Post on 10-Mar-2020
REASSESSMENT OF STANDARDLESS XRF AND PIXE ANALYSIS OF SOME DENTAL MATERIALS USED IN ENDODONTICS
AND ORTHODONTICS
PAULA PERLEA1, IOANA SUCIU1, BOGDAN DIMITRIU1, ELENA PREOTEASA4, EUGEN A. PREOTEASA3, BOGDAN CONSTANTINESCU3, DANIELA STAN3,
CATALINA CHIOJDEANU3, DAN GURBAN3, ADELA SCAFES3, LUCIA GEORGETA DAINA2, RUXANDRA IOANA SUCIU1
1Carol Davila University of Medicine and Pharmacy Dental Medicine Faculty, Bucharest, Romania
2 Faculty of Medicine and Pharmacy, University of Oradea, Romania 3Horia Hulubei National Institute for Physics and Nuclear Engineering,
Bucharest, Romania 4Heliodent
Corresponding author: eugenpreoteasa@gmail.com
Received September 4, 2017
Abstract. A reassessment of our particle-induced X-ray emission (PIXE) and X-ray fluorescence (XRF) analysis of some materials used in endodontic and orthodontic dentistry is presented. The studied dental materials included Ca(OH)2 preparations as well as repair cements and sealers used in endodontics, and glass ionomer cements used in orthodontics. PIXE measurements were done with 3 MeV protons while XRF was performed with portable instruments. By lack of proper reference materials, the matrix effects associated with the thick target measurements and with the heterogeneous and granular structure of the samples could not be completely dealt with. Thus the analyzed relative concentrations were only approximate. A critical evaluation of results showed that XRF measurements were less accurate than PIXE. Therefore a correction procedure of XRF data based on PIXE data was applied. Thus for each element concentration a plausible interval was defined between the initial and the corrected XRF values. The present results may serve for the preparation of reference materials in view of highly accurate analysis. In addition XRF may detect qualitatively high levels of light elements by the relative intensity of Compton vs. Rayleigh scattering of X-rays. The data show highly variable
elemental compositions in the endodontic and orthodontic materials under study, with no class-specific composition profile. In the analyzed dental materials, major elements with low Z appear to play various functional dental roles, while heavy major elements with high Z mainly enhance radio-opacity. Trace elements were probably impurities from raw materials. Further developments in dental research and possible forensic applications are suggested.
Key words: XRF and PIXE standardless multielemental analysis, thick target, approximate corrections, endodontic Ca(OH)2 preparations, repair cements and sealers, orthodontic glass ionomer cements, zinc phospahate cements, specimen and reference materials preparation, biocompatibility, dentistry, forensic applications.
Romanian Journal of Physics 62, 705 (2017)
Article no. 705 Paula Perlea et al. 2
1. INTRODUCTION
A large area in today dental research benefits of nuclear, atomic and
molecular methods for analytic, structural and spectroscopic investigations. They
are used for very diverse applications, from clinical studies to the investigation of
structure and mechanisms in normal and pathological dental tissues and to the
development of new biomaterials. Among them, an important role is accounted for
by various techniques of elemental analysis, which distinguish themselves by
speed, sensitivity, selectivity, versatility and frequently by nondestructive and
noninvasive character. In particular, atomic and nuclear surface and thin layer
analysis methods are of high usefulness in dental research [1], because they allow
the investigation of the alterations of biomaterials in the oral environment,
involving possible adverse effects and interactions with dental tissues. Such
techniques analyze elemental and chemical composition, topography, mineral
particles morphology and crystallographic structure by monitoring the photon,
electron and ion emissions from surfaces of teeth and dental materials when they
are excited by ion, electron or photon bombardment. Among the most popular of
these analytical methods one can mention X-ray fluorescence (XRF), particle
induced X-ray emission (PIXE), particle induced gamma-ray emission (PIGE),
elastic recoil detection analysis (ERDA), Rutherford back scattering (RBS),
secondary ion mass spectrometry (SIMS) and X-ray photoelectron spectroscopy
(XPS).
The first step in such studies consists in the implementation of the methods
for the analysis of the biomaterials and in the evaluation of their potential, which is
followed by more refined and more diversified studies aimed at specific dental
problems. This strategy is exemplified by the evolution of studies of dental
composites and other restorative materials made by mineral micro- and
nanoparticles embedded in an organic polymer resin. Our group analyzed the filler
compositions of auto-polymerizable and light-cured restorative resins using PIXE,
-PIXE, XRF and ERDA [2–5] and then the compositional alteration of a dental
filling during oral use was studied with PIXE and PIGE [6]. The same trend was
evidenced also by other research groups. In a first phase, dental composites were
characterized with various techniques including energy-dispersive X-ray
spectrometry (EDX) and scanning electron microscopy (SEM) [7], EDX/SEM,
PIXE and instrumental neutron activation analysis (INAA) [8], and EDX,
transmission electron microscopy (TEM) and SEM [9]; subsequent research using
XRF and SEM/EDX was performed in view of forensic applications [10], and the
bonding of composites to hydroxyapatite and to hard dental tissues was
investigated with EDX, SEM and X-ray photoelectron spectroscopy (XPS) [11]
and with -PIXE [12]. Thus restorative dentistry illustrates a success story for the
atomic and nuclear surface and thin layer analysis methods.
