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Spectrochimica Acta Part B 6
Characterization of small particles by micro X-ray fluorescence
Thomasin C. Miller a,*, Helen Langley DeWitt b, George J. Havrilla b
a X-ray Optical Systems, Inc., East Greenbush, NY 12180, USAb Chemistry Division, Analytical Chemistry Science, Los Alamos National Laboratory, Los Alamos, NM 87545, USA
Received 2 February 2005; accepted 9 September 2005
Available online 25 October 2005
Abstract
Micro X-ray fluorescence was used to study both homogeneous and heterogeneous particle systems. Specifically, homogeneous glass
microspheres and heterogeneous soil particle samples were prepared by both bulk and single particle sample preparation methods for evaluation
by micro X-ray fluorescence. Single particle sample preparation methods allow for single particles from a collected sample to be isolated and
individually presented to the micro X-ray fluorescence instrument for analysis. Various particle dispersion methods, including immobilization onto
Tacky Doti slides, mounting onto double-sided sticky tape affixed to polypropylene film, or attachment to polypropylene film using 3M Artist’s
Adhesive, were used to separate the sample particles for single particle analysis. These methods were then compared and evaluated for their ability
to disperse the particles into an array of single separated particles for optimal micro X-ray fluorescence characterization with minimal background
contribution from the particle mounting surface. Bulk methods of particle sample preparation, which included pellet preparation and aerosol
impaction, used a large quantity of collected single particles to make a single homogeneous specimen for presentation to the instrument for
analysis. It was found that single particle elemental analysis by micro X-ray fluorescence can be performed if the particles are well separated
(minimum separation distance=excitation source beam diameter) down to a particle mass of ¨0.04 ng and a mean particle diameter of ¨0.06 Am.
Homogeneous particulates can be adequately characterized by micro X-ray fluorescence using either bulk or single particle analysis methods, with
no loss of analytical information. Heterogeneous samples are much harder to characterize, and both single particle as well as bulk analyses must be
performed on the sample to insure full elemental characterization by micro X-ray fluorescence.
D 2005 Elsevier B.V. All rights reserved.
Keywords: Particle analysis; Micro X-ray fluorescence (MXRF); Cascade impactor; Aerosol; Soil; Glass microsphere
1. Introduction
The analysis of particulates is important for many environ-
mental, biological, and forensics applications. Precise chemical
and structural analysis of collected particles might reveal
conditions governing their formation or manufacture, leading
to enhanced characterization and attribution of a given
specimen. For example, particulate samples obtained from
the environment, such as aerosols, fly ashes, soils, and
sediments, can help establish sources of pollution and
conditions of formation. The biological impact of exposure to
various particulates, such as pollutants, irritants or allergens,
can be determined and evaluated. In forensics, the analysis of
particles found on a suspect or at a crime scene, such as fibers,
0584-8547/$ - see front matter D 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.sab.2005.09.003
* Corresponding author. Tel.: +1 518 880 1500x405; fax: +1 518 880 1510.
E-mail address: [email protected] (T.C. Miller).
glass, or gunshot residue, can potentially place the suspect at
the scene of a crime.
X-ray analytical techniques have been an important means
of gathering elemental information about particulate samples.
For example proton-induced X-ray emission (PIXE) has been
employed for the analysis bulk soil samples (e.g., [1,2]) as well
as bulk and individual aerosol particles (e.g., [3–7]). Total
reflection X-ray fluorescence (TXRF) analysis has been used
for the analysis of soil particulate extracts (e.g., [8,9]) as well as
the analysis of aerosol and emission particulates (e.g., [7,10–
18]). Synchrotron radiation X-ray fluorescence (SRXRF) has
been applied to the study of sediment particles [19], municipal
solid waste fly ashes [17], and radioactive particles [20]. X-ray
fluorescence (XRF) has been used in the study of the bulk
elemental properties of aerosols, soils and other particulates
(e.g., [9,21–29]).
