# High-Definition X-Ray Fluorescence: Applications

Authors: Walter M. Gibson; Z. W. Chen; Danhong Li
Journal: X-Ray Optics and Instrumentation
 (2008)

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## Abstract

Energy  dispersive X-ray fluorescence (EDXRF) is a well-established and powerful tool for nondestructive elemental analysis of virtually any material.  It is widely used for environmental, industrial, pharmaceutical, forensic, and scientific
research applications to measure the concentration of elemental constituents or contaminants.  The fluorescing atoms can be excited by energetic electrons, ions, or photons.
A particular EDXRF method, monochromatic microbeam X-ray fluorescence (MμEDXRF), has proven to be remarkably powerful in measurement of trace element concentrations and distributions in a large variety of important medical, environmental, and industrial applications.  When used with state-of-the-art doubly curved crystal (DCC) X-ray optics, this technique enables high-sensitivity, compact, low-power, safe, reliable, and rugged analyzers for insitu, online measurements in industrial process, clinical, and field settings.  This new optic-enabled MμEDXRF technique is known as high-definition X-Ray fluorescence (HD XRF).  Selected applications of HD XRF are described in this paper including air particulate analysis, analysis of body fluid contamination at ppb levels, elemental mapping of brain tissue and bone samples, as well as analysis of toxins in toys and other consumer products.


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## Body

## 1. Introduction

The need for  sensitive, easy-to-use,
accurate, portable, and nondestructive multielement analyzers for
environmental, medical, industrial, and scientific uses has become increasingly
important. Recent development of a high-definition
X-Ray fluorescence (HD XRF) platform shows promise to meet this need. The principles and techniques for this HD XRF
system are discussed in a companion paper [1]. In summary, state-of-the-art
doubly curved crystal (DCC) x-ray optics enables high-sensitivity, compact,
low-power, safe, reliable, and rugged analyzers for insitu, online, measurements in industrial process, clinical,
and field settings. A schematic diagram
of an HD XRF system is shown in Figure 1.
Selected applications of HD XRF are presented in this paper.Figure 1
Schematic diagram of an HD XRF system. The source is a small-spot, low-power
(<100 W) electron bombardment source which with a doubly curved crystal
(DCC) optic and an energy dispersive (ED) detector comprise HD XRF.


