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Application note Received: 29 December 2015

Revised: 1 February 2016

Accepted: 7 February 2016

Published online in Wiley Online Library

(wileyonlinelibrary.com) DOI 10.1002/jms.3757

Detection of polydimethylsiloxanes transferred from silicone-coated parchment paper to baked goods using direct analysis in real time mass spectrometry Andreas Jakob,a Elizabeth A. Crawfordb and Jürgen H. Grossc* The non-stick properties of parchment papers are achieved by polydimethylsiloxane (PDMS) coatings. During baking, PDMS can thus be extracted from the silicone-coated parchment into the baked goods. Positive-ion direct analysis in real time (DART) mass spectrometry (MS) is highly efficient for the analysis of PDMS. A DART-SVP source was coupled to a quadrupole-time-of-flight mass spectrometer to detect PDMS on the contact surface of baked goods after use of silicone-coated parchment papers. DART spectra from the bottom surface of baked cookies and pizzas exhibited signals because of PDMS ions of the general formula [(C2H6SiO)n + NH4]+ in the m/z 800–1900 range. Copyright © 2016 John Wiley & Sons, Ltd. Additional supporting information may be found in the online version of this article at the publisher’s web site. Keywords: direct analysis in real time; DART; polydimethylsiloxanes; PDMS; parchment paper; extraction; baking; cookies; pizzas; accurate mass; quadrupole-time-of-flight mass spectrometer; mass spectrometry

Introduction [1]

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Direct analysis in real time mass spectrometry (DART-MS) provides a quick and facile tool to analyze a multitude of types of samples.[2–6] DART is an ambient desorption/ionization technique[7–9] that provides soft ionization and, thus, results in a low level of fragmentation, if any. Accurate mass data to derive ionic formulas of the analytes is therefore advantageous as it compensates for the lack of fragment ion information in DART mass spectra. Thus, DART has, from the beginning until present, been frequently coupled with high-resolving time-of-flight (TOF)[1,10] and quadrupole-time-of-flight (Q-TOF) instruments.[11,12] DART has also been used in combination with Orbitrap analyzers[13,14] and with Fourier transform ion cyclotron resonance instruments.[15–19] DART-MS is established in food quality and safety analysis.[4] It has, for example, been used to analyze polycyclic aromatic hydrocarbon and polychlorinated biphenyl contaminations in shrimps and fish,[20] to detect aflatoxin in milk,[21] and to observe the formation of acrylamide in biscuits during baking.[22] Positive-ion DART-MS is well-suited for the analysis of polydimethylsiloxane (PDMS) up to about m/z 3000[23] that can also be employed for mass calibration.[16] Further, positive-ion DART-MS can efficiently be applied to analyze PDMS on the surfaces of silicone rubber products of daily use[19] and to track the release of PDMS oligomers from silicone rubber molds into baked goods under the conditions of common kitchen use.[24] Non-stick parchment papers – an everyday aid in hundreds of millions of households – all have in common that they (1) advertise non-stick properties because of coatings, (2) are certified for use up to 220 °C, (3) are designed for multiple use and (4) suggest being environmentally friendly because of use of non-bleached paper and suitability for composting. The non-stick properties of

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parchment papers are generally achieved by coating them with PDMS. As silicones can conveniently be analyzed by DARTMS[19,24,25], this study addresses the question as to whether PDMS is extracted from silicone-coated parchment papers into baked goods during the process of baking. Here, we describe the DART mass spectra of silicone-coated parchment papers and present results concerning the transfer of PDMS onto cookies, dough and pre-baked frozen foods upon baking on the surveyed parchment papers.

Experimental Mass spectrometer A Bruker Impact II mass spectrometer (Bruker Daltonik GmbH, Bremen, Germany), a Q-TOF hybrid instrument was used. The instrument was controlled by the Bruker Compass 1.9 for OTOF Series software, OTOFControl 4.0, and data analysis was performed using the Bruker Compass DataAnalysis software 4.4 (x64).