This paper demonstrates that micro X-ray fluorescence
(MXRF) is a method that shows promise for both bulk and
0 (2005) 1458 – 1467
ww
Table 1
Element emission lines and energies monitored in this study
Element Emission line Emission line energy (keV)
Mg K-L2,3 (Ka) 1.254
Al K-L2,3 (Ka) 1.487
Si K-L2,3 (Ka) 1.740
S K-L2,3 (Ka) 2.308
Cl K-L2,3 (Ka) 2.622
K K-L2,3 (Ka) 3.313
Ca K-L2,3 (Ka) 3.691
Ti K-L2,3 (Ka) 4.510
V K-L2,3 (Ka) 4.952
Mn K-L2,3 (Ka) 5.898
Fe K-L2,3 (Ka) 6.403
Cu K-L2,3 (Ka) 8.047
As K-L2,3 (Ka) 10.543
T.C. Miller et al. / Spectrochimica Acta Part B 60 (2005) 1458–1467 1459
single particle analysis. MXRF employs polycapillary optics
that are used in conjunction with the X-ray source to focus
incident X-rays. This results in smaller beam diameters (i.e.,
10–50 Am) and higher X-ray fluxes (¨107 photons/s) at the
sample surface, increasing detection sensitivity and enhancing
performance over conventional X-ray fluorescence instru-
ments. The smaller X-ray focal spot allows for much smaller
sample features to be observed and characterized than larger
spot techniques such as XRF, TXRF, and PIXE. Unlike
techniques such as SRXRF and PIXE which require specialized
instrumentation rarely accessible to the average user, such as a
synchrotron source, MXRF is readily available as a simple
bench-top instrument. It is a nondestructive technique, leaving
the sample intact for other analyses to be performed on it. It has
advantages over more conventional single particle analysis
techniques, such as scanning electron microscopy (SEM), in
that samples can be analyzed in both air and under vacuum,
and a more complete analysis of the volume of a sample
particle can be obtained due to the penetrating nature of X-rays
(tens to hundreds of micrometers). Furthermore, in addition to
single point analyses, elemental imaging can be performed by
MXRF with the ability to scan over large areas to map the
distribution and elemental characteristics of particles deposited
on or impregnated into a given substrate.
In this study, MXRF was used to study both homogeneous
and heterogeneous particle systems through both single point
and imaging analysis. Single particle sample preparation
methods allow for single particles from a collected sample to
be isolated and individually presented to the MXRF instrument
for analysis. Various particle dispersion methods, including
immobilization onto Tacky Doti slides, mounting onto
double-sided sticky tape affixed to polypropylene film, or
attachment to polypropylene film using 3M Artist’s Adhesive,
were used to separate the sample particles for single particle
analysis. These methods were then compared and evaluated for
their ability to disperse the particles into an array of single
separated particles for optimal MXRF characterization with
minimal background contribution from the particle mounting
surface. Bulk methods of particle sample preparation, which
included pellet preparation and aerosol impaction, used a large
quantity of collected single particles to make a single
homogeneous specimen for presentation to the instrument for
analysis. Bulk analyses were compared to results obtained from
single particle analysis for both homogeneous and heteroge-
neous samples. Size and mass method detection limits were
also calculated for observation of a single particle with the
current MXRF instrumentation available at Los Alamos
National Laboratory (LANL).
2. Experimental
2.1. Reagents and materials
All particle samples were used as received, without further
purification. Stearic acid was obtained from Sigma Chemical
Corporation (St. Louis, MO). 5, 15, and 20 Am borosilicate
uniform glass microspheres as well as 30 Am soda lime glass
particles were obtained from SPI Supplies (Westchester, PA).
