely, as improvements
in this field have been very rapid, and, if the sample is designed around the peculiar
limitations of the energy-dispersive system, a lot of useful information may be obtained
in a very short time.
In general, at the present state of the art, the most valuable single instrument
commercially available would be an automated, programmable scanning electron mi-
croprobe with both wavelength and energy-dispersive spectrometers. Such an instru-
ment is available from several manufacturers at about $125, 000 to $150, 000. If some
compromises are made, an adequate instrument for many applications of particle
analysis could be purchased for about $60,000 to $80, 000.
-------
34
RECOMMENDATIONS FOR FUTURE WORK
It must be emphasized that present methods of collecting air pollution particles
are not adequate to provide the maximum amount of information from single particles,
and an experimental program to develop suitable techniques is necessary. For ex-
ample, it is often easier to disperse particles from a liquid medium; collection tech-
niques involving electrostatic precipitation into a liquid media should be investigated.
There are also many transmission electron microscopy techniques that are applicable
to microprobe analysis and which could be easily utilized.
The aspect of suitably moving specimens from instrument to instrument, yet
keeping track of individual particles, has been mentioned. This technique development
is necessary to provide the maximum information from small particles.
An investigation of the lead-chlorine-bromine correlation in atmospheric and
automobile exhaust samples by the use of intensity ratio techniques would provide very
useful information and should be undertaken as a separate program.
-------
APPENDIX
GENERAL OUTLINE OF THE ELECTRON MICROPROBE
POTENTIAL AND APPLICATIONS TO AIR POLLUTION RESEARCH
-------
A-l
APPENDIX
GENERAL OUTLINE OF THE ELECTRON MICROPROBE
POTENTIAL AND APPLICATIONS TO AIR POLLUTION RESEARCH
The utility of the electron microprobe method in the chemical analysis of micron-
size volumes is well known. The fields of study to which the method has been used
encompass metallurgy, ceramics, geology, biology, electronics, and others. To date,
most applications have been predominantly concerned with the in-situ analysis of
microscopic regions in solid materials where the specimen is essentially of infinite
size compared with that of the electron-bombarded region. Very limited applications
involving the analysis of individual particles in the micron-size range have also been
carried out.
One type of application to which the microprobe method has not been extensively
extended but from which very useful chemical information can be derived is the analysis
of particles of microscopic dimensions. The complexity - and hence the practical
realization - of such an application is critically dependent on the characteristics of the
particles and the information desired. The principal factors determining this com-
plexity include the following:
Specimen collection and preparation requirements
The size range of the particle of interest
The number of elements requiring analysis
The total number of particles requiring analysis
The maximum number of particles which can be analyzed simultaneously
The need for correlating X-ray data with individual particles
The volume and element analysis resolution required
The degree of accuracy required in the chemical analysis.
Thus, in view of the variability to which these factors would be restrictive for a
given problem, a method by which particle analysis could be carried out cannot be
stated in a simple manner. An understanding of the restrictive nature of each of these
factors is necessary to determine the procedures required to obtain the maximum
amount of information from a given situation.
This section is directed to the extension of the microprobe method to the specific
application of particle analysis. The procedures and instrumentation required for this
purpose are discussed. In so doing, the limitations of the microprobe method - in terms
of the basic physics of the technique as well as instrument restraints - are cited.
In spite of all the work being carried out related to air pollution, very little is
known about the composition and fine structure of the individual airborne particles that
-------
A-2
seem to present major problems. This is especially true of the particles in the size
range from about 1 micron and below where classical methods of identification such as
chemical analysis, light microscopy, electron microscopy, and X-ray diffraction are
severely limited. Considering the possible high rate of reactivity because of high
specific surface area and the degree of pulmonary retention of this size particles, it
seems imperative that their characterization be attempted. This information is desir-
able, not only from the basic aspect of identification but also to relate atmospheric
particulates with specific source effluents, to understand more fully the basic aerosol
mechanisms, and to assess the effects on materials, animals, and humans.
