Copyright ©JCPDS-International Centre for Diffraction Data 2008 ISSN 1097-0002
269
BORON IN GLASS DETERMINATION USING WDXRF
Alexander Seyfarth
Bruker AXS Inc., 5465 East Cheryl Parkway, Madison, Wisconsin 53711-5373
ABSTRACT
The application of wavelength-dispersive XRF to the analysis of boron in glass is reviewed with
respect to physical, instrumental, calibration, and maintenance considerations. Advances in XRF
instrumentation have enabled improvement in the analysis of boron. Reduced window thickness
and high-current capabilities of X-ray tubes, as well as new analyzer crystals, produce a higher
intensity boron signal. With correct determination of background and matrix interferences for
successful quantification, only the sample itself limits the analysis. Observed boron migration in
some glasses needs to be understood in order to establish stable, routine quantification in a
process environment.
INTRODUCTION
Boron is an important component of glass. It exerts a strong influence on the viscosity of the
melt, imparting a beneficial effect to the chemical resistance and lowering the expansion
coefficient of the finished article. Boron is widely used in the production of “E” type glasses (7
wt%), neutral glass for pharmaceutical applications (7 to 11 wt%), and Pyrex®-type borosilicate
glasses (13 wt%).
Because of the high volatility of boron in the melting furnace, a very accurate procedure is
needed to meet the strict boron concentration specifications required for the finished product and
to permit any necessary batch adjustment to be implemented almost in real time. XRF is well
suited for glass analyses. Since the 1980s, XRF has been the method of choice because of its
ease of sample preparation and rapid results. In the production control of soda-lime glass, the
technique has already replaced wet chemical techniques because of its high precision and fast
measurement time. Boron, being a very light element, poses challenges to the analyst as well as
to the instrument. Demand for boron analysis in the glass industry prompted the development of
new generation wavelength-dispersive X-ray fluorescence (WDXRF) spectrometers.
ASPECTS OF THE ANALYSIS OF BORON
To put the difficulty of the analysis of boron in glass into perspective, consider its physical and
chemical properties [1–3]. Physical data for boron is given in Table I.
Atomic number (Z)
5
Table I. Physical data for boron.
K shell absorption
Mass absorption
edge (eV)
Kα radiation (eV)
coefficient µλ (cm2/g)
192
183
3,350
Fluorescence yield
(Ω)
1.25 × 10-4
Boron’s low atomic number and low K-shell energy cause various issues for XRF analysis which
will be addressed further.
This document was presented at the Denver X-ray
Conference (DXC) on Applications of X-ray Analysis.
Sponsored by the International Centre for Diffraction Data (ICDD).
This document is provided by ICDD in cooperation with
the authors and presenters of the DXC for the express
purpose of educating the scientific community.
All copyrights for the document are retained by ICDD.
Usage is restricted for the purposes of education and
scientific research.
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Copyright ©JCPDS-International Centre for Diffraction Data 2008 ISSN 1097-0002
EXCITATION AND FLUORESCENCE YIELD
The emitted intensity, IB-Kα, of the characteristic fluorescence radiation is governed by the
overlap of the excitation spectrum and the absorption coefficient µλ of boron [4]. Absorption by
the tube window cuts off the low energy (long wavelength) part of the excitation tube spectrum.
Because of this, boron is excited with the primary X-ray continuum and the rhodium L lines. For
optimum primary continuum, the tube window must be as thin as possible. Currently window
thicknesses range from 150 to 30 µm; 75 µm has become the industry standard since it balances
price, performance, and durability.
Fluorescence yield is another important factor in the analysis of boron. Fluorescence yield is
proportional to Z4 and is therefore close to zero for low atomic number elements. Using the
highest available tube current at the lowest possible voltage, the emission of boron photons from
the sample can be optimized. Starting in 1988, the possible settings for tube current have
increased from 100 mA to today’s maximum of 170 mA [Siemens SRS 303 (1988) to Bruker S8
TIGER 4 kW (2006)].
