REASSESSMENT OF STANDARDLESS XRF AND PIXE ANALYSIS … · 2017-12-11 · reassessment of...

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: [email protected]

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)

mailto:[email protected]

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


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.


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.


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-


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


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.


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


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


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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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

.


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.


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


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


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.

REFERENCES

1. Eugen A. Preoteasa, Elena Preoteasa and Ioana Suciu,. Atomic and Nuclear Surface Analysis

Methods: A Novel Perspective for the Characterization of Dental Composites, Nova Science

Publishers, Inc., NY, 2012, (220 p.).

2. E.A. Preoteasa, C. Ciortea, B. Constantinescu, Daniela Fluerasu, Sanda-Elena Enescu,

D. Pantelica, F. Negoita, Elena Preoteasa, Nucl. Instrum. Methods Phys. Res. B 189, 426–430,

2002.

3. E.A. Preoteasa, Rodica Georgescu, C. Ciortea, Daniela Fluerasu, Livia Harangus, Andreea

Iordan, Feride Severcan, Handan Boyar, Elena Preoteasa, I. Piticu, D. Pantelica, Vl.

Gheordunescu, Analyt. Bioanalyt. Chem. 379, 825–841, 2004.

4. E.A. Preoteasa, Rodica Georgescu, C. Ciortea, Daniela Fluerasu, Elena Preoteasa. CD

Proceedings of the 10th International Conference on Particle-induced X-ray Emission and its

Analytical Applications PIXE2004, 4–8 June 2004, Portoroz, Slovenia 0943.1–0943.3, 2004.

5. E.A. Preoteasa, Elena Preoteasa, L. Harangus, A. Moldovan, M. Dinescu, D. Grambole,

F. Herrmann, X-Ray Spectrom. 39, 208–215, 2010.

6. E.A. Preoteasa, Elena Preoteasa, C. Ciortea, D.D. Marin, D. Gurban, M. Gugiu, Adela Scafes,

X-Ray Spectrom. 38, 548–556, 2009.

7. Y. Asaka, M. Miyazaki, H. Aboshi, T. Yoshida, T. Takamizawa, H. Kurokawa, A. Rikuta,

J. Oral Sci. 46(3), 143–148, 2004.


8. R. Vecchi, G. Valli, M. Chiari, A. Migliori, S. Nava, C. La Porta, G. Guzzi, C. Minoia, Conf. on

Particle-induced X-ray Emission and its Analytical Applications, 4–8 June 2004, Portorož-

Ljubljana, Slovenia, 941.1–941.3, 2004.

9. R.J. Scougall-Vilchis, Y. Hotta, M. Hotta, T. Idono, K. Yamamoto, Dent. Mater. J. 28(1): 102–

112, 2009.

10. M.A. Bush, R.G. Miller, A.L. Norrlander, P.J. Bush, J. Forensic Sci. 53, 419–425, 2008.

11. H.U.V. Gerth, T. Dammaschke, H. Züchner, E. Schäfer, Dent. Mater. 22(10), 934–941, 2006.

12. H. Yamamoto, M. Nomachi, K. Yasuda, Y. Iwami, S. Ebisu, H. Komatsu, T. Sakai, T. Kamiya,

Nucl. Instrum. Meth. Phys. Res. B 260, 194–200, 2007.

13. Ioana Suciu, E.A. Preoteasa, D. Gurban, Ecaterina Ionescu, Dana Bodnar, Rom. Rep. Phys.

58(4), 569–582, 2006.

14. Ioana Suciu, Elena S. Preoteasa, E.A. Preoteasa, Catalina Chiojdeanu, B. Constantinescu,

B. Dimitriu, Paula Perlea, Al.A. Iliescu, Dana Bodnar, Rom. J. Phys. 60(3–4), 528–548, 2015.

15. Ioana Suciu, Elena S. Preoteasa, B. Dimitriu, Oana Amza, Paula Perlea, Anca Raducanu,

Mihaela Tanase, B. Constantinescu, Daniela Stan, E.A. Preoteasa, Rom. J. Phys. 60(9–10),

1490–1500, 2015.