One of the most powerful tools for single-particle identification is the electron
microprobe analyzer. In this device small areas from a few tenths of a micron to
several hundred microns can be examined nondestructively in several different modes
to establish the spatial distribution and the identification of any element from atomic
number 5 (boron) through 92 (uranium). Simultaneous with elemental identification, the
topography of a sample can be photographed to help relate the probe information to light
or scanning electron microscopy examinations. Most elements can be detected in a
homogeneous sample at levels as low as 100 ppm, while isolated particles of an element
can be identified if as little as 1 x 10"15 grams is present in a small area.
There are many problems associated with the analysis of particles below 1 micron
in size in the electron microprobe analyzer. First and foremost is the fact that most
commercial microprobes have a beam size (and thus imaging resolution) on the order of
1 micron, making imaging by the conventional techniques of back-scattered electron or
specimen current images of little value. The problem, of course, is that if the particle
cannot be found by the light microscope incorporated in the analyzer or by one of the
conventional scanning modes, it is impossible to determine in advance where a particle
of interest is located. Even though the X-ray system may be sensitive to as little as
1 x 10-15 gram, it may be impossible to locate and analyze a particle with sufficient
mass using conventional techniques.
It is generally agreed that there is little problem in the qualitative and quantitative
analysis of particles down to a size range of 2 to 5 microns, depending on their density
and conductivity. Below this size, the X-ray intensity falls off, though smaller sam-
ples can be analyzed by ratios or by a known size correction. Nonconductive particles
present problems of their own because of defocusing and deflection of the electron beam
due to the charge buildup on the particle. This effect often can be minimized by coating
the particles with a thin layer of carbon or other suitable material.
The qualitative and quantitative determinations with an electron microprobe de-
pend upon the quality and versatility of the visual optical system, the resolution and
efficiency of the electron optics, and the detection capability of the X-ray spectrometers.
Essentially all electron microprobes available today can be expected to provide about
the same quality of information about a particulate sample. Some microprobes are
more versatile than others and some have special additional features that make them
attractive; however, at present, all have virtually the same electron-imaging resolving
power and X-ray sensitivity. The successful examination of particles of 1 micron in
size and below presently is limited by the characteristics of the current electron micro-
probes and the experience and skill of the operator.
The ability to obtain specific chemical information on small particles with the
electron microprobe is strongly dependent on certain physical limits. These limits are
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A-3
somewhat variable but are basically determined by instrument design and specimen
characteristics as well as basic theoretical considerations. In any practical situation,
knowledge of these various limitations is necessary to provide a sensible approach to
the problem of particle analysis.
Some of the major factors which affect the chemical analysis capability of the
electron microprobe are:
(1) The size (volume) of material from which a chemical analysis can
be made
(2) The precision and accuracy of the analysis
(3) The detection sensitivity for various elements
(4) The degree of specimen alterations due to exposure to the electron
beam and vacuum environment.
These factors, dependent on the physical processes of X-ray excitation and the large
variation in specimen types, all affect the minimum analyzable volume, which can vary
from less than 0. 01 cubic micron to greater than 1000 cubic microns. The error in
the analysis can range from less than 1 percent to greater than 50 percent, the limit
of sensitivity to element detection can vary from 10 to 1000 parts per million, and
specimen alterations can vary from little or negligible change to almost complete de-
terioration. Some of the principal variables are discussed in greater detail below.
Incident Electron Energy
The energy of the bombarding electrons normally is, for various practical and
theoretical reasons, held constant for a given probe analysis. The choice of beam
accelerating voltage is dependent upon the spectral line analyzed and, in the case of
simultaneous multielement analysis, is determined by the spectral line of shortest
wavelength to be analyzed. The incident electron energy is variable in most electron
microprobes from about 2 to 50 kV and its magnitude has a primary influence on the
following:
Electron Excitation Volume. The electron excited volume can be roughly ex-
pressed in terms of beam diameter and the distance penetrated by electrons along the
incident as well as lateral directions.