3 Reassessment of standardless XRF and PIXE analysis of some dental materials Article no. 705
Here we focus on other two fields of dentistry where new dental materials
are developped at a high pace and which can benefit of the potential of the above
elemental analysis methods. The first is endodontics, which is concerned with the
study, diagnosis, prevention and treatment of the dental pulp. The second,
orthodontics, pursuits similar goals but deals with the malpositioned teeth and the
jaws. We previously studied by PIXE calcium hydroxide preparations in
endodontic dentistry [13] and used a portable EDX/XRF spectrometer to analyze
endodontic sealers [14], endodontic repair materials [15] and orthodontic glass
ionomer cements [16]. PIXE has been used by one group for the analysis of
endodontic repair materials [17] but our study was the first to apply this method in
the study of endodontic Ca(OH)2 preparations. Also XRF appears to have not been
used by other groups in the analysis of endodontic sealers and repair materials and
of orthodontic glass ionomer cements.
However endodontic repair materials, which are a special type of cements
with mineral composition, have been largely investigated by various physical
methods. These include scanning electron microscopy equipped with energy
dispersive X-ray analysis (SEM/EDX) [17–22], environmental scanning electron
microscope with energy dispersive X-ray analysis (ESEM/EDX) [23], SEM with
wavelength-dispersive X-ray spectroscopy (SEM/WDS) microanalysis [24], PIXE
[17], XPS [20], X-ray diffraction (XRD) [17, 21, 22], inductively coupled plasma
optical emission spectroscopy (ICP-OES) [20, 25], atomic absorption
spectrophotometry (AAS) [18] and Fourier transform infrared spectrometry
(FTIR) [22].
By comparison the list is significantly shorter for the glass ionomer cements,
a class of materials used not only in orthodontics but also in other dentistry fields
[26, 27]. Glass ionomers have been investigated by XPS [28], SEM and electron
probe microanalysis (EPMA) [29], SEM and chemical analysis [30], AAS and ion-
sensitive electrodes [31] and by ICP-OES [32]. Endodontic sealers have not been
approached with physical analysis methods before our study.
Both PIXE and XRF are methods of X-ray spectrometry for multielemental
analysis based on the detection of characteristic X-rays. These radiations are
emitted by the elements in the sample when irradiated with accelerated protons or
heavier ions (PIXE) or with X-rays (XRF) and detected by a spectrometric chain.
PIXE is a particularly sensitive method, exceeding most other ion beam analysis
techniques by its sensitivity in trace element detection. In our studies we used a
portable XRF spectrometer, which has a lower sensitivity, but sufficient for the
analytical examination of the dental materials. Both PIXE and XRF are
conveniently specific, and relatively nondestructive techniques, which can cover a
Article no. 705 Paula Perlea et al. 4
high dynamic range of values in the spectra and thus can give relevant insight on
major to trace elements in biomedical applications [1, 33, 34].
In our investigations by both PIXE and XRF the X-ray emission spectra
were collected with energy dispersive semiconductor detectors which allow a
simultaneous multielemental analysis; therefore the spectra give at a glance the
composition without the need of prior knowledge about the sample. In our
experimental conditions PIXE can “see” almost all elements starting from P up to
U (Z = 15 – 92) and XRF can detect most elements starting from K (Z = 19). Each
method excepts a few elements, depending of the sample nature and experimental
setup. Both methods explore a thin layer at the surface of the sample with a
thickness of the order of a few tens of μm. This represents an important advantage
for studies of the interactions between dental materials and hard dental tissues,
because the most important processes occur at their interface. However, the
analysis of the surface layer of thick samples presents conceptual and
computational difficulties due to the complexity of interactions between the
incident beams and emergent radiations with the samples [35–37]. As a
consequence, the evaluation of absolute concentrations in thick samples starting
from prime principles could be affected by important systematic errors, and relative
concentrations are more reliable; in most cases this is completely satisfactory in
biomedical research. For the measurement of absolute concentrations, reference
materials with known composition and with physical structure similar to the
analyzed sample – specially prepared with the aim of reducing as much as possible
the so-called matrix effects – are needed. Highly specialized laboratories such as
those of the International Atomic Energy Agency and of the National Institute of
Standards and Technology provide a large diversity of standard materials for PIXE
and XRF analysis of many types of materials.
However, standard reference materials of dental materials are lacking,
which is a major problem in their study. On one hand, these preparations develop at
a high rate; on the other, the manufacturers do not disclose the composition of their
products for commercial reasons. This difficulty associated with missing standards
confronted also our studies of dental materials. In particular, for dental composites
we found significant differences for the concentrations as evaluated by PIXE and
by XRF. Based also on concentrations in composites communicated by other
authors [8] which obtained results close to ours [5, 6], we developed a correction
procedure (Preoteasa et al., in preparation).
In the present paper the corrected relative concentrations of the mineral
elements detected by EDX-XRF were evaluated, assuming that the correction
factors of composites are valid also for the endodontic and orthodontic materials.
5 Reassessment of standardless XRF and PIXE analysis of some dental materials Article no. 705
The outcomes include the ensemble of initial and corrected XRF results and of the
PIXE data. The results are discussed comparatively in search of general rules and
of a ‘big picture’ on PIXE and XRF standardless analysis of endodontic and
orthodontic materials.