## 2. Trace Element Analysis

## 2.1. Environmental Particulate Analysis

Air quality
has long been identified as a major public health issue and is the object of
ongoing study and remediation efforts.
Epidemiologic studies dating as early as the 1930s and extending through
the 50s and 60s [2–4] indicate that human exposure to high concentrations of
particulate matter (PM) results in serious health effects including higher
death rates, particularly in children, elderly, and other physically vulnerable
members of society. As a result, the Environmental
Protection Agency (EPA) has regulated the levels of ambient PM since 1971.  More recent studies indicate that even small,
short-term increases in PM levels can induce significant increases in both
mortality and morbidity from respiratory and cardiovascular diseases [5, 6]. Extensive air particulate monitoring (Federal
Reference Method (FRM)) networks have been established with increased attention
to finer particles (ambient particles are defined by their aerodynamic
diameter: ultrafine, less than 1 μm;
fine,1–2.5 μm;
and coarse, 2.5–10 μm,
undifferentiated or total PM is sometimes collected which includes particles
up to ~50 μm). Since 1978, the National Ambient Air Quality
Standards (NAAQS) have regulated “inhalable” particles, sorted into particles
with an aerodynamic diameter of 2.5 μm
or less (PM2.5), 10 μm
or less (PM10), or 2.5–10 μm
(PM2.5–10). Medical and other
effects tend to be studied in terms of these size designations, with PM2.5 (and more recently PM1.0 or even PM0.1) singled out for
particular attention.General air
particulate monitoring in the (US), is concentrated principally in two networks: National Air
Monitoring Stations (NAMS), and State and Local Air Monitoring Stations (SLAMS). These systems include over 5,000 monitors at
approximately 3,000 sites. Recently, EPA
has proposed combining these and other networks into the NCore Monitoring
Network, a three-tier system of monitoring sites [7]. Of these, Level 2 would be a national network
that would provide near real-time, high-quality PM2.5 data plus
routine chemical speciation.The Center
for Disease Control has published two of three projected National Reports on
Human Exposure to Environmental Chemicals, the latest in 2005 [8]. Along with a large number of metabolites of organophosphate pesticides, cotinine,
phthalates, aromatic hydrocarbons dioxins, herbicides, and so forth, a large
number of metals such as Pb, Cd, Hg, Co, U, Sb, As, Ba, Be, Cs, Mo, Pt, Th, and
W were included.  The route of most of
these toxins into the body is by ingestion and for some of them,
the origin has been identified as principally geological, carried to humans or
to the food chain through ground water.
Others, however, have been associated with PM contamination. For example, Hg, which is a major health
danger for children, is known to be introduced into the environment almost
exclusively by combustion of coal.
Although distribution and concentration in the food chain and subsequent
ingestion is believed to be the most serious pathway to humans, the initial
role of PM contamination as a primary source is important. PM involvement is also believed to be
connected with Pb and Cd toxicity.Little
information is available on the direct and indirect involvement of PM on
elemental (especially transition and heavy metal) toxicity. Therefore, direct, sensitive, and remote
monitoring of trace metal PM contamination is of great importance.In addition
to possible toxic involvement, elemental speciation of PM in nearly real time
can provide a sensitive signature on the sources of PM. Therefore, the shortest time resolution and
widest possible distribution of trace element PM speciation monitors are important. The remarkable sensitivity, small size, low
power, and low cost of HD XRF-based analysis has resulted in EPA and New York
State Environmental Research Authority (NYSERDA) funding for development of a
remote, near real-time HD XRF analyzer for PM speciation applications (EPA
EP-D-06-089, NYSERDA Agreement no. 9392 PON944).In addition
to air particulate monitoring and analysis of general urban and remote air
particulate exposure, there is intense current interest in determination of
personal exposure to airborne particulate contamination. Two large government initiatives, the
Genetics-Environment Initiative (GEI) [9], and the National Children’s Study
(NCS) [10], have been recently announced by the Department of Health and Human
Services. These initiatives include
development of personal air particulate samplers with time- and even position-resolved
measurement of the amount and composition of the personal exposure of targeted
subjects. Miniaturization of air
particulate collectors involves significantly reduced sampled air volumes and
sample size both of which virtually preclude use of current laboratory-based elemental
analysis techniques [11]. The high
sensitivity of HD XRF is well suited for such analyses. Techniques and instruments for this purpose
are being actively explored by X-Ray Optical Systems, Inc. (XOS) (East Greenbush, NY, USA) and Research Triangle Inc. (RTI) (Raleigh, NC,
USA). An HD XRF spectrum for a small-area filter
collected air particulate sample is shown in Figure 2. This spectrum illustrates the ability of HD
XRF to measure many elements at the same time.Figure 2
HD XRF spectra from total air particulate sample and background
filter collected by miniature personal air particulate sampler with air
collection volume of 0.500 L/min with a monochromatic Mo Kα excitation beam
(courtesy of RTI, Raleigh, NC, USA).A tabulation
of collected mass values for this sample from fundamental parameter (FP)
analysis [1] is shown in Table 1. It is
instructive to comment on some features of Figure 2 and Table 1 as follows. (1) The measured HD XRF intensities are
plotted as the square root of the number of measured counts. This is convenient because it allows a large
range of intensities to be shown on the same graph and it also makes it easy to
estimate the statistical uncertainty from the Poisson statistics in the measurement. For example, the sqrt of the Fe Kα peak is ~130 so the statistical
uncertainty is 130/(130)2=1/130=0.008 or 0.8%, and for the Ti
Kα peak it
is 1/20=5%. (2) The background measurement (shown in red), taken by shifting
the sample so that the exciting beam is incident on a part of the filter
displaced from the well-defined area on which the particles were collected,
shows a low and nearly constant background.
It also shows the Rayleigh (coherent) and Compton
(incoherent) scattering of the