The size of the excited volume is a function of the range of the primary electrons
and hence their accelerating potential, and also the critical excitation potential of the
particular characteristic X-ray line utilized for the analysis. Two major dimensions,
the depth and width, of the X-ray-producing volume have been measured experimentally
as a function of accelerating potential and found to vary according to the well-established
electron range equation:(2)
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A-4
where R is the electron range expressed as a distance times the sample's'density an:'.
Eo is the primary accelerating potential. This range equation, suitably modified to ta«.e
into account the electron energy required to produce the characteristic X-ray under
investigation, has been used to calculate the volume contributing to the analysis througi.
direct electron excitation. The important fact is that the volume contributing to the
analysis is a rapidly changing function of accelerating potential and that the analysis of
low-density specimens, with good spatial resolution, such as some types of air pollution
particles, requires the use of low accelerating potentials. At an accelerating potential
of 5 kV, the analytical volume in such a specimen is approximately three orders of
magnitude smaller than that which would result from the use of an accelerating potential
of 20 kV. Reducing the diameter of the electron probe at the low accelerating potential
will further reduce the size of the analyzed volume and improve spatial resolution, al-
though not nearly so substantially so as obtained by the reduction of the primary
accelerating potential.
The above paragraph should be modified, since this may not be necessarily true
for all preparations of air pollution particles. A more general statement would be that
in order to preserve spatial resolution (see sections on microprobe analysis of parti-
cles), one must either use a short-range, low-voltage probe or study thin or widely
dispersed specimens of small size. The use of low voltages for analyzers has been
covered by Anderson(2) and by Anderson and Hasler(3), which, for certain specimens,
have very high sensitivity. Hall(4), on the other hand, has some very convincing argu-
ments in favor of high-voltage operation when examining thin sections of biological
material. The important point is that, by proper specimen collection and preparation
methods, samples that are similar to thin biological specimens can be produced from
air pollution collections. This is precisely the case if one is interested in very small
(less than 1 /LI) particles and it is possible to disperse these particles (for example, in
a thin carbon film) so that X-ray background from the substitute does not interfere with
the analysis.
The effect of the critical excitation potential of the characteristic X-ray line of
the element under study on the volume analyzed is important also and it can be shown
that large increases in spatial resolution can be attained with respect to a particular
element by using an accelerating potential close to its critical excitation potential. Al-
though this technique produces very small analytical volumes for the element in question,
it also produces the generally undesirable consequence that the characteristic X-ray
emission of other elements of lower critical excitation potential studied in connection
with the first element will come from comparatively large volumes resulting in the re-
ported elemental analyses not representing the same volume. For example^), at
11 kV a zinc analysis using the ZnK& line (Ec = 9- 66 kV) originates from a volume of
about 2 ju3, but at the same kV sodium analysis using its Ka line (£c = 1. 07 kV) originates
from a volume of 75 /j3. This situation can be improved by using the La line of zinc
(Ec = 1. 02 kV) and reducing the accelerating potential still further. At 5 kV the volumes
contributing to the analyses of zinc and sodium as well as all the lighter elements will
be about 2 jiP.
From the above discussions it is clear that, generally, to attain the best possible
spatial resolution in a bulk specimen while remaining within the bounds of X-ray emis-
sion microanalysis as commonly employed and yet maintaining the equality of the
spatial relationships of the elemental analyses, the longest wavelength characteristic
X-ray lines of the elements which are detectable should be used at the lowest accelerating
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A-5
potential compatible with the other basic aims of the analysis. This consideration is
especially important when dealing with the light elements (Z <12) for which the critical
excitation potential is low (of the order of 1 kV or less).
Investigations of the effects of such parameters as accelerating potential and
critical excitation potential are necessary to demonstrate the maximum sensitivity ob-
tainable when air pollution particles are examined.
Target Absorption. The amount of absorption of X-rays escaping from the sample
increases as the accelerating voltage increases. Primarily, this is due to the greater
depth of electron penetration, and, consequently, the correction factors required for
quantitative analysis to account for absorption are also increased, thus decreasing the
correction's accuracy.