* Correspondence to: Jürgen H. Gross, Institute of Organic Chemistry, Heidelberg University, Im Neuenheimer Feld 270, 69120 Heidelberg, Germany. E-mail: [email protected] a Bruker Daltonik GmbH, Fahrenheitstraße 4, 28359, Bremen, Germany b Institute of Bioanalytical Chemistry, Saarland University, Campus B2 2, 66123, Saarbrücken, Germany c Institute of Organic Chemistry, Heidelberg University, Im Neuenheimer Feld 270, 69120, Heidelberg, Germany

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DART detects silicone on food baked on parchment Final averaged spectra in the range m/z 50–3000 were acquired by adding 16 individual summed spectra each consisting of 6779 single spectra accumulated in 1-s acquisition time. Detailed instrument settings: end plate offset: 0 V, capillary: 0 V, nebulizer: 0 bar, dry gas: 4.0 l min–1, dry temperature 50 °C; funnel 1: RF 150 Vpp, funnel 2: RF 200 Vpp, isCID: 0 eV, hexapole RF: 50 Vpp, ion energy: 4 eV, low mass: 90 m/z, collision energy: 7 eV, collision RF: 650 Vpp, transfer time: 80 μs, pre-pulse storage: 5 μs. DART analyses A DART-SVP ionization source (IonSense Inc., Saugus, MA) was mounted on the electrospray ionization (ESI) interface of the ion source via the Vapur Interface (IonSense, Inc., Saugus, MA) that provides an additional pumping stage at the MS inlet. The DART source was positioned at an angle of 40° relative to the axis of the ceramic inlet tube of the Vapur Interface. The DART source was positioned at the 23 mm marking on the sliding y-axis rail, and the ionization source was shifted vertically to the upmost position (70 mm, Supporting information, Fig. S1). The distance from the DART source exit to the end of the ceramic inlet tube of the Vapur Interface was fixed at 10 mm. Internal mass calibration in positive-ion DART mode was established on omnipresent background ions starting from m/z 57.0698, [C4H9]+, as the lowest mass and, by inclusion of some silicone background ions, going up to m/z 758.2217, [(C2H6SiO)10 + NH4]+.[16] The m/z 700–1900 range, which was of highest relevance for this study, was nonetheless free of silicone background ions and mass calibrated by extrapolation. Pieces of parchment paper (ca. 2 × 3 cm) were positioned in the ionization zone using tweezers. Baked foods (pieces of ca. 2–3 cm2 from the bottom) were placed on a stainless steel teaspoon and positioned manually about halfway between helium exit and capillary entrance to expose them to the ionizing gas. For best sensitivity, the DART helium gas was set to 350 °C during run mode and the gas flow was switched to nitrogen during the standby mode in between measurements. The grid voltage at the exit of the ion source was held at +350 V. Repeated measurements of the same object showed acceptable variation in intensity and m/z distribution of the signals. Samples were measured in triplicate to ensure that the data were representative of the respective parchment paper or food sample. Typically, variations in PDMS peak intensities were on the order of 10% on average in the case of parchment papers and on the order of 20–30% in the case of the less homogeneous food samples. Parchment paper and food samples Five brands of non-stick parchment paper (Table 1), cookies, and all of the ingredients of a simple dough were obtained from local supermarkets. Food samples were either baked directly on the heating plate of a magnetic stirrer (IKA RH basic) to obtain a blank reference or on different silicone-coated parchment papers. The

temperature of the heating plate was regularly controlled using an infrared thermometer (Fluke Deutschland GmbH, Glottertal, Germany). During baking, the food samples were covered with a compact stainless steel hood. (1) Commercially available cookies (Walkers Shortbread Ltd., Aberlour House, Aberlour on Spey, GB) were heated to 160 °C for 5–10 min. (2) A spritz biscuits dough was prepared in a stainless steel bowl (KitchenAid, Greenville, OH) from wheat flower (250 g), sugar (100 g), margarine (150 g), eggs (one piece) and a tablespoon of baking soda. The dough ingredients were stirred (ca. 5 min) and stored in a bowl. The dough was baked at 180 °C for 15–20 min. (3) Frozen food (mini pizzas, brand: Wagner Piccolinis, Nestlé GmbH, Frankfurt, Germany) was allowed to thaw inside the packaging material to avoid ice crystals on the surface and baked at 200 °C for 15–20 min.