incident monochromatic Mo Kα (17.6 keV) X-ray beam. The difference in the Rayleigh and Compton
scattering between the observed particulate sample and the measured background
filter sample is due to scattering from the observed (and unobserved) elemental
constituents. This can be used to
estimate the mass of low-mass elements in the sample [1]. (3) It is seen in Figure 2 that  the Argon (Ar)
peak is the same for the sample and
background measurement. This peak is due
to excitation of Ar in the volume of the air path in front of the sample that
is exposed to both the exciting beam and the detector measurement cone. This does not change when the position of the
filter is moved and provides a very convenient internal energy and intensity
standard that shows that the geometry, incident beam intensity, and the
detector response function were constant for the two measurements. (4) Finally, it is observed that three small
peaks corresponding to Fe, Cu, and Pb are evident in the background
spectrum. The Fe and Cu peaks are
artifacts arising from small levels of contamination in the Aluminum collimating
aperture in front of the detector and the Pb has been tentatively traced to
contamination of the filter from handling and storage  [11].Table 1
Mass
per unit area of elements shown in Figure2 from FP analysis [1]. Uncertainties shown are statistical
Poisson statistics from peak intensities.
ElementPbFeZnTiCaCuAsKMeas. Data(ng/cm2)2080±29.37578±80.3693±7.2763±7.912096±127971±9.8139±1.82080±29.3ElementCrMnNiVRbSrBrMeas. Data(ng/cm2)42+0.8139±1.855±0.921±0.77±0.744±1.435±0.9The same
RTI collector used to collect the total particulate fraction in a small (2 mm)
spot on the filter used for the measurement shown in Figure2 also collected a
more biologically important PM2.5 fraction in a larger spot on a
different filter. The mass collected in
this fraction is much smaller and the collection was over a larger area, ~10 mm
diameter. Measurement with a nearly
normal incident beam as done for Figure 2 would require longer measurement time
in order to obtain useful measurement statistics. The small analyzer incident beam spot leads
to less efficient use of the exciting beam and lower measurement efficiency for
large sample areas. However, changing
the incident beam direction to a more glancing angle makes a significant
difference. This is possible because of
the small divergence of the incident beam.
Figure 3 shows the spectrum and Table 2 shows the calculated masses for
observed constituents for an incident angle of five degrees.Table 2
Mass per unit area of elements shown in Figure3 from FP analysis
[1]. Uncertainties shown are statistical Poisson statistics from peak
intensities.
ElementPbFeZnTiCaCuAsKMeas. Data(ng/cm2)85.4±1.2237.1±3.162.6±1.025.6±0.9166.4±2.711.4±0.75.7±0.9204.3±3.1ElementCrMnNiVSrBrSeMeas. Data(ng/cm2)8.5±1.010.3±0.92.3±0.72.0±0.71.8±0.78.5±0.914.2±1.0Figure 3
PM2.5 fraction from miniature personal particulate
environmental analyzer. Grazing
angle HD XRF spectrum.The important question is the following. How reliable are these results? To check this, measurements were made with an NIST
air particulate standard SRM 2783. This
standard was developed for standard particulate collection on 2.4 cm diameter
filters. Measured and reference results
are shown in Table3. The measured and
reference values are within the NIST-reported uncertainty in the reference and
the statistical uncertainty in the measurement except for Ca and K for which
the measured values are ~26% and 43% lower than the reference values,
respectively. The reasons for these
discrepancies are not clear but may arise from a systematic error in the
excitation probability adjustment (which is very large for such low Z elements
for the Mo Kα excitation beam)
or possibly from particle size effects, air absorption, or medium absorption
(from penetration of the captured particles into the filter) due to the
low XRF energy for these elements.Table 3
Reference data and measured data of air particulate on filter media from NIST (SRM 2783) with power of 50 kV 25 W, measurement time, 2 hours.
AsCaCrCuFeKMnRef. data(ng/cm2)1.18±0.121325.3±170.713.6±2.540.6±4.22660.6±160.6530.1±52.232.1±1.2Meas. Data(ng/cm2)1.25±0.19979.3±10.511.9±0.936.9±1.02516.6±26.2302.2±5.535.3±0.7NiPbRbTiVZnRef. data(ng/cm2)6.8±1.231.8±5.42.4±0.6149.6±24.14.9±0.6179.7±13.1Meas. Data(ng/cm2)7.7±0.333.8±0.92.7±0.2130.7±2.95.4±0.2172.5±1.8Conventional air particulate
samplers use a sampling volume of 14.6 L/min onto filters typically 2.4 cm
diameter. This has, in part, been
motivated by direct-excitation XRF analysis that has a low flux per unit area and cannot
concentrate the X-ray beam. The long
measurement time was necessitated by the small spot size and the large
collection area for the standard. Even
so, the excellent S/B ratio allows statistically meaningful measurements and
shows good agreement over a wide range of elements.Personal
environmental monitors needed for large new national health initiatives such as
the GEI [9] and NCS [10] will require lower collection volumes (perhaps 0.5-l L/min), therefore,
smaller pumps and lower power, and will involve much smaller filter or impact
collector areas, in part to allow many collection positions on the same
collecting substrate. New approaches for
concentration of the aerosol flow stream are being explored in order to achieve
desirable collector loading for shorter collection times. Because of its high sensitivity for
measurement of low-sampling area, HD XRF may be an enabling analysis technology
for such systems. This could result not
only in personal air particulate monitoring for biologically interesting PM2.5,
PM1, or even PM0,2 constituents, but also for smaller, less-expensive,
and higher-sensitivity rural, urban, and workplace monitoring in nearly real
time and also for stack- or exhaust-emission monitoring. Such possibilities are currently under
investigation.It should be noted that the most
desirable quantitative information from air particulate studies is the
concentration per unit volume of air sampled.
This can be obtained from the measured mass per unit area on the
sampling medium by application of information on the sampling time, the amount
of air per unit time through the sampler, and any necessary corrections for
capture and retention efficiency of particular elements on the sampling medium.These air particulate measurements
were chosen to illustrate the broad elemental range and high sensitivity of HD
XRF as well as the quantitative application for thin samples in general.  These properties are also valuable for other
important applications.