China Loess CRM CJ-2 simulated Asian mineral dust was
obtained from the National Research Center for Environmental
Analysis and Measurement (China). K-411 Glass Microspheres
(1 Am–40 is Am in diameter) SRM 2066 was obtained from the
National Institute of Standards and Technology (NIST)
(Gathersburg, MD). The following particle mounting supplies
were used as received: Tacky Doti slides and related materials
(SPI Supplies, West Chester, PA), polypropylene film (4 Amthickness) (Spex CertiPrep, Metuchen, NJ), double-sided clear
sticky tape, and 3M Artist’s Adhesive (3M, Minneapolis, MN).
Parafilm and Fisherbrand Superfriendly Air-It were used for
the air dispersion method of particle sample preparation and
were both obtained from Fisher Scientific (Houston, TX).
2.2. Instrumentation and sample preparation
MXRF analysis of individual particles and bulk pellet
samples was performed in vacuum using an EDAX Eagle II
MXRF system equipped with a Rh target excitation source
and a SiLi detector (EDAX, Mahwah, NJ). The X-ray source
was equipped with a polycapillary X-ray focusing optic
having a 60 Am nominal X-ray spot size at Cu Ka, 8.04 keV
(X-ray Optical Systems, East Greenbush, NY). The optic on
the instrument could also be defocused to an X-ray focal spot
size of ¨360 Am. Table 1 lists the element emission lines and
their energies that were monitored in the experiments outlined
in this study. Particle characterization was performed with a
nominal X-ray spot size of 60 Am while bulk analysis of
particles pressed into pellets was performed with an X-ray
spot size of 360 Am.
Microscope observation was performed using a Leica Micro
Star IV stereomicroscope (Leica Microsystems, Bannockburn,
IL). The microscope was equipped with a 21 mm Reticle X-
Scale 5 mm Crossed Micrometer Scale-5 mm/100 Div.
(Edmund Optics, Barrington, NJ) calibrated with a 1 mm/100
divisions stage micrometer to determine or verify particle sizes.
A Carver 4350 Manual Pellet Press (Chemplex Industries,
Inc., Palm City, FL) with a 4-in. platen diameter and a 13 mm
supplied die was used to apply force to samples mixed with
stearic acid to create bulk sample pellets. Bulk particle samples
T.C. Miller et al. / Spectrochimica Acta Part B 60 (2005) 1458–14671460
were prepared by forming pellets from a homogenized mixture
of ¨40 mg of mixed particles and equivalent amounts of the
binder stearic acid. A Wig-L-Bug was used for mixing
homogeneous particle/binder samples prior to pellet formation
(Reflex Analytical Corp., Ridgewood, NJ). Approximately
14,000 lbs of force was applied to the pellet during pressing.
A rotating micro-orifice uniform deposit impactor
(MOUDI)i, model number 110, (MSP Corporation, Minnea-
polis, MN) was used to separate and collect different particle
sizes of CRM-CJ-2, a simulated Asian mineral dust sample for
MXRF imaging and analysis. Polypropylene film (4 Amthickness) was used as the impaction substrate.
Dispersed particle samples were prepared using a variety
of methods to both reduce the background contribution of the
mounting surface and to disperse the particles into an array of
individual, separated particles. For effective single particle
detection, the particles must have a minimum separation
distance of the diameter of the X-ray focal spot on the sample
surface (¨60 Am). The different particle mounting methods
used were immobilization onto Tacky Doti slides, mounting
onto double-sided sticky tape affixed to polypropylene film,
or attachment to polypropylene film using 3M Artist’s
Adhesive.
The Tacky Doti array is a unique adhesion system, in
which a 25 Am thick layer of transparent polymer containing a
controlled pattern of ‘‘tacky’’ dots of precisely determined size
and location, that allows the user to produce a highly regular
array of separated particles. For this study, the tacky polymer is
affixed to a normal glass microscope slide by the manufacturer
(SPI Supplies). Immobilization of particles onto the Tacky
Doti slide mount is described in detail in a report generated
by SPI Supplies [30]. Briefly, using the Tacky Doti slide
holder, a few milligrams of particle sample were placed into the
slide holder and the ensemble was agitated from side to side to
distribute the particles across the adhesive dots.