2. MATERIALS AND METHODS
2.1. ANALYZED DENTAL MATERIALS
The following dental materials were included in the study: 1) calcium
hydroxide commercial mixtures – Calasept (Nordiska Dental, Angelholm,
Sweden), a one-component ready-to-use paste product and Cikal (DOD, DCA –
Leskovac, Yugoslavia), consisting of a paste accompanied by a setting fluid (only
the paste of the Cikal product was examined); 2) endodontic repair materials
(MTA cements) – ProRoot MTA (Dentsply), MTA-Angelus (Angelus), MTA Plus
noted also MTA+ (Avalon Biomed Inc.) and Biodentine (Septodont);
3) endodontic sealers – a polymethacrylate resin-based sealer Real Seal (Pentron
Clinical Technologies LLC, Wallingford, CT) and two epoxy resin-based sealers,
AH Plus and AH26 (Dentsply International Inc, York, PA); and 4) orthodontic
glass ionomer cements – Fuji Ortho LC light cured glass ionomer cement for
orthodontic bonding (GC), Transbond Plus XT light cured adhesive for metal and
ceramic brackets bonding to tooth surfaces (3M Unitek), Ketac Cem easily
photopolymerizable resin modified glass ionomer (3M ESPE). The conventional
zinc-phosphate self-curing cement Adhesor (Spofa Dental), which is used in
restorative dentistry, was also analyzed for comparison with the later three glass
ionomer cements.
2.2. PREPARATION OF TARGETS FOR PIXE AND XRF
Disk shaped samples of 7–8 mm diameter with a flat surface were prepared
on polished glass plates by drying in air at room temperature, by selfcuring or by
photopolymerization as described before [2, 3, 13–16]. When the solidification was
slow, the samples were let to solidify in Petri dishes partially covered and
preserved in a closed space to avoid dust deposition. For specific materials, the
solidification process was triggered chemically by mixing materials’ components
followed by vibration until setting, or photochemically with intense blue light
(420–500 nm) from a halogen lamp with glass fiber optics.
Crack formation occurred in the calcium hydroxide samples during drying.
Endodontic repair materials were adherent to the glass surface and brittle after
setting. Endodontic sealers required long setting times and were not completely
solidified (semi-solid) and adherent after solidification, even after use of a
photopolymerization light or incubation at 40 oC.
Article no. 705 Paula Perlea et al. 6
For PIXE the solid specimens were fixed with the flat surface up on
aluminum diaphragms. Covering the specimens’ surfaces with a thin film of carbon
picked up from an air-water interface was not possible due to the hydro-solubility
of calcium hydroxide. For XRF the disk shaped samples were measured as such,
without further preparation. Because the endodontic sealers were semi-solid and
adherent, a special Teflon gondola with a thin Mylar film at the bottom was used
for the measurements. The samples were put in the gondola on the Mylar
membrane bottom with their flat surface down and the gondola was positioned on
top of the portable EDX spectrometer.
2.3. REFERENCE MATERIALS
A pellet of compacted hydroxyapatite powder (a gift from
Forschungszentrum Rossendorf, Germany) and disk shaped samples prepared as
above from the light-curing dental composites Tetric Ceram and Ariston (Ivoclar-
Vivadent, Switzerland) were used as reference materials. In hydroxyapatite, the
relative concentrations estimated previously for this specimen with the GUPIX
code [38] and corrected for the nominal P/Ca ratio of 0.464 were the following: P,
18.5 ± 8.9 %; Ca, 39.9 ± 0.3 %; Fe, 432 ± 104 ppm; Cu, 224 ± 72 ppm; Zn,
270 ± 122 ppm; Sr, 1248 ± 799 ppm; Ba, 512 ± 108 ppm; and Pb < 516 ppm (w/w;
1 ppm = 1 mg/kg = 10-4
%). The concentrations evaluated with the Gupix code
from the PIXE spectra of the Tetric Ceram dental composite were the following:
Ca, 0.197 ± 0.024%; Fe, 64 ± 10 ppm; Sr, 200 ± 52 ppm; Zr, 0.454 ± 0.145%; Ba,
3.97 ± 0.53%; Yb, 5.36 ± 0.39%; Hf, 47 ± 21 (Preoteasa et al., in preparation;
mean values from [5, 6]).
2.4. PIXE MEASUREMENTS
Wide beam PIXE measurements were performed with 3 MeV protons at the
8.5 MV NIPNE-HH tandem Van de Graaff accelerator as described before [2, 3,
13]. The proton beam hit the target at 45o with respect to the surface normal. The
X-rays emerging from the target were collected with a Canberra hyperpure (HP)
Ge detector cooled with liquid nitrogen and having a crystal surface of 100 mm2
and an energy resolution of 180 eV at 5.9 keV, placed perpendicularly to the beam.
The X-rays passed through the Be windows of the scattering chamber (0.25 mm
thick) and detector and a 2.4 cm air gap. The spectra were collected without
additional absorber. The spectroscopic chain consisted of detector preamplifier, a
Tennelec amplifier, a Canberra analog-to-digital converter and a Canberra S100
counting system connected to a computer.
Because the Ca(OH)2 dental preparations are Ca rich samples, our standard
PIXE measurements could not detect traces of Ni if present, because its
characteristic lines are superimposed with Ca+Ca and Ca+Ca artefactual (pile-
7 Reassessment of standardless XRF and PIXE analysis of some dental materials Article no. 705
up) lines. Spectra were processed for background subtraction and intensity
evaluation of characteristic lines with the LEONE program.