## 2.2. Trace Element Measurements in Bulk Materials

A much broader range of applications
for HD XRF other than air particulate measurements includes trace and major
constituent concentration measurements in natural, manufactured, and
environmental materials for forensic, environmental, industrial, health,
regulatory, and other applications.   We
will not attempt to review all current and potential applications but will
indicate the current status for some representative cases.  As noted in the FP calculation discussion in [1],
after thin samples, it is most straightforward to obtain quantitative results
for homogeneous samples that are infinitely thick to the incident X-ray beam
and/or the fluorescence X-rays.  This
includes many important applications such as measurements of trace elements in
liquids such as water, fuel, and body fluids, and in homogeneous solid
materials such as plastics or metals used in consumer products of interest for
environmental and medical studies.  For
thick samples, additional factors such as incident X-ray and fluorescent X-ray
absorption, X-ray scattering, and secondary excitation effects must be taken
into account (1).


## 2.2.1. Ultratrace Elements in Body Fluids

Measurements of the
composition of body fluids (urine, blood, plasma) are among the most ubiquitous
and powerful tools for disease detection and control.  Simple physical measurements such as
turbidity, sedimentation rate, color, and clarity, as well as sophisticated
measurements of cholesterol, glucose, cell type, prostate-specific antigen
(PSA), and so forth, are used to indicate current or potential disease or
disorder. Self-testing is routinely
used as a primary control guide for diseases such as diabetes.  One class of measurement, less routinely
applied but of increasing importance, is determination of trace element
concentrations.  Measurement of toxic
metals, such as lead, cadmium, and arsenic, or radioactive isotopes is carried
out when exposure to these materials is suspected.  In addition, it is increasingly recognized
that the appearance of even common metals in places where they should not be,
such as iron in urine or manganese in blood, can indicate organ malfunction or pathogen-initiated
disease.  In some cases, absence or low
levels of some elements such as zinc or selenium can signal danger.  Often, the concentration level of trace
elements of significance is in the range of 10–100 part-per-billion (ppb) and
simple, fast measurement techniques at this level of sensitivity are not
readily available.In an NIH-supported study (NIH 2R44RR021797-02), development of an HD XRF-based
analyzer for measurement of trace elements of importance for understanding and
potential diagnosis and treatment of neurodegenerative diseases has recently
begun at XOS in collaboration with the Alzheimer’s Center (Albany Medical
Center, Albany, NY, USA)
and the Wadsworth Center (New York State Department of Health, Albany, NY, USA).  The object of this study is a clinically
based analyzer for rapid, nondestructive analysis of trace element concentrations
at ppb levels.  Currently, only complex
and expensive chemical analysis such as inductively coupled plasma mass
spectroscopy (ICP-MS) is capable of such measurements at the low ppb levels
required [12].  This results in expense and
delay (typically up to three weeks) such that desired analyses are often not
carried out for diagnostic evaluation.
Furthermore, the rapid results needed to monitor chelation therapy are
impractical.  The goal of this funded
program is to permit inexpensive and accurate measurements of trace metals such
as Fe, Cu, and Zn at the clinical point of care at concentration levels less
than 10 ppb.  Preliminary measurements
have already shown good agreement with ICP-MS at levels as low as 40 ppb as
shown in Figure 4 and Table 4.  In
addition to Alzheimer’s Disease and other neurodegenerative disorders that are
the focus of the current study [13, 14], such measurements are important to a variety of
metabolic disorders such as Wilson’s disease (a copper overload disease) [15],
hemochromatosis (a family of Fe overload disorders) [16], and toxic effects of
Pb [17], As [18], Hg [19], Cd [20], and Sb [21].  It is expected that such point-of-care
analyzers will also be important for renal diseases and a variety of autoimmune
diseases.  It is interesting to note that
for many of these cases, the elements of interest are not the traditional
toxins but are often bioessential elements, which makes the broad elemental
range of HD XRF particularly valuable.Table 4
Measured As and Pb levels in five
reference urine samples compared with the results of an interlaboratory
study by ICP-MS measurements.
SampleAs  (μg/L,
ppb)Pb (μg/L, ppb)HD XRFICP-MSHD XRFICP-MSEU03-1<3511.8±2.180±2579.4±4.4EU03-2<353.2±0.9<40<0.7EU03-3154±15158.9±7.7524±25528±26EU03-440±1256.8±3.0212±20224.8±8.3EU03-5104±18114.8±5.0<405.2±1.5HD XRF spectra for three standard urine samples. (b) is an expanded
part of (a).  Measurement time was
1600 seconds with a source power of 40 Watts  with an Mo X-Beam source of the type
discussed in [1].
(a)(b)Small peaks corresponding to Au, Cu, and Zn are observed in Figure3.  These are all due to detector-related
artifacts.  The Au is from secondary
excitation of Au in the electrode on the front face of the detector and the Cu
and Zn have been identified as secondary excitation of impurities in the Aluminum
collimator in front of the detector.
These measurements were taken with a preliminary prototype system. The NIH-funded
project, which started in October 2007, is designed to improve the measurement
sensitivity to allow minimum quantifiable limits for Pb, As, Zn, Cu, and Fe
down to 5 ppb or less.  This will include
application of multiple optic excitation as well as detector resolution,
counting rate, and artifact removal improvements.  No other laboratory-based  multielement XRF
system can achieve such low detection limits.The detection
limits for Pb and As in urine was estimated based on the above preliminary
data.  The minimum
detection concentration is given by(1)MDC=3BackgroundSignalWi=3BStWi=3B/SS×tWi, where B is the background
count rate in cps, S is the signal count rate in cps, t is the
measurement time in seconds, and Wi
is the reference sample
weight concentration.  For the sample
EU03-5, the As net counting rate was measured to be 1.2 cps and the background
counting rate was 25 cps.  Therefore, the
detection limits for 1600 seconds for As=3×5×117/1.2/40∼36ppb. Similarly,
detection limits for Pb were found to be about 45 ppb for 1600-second measurements
with this early prototype system based on the signal and background level of
sample EU03-3.