Results and discussion DART spectra of parchment papers Five silicone-coated parchment papers were arbitrarily selected from local stores (Table 1). First, the positive-ion DART mass spectra of these parchment papers were acquired to characterize the parchment coatings. For compactness, we focus on the silicone components relevant for this study and discuss the general appearance of this group in the DART spectra of 1_Paclan parchment as an example (Fig. 1). A compilation of the DART spectra of all five parchment papers is provided in the Supporting information (Figs. S2–S6). In the case of 1_Paclan parchment paper, the series of PDMS ions begins to show noteworthy intensity with the 11mer at m/z 832.2398, [(C2H6SiO)11 + NH4]+ (calc. m/z 832.2405) (Fig. 1(a)). The monoisotopic ions belonging to this series up to the 27mer, m/z 2016.5434, [(C2H6SiO)27 + NH4]+ (calc. m/z 2016.5412), are all listed in Table 2 along with the respective calculated m/z values and mass errors. As expected for PDMS, the peaks were spaced at Δm/z = 74.0185 on average, which is characteristic of the [(CH3)2SiO] repeat unit of PDMS (Fig. 1(b)). The most abundant PDMS ions in positive-ion DART-MS are formed as ammonium adducts and correspond to a series of the general formula [(C2H6SiO)n + NH4]+ yielding peaks at nominal m/z = (n × 74) + 18. These ions do not necessarily represent intact PDMS molecules.[16,19,23] The isotopic patterns also reveal the presence of multiple Si atoms. For example, the signal at m/z 1202.3332 is shown in expanded view and compared with the isotopic pattern calculated for [(C2H6SiO)16 + NH4]+ (insert Fig. 1(c)). The general appearance of the spectrum and the signals listed in Table 2 are in full accordance with previous DART measurements of PDMS samples[16,19,24] and, thus, do not require a more detailed examination to prove their identity. Typically, the DART spectra of the five parchment papers show the maximum intensity of PDMS ion peaks in the m/z 1000–1300 range (Supporting information, Figs. S2–S6). Obviously, the DART spectra of these parchment papers exhibit close similarity in terms

Table 1. Parchment papers analyzed No.

Paclan Backpapier Toppits Back-Bögen Selex Selection Backpapier-Zuschnitte alio Backpapierzuschnitte ja! Backpapier-Zuschnitte

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Abbreviated name 1_Paclan 2_Toppits 3_Selection 4_Alio 5_Ja

Supplier CeDo S.A.S., Palaisseau, France Cofresco Frischhalteprodukte GmbH & Co. KG, Minden, Germany Zentrale Handelsgesellschaft-ZHG-mbH, Offenburg, Germany Metsä Tissue GmbH, Düren, Germany REWE Markt GmbH, Köln, Germany

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1 2 3 4 5

Parchment brand

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Figure 1. Positive-ion DART mass spectrum of 1_Paclan parchment in the m/z 700–1900 range. (a) The series of PDMS ions begins at m/z 832.2398 and proceeds beyond m/z 1800; they are marked with arrows. (b) Peaks are spaced at Δm/z = 74.0185 because of the C2H6SiO repeat unit of PDMS. (c) The expanded view shows the isotopic pattern of the 16mer, and the insert allows for comparison of measured spectrum and calculated isotopic pattern of + + [(C2H6SiO)16 + NH4] , i.e. [C32H100NO16Si16] .