Particles were mounted onto double-sided sticky tape using
two different methods; direct deposition and air dispersion. For
both processes, a piece of double-sided tape was first affixed to
the surface of polypropylene film which had been previously
affixed to a 35 mm slide mount. Direct deposition was
achieved by using a small spatula to transport a few milligrams
of particle sample onto the tape surface. The sample was then
shaken upside down to remove any unfixed particles. For the
air dispersion method, the tape/film mount was placed into the
bottom of a 500 mL beaker and a few milligrams of particles
were placed beside it. The beaker top was sealed with Parafilm
and then punctured with a small hole. The nozzle of a
Superfriendly Air-It air spray can was fitted into the small
hole and used to disperse the particles in the beaker. The
separated particles were then allowed to settle onto the tape-
covered slide at the bottom of the beaker [31].
3M Artist’s Adhesive was also used to prepare a sample of
dispersed particles. 3M adhesive was sprayed onto polypro-
pylene film attached to a 35 mm slide mount. The particles
were then affixed to the adhesive prepared slide using the same
air dispersion method described above for the double-sided
sticky tape.
3. Results and discussion
3.1. Effect of sample mounting on single particle dispersion
and MXRF detection
Fig. 1 shows white light images of 20 Am glass microspheres
prepared with each of the different mounting methods described
above. In terms of particle separation, the Tacky Dot, aerosol
method with tape, and aerosol method with 3M adhesive are all
adequate for particle dispersion. The direct deposition method,
pictured in Fig. 1b, does not adequately separate the glass
spheres from one another. The particles are clumped very close
together on the tape surface. The Tacky Dot array allows for the
particles to be separated; however, much depends on the particle
size. According to the SPI sample preparation report [30], the
‘‘tacky’’ dot size needed to hold a smooth, roughly spherical
particle should have a diameter at least 25% that of the longest
dimensions of the particles present for highest array formation
efficiency. The diameter of the glass microspheres is 20 Am.
According to the specifications given above, a 5 Am dot array is
optimal for the 20 Am spheres. At this time, the smallest Tacky
dot size available is 15 Am. In Fig. 1a, one can see that even with
the non-optimal 15 Am dot size array, the particles can be
separated from one another. However, if one looks closely, one
can see that the particles are immobilized on the tacky surface,
but not necessarily in the dot wells. As a comparison, the inlaid
picture in Fig. 1a shows 100 Am glass microspheres immobi-
lized onto a Tacky dot array; they are nicely immobilized as a
regular array on the tacky surface. Since the smallest Tacky dot
size currently available is 15 Am, this method is only efficient
for particles of �60 Am in diameter. Using the Tacky Dot array
even becomes more problematic with a particle sample
containing many different particle sizes, as is often the case
with environmental samples of interest, such as soil particulates.
The remaining two methods of sample preparation are those
using the air dispersion method combined with either double-
sided sticky tape or 3M adhesive on polypropylene film. Both
show that the particles can be well separated from one another.
The largest separation is achieved with the 3M adhesive,
although the difference is probably due to the amount of
sample introduced into the beaker prior to dispersion as well as
how the particles settled out on the substrate surface.
Another factor that will affect particle analysis is the
background produced by the mounting substrate. Fig. 2 shows
MXRF spectra of each of the different substrate materials; the
Tacky Dot slide, double-sided sticky tape on polypropylene,
and 3M adhesive on polypropylene. The spectra were normal-
ized by dividing by the highest spectral peak intensity in each of
the respective spectra. The Tacky Dot slide shows the most
complex spectrum containing large elemental peaks from Si and
Ca. Smaller peaks of S, Cl, K, and As are also present.