## 2.2.2. Toxins in Toys and Other  Consumer  Products

There is a great deal of interest in
control of toxic elements, particularly Lead, in toys, children’s play
equipment, furniture, and all items that could contribute to exposure of
children to harmful substances.  In
addition, the European Union has, under the EU directives: Restriction on the
use of certain Hazardous Substances (RoHS) (2002/96/EG) and Waste Electrical and Electron Equipment
(WEEE) (2002/95/EG), restricted the amount
of a number of elements and compounds in electronic materials and electrical
equipment provided to the European market as of June, 2006.  These include, Pb, Hg, Cd,Cr+6, and
certain compounds of Br.
In addition Ba, Sb, and Se have been
identified as potentially harmful at high levels.  These elements are also of concern for toys
and children’s and household furniture, as well as other consumer
products.  Many of the items of interest
are made of molded or painted plastic and often of painted wood.2.2.2.1. Homogeneous Solids
Analysis of homogeneous plastic
materials can be carried out by the same FP approach as that used for the
liquid samples discussed in the previous subsection.
Comparisons of such analyses for homogeneous polyethylene (PE) plastic
standard samples are shown in Figure5 and Table 5.
The analytical simulation
fundamental parameter (SFP) analysis [1] used in this example is fast but not
automated.  It requires fitting of the
peak intensity for both the Rayleigh and Compton scattering peaks (even though
the shape of the Compton
peak is not matched) and for the major peaks in the spectrum for each observed
element.  The simulation takes into
account secondary excitation, peak overlap, and absorption effects even for
unobserved elements (such as H, C, O, and N).
An automated reverse fundamental parameter (RFP) analysis is more desirable
and is possible for simple homogeneous samples as shown in what follows.  This case is shown because the SFP analysis
is still available for quantitative concentration determinations even while the
more complex RFP algorithms are being developed (especially for multilayer
samples).
Plastic toys and other plastic-consumer
products are principally made from polyethylene (PE), polyethylene terephalate (PET),
or acrylonitrile butadiene styrene (ABS). These plastic matrices are very
similar in their behavior to FP analysis.
Figure6 shows HD XRF spectra for two elementally doped ABS standards
with markedly different Pb content, and Table 6 shows the results of automated
RFP analysis for four NMIJ standards.
The measurement time for each measurement was 120 seconds and the
analysis time was <10 seconds.  As
with Figure 5, it is observed that the Rayleigh and Compton scattering peaks
are very strong for these low Z matrices.
Another plastic used extensively for consumer products is polyvinyl
chloride (PVC).
Detailed
discussion of the methods used to obtain concentrations shown in Tables1–9 is
given in the companion paper on HD XRF principles and techniques [1].
Even
though the analysis software development is not yet completed for general
matrices, it is possible for a variety of homogeneous plastic solid and
homogeneous liquid samples to carry out an RFP analysis in a few seconds.  The rapid analyses and the low level of
detection are enabled
by the use of monochromatic excitation and the high beam intensity in the HD
XRF system.  Analysis refinements that
are underway are connected with spectrum preprocessing and background and peak
overlap corrections.   The minimum detection concentration calculated
by (1) is shown for Cr, Hg, and Pb in Table 6.
It can be seen that MDC values achievable by the prototype HD XRF system
for toxins in homogeneous plastic products are in the low ppm range.Table 5
Reference and measured values for three polyethylene standards.  The experimental uncertainty (at 95%
level) includes estimated systematic uncertainty plus Poisson counting
statistical error.
SampleCrHgPbBrErrorPE_5398_NReference1001100010005004% (95% level)Measured102095098144210% (95% level)PE_H_18AReference10001100120011004% (95% level)Measured102011501220109010% (95% level)PE_LReference4002004005004% (95% level)Measured40118541950810% (95% level)Table 6
RFP analysis for ABS plastic standards.
SampleCr (ppm)Hg (ppm)Pb (ppm)ReferenceMeasuredReferenceMeasuredReferenceMeasuredNMIJ CRM 8105a27.5±0.525.0±3.2N/AN/A108.3±1.2105.8±4.4NMIJ CRM 8106a268.1±2.1276±17N/AN/A1077±6.71081±14NMIJ CRM 8115a94.3±1.191.2±9.093.8±2.891±4.294.2±1.0109.9±4.5NMIJ CRM 8116a944±12946±32939±27853±14941±9973±14Minimum detection concentration