Table 2. Identification of ionic formulas of polydimethylsiloxanes (PDMS) from the surface of parchment paper 1_Paclan by DART-Q-TOF-MS Measured m/z 832.2398 906.2583 980.2771 1054.2958 1128.3145 1202.3332 1276.3519 1350.3705 1424.3893 1498.4083 1572.4271 1646.4461 1720.4653 1794.4850 1868.5048 1942.5248 2016.5434

Ionic formula

Compound group +

[(C2H6SiO)11 + NH4] + [(C2H6SiO)12 + NH4] + [(C2H6SiO)13 + NH4] + [(C2H6SiO)14 + NH4] + [(C2H6SiO)15 + NH4] + [(C2H6SiO)16 + NH4] + [(C2H6SiO)17 + NH4] + [(C2H6SiO)18 + NH4] + [(C2H6SiO)19 + NH4] + [(C2H6SiO)20 + NH4] + [(C2H6SiO)21 + NH4] + [(C2H6SiO)22 + NH4] + [(C2H6SiO)23 + NH4] + [(C2H6SiO)24 + NH4] + [(C2H6SiO)25 + NH4] + [(C2H6SiO)26 + NH4] + [(C2H6SiO)27 + NH4]

PDMS PDMS PDMS PDMS PDMS PDMS PDMS PDMS PDMS PDMS PDMS PDMS PDMS PDMS PDMS PDMS PDMS

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of average mass and m/z range of PDMS ions and show only some variance in overall intensity of the corresponding PDMS signals. While the spectrum of 1_Paclan shows signals up to 2.3 × 105 counts, that of 2_Toppits reaches the highest value among those examined of 5.1 × 105 counts, and 5_Ja has the lowest at 1.2 × 105 counts. The intensities stated here are averages of three repeat measurements per parchment paper that typically showed variations on the order of 10% on average. It may thus be inferred that the selected parchment papers are typical representatives of this

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Calculated m/z 832.2405 906.2593 980.2781 1054.2969 1128.3157 1202.3345 1276.3533 1350.3721 1424.3908 1498.4096 1572.4284 1646.4472 1720.4660 1794.4848 1868.5036 1942.5224 2016.5412

Error [mu] 0.7 1.0 1.0 1.1 1.2 1.3 1.4 1.6 1.6 1.3 1.3 1.1 0.7 0.2 1.2 2.4 2.2

Error [ppm] 0.9 1.1 1.0 1.0 1.1 1.1 1.1 1.2 1.1 0.9 0.9 0.7 0.4 0.1 0.6 1.2 1.1

class of products. The price of the product is coarsely reflected by the amount of silicone coating, i.e. more expensive brands tend to show more intense peaks. DART spectra of shortbread Based on the results from the previous section, 2_Toppits and 3_Selection were chosen to perform a simple preliminary experiment. Shortbread was baked at 160 °C for 10 min on the cleaned

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DART detects silicone on food baked on parchment bare heating plate (blank) and then on each of the two selected parchment papers. Analogous to results from a previous study,[24] the spectrum from the undersurface of this cookie blank in the m/z 450–1900 range mainly showed signals because of [M + NH4]+ ions of low-mass triacylglycerols (TAGs) from butter,[26–28] a major ingredient of shortbread (Fig. 2). Among these, the most intense

peaks were correlated to [C29H58NO6]+, m/z 516.4256 (calc. 516.4258) [C31H62NO6]+, m/z 544.4568 (calc. 544.4572) [C33H66NO6]+ and m/z 572.4879 (calc. 572.4885). In addition, higher mass TAGs and cluster ions of the general composition [2M + NH4]+ were observed. This positive-ion DART spectrum coincides in its general appearance with one of cow milk fats in an independent

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Figure 2. Positive-ion DART spectra of shortbread undersurfaces after baking. (a) Spectrum of the blank sample in the m/z 700–1900 range, (b) expanded view of m/z 450–1100 showing the TAG ions in detail and (c–e) further expanded to m/z 1195–1220. Spectrum (c) is taken from the blank shown in (a). Spectra (d) and (e) are obtained after baking on 2_Toppits and 3_Selection, respectively. The specific mass defects of TAG and PDMS enable good separation of peaks because of PDMS ions from those by TAGs (marked with arrows).

