Abstract
Objectives
Ethoxylated bisphenol A dimethacrylate (bisEMA) is a basis monomer in several dental resin composites. It was the aim of the present study to develop a method allowing detection of bisEMA and its different degrees of ethoxylation eluted from polymerized resin composites.
Methods
High-temperature gas chromatography/mass spectrometry (HT-GC/MS) by direct on-column injection was used to identify ethoxylated bisEMA in ethanol/water (3:1) eluates from polymerized specimen of four bulk-fill resin composites – Venus ® bulk fill, Surefil ® SDR™ flow, Filtek™ Bulk Fill and Sonic Fill™. Additionally, the unpolymerised pastes were analysed.
Results
The developed method allowed identification of a homologous series of bisEMA up to twelve ethoxy groups in the unpolymerised materials. The molecular masses of the homologous bisEMA varied between 452 g/mol and 892 g/mol and were detected for retention times from 9.43 min to 13.36 min. Analysis of eluates from polymerised materials identified bisEMA monomers with less than 6 ethoxy groups. Chromatograms showed larger peak areas for the lower volatile bisEMA with 4–6 ethoxy groups compared with higher volatile bisEMA with 2 or 3 ethoxy groups, thus indicating that the amounts of these homologues in the pastes were higher.
Significance
Ethoxylated bisEMA with up to twelve ethoxy groups can be identified by HT-GC/MS. In all eluates bisEMA was found. The higher the number of ethoxy groups the lower are the peak areas from bisEMA in the gas chromatogram. These findings may be significant for toxicological analysis of resin-composites incorporating bis-EMA.
1
Introduction
The chemical composition of dental restoratives influences the material properties, the biocompatibility and the indication of use and is closely related to the identity and quantities of released substances . Identifying and quantifying these substances allows for the assessment of the toxicological risk of dental restoratives. However, this cannot be solved by a single analytical method and represents a challenge for analytical chemistry with its extensive range of preparation and measuring methods .
Common monomers released from resin based dental materials are bisphenol A glycidyldimethacrylate (bisGMA), ethoxylated bisphenol A dimethacrylate (bisEMA; bisphenol A ethoxylate dimethacrylate), urethane dimethacrylate (UDMA), triethylene glycol dimethacrylate (TEGDMA), or 2-hydroxyethyl methacrylate (HEMA) . But also substances such as 2-hydroxy-4-methoxy benzophenone (HMBP), camphorquinone (CQ) or impurities from bisGMA synthesis such as triphenyl phosphane (TPP) were identified in eluates from polymerized specimen . Moreover, ions such as Cu 2+ , Fe 2+ , Zn 2+ , Ca 2+ , Sr 2+ , Al 3+ and F − can be released from resins, glass ionomer or glass polyalkenoate cements . To identify all these compounds, different analytical methods are needed. Larger molecular size monomers, such as bisGMA, UDMA or bisEMA, are well detected by high-performance liquid chromatography (HPLC) or HPLC mass spectrometry (HPLC/MS; the method is also called liquid chromatography/mass spectrometry (LC/MS)). After derivatisation of bisEMA, the different degrees of ethoxylation can be also be separated and determined by capillary zone electrophoresis (CZE) . Due to its complexity this method is less used in the analysis of dental resin composites . The main deficiency of this method compared with LC/MS or gas chromatography/mass spectrometry (GC/MS) techniques is the need for intermediate preparation steps of the eluates, prior to analysis of lower molecular size monomers, such as TEGDMA, HEMA, HMBP, CQ or TPP with detection and identification by GC/MS. For identifying metal ions, such as Zn 2+ , Cu 2+ , Al 3+ and Ca 2+ , atomic spectroscopy techniques, inductively coupled plasma mass spectrometry (ICP-MS) or some photometric techniques after complexation to a colored complex, are needed. However, it would be desirable to have one technique to identify and quantify all the substances in the eluates from dental restoratives.
The importance of GC/MS in detecting elutable and toxicological relevant substances is steadily increasing. The method allows detection of a variety of the elutable and toxicological relevant substances (an overview is given in ). But it was not possible until now to establish an analytical procedure to detect low volatile monomers like bisEMA ( Fig. 1 and Table 1 ) and its different degrees of ethoxylation. However, their identification is important since they have been shown to have a cyto- and genotoxic potential . The regular GC/MS methods, described so far in the dental literature for identifying larger and low volatile methacrylate based monomers, include the insertion of the eluates in the injector, before reaching the column. The drawback of this method is that these molecules are only detectable as fragments and not as parent molecules, leaving doubts in the interpretation of the measured data. Moreover, the detection of a molecule as fragments does not allow a quantification of the intact substance. Therefore, methods for detection of parent molecules are required. This might be achieved by impeding the fragmentation of the molecules prior to reaching the column. The techniques to avoid a fragmentation include either a direct introduction of the substances on the column or derivatisation of the parent molecule. The last method includes an additional preparation step and must consequently take into account reduced detection sensitivity due to incomplete derivatisation of the initial molecules . Moreover, the derivatisation reaction might involve, besides the target molecules of interest, reaction with other low volatile molecules present in the eluates, making the interpretation of the chromatograms more difficult.