(MDC)7.41.41.4Table 7
Measured concentration of the black lettering from “I Love NY” cup.
ElementCaTiVCrMnFeCoZnPbConcentration (%)0.64±0.030.21±0.040.34±0.051.79±0.020.58±0.022.21±0.033.53±0.041.68±0.0220.5±0.2Table 8
Measured concentration of the red heart from “I Love NY” cup.
ElementKCaTiVFeCuZnSePbConcentration (%)1.98±0.020.64±0.030.42±0.030.15±0.030.07±0.020.19±0.030.81±0.020.78±0.033.69±0.04Table 9
Measured concentration of the white substrate from “I Love NY” cup.
ElementKCaVMnFeCoZnRbSrBaPbConcentration (%)1.982.520.050.030.060.021.650.020.610.980.01±0.02±0.03±0.03±0.02±0.03±0.01±0.02±0.01±0.01±0.02±0.01Figure 5
HD XRF spectrum from polyethylene standard PE 5398 L sample and analytical simulated spectrum from fundamental parameter analysis.   A background subtraction was made for the spectrum below 15.2 keV.Figure 6
HD XRF spectra for ABS plastic standards.2.2.2.2. Layered Solids
Layered materials present a much more
difficult problem, for which the analysis is at an even earlier stage of development.  The special features of HD XRF of
monochromatic excitation and small analysis spot size are particularly
important for analysis of layered materials.
It is instructive to consider a specific example that illustrates the
power of HD XRF in measuring elemental concentration in layered samples.  This example is a well-known promotional
ceramic cup with the “I Love NY” slogan.
A  photograph of such a cup with HD XRF spectra for the white ceramic
cup, the printed black lettering, and the printed red heart is shown in Figure7.  Superposition of the three spectra is shown in Figure 8
where the square root of the number of measured counts is used to emphasize
some of the low-intensity features.
A number of
features are worthy of note in the superposed spectra shown in Figure8 and
described as follows.(i)
The elastic
(Rayleigh) and inelastic (Compton) scattering peaks are much lower, relative to
other features than observed for plastic matrices in Figures4 and 5.  This is due to the lower concentrations of
low Z elements (particularly hydrogen) in the ceramic cup and in the painted
layers.(ii)
Pb, Co, Fe, Cr are present in the black lettering but are at lower levels
or absent in the white cup.(iii)
Pb and Se are present in the red
heart.(iv)
Ca, Sr, and Zn are principle constituents of
the white cup, probably as part of the ceramic which also, of course, is
expected to contain oxygen, carbon, and nitrogen and may contain other major
constituents such as Si and Al which are not observed.(v)
Of special note is the observation of four characteristic lines from Pb
(Pb M, Pb Lα, Pb Lβ, and Pb Lγ).  The
relative excitation probability for these lines can be looked up from tabulated
excitation tables, and absorption coefficients are also available from
tabulations using the material  density derived in the iterative FP calculation.  Therefore, their relative observed
intensities (particularly the Pb M line) can be used in the FP analysis to
determine the thickness of the black paint layer.(vi)
Similarly, attenuation
of the Sr and Zn peaks can give information on the thickness of the black and
red paint layers.
Taking these effects into account, by
using the analytical simulation calculation, the concentration of elements in
the white cup and in the black and red paint layers have been calculated and
are shown in Tables7, 8, and 9.  The very
high Pb concentration in the black lettering is of special interest.  The uncertainties shown are from the Poisson
counting statistics.
It is clear that for single paint
layers on a plastic or other substrate, it is very useful to be able to measure
the substrate independently.  Since the
spot size for the HD XRF measurement is small (<1 mm), removal of the surface
layer in a small 2-3 mm spot can give this important information without
destroying or even seriously defacing the sample.  General automated FP analysis for layered
samples is currently being developed and we believe measurements will be able
to be quantified with uncertainty of 1–5%.   Currently, quantification of layered samples
by automated FP analysis is accurate to about 10–20%.Figure 7
Photograph of promotional “I Love NY” cup and HD XRF spectra
for the white ceramic, the black printed
lettering, and the red printed heart.Figure 8
Superposition of spectra for three features on “I Love NY”
promotional cup.