2.5. ANALYSIS OF PIXE SPECTRA USING THE X-RAY YIELD CURVE
To evaluate element concentrations from the peak areas of the thick target
PIXE spectra, we constructed the X-ray yield curve (line area/concentration ratio
vs. atomic number Z) of hydroxyapatite reference material. The curve showed a
deeply asymmetric bell-shape for Z = 15 – 40. Although it may be affected by
considerable inaccuracy for low Z elements, it allowed an acceptable analysis of
the elements from Ca to Sr. For the higher Z elements yielding intense L lines (Ba
and Pb), a similar procedure was applied, however we referred not only to
hydroxyapatite but also to a previously analyzed Ca- and Ba-containing dental
composite and to a dental enamel sample containing traces of Pb [3]. The yield
used for Pb evaluation was adapted from a dental enamel spectrum [3].
2.6. XRF MEASUREMENTS
Two portable EDX-XRF X-MET 3000 TX+ spectrometers (Oxford
Instruments) with X-ray tubes having Rh and Ag anodes and with Si PIN diode
semiconductor detectors cooled with Peltier elements were used. The tubes were
operated at 40 kV and 0.006 mA anode current for 60 sec. The energy resolution
was of 275 eV for the Mn K line, that is, not so good as that mentioned above for
liquid nitrogen-cooled detector.
These instruments could analyze all elements from a light element like K on,
except those elements having characteristic radiation in the energy interval of
anode’s radiation coherently and incoherently scattered by the samples. This
interval is defined between the Compton scattered K line and the Rayleigh
scattered K line of the anode: Rh, Pd, Ag, Cd, In for the Ag anode (K ~22.2 keV,
Compton ~20.6 keV, K ~24.9 keV) and Ru, Rh, Pd, Ag for the Rh anode (K
~20.2 keV, Compton ~19.0 keV, K ~22.7 keV).
To evaluate the concentrations, the spectra were processed with the
WinQXAS version 1.30 software (IAEA, Vienna) in the “thick sample” mode in
order to make corrections for matrix effects, and with the Alloy FP (Fundamental
Parameters) program incorporated in the instrument, also in the “thick sample”
mode; the later program does not analyze Ca and lighter elements.
2.7. ANALYSIS OF XRF SPECTRA USING THE X-RAY YIELD CURVE
Although the softwares made some corrections for matrix effects, they did
not account for the granularity of the materials. By lack of reference materials, in
Article no. 705 Paula Perlea et al. 8
the case of endodontic sealers for which the WinQXAS software was used, the
considered matrix effects corrections proved insufficient and resulted in unreliable
concentration values for some elements, especially for Ca, while for other the
results were presumably distorted by unknown inaccuracy [14]. The granularity
effect is more important for L lines than for K lines. Matrix effects due to
granularity as measured by the shift from linearity increase steadily with the
particle size. For instance, for particles of 12 μm diameter and of mean atomic
number 28, they are about 10% for K lines and about 80% for L lines [1, 39]. For
YbF3 granules of mean atomic number 24 and of 10-50 μm size as seen by -PIXE
in the dental composite Tetric Ceram, we estimated granularity corrections of about
50% of the evaluated concentrations [1].
In order to minimize as much as possible these errors, we applied a
procedure similar to that used for PIXE spectra of dental calcium hydroxide
preparations [13]. Mean (“smoothed”) yield values were calculated by fit of the
empirical yields resulting directly from the software output vs. atomic number Z.
The empirical yield was given by the software output of concentrations and count
numbers of the corresponding lines. The empirical data were mean values for the
three endodontic sealers pooled together with five dental composites (Preoteasa et
al., to be published). The composites’ (and the hydroxyapatite) data were necessary
in order to increase the number of elements in the yield plot of L lines to at least
four by including Yb and Hf from the former (and Pb from the later).
The whole set of K-lines data form Ca (Z = 20) to Ba (Z = 56) could be
approximately fitted with a single log-normal function, but a more precise fit could
be done using exponential functions on the low-Z domain (from 20 to 30, Ca to Zn)
and on the high-Z domanin (from 40 to 56, Zr to Ba), with the log-normal function
in between from Zn to Zr [14]. The ensemble of L-lines data could be fitted either
with the Gaussian function or with the more exotic so-called “extreme” function,
and for a better fit we used the Gaussian function from Z = 56 (Ba) to Z = 70 (Yb),
and an “extreme” function extrapolated from Z = 70 to Z = 83 so as to include Bi.
The procedure described is not a rigorous solution, because it does not take
into account the absorption edges and because it uses extrapolation for the lighter
elements like Ca and for the heavier ones like Bi. The lack of reference materials
inherently resulted in significant limitations of our attempts of quantitative
analysis. However the procedure gives improved Ca values as compared to the
direct estimates and makes possible a quantitative estimate for Bi. The relative
concentrations thus obtained, expressing percent values of the total detected
elements (while the light XRF-invisible elements are neglected) are more reliable
than absolute concentrations. No similar procedure was applied for the endodontic
repair materials [15] and for the orthodontic cements [16] because the output data
of the Alloy FP (Fundamental Parameters) program were less complex.