According to SPI Supplies [30], the S and Cl are from the
polymer Tacky surface and the other elements are present in the
glass slide on which the polymer is affixed. Due to the presence
of so many elements, this background may not be suitable for
MXRF analysis of particles, especially those containing these
background elements. The double-sided sticky tape on poly-
Fig. 1. 20 Am glass microspheres mounted on a) a Tacky Doti slide (picture inlay shows 100 Am glass microspheres mounted on a on a Tacky Doti slide for
comparison), b) double-sided sticky tape on polypropylene film using direct depositions, c) double-sided sticky tape on polypropylene film using the dispersion
method, and d) 3M Artist’s adhesive on polypropylene film using the dispersion method.
T.C. Miller et al. / Spectrochimica Acta Part B 60 (2005) 1458–1467 1461
propylene shows significant S and K peaks as well as a higher
Bremsstrahlung background. This is unsuitable for particles
containing either S or K, or traces of any elements found in the
high background region ranging from ¨4 to 12 keV. The 3M
adhesive on polypropylene gives the best background trace. It
only contains minor Si and Ca contaminants.
Due to its low elemental and Bremsstrahlung background as
well as its good particle dispersion characteristics, the 3M
adhesive on polypropylene used in conjunction with the air
dispersion method was used as the sample mounting method
for the remaining experiments.
0
0.2
0.4
0.6
0.8
1
0 5
Ener
No
rmal
ized
Inte
nsi
ty (
cou
nts
)
Ca
K
Si
S
Rh
Cl
Fig. 2. Spectra of double sided sticky tape on polypropylene film, 3M Artist’s adhes
kV, 1000 AA for sticky tape and 3M adhesive, 40 kV, 330 AA for Tacky Doti sli
3.2. Particle size detection limit
To determine the smallest relative particle size that can be
detected with our MXRF instrument, a series of isolated
uniform borosilicate glass microspheres of different diameters
smaller than the X-ray beam size (5 Am, 15 Am, and 20 Am;
SPI Supplies, West Chester, PA) were imaged with MXRF. The
Si intensity for each particle size is listed in Table 2. From this
calibration of particle diameter versus elemental intensity, the
particle size limit of detection for MXRF is calculated to be
¨0.06 Am. The intensity is directly proportional to particle
10 15 20
gy (KeV)
Sticky Tape
3M Adhesive
Tacky Dot
As
Scatter
ive on polypropylene film, and a Tacky Doti slide. X-ray tube conditions: 40
de, Spectral dwell time=100 live seconds.
Table 2
Particle diameter versus Si intensity for uniform glass microspheres (n =3)
Particle diameter (Am) Si intensity (cps) Particle mass (ng)
4.9T0.5 43T3 0.157T0.047
14.5T1.0 111T11 4.45T0.83
19.9T1.4 185T5 10.5T2.2Calculated limit
of detection
Particle diameter
(Am) 0.06
Particle mass
(ng) 0.04
T.C. Miller et al. / Spectrochimica Acta Part B 60 (2005) 1458–14671462
diameter. Similarly, the mass for each particle was calculated
assuming a spherical shape and known particle density of 2.50
g/cm3. By plotting a calibration curve of Si intensity versus
particle mass, the particle mass limit of detection was
calculated to be ¨0.04 ng. Conversely, the largest diameter
Si particle that can still be analyzed with minimal absorption-
enhancement effects (i.e., primary X-rays penetrate and analyte
line X-rays emerge substantially unabsorbed) can be estimated
by calculating the critical depth of penetration for Si. The
critical depth was calculated to be ¨50 Am by Eq. (1) [32]:
d ¼ 46; 000
lqsinb ð1Þ
where d is the critical depth in Am, l is the mass attenuation
coefficient in cm2/g (lSi=3.192�102 cm2/g [33]), q is the
density of the sample in g/cm3 (2.50 g/cm3), and b is the
takeoff angle of the spectrometer (for our instrument, b =60-).Therefore the largest Si particle can be ¨50 Am in diameter
and would correspond to a mass of ¨90 ng (assuming a
density of 2.50 g/cm3).