## 3. Mapping of Elemental Distributions with an HD XRF Analyzer

The concentration and
distribution in tissue of bioessential elements such as Fe, Cu, and Zn, so-called
Redox metals, have been identified as major players in cardiovascular diseases
such as Friedreich’s ataxia (FRDA) and of neurological diseases including
Alzheimer’s disease (AD), Parkinson’s disease (PD), multiple sclerosis (MS),
Huntington’s disease (HD), aceruloplasmanemia, amyotrophic lateral sclerosis
(ALS), and others.  More than two dozen
human nervous system disorders have been associated with oxidative tissue
damage in which redox metals are believed to play an important role.Although abnormal iron
deposition has been associated with many neurodegenerative diseases since the
association was discovered more than 50 years ago [22], its role and origin are
still not well understood.  A long-term
objective is to correlate the location and, if possible, the oxidation state
distribution of metal deposits at a subcellular level with structural
abnormalities such as betaamyloid plaque (Aβ), neurofibrillary tangles, and so
forth.  Iron staining is widely used to
determine regions of iron overload, and the distribution of nonheme iron [23, 24] and some information about the redox state of the iron can be obtained with
modified staining techniques [25].
Magnetic resonance imaging (MRI) is important for study of diseases
exhibiting region-specific brain-iron accumulation, and progress continues to
be made in development of this technique as a diagnostic tool [26, 27].  A major benefit of nondestructive HD XRF
mapping of elemental distributions is that subsequent to this measurement, staining
microscopy can be carried out on the same samples to study detailed correlation
of metal deposits with anatomical and pathological features.Recently, a number of
Synchrotron-based micro-X-ray fluorescence (SμXRF) studies of brain sections
from AD, PD, and other neurodegenerative diseases have been reported [28–33].  These studies have demonstrated the power of
micro-X-ray fluorescence in such studies and have also shown that Fe, Cu, and
Zn distributions are associated with β-amyloid plaque (Aβ) [18].  Unfortunately, application of the Synchrotron
tool is expensive and it has limited accessibility.There are many other
instances where elemental distributions play an important role.  These include measurement of lead in bone,
minerals in ore and stone deposits, art history, document authentication, and
so forth.  Only two cases (metals in
brain tissue and lead in bone [24]) will be shown as examples in this paper.  These have been chosen in part because of the
award of an NIH grant (NIH 2R44RR022001-02) to XOS for development of
an instrument for these medical research applications.A diagram of a setup for
measurement of elemental distributions by HD XRF is shown in Figure9(a).  A photograph of the setup is shown in Figure
9(b).  In this setup, an Oxford Mo Apogee
tube was used with a power setting of 45 W.
The Apogee tube has a source spot size of 90 μm.  An Si(220) asymmetrical DCC was used to form
a focused monochromatic Mo Kα beam.  The parameters of the Si(220) DCC optic are
listed in Table 10. The X-ray source was demagnified to form a 50 μm spot on the sample.  The sample was mounted on a PC-controlled XYZ
stage.  The sample surface was parallel
to the XY plane of the stage. A silicon drift detector (SDD) was used to
measure the fluorescent X-rays from the sample.Table 10
Asymmetrical
Si(220) DCC optic parameters.
CrystalFocusing ESource-optic dist.Optic-Focus dist.Flux @ 45wMeasured spot sizeSi(220)17.5 keV160 mm80 mm1×10750μmSchematic (a) and photograph (b) of a
laboratory HD XRF setup for elemental mapping.
(a)(b)A
PC Window-based program was used for scanning and spectrum analysis.  The program can   integrate the intensity under
a number of selected fluorescent peaks in the XRF spectrum.  The scanning was done by moving the sample in
a raster pattern. The integrated intensities for selected peaks in the X-ray
fluorescence spectra versus pixel location of the sample are used for analysis and
creating multielement maps.


## 3.1.  Scanning Measurements for Selected Samples

## 3.1.1.  Scanning of Mouse Brain Sections

Figures10, 11, 12, 13, and 14 show the
results of scanning a 100 μm thick transgenic amyloid precursor
protein (APP) (sw695) mouse brain section.  It should be
noted that the general brain distribution scans (with all elements being measured in the
same scan) were made with a beam spot of 100 μm diameter by displacing the sample from the focal
spot (along the beam direction), while selected area scans were with a 50 μm spot size as listed in
Table 10.Raster scans for an APP mouse brain section
from integration of the Compton scattering peak (a) (which shows the brain
section density distribution) and the Zn distribution (b).
(a)(b)Figure 11
Raster scans of the Fe peak intensity for the
APP mouse brain section.  The entire
section is shown on the left with 5 sec/0.1 mm step.  An expanded region is shown on the right with
10 sec/0.05 mm step.  Note that the
number of counts per step has been converted to Fe concentration inμg/g in
this figure.Figure 12
X-ray spectra for selected
points on the Fe map of APP mouse brain section.Figure 13
Raster scans of the Cu peak intensity for the
APP mouse brain section.Quantitative scans of selected slices of Fe and Cu distributions of
APP mouse brain scans.
(a)(b)The 100μm thick mouse brain section was
mounted on a 4 μm
thick Mylar film.  Analysis was carried
out with the same FP analysis program used for the thin air particulate samples
discussed in Section 2.1.  By following
the DCC focusing optic with a strongly focusing polycapillary optic, it is possible
to achieve a focal spot size as small as 10 μm with zooming capability to ~300 μm scans, allowing rapid general
scanning of 2cm×2cm  samples (in ~2 hours) and ultrahigh resolution of
specific areas of interest.