| Materials | Manufacturer color, batch | Resin matrix | Filler | Filler wt./vol. |
|---|---|---|---|---|
| Venus ® bulk fill | Kulzer, Universal 010030 | UDMA, bisEMA | Ba–Al–F–Si–glass and SiO 2 | 65/38 |
| Surefil ® SDR ® flow | Dentsply Caulk, Universal, 01082 | Modified UDMA, TEGDMA, bisEMA | Ba–Al–F–B–Si–glass and St–Al–F–Si–glass | 68/44 |
| Filtek™ Bulk Fill | 3M-ESPE, A3 N414680 | bisGMA,UDMA, bisEMA, Procrylat resins | ZrO 2 /SiO 2 , YbF 3 | 64.5/42.5 |
| Sonic Fill™ | Kerr, A3 3815160 | bisGMA, TEGDMA, bisEMA | SiO 2 , Glass, oxide | 83.5/n.s |
Therefore the aim of the present study was to develop a new GC/MS method allowing for a direct introduction of the eluated substances on the column, by eliminating the risk of fragmentation in the injector and thus the identification of bisEMA as a parent molecule. Additionally, the method should also be able to separate and identify the different degrees of ethoxylation in bisEMA.
2
Methods
2.1
Specimen preparation
Three low-viscosity and one high viscosity bulk-fill resin composites, known to contain bisEMA, were analysed ( Table 1 ). Cylindrical specimens were prepared in Teflon molds of 4 mm height and 3 mm diameter ( n = 6 from each material). The molds were filled in one (bulk) increment, covered by a thin glass slide and cured by applying a high irradiance light curing unit (Bluephase 20i, High power mode, 20s, Ivoclar Vivadent, Schaan, Liechtenstein) placed directly on the specimen surface. Specimens were incubated 5 min after initiating the polymerisation in an ethanol/water (3:1) solution at 37 °C for 24 h (Ethanol absolute EMSURE ® , water for chromatography LiChrosolv ® ; both solvents were obtained from Merck, Darmstadt, Germany). Each aliquot was analyzed with a gas chromatography unit, coupled with mass spectrometry (GC/MS). Moreover from each uncured material paste 100 mg were dissolved in 1000 ml ethanol/water (3:1) and measured ( n = 6).
2.2
GC/MS analysis
The analysis of the eluates was performed on a Trace GC 1310 gas chromatograph connected to an ISQ mass spectrometer (Thermo Fisher Scientific, Dreieich, Germany). A MXT ® -HT SimDist capillary column (length 15 m, inner diameter 0.25 mm, coating 0.1 μm; Restek Corporation, Bellefonte, PA, USA) and MXT ® Retention Gap (length 5 m, inner diameter 0.53 mm, Restek Corporation) was used as the capillary column for GC separations. The GC oven was heated from 50 °C (1 min isotherm) to 400 °C (1 min isotherm) with a rate of 30 °C/min and 1.0 μl of the solution was injected directly on-column. Helium was used as carrier gas at a constant flow rate of 1.5 ml/min. The aliquot from the eluate was directly applied on the column by a programmed temperature vaporization (PTV) injector system at 50 °C. The temperature of the direct coupling (transferline) from the GC to the mass spectrometer was 350 °C. The MS was operated in electron ionisation mode (EI, 70 eV), with the ion source operated at 350 °C; only positive ions were scanned. Scans were made over the range m / z 40–1100 at a scan rate of 3 scan/s for scans operated in full scan mode for identification and quantitation of the analytes. In the single ion mode (SIM) targeted analysis of the bisEMA homologues was performed for the following m / z : 69, 113, 437, 481, 525, 563, 569, 613, 657, 701, 745, 789, 833 and 877.
The integration of the chromatograms was carried out over the base peak or other characteristic mass peaks of the compounds, typically M + and M + -15. Identification of the various substances was achieved by interpretation of their fragmentation patterns .
For each degree of ethoxylation of bisEMA, the ratio between the area of the molecule specific mass peak detected in eluates of the polymerised and unpolymerised material was calculated and presented in parts per mille (0/00).
<SPAN role=presentation tabIndex=0 id=MathJax-Element-1-Frame class=MathJax style="POSITION: relative" data-mathml='x=AreabisEMA(i)polymerisedAreabisEMA(i)unpolymerised*1000(0/00)’>x=AreabisEMA(i)polymerisedAreabisEMA(i)unpolymerised*1000(0/00)x=AreabisEMA(i)polymerisedAreabisEMA(i)unpolymerised*1000(0/00)
x = Area bisEMA ( i ) polymerised Area bisEMA ( i ) unpolymerised * 1000 ( 0 / 00 )
2.3
Calculations and statistics
The results are presented as means (standard deviation) (SD).
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