Secondary Emission (Fluorescence). The percentage of secondary emission
(caused by the continuum) as compared with that of primary emission increases with the
incident electron beam energy, thus increasing the total X-ray excitation volume and
decreasing the accuracy of corrected intensity data for quantitative analysis.
Signal-to-Background Ratio. The signal-to-background ratio, which affects the
minimum, level of element detectability, initially increases as the accelerating voltage,
EQ, excee'ds the critical excitation energy, Ec, of a given spectral line, and then de-
creases for large values of the ratio, EO/EC.
Electron Background. Total electron background (single or multiple scattered
electrons) increases as the accelerating voltage increases. Since it contributes to the
measured background of the X-ray detection system, the minimum level of detectability
is reduced.
Specimen Surface Conditions. A very complete understanding of the nature of the
surface of a specimen is necessary for the proper interpretation of measured data.
High accelerating potentials cause greater electron penetration and decrease the sen-
sitivity of the analysis as far as surface conditions are concerned. This decreased
sensitivity is, of course, highly dependent on the magnitude of absorption which the
excited X-rays undergo and can be negligible in the case of a very-high-absorption
situation.
Specimen Characteristics
Characteristics of the specimen may be the most important controlling factor(s)
in the analytical capabilities of the microprobe. Some of the important characteristics
include:
Density. For a given accelerating voltage and analyzed spectral line wavelength,
the extent of electron penetration is determined by the density of the bombarded region.
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A-6
Conductivity. The thermal and electrical conductivity of the specimen determir
the stability of the electron beam and the ability to withstand the total energy in the
incident beam without deterioration.
Distribution of Element Analyzed. A large concentration of the element being
analyzed in regions surrounding the bombarded region is a very significant factor
affecting the minimum detectable limit.
Spectral Interferences. The presence of other elements, which produce spectral
lines that interfere with the desired element analysis, is a very significant factor
affecting the minimum limit of detectability. Misidentification can occur when lines
from two different elements in the sample appear at the same wavelength position in the
spectrum. One example of this is the near coincidence of Ti K£ and V K
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A-7
Focused Beam Size. The minimum usable diameter of the focused beam for X-
ray analysis is limited by mechanical design compromises necessary to incorporate
X-ray spectrometers and light microscope systems for the analysis of opaque speci-
mens. The minimum diameter is about 0. 1 to 0. 5 micron at 20 kV. At low accelerating
voltages, below approximately 5 kV, the thermal spread of the thermionic emitting
source greatly affects the minimum beam diameter because of the chromatic aberration
sensitivity of magnetic lens focusing, and very small beam sizes generally cannot be
obtained.
Probe Current Density. The current density of the focused beam is determined
by the brightness of the electron source, the stability of the lens and accelerating-
voltage power supplies, the spherical aberration of the final demagnifying Lens, and
the spread in thermally emitted electrons from the source. The limiting factor at the
present time, from a practical point of view, is the brightness of the emitter source.
X-Ray Analysis System. Dispersive Type. The efficiency of X-ray detection and
selection varies with the spectral line analyzed and is dependent upon the mechanical
precision of the spectrometer, the perfection and efficiency of the analyzing crystal, the
type of spectrometer, and the efficiency of the X-ray detector. The basis for dispersive
spectrometer design is a practical compromise for the requirements of efficient X-ray
detection for the total spectrum of X-rays generally analyzed, i. e. , 1 to 100A.
Energy-Dispersive Type. Energy-dispersive X-ray analysis can be carried out
using solid-state lithium-drifted silicon detectors in conjunction with multichannel pulse-
height analyzers. The performance of this type of X-ray spectrum analyzer is limited
by the energy resolution of the detector, the maximum counting rate of the detector/
electronics system, and the minimum photon energy it is capable of detecting. It may
be used for high concentration levels of element analysis, but its poor signal-to-
background performance limits its usefulness for low-concentration analyses. How-
ever, its higher efficiency for photon collection and its ability to analyze the complete
spectrum (lower limit of about 4A) simultaneously can, in certain cases, substantially
decrease the time of analysis. This efficiency also can be an overriding factor in de-
termining which type of detection system to employ (dispersive or energy-dispersive)
in the measurement of low signal levels where background is not predominantly of the
X-ray type (see following item). This is an important consideration in the examination
of air pollution particles and may point the way to use of a scanning electron microscope
equipped with an energy-dispersive detector as a way of investigating very small parti-
cles. This will be discussed in detail in a later section. Use of energy-dispersive
detectors of the gas-flow proportional type can also be used in a similar manner for
low-energy photon detection that is inaccessible to the solid-state detector, i. e. ,
greater than 4A.