9 Reassessment of standardless XRF and PIXE analysis of some dental materials Article no. 705
2.8. TOWARDS A CORRECTION PROCEDURE FOR THE ANALYSIS OF XRS DATA
Due to the lack of proper reference materials, it was plausible that the concentration of any given element determined by XRF was affected by an unknown inaccuracy dependent of the analysis method itself. To reduce the method-dependent inaccuracy, it would have been recommended to perform the analysis not only by XRF but also by another method. In particular, this approach has been applied for dental composites which were analyzed both by XRF, PIXE and ERDA [2]. In general the concentrations measured by different methods showed significant differences. We noticed that for the Tetric Ceram composite the
mean values of concentrations measured by PIXE [6] and -PIXE [5] are very close to the PIXE values communicated by other authors [8], while they were different with respect to XRF concentrations (Preoteasa et al., in preparation). The good convergence of different PIXE analyses is consistent to the postulate that they are more close to the real concentrations than the values measured by XRF. Therefore we defined a correction factor by the PIXE/XRF ratio. The values for various elements in the K-lines and L-lines series were interpolated from its dependence on the atomic number Z. For the purpose of the present study, we assume that within acceptable approximations the same correction factor can be used for the XRF concentrations measured in of endodontic and orthodontic materials. Provisional values for the correction factor of 0.2–1.2 in the K series for Z = 20–40 (Ca to Zr) and of 0.7–2.4 in the L series for Z = 56–83 (Ba to Bi) were determined (Preoteasa et al., in preparation) and used here. Thus “PIXE-equivalent” concentrations were evaluated. Because we do not have additional information to decide between the corrected and uncorrected data, we are compelled to assume that the XRF and PIXE-equivalent concentrations define limits for the real concentrations.
2.9. INVISIBLE LIGHT ELEMENTS: COMPTON VS. RAYLEIGH SCATTERED RADIATION IN XRF SPECTRA
Although light elements could not be detected in XRF by their characteristic X-rays of very low energy, one can gather some general information of them by looking at the peaks of coherent (Rayleigh) and incoherent (Compton) scattering of the X-ray tube anode characteristic radiation which is “reflected” by the sample. The relative intensity of Compton vs. Rayleigh scattering of X-rays decreases exponentially with the atomic number Z of the target, so the higher concentration of light elements the higher is this ratio [40].
For example, AH Plus and AH 26, in contrast with Real Seal, the intensity of the Compton scattering of radiation is greater than that of the Rayleigh scattering of the primary excitation X-rays. This suggests a higher concentration of light elements in the former two biomaterials as compared to the last one. It is known that AH Plus contained silica [41] and it is plausible that the same was true also for
Article no. 705 Paula Perlea et al. 10
AH 26, while we may suppose that SiO2 was not present in Real Seal in high concentration. This promising procedure will not be used in the following, as it has to be refined in further studies.
One may hope that after developing this technique XRF could have some edge on standard configuration PIXE (with outside detector), hereby providing a possibility of detecting (at least qualitatively and undiscriminately) high levels of very light elements such as Si (Z = 14) and even O (Z = 8). However, high-performance PIXE with the detector inside the reaction chamber can analyze by their characteristic X-rays light elements starting from Mg (Z = 12).
3. RESULTS AND DISCUSSIONS
3.1. OVERVIEW OF RESULTS
The PIXE and XRF analysis of materials used in endodotics and orthodontics (and for comparison, of one material used in restorative dentistry) detected 15 elements (Table 1), all of them metals (Ca, Mn, Fe, Co, Cu, Zn, Sr, Zr, Ba, La, Hf, W, Au, Pb, Bi). Of them, Mn could be artefactual, and Fe, Co, Cu and Pb were present only as traces, probably impurities from the raw materials or from the fabrication process. Note that except Pb, a post-transition metal, these are transition metals of the first series and they do not play a role in the studied dental materials. Their detection can serve only for tracing a particular brand of a material. By the contrary, Zn, which is at the interface of transition and post-transition metals, is essential in the zinc-phosphate cement Adhesor (M). Hf is only an impurity which accompanies Zr, both transition metals of the same group.
In principle, the detected major (and even minor) elements can play two types of roles in the analyzed dental materials. First, there are light and medium weight metals with a role in the dental function of the material (Ca, Zn, Sr, Zr; and probably Ba in Real Seal-I); second, medium and heavy metals with radiological opacity role (probably Ba in Calasept-A, and La, W, Au, Bi). The analyzed dental materials were noted with capital letters in Table 1.
The concentrations evaluated by PIXE and XRF in the analyzed dental materials are presented in Table 1. Relative w/w concentrations are given in ppm and % with respect to the total detected elements (normalized to 100 %). PIXE analyzed concentrations are given as means with corresponding standard deviations. XRF determined concentrations are given by limits which are defined by the uncorrected (normal characters) and corrected (bold characters) values, respectively.
3.2. ACCURACY OF XRF DATA
In the absence of proper reference materials, the presence of Hf is one of the few hints allowing an evaluation of the accuracy of the XRF analysis. The Hf/Zr
11 Reassessment of standardless XRF and PIXE analysis of some dental materials Article no. 705
ratio found in MTA Plus (E) and Biodentine (F) was 0.4–0.7 %, which is 2–4 times lower than its value in commercial zirconium metal (1–2.5 %) but fits well as order of magnitude. Assuming that the real concentration of Hf in Zr was as in commercial Zr, the departure of our results by a factor of 2–4 is acceptable, given that in the X-ray spectrum the Zr is a K-line element while Hf is a L-line element. Because the yield vs. atomic number curves as well as of the correction factor curves (Preoteasa et al., in preparation) used for calculating the Zr and Hf concentrations are different for K and L series elements, the respective differences may explain the limited accuracy.