3.3. MXRF analysis of homogeneous particulates
MXRF point spectra and imaging analysis can be used to
detect and differentiate particulates of homogeneous composi-
tion, such as glass fibers or particles, by the elemental
0
0.2
0.4
0.6
0.8
1
1.2
0 2 4
Energ
No
rmal
ized
Inte
nsi
ty (
cou
nts
) CaSi
Al
Mg
Ti
Rh scatter
Fig. 3. Point spectra of a SRM 2040 K-411 glass microsphere, a 30 Am soda-lime gl
1000 AA, Spectral dwell time=100 live seconds.
‘‘fingerprint’’ of the material of interest by both single point
spectra as well as elemental imaging. For example, Fig. 3
shows the spectra of three different kinds of glass particles; K-
411 glass spheres, 30 Am soda lime glass, and 20 Amborosilicate glass. The spectra have all been normalized to
their largest intensity for ease of comparison. Each has a
specific MXRF elemental signature. The K-411 glass is
primarily made up of elemental Mg, Si, Ca, and Fe and has
relative intensities in the order of Fe>Ca>Si>Mg. The soda
lime glass spheres primarily contain Si, Ca, and Fe in the
intensity order Si>Ca>Fe (the opposite trend compared to the
K-411 particles). Borosilicate glass consists of Si and Ca with
traces of Al and Ti. The Ca relative intensity is much greater
than that of Si.
Fig. 4a shows MXRF elemental images of a sample
prepared from a mixture of the three different types of glass
particles. By knowing their different elemental signatures, the
different glass particles can easily be identified in the mixture
sample. By examining the Mg, Si, Ca, and Fe elemental maps,
particle 1 shows very high Si, Ca, and Fe intensities and also
contains Mg. This elemental signature corresponds to the K-
411 glass. Particle 2 displays high Si and Ca intensity with
small Fe and minimal Mg, corresponding to the fingerprint of
soda-lime glass. Particles 3, 4, and 5 all contain Si and Ca, but
lack any Mg or Fe intensity. They are all borosilicate glass
particles. Fig. 4b is a cartoon identifying each of the different
types of glass spheres based on their elemental image
signatures.
One advantage of analyzing particles of homogeneous
composition is that each particle should be elementally the
same, both qualitatively and quantitatively, as other particles of
the same types as well as the bulk material. For example, Fig. 5
shows a spectrum of an individual 20 Am borosilicate glass
microsphere as well as a spectrum of a pellet made from 40 mg
of bulk borosilicate glass microspheres. Notice that the spectra
6 8 10
y (KeV)
K-411
Soda Lime
Borosilicate
Fe
ass sphere, and a 20 Am borosilicate glass sphere. X-ray tube conditions: 40 kV,
Fig. 4. a) MXRFMg, Si, Ca, and Fe elemental images of a mixture of K-411, soda lime, and borosilicate glass microspheres mounted on polypropylene film with 3M
adhesive using the air dispersion method. X-ray tube conditions: 40 kV, 1000 AA; Imaging conditions: 64�50 pixel matrix, 200 ms dwell time per pixel. b) A
cartoon identifying each of the different types of glass spheres based on their elemental image signatures. Particle 1 is K-411 glass, particle 2 is soda-lime, and
particles 3–5 are all borosilicate glass.
T.C. Miller et al. / Spectrochimica Acta Part B 60 (2005) 1458–1467 1463
show the same elements in the same approximate ratios. No
information is lost by examining one particle or a bulk quantity
of particles.
3.4. MXRF analysis of heterogeneous particulates
The analysis of heterogeneous particulate systems, such as
soils and environmental air samples, is much more complex
than for homogeneous systems. Specifically, the samples are
made up of more than one type of particle and cannot be
characterized by one particle alone. For example Fig. 6 shows
MXRF elemental images of a simulated Asian mineral dust
sample, CRM-CJ-2. Notice that not all of the particles in the
sample have the same elemental composition. For example,
particle 1 and particle 2 contain Al, Si, K, Ca, and Fe.