16. Ioana Suciu, Ruxandra Bartok, Elena S. Preoteasa, B. Dimitriu, E.A. Preoteasa,

B. Constantinescu, Daniela Stan, Georgiana Moldoveanu, Ileana Ionescu, Dana Cristina

Bodnar, Rom. J. Phys. 60(9–10), 1483–1489, 2015.

17. I.A. Belío-Reyes, L. Bucio, E. Cruz-Chavez, J. Endod. 35(6), 875–878, 2009.

18. A.H. Borges, O.A. Guedes, M.C.G.O. Dorilêo, M.C. Bandeca, C.R.A. Estrela, C. Estrela,

OHDM 13(4), 1007–1012, 2014.

19. M.G. de Oliveira, C.B. Xavier, F.F. Demarco, A.L.B. Pinheiro, A.T. Costa, D.H. Pozza, Brazil.

Dent. J. 18(1), 3–7, 2007.

20. T. Dammaschke, H.U. Gerth, H. Züchner, E. Schäfer. Dent. Mater. 21(8), 731–738, 2005.

21. J.S. Song, F.K. Mante, W.J. Romanow, S. Kim, Oral Surg. Oral Med. Oral Pathol. Oral

Radiol. Endod. 102(6), 809–815, 2006.

22. L. Grech, B. Mallia, J. Camilleri, Int. Endod. J. 46(7), 632–641, 2013.

23. M.G. Gandolfi, F. Siboni, C.M. Primus, C. Prati, J. Endod. 40(10), 1632–1637, 2014.

24. L. Han, T. Okiji, Int. Endod. J. 44(12), 1081–1087, 2011.

25. K.Y. Kum, E.C. Kim, Y.J. Yoo, Q. Zhu, K. Safavi, K.S. Bae, S.W. Chang, Int. Endod. J. 39(4),

497–500, 2013.

26. R.G. Craig, Restorative Dental Materials, Mosby-Year Book Inc., 1997.

27. P. Hunt, Glass-ionomers: The Next Generation, Philadelphia, 1994.

28. H.E. Sennou, A.A. Lebugle, G. Grégoire, Dental Materials 15, 229–237, 1999.

29. H.C. Ngo, G. Mount, J. Mc Intyre, J. Tuisuva, R.J. Von Doussa, J. Dentistry 34, 608–613,

2006.

30. P.Z. Gao, S. Matsuya, M. Ohta, J.Z. Zhang, Dent. Mater. 16, 170–179, 1997.

31. H. Forss, J. Dent. Res. 72, 1257–1262, 1993.

32. B. Czarnecka, J.W. Nicholson, J. Dentistry 34, 539–543, 2006.

33. V. Valkovic, Analysis of Biological Material for Trace Elements Using X-Ray Spectroscopy.

CRC Press Inc., Boca Raton, Florida, 1980.

34. M. Thellier, C. Ripoll, C. Quintana, F. Sommer, P. Chevalier, J. Dainty, Meth. Enzymol. 227,

535–586, 1993.

35. J. L. Campbell, J. A. Cookson, Nucl. Instr. Meth. Phys. Res. B3, 185–197, 1984.

36. S.A.E. Johansson, J.L. Campbell, K.G. Malmqvist, editors. Particle-Induced X-ray Emission

Spectrometry (PIXE). New York – Chichester – Brisbane – Toronto – Singapore: John Wiley &

Sons; 1995.


37. R. Jenkins, An introduction to X-ray Fluorescence Spectrometry. London – New York –

Rheine: Heyden; 1976.

38. S. Gomez, E.A. Preoteasa, L. Harangus, A. Iordan, D. Grambole, F. Herrmann, Nucl. Instr.

Meth. Phys. Res. B 249, 673–676, 2006.

39. N.G. West, Trends Anal. Chem. 3, 199–204, 1984.

40. E.P. Bertin, Principles and Practice of X-ray Spectrochemical Analysis, Plenum Press, New

York – London, 1978.

41. Dentsply International Inc, York, PA, AH Plus Product information.

42. G.J. Mount, W.R. Hume (eds.), Preservation and Restoration of Tooth Structure, Mosby

International Ltd., 1998 [Romanian transl. 1999].

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