Another reference value is the Ca/W ratio in AH Plus (G) because both Ca
and W are contained in the one and only single compound (CaWO4) with a precise
stoichiometry, thus Ca/W = 40.078/183.85 = 0.218. With the new corrected values
the Ca/W ratio is too small, 1.74/77.97 = 0.022; with the older values it was too
large, in the range 0.80–1.12. This shows that neither the new (corrected) and the
old (initial) values are the real values, and that the later are somewhere between the
two limits, namely Ca = 1.7 – 14 % and W = 62 – 78 %, confirming therefore our
basic assumption at least in the particular case of AH Plus (G).
Finally, the Fe/Bi ratio can be compared to literature values for MTA Pro-
root (C) and MTA Angelus (D) [17–21]. In the two materials we found a Fe/Bi
ratio of 0.008 and 0.004 with the old values and of 0.0009 and 0.00045 with the
new ones, respectively. Other authors found in MTA Pro-root (C) a Fe/Bi ratio of
0.26 [19], < 0.25 [20] and < 0.028 [17]. This suggests that in our results Fe was
underestimated and Bi overestimated, or both – but most probably, Ca was
underestimated. In the case of MTA Pro-root (C) and MTA Angelus (D) the old
values seem more reliable, and probably correct as order of magnitude.
In brief, the ratios between concentrations of K-series light elements and
L-series heavy elements like Zr/Hf (or Hf/Zr), Ca/W and Fe/Bi sustain the
postulate that the real values could be expected to be located somewhere between
the old values and the new ones. The difficulty in using the K-series/L-series ratios
is related to the fact that the yield vs. Z curves as well as of the correction factor
curves are different for the two series; this leads in itself to unknown inaccuracy. In
addition, we used the above ratios in a number of particular analyzed materials –
MTA Plus (E) and Biodentine (F); AH Plus (G); MTA Pro-root (C) and MTA
Angelus (D) – and extrapolated the conclusions for the rest, while the matrix
effects are different from a material to another. Accordingly, we suppose by
caution that we should trust in the first place the order of magnitude of
concentrations. A more generous and less restrictive solution would be to extend
the limits with the standard deviations (incertitudes) of the corresponding
concentrations, and thus to enlarge the domains of plausible concentrations. In
general, the statistical errors are within 10 % for major elements and up to 70 % for
minor and trace elements (footnote to Table 1). PIXE analysis has the advantage of
higher sensitivity as compared to XRF, but the later is simple and convenient.
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3]
Rep
air
cem
ents
[15
] M
iner
al m
atri
x
Sea
lers
[14]
Min
eral
+ o
rgan
ic m
atri
x
Gla
ss i
on
om
er c
emen
ts [
16]
Min
eral
+ o
rgan
ic m
atri
x
Zn
ph
osp
hat
e
cem
ent
[16]
Den
tal
mat
eria
l
A
Cal
a-se
pt
B
Cik
al
C
MT
A
Pro
-
Root
D
MT
A
An
gel
us
E
MT
A
Plu
s
F
Bio
-
den
tin
e
G
AH
Plu
s
H
AH
26
I
Rea
l
Sea
l
J
Fu
ji
Ort
ho
LC
K
Tra
ns-
bond
Plu
s
L
Ket
ac
Cem
M
Ad
hes
or
1
2
3
4
5
6
7
8
9
10
11
12
13
14
Ca
85.7
0
.1 %
9
9.8
0.1
*
0.0
9 –
1.0
%
0.1
0 –
1.2
%
0.0
8 –
0.7
0 %
0.3
–
2.0
%
1.7
–
14 %
0.0
04
–
0.0
50
%
0.2
3 –
2.5
%
0.0
3 –
0.1
0 %
0.0
3 –
0.1
0 %
20 –
48 %
0
.05 –
0.1
0 %
Mn
600
2
60
Fe
100
2
6
60
10
0.0
9 –
0.8
0 %
0.0
5 –
0.4
0 %
0.0
7 –
0.5
0 %
0.1
5 –
0.7
0 %
0.1
6 –
0
.95
%
0.0
3 –
0.2
6 %
0.0
9 –
0
.15
%
0.8
–
2.0
%
0.8
–
2.0
%
0.5
5 –
1.0
%
0.3
3 –
0.5
0 %
Co
0.2
8 –
0.4
0 %
Cu
< 1
70
< 1
00
600
–
2500
Zn
150
90
0
.82 –
1
.0
99.0
– 9
9.3
%
Sr
2570
600
500
560
–
2000
560
–
2000
1
100
–
2000
9
0 –
95 %
90 –
95
%
1.0
–
1.4
%
Ta
ble
1
(co
nti
nu
ed)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
Zr
0.5
– 1
.0
%
0.6
– 1
.2
%
30 –
46
%
95 –
97
%
13 –
18
%
Ba
14.0
0.1
%*
200
6
0
2.0
– 6
.6
%
La
0.5
0 –
1.2
%
49 –
77
%
Hf
0.2
0 –
0.2
2 %
0.4
0 –
0.6
3 %
W
62 –
78
%
Au
0.6
- 1
.1
%
3
.0 –
9.6
%
3.0
– 9
.6
%
Pb
< 1
37
0
<
800
Bi
98.0
–
99.3
%
98.2
–
99.2
%
52 –
69
%
1.1
- 2
.1
%
4.4
- 6
.5
%
99.7
–
99.9
%
90.8
–
97.8
%
2
0.4
768
7
0.4
748
8
0.3
503
7
0.4
849
0.4
020
9
0.5
429
8
0.3
690
4
1.4
231
9
1.4
231
9
0.2
123
4
Note
: R
elat
ive
mas
s co
nce
ntr
atio
ns
of
the
tota
l d
etec
ted
ele
men
ts (
in p
pm
an
d %
). P
IXE
con
cen
trat
ion
s: m
ean
± S
D.