Noise Level. In the analysis of low X-ray intensity signals, two factors which can
restrict measurement precision - and which are not related to the physical processes
involved in the electron beam/target interactions - are electronic noise and cosmic
radiation. Electronic noise may be related to random pulses caused by limitations in
the X-ray detector/preamplifier system. The degree to which cosmic radiation inter-
feres is dependent upon the amount of shielding present. With little shielding, a
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A-8
count-rate level of approximately 0. 3 cps can be experienced. Instrument location ir.
the center of a large building (preferably, the basement), could reduce this level to
0. 1 cps or less.
Microprobe Analysis of Particles
From the preceding discussion, it is easily seen that spatial resolution and detec-
tion sensitivity are extremely important topics, considering that the basic goal of
microanalysis is chemical analysis of the smallest possible volume with the greatest
possible sensitivity. The pursuit of this ideal is important, especially, in the field of
small air pollution particles where the microstructures of interest are generally much
smaller and their spatial interrelationships more complex than those most commonly
investigated in the other fields of electron probe application. Spatial resolution(2) is
determined by the volume of the specimen which, through excitation by the primary
electron beam, contributes to the analysis. Detection sensitivity is commonly con-
sidered in two categories. The absolute detection limit gives the smallest number of
atoms, commonly expressed in grams, necessary for the analysis. The weight frac-
tion or relative detection limit describes the smallest fraction by weight detectable in
the sample irrespective of the volume analyzed.
The weight-fraction detection sensitivity of an element is determined by the inten-
sity of the characteristic X-ray line of the element utilized for the analysis and the
intensity of the background radiation associated with the characteristic line. These in-
tensities integrated over time are dependent, as mentioned previously, on the accel-
erating potential used, the primary electron beam current, the efficiency of the X-ray
crystal and detector used in the X-ray detection system, and the matrix in which the
element is found. The weight fraction detection limit is defined here as the concentra-
tion of the element that is required to produce an X-ray intensity greater than the in-
tensity of the background at the position of the analytical line by an amount equal to
three times the standard deviation of the background intensity. The absolute detection
limit is determined by the weight-fraction detection sensitivity and the mass of material
excited.
In light of the previously stated ideal of detecting the smallest number of atoms
of an element in the smallest possible volume, it is of primary importance-.to consider
the absolute detection limit that is the analytical definition of this ideal. The absolute
detection limit is the smallest number of atoms of the element required for the analysis
and is calculated by multiplying the weight-fraction detection sensitivity by the product
of the excited volume of the sample and the sample's density. The size of the excited
analytical volume, which is a function of the range of the electrons and therefore of the
primary accelerating potential as discussed above, plays a dominant role in determining
the number of atoms detected, with the result that those general conditions that produce
the best spatial resolution also contribute to the best absolute detection limits.
The choice as to which of the characteristic X-ray lines of an element to be em-
ployed in a particular investigation should be carefully considered in light of the detec-
tion sensitivities achievable with each line. Each characteristic line of an element will
in principle have a different detection sensitivity as the accelerating potential, sample
composition, and efficiency of the detection system for each line are changed.