## 3.1.2. Measurements of Goat Tibia Bone Section

In collaboration
with Wadsworth Laboratory (New York State Department of Health), a series of
measurements were carried out on a section of the tibia of a sacrificed goat
that had been fed a diet containing lead [34].
These measurements are summarized in order to show application of the
technique for a thick solid material and to allow the results to be compared to
measurements carried out by a different technique, laser ablation, inductively
coupled plasma mass spectroscopy (LA-ICP-MS).Figures15, 16, and 17 show the
results of scanning of a 5 mm thick goat bone tibia section.Elemental maps
for the goat bone section.
(a)(b)(c)(d)Figure 16
X-ray fluorescence spectra from selected
positions on the bone section.Figure 17
Elemental distributions
along a selected path in the goat bone section.These bone scans show a
great deal of detail in the elemental distributions. They also show that it is
possible to detect unexpected anomalies such as the interesting copper seam
shown in Figure15.  They also show that
it is possible to use the HD XRF scanning with different types of material  and
with thick samples.  A comparison of the HD
XRF scans with scans using LA-ICP-MS shown on Figure 18 gives good and even
quantitative correlation.  Indeed, the X-ray
fluorescence results show more detail in the distributions and are more stable
than the LA-ICP-MS technique.
Furthermore, they are easier to do, faster, less expensive, and can be
repeated, perhaps at higher resolution since they are nondestructive compared
to the LA-ICP-MS method which erodes the sample.Laser ablation mass spectrometry-based
elemental mapping of goat tibia bone crosssection [24].
(a)(b)(c)


## 4. HD XRF as a Platform Technology

The trend toward increasing global regulation of toxins
presents an opportunity for HD XRF as a platform technology to address a number
of currently important applications.  HD
XRF brings several advantages over previous toxin detection technologies with
the combined ability to nondestructively detect very low levels, isolate small
features, and give reliable results across a wide range of toxic elements as
demonstrated in Sections2 and 3.
Conventional XRF analyzers and more standard analytical chemistry
techniques do not possess the needed combination of reliable results and low
cost per test needed in today’s tightening regulatory environment.  Areas that are in need of these attributes
include consumer products, electronics, air quality monitoring, body fluids,
food products, and pharmaceuticals.
Many of these applications can, in principle, share a common HD XRF
hardware and software platform to hasten new product introductions and lower
manufacturing costsA prototype HD XRF analyzer has been developed.  A schematic drawing of this prototype analyzer
is shown in Figure19.Figure 19
Schematic drawing of a portable HD-XRF analyzer measurement head.


## 4.1. Lead in Toys

As noted in Section2.2.2, a topic of much current interest is the presence of lead in
toys and other items to which children  are exposed.  Infants and young children are particularly
susceptible to lead poisoning.  A
nurturing child absorbs up to 50% more lead.
Infants and young children have the tendency towards sucking on fingers
and chewing on inedible products.  Lead
damages the central nervous system, causing delayed mental and physical
development, attention, and learning deficiency.  Any unnecessary exposure of children to lead should
be avoided.  The large number of toy recalls in the past two years
and growing concern over continued existence of contaminated toys in use and in
the distribution channel emphasize the need for a sensitive, quantitative, and
efficient method for control of toxins in consumer products.  The inefficiency and expense of current,
largely chemical techniques available to measure contamination at the desired
levels and with the necessary precision and accuracy have led to development of an easy-to-use
HD XRF instrument for screening and quantitative evaluation of toys, jewelry,
and other consumer products.  Figure 20
shows photographs of such an analyzer currently under evaluation.Photograph of an HD XRF analyzer showing possible positioning boom.
(a)(b)It is important to be able to measure
small features as well as thin layers on toys.
Examples of toys showing that lead is present in small features are
shown in Figure21.  It is clear that the
concern for lead in children’s goods will soon extend to other toxic elements
and to other consumer goods.  The broad
applicability of HD XRF for accurate and sensitive measurements of more than
two dozen elements in virtually all materials and matrices make it a natural
and powerful platform for such measurements.Figure 21
Examples of toys on which small features
need to be tested for presence of toxic materials.


## 4.2.  Toxins in Electronic Materials

RoHS
Electronics manufacturing has also
attracted significant regulatory control.  RoHS  (2002/96/EG) and WEEE (2002/95/EG) regulations have created the need to inspect
electronics for global markets.  This
application is very attractive to HD XRF since the small spot capabilities for
analysis of circuit board components of the type shown on Figure22 offer a big
advantage over handheld XRF and significant cost and time savings over bench top
XRF and chemical digestion methods.  The actual
HD XRF instrument may be a stand-alone manually operated system similar to the
HD XRF analyzer shown in Figure 20, or a programmable automated system
integrated with other inspection technologies within a factory.Figure 22
An example electronic circuit board showing small and tightly packed
components that need to be separately evaluated for RoHS compliance.


## 5. Conclusions

Selected examples show the
usefulness of HD XRF in a wide range of applications in science, medicine, and
industry.  By judicious choice of the
source, optics, and detector type, and configuration, it is possible to reach
previously unattained levels of sensitivity (down to a few ppb), elemental
specificity, and spatial definition in compact, low-power, reliable, rugged,
and nondestructive analyzers for clinical-, field-, online-, and laboratory-based
applications.



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*Source: 709692-2008-09-30.xml*