XR
F c
on
cen
trat
ion
s: l
imit
s d
efin
ed b
y u
nco
rrec
ted
(norm
al c
har
acte
rs)
and
corr
ecte
d (
bold
) val
ues
. In
both
cas
es s
tati
stic
al i
nce
rtit
ud
es a
re w
ith
in 1
0 %
for
maj
or
elem
ents
and
up t
o 7
0 %
for
min
or
and
tra
ce e
lem
ents
.
Article no. 705 Paula Perlea et al. 14
Within the enlarged concentration ranges defined this way, one can
formulate and prepare reference materials with selected compositions, which would
allow more accurate and precise analysis of the endodontic and orthodontic
materials. For a high precision analysis, this procedure may be iterated, and thus
our present results may guide further research aimed at obtaining most reliable
results within the potential of PIXE and XRF.
As a measure of the accuracy of the initial (uncorrected) XRF concentrations –
or of the size of the needed corrections – we defined for a given dental material the
following amount:
rel2 = (1/N) (ci
cor – ci
ini)
2/(ci
ini)
2, (1)
where ciini
and cicor
are the initial and corrected concentrations of element i,
respectively, and N is the number of analyzed elements in all samples (including
elements not detected in a specific sample, i.e. with zero concentrations). Thus we
found for rel2 the following mean values for the relevant three classes of dental
materials: 0.447 ± 0.064 for endodontic repair cements, 0.438 ± 0.092 for the
endodontic sealers, and 1.02 ± 0.70 for orthodontic glass ionomer cements. Due to
the relatively large SD values, the differences between the three means are not
rigorously significant, but there is a statistical trend for a minimum value of rel2 in
the case of endodontic sealers. (However in particular cases of certain endodontic
repair cements and orthodontic glass ionomer cements there are exceptions to this
rule, which is not verified in general.) We note that the XRF spectra of the
endodontic sealers were processed with the WinQXAS code, in contrast to the
other dental materials which were analyzed with the Alloy FP program. The above
results suggest that WinQXAS may give more accurate concentration values. Of
course this preliminary observation is only connjectural and needs further
verification. But WinQXAS gives anyway superior results in the analysis of XRF
spectra, e.g. concentrations of more analyzed elements and more trace values. The
Alloy FP program can be properly used only for dental alloys, where it gives good
results (Preoteasa and Chiojdeanu, to be published). As a more general observation,
the standardless XRF analysis of dental materials with complex composition and
granular structure using portable spectrometers can’t evaluate concentrations with
an accuracy comparable to that obtained with dental alloys.
3.3. ‘THE BIG PICTURE’ OF THE DENTAL MATERIALS’ ELEMENTAL COMPOSITION
Some general observations can be made on the 15 elements detected in 13
dental materials by PIXE and XRF. There are major (dominant) and minor/trace
elements. Most trace elements occur both as major elements in different materials.
When not being a major element, Ca is ubiquitous as a trace element. The same is
true for traces of Fe (< 1 %), which have been detected in all materials; it may be a
widespread impurity from raw materials or, in some cases, an iron oxide colorant.
15 Reassessment of standardless XRF and PIXE analysis of some dental materials Article no. 705
Other trace elements (Mn, Co, Cu, Zn, Sr, Zr, Ba, La, Hf, Au, Pb, Bi) are probably
of accidental origin, i.e. impurities from raw materials or production processes. As
already mentioned, Mn and Co traces may be artifacts from the spectra. Thus the
only confirmed elements which occur only in traces are Cu, Hf and Pb. Major
elements detected in the analyzed materials are Ca, Zn, Sr, Zr, Ba, La, W, Au and
Bi. Of them, W is the only one which does not occur in traces but only as a major
element. The qualitative distribution of detected elements in the dental materials is
shown in Table 2.