0
500
1000
1500
2000
2500
3000
3500
4000
4500
0 2 4
Ene
Inte
nsi
ty (
cou
nts
)
Si
Ca
Rh scatter
Al
Fig. 5. Point spectra of a bulk pellet and a single particle of 20 Am borosilicate glass
kV, 220 AA for the pellet, Spectral dwell time=100 live seconds.
Additionally, particle 2 contains Cu. Particle 3 only contains
Al, K, Ti, and Fe. One individual particle obviously is not
representative of the bulk mineral dust sample.
Creating a homogeneous bulk pellet out of the heteroge-
neous particles can allow for a more representative sample to
be prepared in order to gain additional information about a
given sample. For example, Fig. 7 shows the spectrum of a
single particle of CRM-CJ-2, as well as a pellet made out of
¨40 mg of the simulated Asian mineral dust. The pellet shows
the presence of Al, Si, K, Ca, Ti, V, Mn, and Fe. The intensities
of these elements in the single particle spectrum are much less
intense, and the trace quantities of V and Mn are almost non-
existent. Interestingly, the element Cu is present in the single
particle, but is absent in the bulk pellet sample. Cu may only be
present in a small amount of particles in the sample, and its
6 8 10
rgy (KeV)
pellet
particle
Fe
microspheres. X-ray tube conditions: 40 kV, 1000 AA for the single particle, 35
Fig. 6. MXRF elemental images of CRM CJ-2 mounted on polypropylene film using 3M adhesive and the air dispersion method. The elemental signatures of
particles 1, 2 and 3 in the sample are all different. X-ray tube conditions: 40 kV, 1000 AA, Imaging conditions: 64�50 pixel matrix, 200 ms dwell time per pixel.
T.C. Miller et al. / Spectrochimica Acta Part B 60 (2005) 1458–14671464
intensity may not be enough to overcome background in the
bulk sample. This shows the importance of analyzing both the
bulk and single particle composition of a sample to completely
characterize a given specimen.
In addition to different composition, heterogeneous samples
are often made up of particles of different sizes and morphology.
It is advantageous to separate and elementally characterize
0
1000
2000
3000
4000
5000
6000
0 2 4
Ene
Inte
nsi
ty (
cou
nts
)
Si
Ca
K
T
Al
Rh scatter
Fig. 7. Point spectra of a bulk pellet and a single particle of CRM CJ-2 simulated As
35 kV, 220 AA for the pellet, Spectral dwell time=100 live seconds.
particulate size fractions for determination of chemical compo-
sition and source attribution of a given specimen. Size
fractionated collection of particulates is usually carried out with
an impactor. Cascade impactors segregate aerosols by size based
on their particulate inertial characteristics [34]. The MOUDI,
used in this study, is a 10-stage cascade impactor with stages
having 50% cut-points ranging from 0.056 to 18 Am in
6 8 10
rgy (KeV)
Pellet
Particle
i
Fe
Cu
V Mn
ian mineral dust. X-ray tube conditions: 40 kV, 1000 AA for the single particle,
Fig. 8. MXRF Ca elemental images of CRM CJ-2 particulates collected on polypropylene film supports from 8 of the different stages of a MOUDIi cascade
impactor. No Ca particles were observed for stages 9 and 10. X-ray tube conditions: 40 kV, 325 AA, Imaging conditions: 128�100 pixel matrix, 200 ms dwell time
per pixel. Particle sizes at each stage are 1) �10 Am, 2) 5.6–10 Am, 3) 2.5–5.6 Am, 4) 1.8–2.5 Am, 5) 1.0–1.8 Am, 6) 0.56–1.0 Am, 7) 0.32–0.56 Am, 8) 0.18–
0.32 Am, 9) 0.10–0.18 Am, and 10) 0.056–0.10 Am.
T.C. Miller et al. / Spectrochimica Acta Part B 60 (2005) 1458–1467 1465
aerodynamic diameter. The principle of operation is straightfor-
ward and has been discussed in detail in references [35] and [36].