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A-9
Two basic questions may be asked in an elemental analysis. The first question
ie qualitative and seeks the elements present. The second question, predicated on the
first, is quantitative and seeks how much of each of the elements is present. A qualita-
tive analysis by X-ray emission, therefore, depends on correctly identifying the Lines
of the characteristic X-ray spectrum as to their source elements, while a quantitative
analysis depends on an accurate measurement of the X-ray line intensities as related
to a reference standard of known composition. A qualitative analysis is, therefore,
accomplished by examining the X-ray spectrum of the specimen and comparing the wave-
lengths of all the characteristics lines present with one of the several available refer-
ence compilations of X-ray wavelength versus elemental atomic number. Since the
microprobe has crystal spectrometers which through suitable gearing give the wave-
length directly in angstrom units, or in any case, since the wavelength is easily deriv-
able from the & angle, this task becomes one of simply comparing the wavelength read-
ing of the peak of the spectral line with the reference list in order to identify the element
that is responsible for that particular line. In most cases this is rather easy, consider-
ing that the X-ray spectra of the inner shell transitions are comparatively uncomplicated.
In this way, information about a number of elements suspected of playing an im-
portant role in the chemistry of the sample can be quickly obtained. In the general case,
when a complete qualitative analysis is desired, the X-ray spectrometers are run
through the complete wavelength range so as to detect as many of the elements present
in the sample as possible. The practice of surveying as much of the wavelength spec-
trum as possible is a good one to adopt even for so-called known samples. Once the
complete spectrum has been recorded the problem of identifying the source elements
may be attacked by identification of the individual lines.
General Aspects of Particle Analysis
The microprobe is capable of providing chemical information from particles which
in many cases is beyond the limitations of other analytical methods. For example, this
situation may be true when
The sample size is very small
The analysis of individual particles is desired
Nondestructive analysis is required
Increased sensitivity and accuracy in the detection and analysis of
minor constitutes is necessary
The valence state of the elements, i. e., the state of chemical com-
bination, is important
Improved quantitative analysis is required.
The capability of most overriding importance is that of mass sensitivity, i. e. ,
the ability of derive useful information from material of microgram quantities or less.
This capability provides the basis for identifying constituents in a specimen which, in a
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A-10
macroaample, occur in very low average concentrations but, in a microsample, occur
in a percentage amount which fluctuates from high to low values. The points to be con-
sidered for detecting such "needles in the haystack" and characterizing them within the
limits of the microprobe method are discussed below. In this discussion, the particles
being sought are assumed, for the sake of simplicity, to be composed of a single ele-
ment. The arguments presented can be easily extended to the multielement case using
the weight fraction of the element being analyzed as a modifying factor.
Definition of Analysis Sample
The first point to be determined is the sample size (as distinguished from parti-
cle size), which must undergo analysis to provide the desired information. This deter-
mination requires a definition of the minimum number of particles which will give a
satisfactory statistical representation of the total particle aggregate. The definition
must take into account the relative occurrence of the particle types of interest. In this
respect, the minimum sample then will be defined by the particle type which occurs
least frequently and should include a number of such particles. Arbitrarily, for the
purposes of discussion here and below, ten such particles will be considered sufficient
for statistical purposes. Thus, if the least frequent particle type of interest occurs
once in n particles, then the minimum number of particles requiring analysis is lOn.
If the frequency of occurrence of a given particle type is unknown and is the informa-
tion being sought, then an assumed minimum level that is based on some practical
criteria must be used. In so doing, the criteria used must recognize the complexity
and cost of the analysis. In the event that the number of particles is limited, i. e. ,
below the lOn required, then the statistical characterization of the aggregate will of
necessity be determined by the total number available.
The above comments point out one of the major problems associated with particle
analysis; that is, the detection of a minor constitutent requires the analysis of a very
large number of particles. For example, if a constituent is being sought which occurs
once in every 10,000 particles, the analysis of 100,000 particles is required. Although
individual analysis of all particles is not necessary, the analysis will in any event be
very repetitive in nature, thus making it advisable to pursue automated procedures.
The possibility of sample treatments that will concentrate the particle types of interest
should also be evaluated thoroughly in the event that analysis cost is excessive. Also,
the possible use of other techniques for pinpointing certain particle types should be
pursued, e.g., detection by luminescence behavior under electron bombardment (visible,
infrared, ultraviolet).