Table 2
Distribution of elements in the endodontic and orthodontic materials analyzed by XRF and PIXE
Element Major Trace or minor
Ca Calasept, Cikal, Ketac Cem
MTA Pro-root MTA Angelus MTA Plus Biodentine
AH Plus AH 26 Real Seal Fuji Ortho LC Transbond
Plus Adhesor
Mn Calasept
Fe
Calasept, Cikal, MTA Pro-root MTA Angelus MTA
Plus Biodentine AH Plus AH 26 Real Seal Fuji
Ortho LC Transbond Plus Ketac Cem Adhesor
Co Adhesor
Cu Calasept, Cikal, AH Plus
Zn Adhesor Cikal, Ketac Cem
Sr Fuji Ortho LC, Transbond Plus Calasept, Cikal, MTA Pro-root, MTA Angelus,
Biodentine, Ketac Cem
Zr MTA Plus, Biodentine, AH Plus MTA Pro-root
Ba Calasept Cikal Real Seal
La Ketac Cem MTA Plus
Hf MTA Plus, Biodentine
W AH Plus
Au Fuji Ortho LC, Transbond Plus Biodentine
Pb Cikal, Calasept
Bi MTA Pro-root, MTA Angelus,
MTA Plus, AH 26, Real Seal
Biodentine, AH Plus
With the possible exception of Ca(OH)2 endodontic products and of the zinc-
phosphate cements, no class of analyzed dental materials – endodontic repair
cements, endodontic sealers and orthodontic glass ionomer cements – is
characterized by a certain composition profile, e.g. by a given major element or a
pair of major elements. Each class of materials contains products of different
composition. At the same time there are materials from different classes with
similar compositions, e.g. MTA ProRoot (C) and MTA Angelus (D) on one side
and AH 26 (H) and Real Seal (I) on the other, which all show high concentrations
of Bi. Two materials are practically identical within the possibilities of XRF used
for their analysis (Fuji Ortho LC-J and Transbond Plus-K), but the rest can be
Article no. 705 Paula Perlea et al. 16
identified by a certain composition. This characteristic of the later could be of
potential forensic relevance.
An important general observation emerging from Table 1 may distinguish
between major elements with Z = 20 – 40 which play a diversity of functional
dental roles (Ca, Zn, Sr, Zr), and high-Z (56 – 83) major elements which play
mainly a role for enhanced radio-opacity (Ba, La, W, Au, Bi). The first are detected
in the X-ray spectra by their K lines and the second by their L lines. For the later,
note that while Ba was used since long as an radio-opacity additive in dental
materials, the presence of other heavy, rare and even precious metals like La, W,
Au, Bi expresses a present trend in dentistry [26, 27, 42]. In our previous papers [2,
3, 13–16] we discussed the possible toxicity of detected elements, including the
‘exotic’ ones recently introduced in the dental materials. Our results showed that
the presence of these elements in insoluble compounds at the estimated
concentrations do not present health risks. However, long-term studies, including
by our group, are necessary for assessing the biocompatibility of these elements,
mostly metals.
4. CONCLUSIONS
A critical evaluation of standardless PIXE and XRF analysis of endodontic
and orthodontic materials has lead to a better understanding of the accuracy of this
approach with emphasis on XRF. Although standardless XRF analysis with a
portable EDX instrument presents the advantage of the highest simplicity, our
results obtained in the study of various dental materials were in general less
accurate as compared to PIXE. Divergence between methods was substantial in
particular for dental composites. For the later we devised a correction method of
XRF concentrations which uses PIXE data as a reference. We applied the same
method to our XRF results on endodontic and orthodontic materials and we
estimated corrected concentrations, assuming to a first approximation the validity
of the same correction factors as for composites. Thus ranges of elements’
concentration were defined between the initial and the corrected values. Using
these two limiting values and their respective standard deviations, the confidence
intervals of these ranges can be extended to account for the different matrix effects
in the different endodontic and orthodontic materials. The results suggest that the
WinQXAS code may give more accurate concentration values than the Alloy FP
program, but this observation has to be verified further. However, the accuracy of
concentrations evaluated by standardless XRF analysis with portable instruments
remains lower in the case of dental materials with complex composition and
granular structure as compared to the analysis of dental alloys. The results may be
used in future studies for the preparation of proper reference materials with
concentrations in the ranges defined by the present results. Increasingly accurate
17 Reassessment of standardless XRF and PIXE analysis of some dental materials Article no. 705
results could be thus obtained by successive iterations of dedicated standards’
preparations. In addition the ability of XRF to detect qualitatively high levels of
light elements by the relative intensity of Compton vs. Rayleigh scattering of X-
rays was explored. Preliminary results were encouraging and recommend
developing of this technique in further studies.
The overall PIXE and XRF results show highly variable elemental
compositions in the endodontic and orthodontic materials under study. No class of
the analyzed dental materials was characterized by a certain composition profile.
The high variability of composition implies the necessity of preparing different
specific standards for almost all of the preparations. The present data already show
that the concentrations of ‘exotic’ metals evidenced in the investigated dental
preparations (Sr, Zr, Ba, La, Hf, W, Au, Bi) do not present health risks. They may
be correlated to biocompatibility studies (Suciu et al., to be published). As a
general observation, major elements with Z = 20 – 40 (Ca, Zn, Sr, Zr) play various
functional dental roles, while heavy major elements with Z = 56 – 83 (Ba, La, W,
Au, Bi) were added mainly for enhancing radio-opacity. Trace elements include
Cu, Zn, Sr, Zr, Ba, La, Hf, Au, Pb, Bi in a number of materials, while some of them
are major in other preparations. The more accurate results which can be expected in
future investigations will allow the approach of specific interface phenomena
occurring in the oral environment, such as long-term dissolution, leakage and
diffusion of metals from the endodontic and orthodontic materials. Possible
forensic applications are also considered.
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