Figure 1. Comparison of boron intensity in 3 kW and 4 kW WDXRF systems.
As can be seen in Figure 1, 40% greater boron intensity was obtained from a
boronphosphosilicate glass (BPSG) sample (4 wt% B) analyzed on a 4 kW instrument in 1999
than a 3 kW instrument in 1988 [SRS 3000 with 75 µm end window X-ray tube and 3 and 4 kW
generator (1999)].
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ANALYZED LAYER, SAMPLE PREPARATION, AND X-RAY BEAM PATH
Because the energy of boron Kα radiation is so low (183 eV), only a thin 0.6 µm (600 nm)
surface layer of the sample can be analyzed. The density and composition of the glass can, in
some cases, reduce this down to 200 nm. The glass samples to be analyzed must therefore be
prepared with a good surface finish. Mellott compiled and investigated the surface roughnesses
of commercial glasses using Atomic Force Microscopy (AFM) and found that many are in the
range from 2 to 0.2 nm [5]. Quantitative boron analysis on non-polished glasses was much less
accurate and repeatable than on the same polished material (private communication in 2004 with
Brian Simpkins, Alcan Packaging, formerly Wheaton Glass, Millville, New Jersey).
Reducing the path length of the X-ray beam (tube-to-sample and sample-to-detector) and using
thin-window flow proportional detectors, combined with a controlled vacuum of better than 0.1
mbar, enables higher yield of detected boron radiation. Comparing the published sensitivities
from a 1998 dual-turret spectrometer with those from a direct-loading S4-type system using the
same X-ray tube, the type of generator shows a substantial effect, attributable to the optimized
beam path and window design [6–8]. Changes in the vacuum during the measurement affect the
count rate of the detected boron and needs to be taken into consideration for those systems that
have large sample compartments (e.g., dual-turret systems). In contrast, direct-loading systems
can usually achieve a stable vacuum without changeover during the whole measurement period.
These direct-loading systems feature pressure-controlled sample introduction. Measurement is
started only when the pressure in the sample compartment reaches a certain level, resulting in
constant pressure and higher, more stable count rates for boron.
ANALYZER CRYSTALS
Ricardo et al. showed in 2001 that with a
new type of patented multilayer detector
crystal, the boron signal can be raised by
28% [9].
The XS-B™ crystal depicted in Figure 2 is
a LaB4C multilayer and is available from
Incoatec GmbH (Gesthacht, Germany).
Schuster et al. performed a feasibility
study for the analysis of boron in a coating
of BPSG, a common semiconductor
material, in 1987 [10]. Using a WDXRF
Figure 2. Comparison of boron intensity collected with a
system equipped with a 125 µm endtraditional crystal versus multilayer crystal.
window X-ray tube, 80 mA tube current,
and early multilayer crystal (OVO 160), he
obtained a sensitivity of 2.7 counts/s per wt% B. The detection limit was reported to be 0.04 wt%
for 600 s measurement time.
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Copyright ©JCPDS-International Centre for Diffraction Data 2008 ISSN 1097-0002
Using a 200 Å Mo-B4C multilayer crystal (OVO-B) and an S4 Series WDXRF system optimized
for beam path, Mauser achieved a detection limit of 0.02 wt % B in 100 s and a sensitivity of 330
counts/s per wt% B on the same type of material [7]. Using the XS-B multilayer crystal on the
same S4 instrument further improved the sensitivity to 420 counts/s per wt% B as was shown by
Behrens in 2004 [11].
CHOICE OF COLLIMATORS AND BACKGROUND POSITIONS
Multilayer crystals produce wide peak profiles and should be used with coarse collimators. The
rule of thumb in WDXRF is to use the coarsest collimator for the lightest element detection.
Since achieving a high count rate is paramount for the analysis, peak resolution and interference
corrections are not an issue. This holds true for BPSG, but in order to create a meaningful widerrange calibration for commercial borosilicate and E glass as well, count rate needs to be balanced
against resolution [12].