Specimen Preparation
The preparation of particles for microprobe analysis can be carried out in several
different ways; for example:
Metallographic mounting, in which the particles are embedded in a
resin binder material such as bakelite or epoxy (limited to fairly
large particles)
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A-ll
Briquetting, in which the particles are compressed to yield a dense
composite (when a large number of particles are available)
Substrate mounting, in which the particles are placed on a substrate
and affixed to it by the use of collodion or a vacuum evaporated
coating
Carbon-film embedding, in which the particles are placed on a near-
dry collodion-amyl acetate film, carbon coated, and the collodion
film dis solved.
Of these methods, metallographic mounting is best from an analysis standpoint but is
limited to particles of a few microns in size or larger and materials that are not soluble
in resins. Where this mounting method is applicable, quantitative chemical analysis
can be successfully carried out using standard correction procedures. Briquetting is
useful for submicron-size particle aggregates where quantitative analysis on a micron
scale is desired, sufficient specimen material is available, and the analysis of con-
stituents of very low concentration is not required. Substrate mounting or carbon-
film embedding is necessary when the amount of sample material is limited, the mech-
anical alteration of the particles is not allowed, the identification and analysis of
individual submicron particles is required, or when only qualitative or semiquantitative
chemical analysis is needed. Carbon-film embedding may be necessary to provide
good thermal contact for dissipation of heat generated by the beam, and, in addition, it
provides maximum sensitivity to the particle analysis through the reduction of X-ray
background and electron diffusion. The factors affecting the particle density per unit
area for optimizing the efficiency of the analysis problem are discussed further below.
Single -Particle Detection Limits
One aspect of particular concern in particle analysis is the size of the smallest
particle that can be detected as well as the ease with which the detection can be ac-
complished. Many factors must be considered to answer this question including the
photon excitation rate produced by the probe, the composition of the particle in ques-
tion, the photon-detection efficiency of the spectrometer, the background level, and the
statistical character of photon production and measurement. The discussion in the
remainder of this section will cover these points, with the exception that only pure-
element particles will be considered.
Typical Instrument Performance. ^) Consider a target of copper bombarded by a
30 keV electron beam of a current density equal to 100 amperes /cm . This current
density is equivalent to 0. 1 jua in a 1 -micron-diameter beam and is representative of
typical microprobe operation. (This current density can be increased by a factor of
about 5 if all conditions are optimized. ) Under these conditions, a Cu Ka counting
rate of about 100, 000 cps can be measured by a focusing -type dispersive spectrometer
with a peak to background of about 1000. Assuming the excited volume is about 3 cubic
microns, then the counting rate that would be measured from a cubic particle of 0. 1-
micron dimension will be 30 cps. In other words, if a copper particle of this volume
were bathed in an electron beam under the conditions given and no X-ray background
was produced by the electrons not striking the particle, the counting rate of 30 cps would
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A-12
be recorded by the spectrometer. A copper particle of this volume represents apprc :-. i
mately 10-14 grams. This calculation provides an indication of the practicality of de-
tecting particles in the submicron-size range. Thus, the experimentally measurable
counting rate per picogram of copper, using a dispersive focusing spectrometer for the
conditions stated, is approximately 3000 cps. The dependence of counting rate per
picogram on critical excitation and accelerating voltages, spectral series, and specimen
density has already been discussed.