The MOUDI collects the aerosol in 10 discrete size-fractionated
samples so as to provide information on the distribution of
chemical components as a function of particle size.
Fig. 8 shows the Ca elemental images of particulates
collected at each MOUDI stage, 1 through 8. No images are
shown for stages 9 and 10 because no Ca particles were
collected above stage 8. MXRF can be used to both image and
take point spectra of the particulates collected at each of the
stages. Imaging is used to observe the impaction pattern of the
particles at each impactor stage as well as give the relative
intensities (abundance) of the elements in the collected
0 2 4 6 8
Energy (keV)
AlSi
Cl CaTK
Fig. 9. Point spectra of the center impaction spot for CRM CJ-2 particulates separa
Spectral dwell time=100 live seconds.
fractions. Fig. 9 shows point spectra taken at the particle
impaction spot closest to the center of the impaction substrate
for each of the impaction stages. The point spectra show
changes in elemental intensities between the different particle
sizes. For example, the Fe intensity steadily decreases from
stage 1 to stage 10. Other elements, such as Si, show varying
intensity over the different fractions.
4. Conclusion
Micro X-ray fluorescence is a new tool that can be used for
particle sample characterization. Through the incorporation of
polycapillary X-ray focusing optics in readily available
10
0
5000
10000
15000
20000
25000
30000
35000
01
23
45
67
89
10
Stage
Inte
nsi
ty (
cou
nts
)
Fe
i Cr
ted in 10 stages by cascade impaction. X-ray tube conditions: 40 kV, 325 AA,
T.C. Miller et al. / Spectrochimica Acta Part B 60 (2005) 1458–14671466
laboratory bench top instrumentation, the technique overcomes
many of the drawbacks of more conventional X-ray methods
for particle analysis. The smaller beam diameter and high X-
ray flux achieved with MXRF allows for very small sample
features to be observed and characterized. Both homogeneous
and heterogeneous particulate systems can be nondestructively
analyzed in both air and under vacuum using both single point
analyses and elemental imaging with the ability to analyze
small features as well as scan over large areas to map the
sample elemental characteristics.
Single particle elemental analysis can be performed if
samples are prepared such that the particles are well separated
with minimum separation distance greater than or equal to the
excitation source beam diameter. The best methods for
preparation of single particles are air dispersion methods where
the particles are separated and immobilized in an array on a low
background support material such as polypropylene film.
Individual particles can be detected with the instrumentation
used in this study down to a particle mass of ¨0.04 ng and a
mean particle diameter of ¨0.06 Am. Bulk particle samples,
which use a large quantity of collected single particles to make
a single homogeneous specimen, were successfully prepared
for MXRF analysis using both pellet preparation and aerosol
impaction. Homogeneous particles, such as glass microspheres,
and heterogeneous systems, such as soil particles, were both
successfully prepared by bulk and single particle methods and
analyzed by both single point spectral analysis and elemental
imaging. The elemental signatures of homogeneous particu-
lates can be determined using MXRF for either bulk and/or
single particle analysis. Heterogeneous samples are much
harder to characterize, and both single particle as well as bulk
analyses must be performed on sample specimens to be fully
described by MXRF.
Future work will focus on integrating MXRF analysis with
other spectroscopy techniques as particle samples will often
contain both inorganic and organic components. MXRF is a
nondestructive technique and leaves samples intact for other
types of analyses, such as IR or Raman spectroscopies or X-ray
diffraction, that will allow for characterization of the molecular
components of such samples. The use of MXRF in conjunction
with these established methods of molecular analysis will allow
for a more complete characterization of the particles.
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