Cl Ll, B KA1
O (3) Zr Mz
K Ll, Ca Ll
Si LB/ S Ll Ln Ca
Ca and K Ll lines, as well as Ca Ln and Zr M
lines, should be separated as shown in Figure 3.
The effect of higher order reflections of O
cannot be reduced much by using pulse height
discrimination and needs to be minimized by
collimation. With Ca and Zr present in the
analyzed material, the selection of background
positions becomes difficult. The approach from
Feather and Willis is particularly useful in this
regard, as it can be used to perform interference
correction on the background [13].
CALIBRATION APPROACH
<21.778
Y : 0 to 2.260 KCps
61.978>
Figure 3. Interference on and around the boron K
signal.
Obtaining a sufficient number of glass
standards, as well as including enough samples
to perform the interference corrections needed,
pose the biggest challenge to the analyst.
The quality of the boron certification by Manitol-based volumetric means is very much
dependent on the experience of the analyst with that methodology. Few commercial labs now
offer Manitol-based boron analysis. Inductively coupled plasma (ICP) methods are increasingly
being offered instead, but these methods are time- and cost-prohibitive because of the
requirement for sample digestion.
Few glass samples of defined boron composition are commercially available. Many borosilicate
glass samples, such as those available from Breitlaender GmbH (Hamm, Germany), are just premelt batch and not chemically certified. Borosilicate glass suppliers are the best source of sample
material. They can produce a range of experimental matrix for the calibration based on past
batch-melt samples. A comparison and discussion of the quality of certification techniques for
boron in glass is unfortunately absent from the literature.
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Using over 40 glass samples of standard 32 mm size and a small 28 mm aperture, a calibration
for boron was established from 0.00 to 19.35 wt%. The calibration has a standard error of
calibration (SEC) of 0.266 wt%. The sensitivity is 94 counts/s per wt% B2O3. An empirical
calculation using a concentration-based correction approach produced better results than any
fundamental parameters (FP) model, even with variable influence coefficients. FP for boron, as
taken from the NIST-compiled tables, are not well characterized and would need further
improvements. Of greatest importance is interference correction of both the background and
peak position to obtain a good net intensity for boron.
MAINTENANCE OF CALIBRATION
In order to maintain the calibration, drift correction is needed to account for changes in detector
gas, foil, and tube signal that can influence the intensity and therefore the analysis.
A very interesting study by Guadagnino, Sundberg, Michiels, and Brochot showed that some
glass types exhibit a migration of boron when measured repeatedly [14]. Figure 4 shows the
change in B2O3 content in Pyrex-type and E-type glass as a function of the number of WDXRF
measurements. Borosilicate glass should therefore be used as a drift correction and QC sample
only after it has been conditioned.
Figure 4. Long-term stability of boron in E-type and Pyrex-type glass
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Copyright ©JCPDS-International Centre for Diffraction Data 2008 ISSN 1097-0002
CONCLUSION
The analysis of boron in glass using WDXRF has dramatically improved since its first
application as a process control tool in the late 1980s. Nevertheless, it is still one of the most
challenging applications, since the analyst must gather enough reference material and optimize
conditions to fit the spectrometer capabilities. The choice of the right collimator, background and
peak counting positions, correct pulse height analysis settings, and interference correction
method greatly influences the range and durability of the resulting calibration. Maintenance of
the calibration and a daily check of the system and calibration need to take the stability of the
glass standards into account.
ACKNOWLEDGMENTS
The author thanks Brian Simpkins (Alcan Packaging, formerly Wheaton Glass, Millville, New
Jersey) for his time performing the scans and tests on the borosilicate glass samples, as well as
for evaluating the XS-B and OVO-B crystals side-by-side in a process environment. Many
thanks as well to my colleagues at Bruker AXS Larry Arias, Kai Behrens, Karl Mauser, and
Karen Roscoe for contributing to the presentation and article.
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