Probe Diameter and Current Density. (°) The question of the improvement that can
be obtained by reducing the probe diameter deserves comment at this point. In this
respect, it should be noted that, for a fixed electron-source brightness and spherical
aberration constant of the final probe forming lens, the maximum current obtainable in
the focused probe is proportional to the 8/3 power of the probe diameter. Thus, the
current density varies directly with the 2/3 power of the probe diameter. This cir-
cumstance thus will decrease the estimated counting rate given above if the probe
diameter is decreased and will increase it if the reverse is true. Three additional
factors affecting the most useful probe size are (1) the ability of the prepared specimen
to dissipate the heat generated by the beam, (2) the X-ray background generated by
electrons not striking the particle of interest, and (3) the real particle density of the
prepared specimen. All of these factors are controllable, at least to some extent, by
specimen preparation procedures. It should be noted that, although the probe current
density varies directly with the 2/3 power of the probe diameter, the 8/3 -power de-
pendence of the total current on the probe diameter will cause the X-ray background -
from other than the particle of interest - to increase in the same manner, i. e. , 8/3
power. Thus, any benefit from increasing the current density can be realized only if
conditions for minimizing this background are rigorously adhered to. Finally, it should
be mentioned that improvement in the typical brightness of the electron source and
spherical aberration of the final probe forming lens are possible but involve major de-
sign innovations in current microprobe instrumentation.
Energy -Dispersive X-ray Detection. W In the above discussion, the X-ray detec-
tion system was assumed to be a dispersive -focusing- spectrometer type. The use of an
energy-dispersive X-ray detection system could, in certain cases, allow an increase in
the efficiency of total X-ray counting by a factor of 10 or more. Energy-dispersive
systems using solid-state, lithium-drifted silicon detectors are limited in their total
count- rate capacity (energy resolution decreases with counting rates above about
25,000 cps). In addition, their peak-to-background performance in the measurement
of X-rays from pure elements is quite low, varying from 5 to 25 in the wavelength
range 1 to 4A. Recent availability of solid-state detectors, with a factor -of -2 im-
provement in energy resolution, should increase these peak-to-background values also
by a factor of 2. Energy-dispersive detectors of the gas -flow proportional type have
poorer photon-energy resolution above 3 keV (4A). Below this energy, however, their
resolution can be comparable or better. The relative mechanical simplicity of the gas-
flow type compared with the solid-state type is also a favorable practical characteristic.
It should be noted that the peak-to-background characteristic of the dispersive
focusing spectrometer may not be applicable at low counting rates. Under conditions
where the X-ray signal is 10 cps, a calculated background -count level of 0. 01 cps
would be obtained if a 1000 to 1 peak-to-background ratio applied. Since the cosmic
radiation background can be 0.3 cps if little shielding is present, the effective peak to
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A-13 and A-14
background is reduced to 33, thus making the dispersive and nondisperaive systems
comparable in this respect. The higher photon-collection efficiency of the energy-
dispersive system may thus be quite practical in certain aspects of particle analysis.
Hall' ' haa also commented on the possibility of favorable energy-dispersive
detection using very small probe diameters (0. 1/u to 0. 03/u) and thin samples. This
could be the case if a scanning electron microscope equipped with an energy-dispersive
detector were used on an air pollution sample suitably dispersed on or in a material of
low X-ray background. The lower elemental resolution would still be a major limita-
tion, however.
REFERENCES
(1) White, E. W. , Denny, P. J. , and Irving, S. M. , "Quantitative Microprobe Analysis
of Micr©crystalline Powders", in The Electron Microprobe, proceedings of the
symposium sponsored by the Electrochemical Society, Washington, D. C.,
October, 1964. (Editors: T. D. McKinley, K. F. J. Heinrich, and D. B. Wittry;
John Wiley and Sons, 1966, pp 795-796).
(2) Anderson, 'C. A. , "An Introduction to the Electron Probe Microanalyzer and Its
Application to Biochemistry", chapter in Methods of Biochemical Analysis, Vol XV,
pp 147-270, John Wiley, Interscience, 1967.
(3) Anderson, C. A. , and Hasler in X-Ray Optics and Microanalysis (Eds. , Castaing,
Descamps, and Philibert), Hermann, Paris, 1966, p 310.
(4) Hall, T. , in Quantitative Electron Probe Microanalysis (Ed. , K. F. J. Heinrich),
U. S. Department of Commerce, National Bureau of Standards, Special Publication
298, October, 1968, pp 286-287.
(5) Macres, V. C. , Materials Analysis Company, private communication.
(6) Ibid.
(7) Ibid.
(8) Hall, T. , op cit. , pp 295-296.
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