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publication.[29] Clearly, the section of the spectrum where the most abundant PDMS ions would be expected only shows TAG-related peaks (Fig. 2(c)). After heating the shortbread on parchment papers 2_Toppits and 3_Selection, the appearance of the spectra changes in those PDMS ions are observed in addition to TAG ions. Comparison of Fig. 2(c–e) reveals the difference. Even though the signal at m/z 1202.3321, [(C2H6SiO)16 + NH4]+ (calc. m/z 1202.3345), is rather weak in spectra d and e, the resolving power of the instrument is nonetheless sufficient to clearly separate it and the corresponding isotopic peaks from the TAG background, and the mass accuracy allows to assign its formula based on the measured accurate mass. In both spectra d and e, the complete isotopic pattern of the PDMS 16mer is fully visible and separated from TAG ions by about 0.3 u difference in mass defect between TAG and PDMS 16mer at about m/z 1202. This provides first evidence that PDMS can be extracted from parchment paper and transferred on food during baking even though the contact between a piece of shortbread and the parchment paper is not very intense. DART spectra of freshly baked dough A soft dough, in contrast to the pre-formed shortbread biscuit, comes into much more intimate contact with the parchment paper. During baking, fat from the dough may soak into the parchment and, in this way, assist in the extraction of silicone from the parchment into the baked good. Thus, a spritz biscuit dough was baked at 180 °C for 15–20 min on either the cleaned blank heating plate or on the selected parchment papers. After

baking, the undersurfaces of the baked biscuits were directly subjected to DART-MS. While an increase in PDMS was expected, the amount of PDMS on the freshly baked biscuits was still surprising as the intensity of PDMS ions even exceeded those of the TAGs (Fig. 3). It should be kept in mind, however, that the DART spectrum only approximately reflects the relative amounts of TAGs and PDMS at the surface, while it does not represent the concentration of PDMS of the entire biscuit. One may assume that some enrichment of PDMS on the crust did occur as was the case when muffins had been baked in silicone rubber molds.[24] The positive-ion DART spectrum of freshly baked biscuit blank undersurfaces only shows TAG ions in the m/z 1195–1220 range. In contrast, spectra b and c obtained after baking on 2_Toppits and 3_Selection, respectively, are dominated by PDMS ion signals. The PDMS isotopic pattern signals caused by 16mer ions from m/z 1202.3358 to m/z 1207.3339 were of higher (Fig. 3(b)) or at least the same intensity as those because of TAGs. While this discussion for simplicity focuses on the signals of the 16mer, the full spectrum of the biscuit baked on 2_Toppits also reveals a series of signals by silicones having shorter and longer chains, respectively (Supporting information, Fig. S7). It can be noted that the peaks of the isotopic pattern of [(C2H6SiO)16 + NH4]+ are sharp and symmetric whereas peaks related to TAG ions are broadened and non-symmetric. We suppose that this broadening is caused by superimposition of isobaric TAG ions, e.g. [M + NH4]+ of TAGs with long-chain fatty acids, [2TAG + NH4]+ cluster ions of short-chain fatty acids, superimposition of isotopic peaks with those of TAGs of a lower degree of unsaturation. In contrast, each PDMS group of signals is strictly

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Figure 3. Positive-ion DART spectra of freshly baked biscuit undersurfaces in the m/z 1195–1220 range. (a) Spectrum of the blank sample. Spectra (b) and (c) are obtained after baking on 2_Toppits and 3_Selection, respectively. The PDMS isotopic pattern from ions ranging from m/z 1202.3358 to m/z 1207.3339 dominates the spectra. PDMS ions are well separated from TAGs and marked with arrows. For ease of comparison and a better estimate of the intensity of the PDMS ions across the samples, the spectra are uniformly scaled to an intensity of 7000 counts.

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DART detects silicone on food baked on parchment

Figure 4. Positive-ion DART spectra of mini pizza (Wagner Piccolinis) undersurfaces after baking. (a) Spectrum of the blank sample in the m/z 1195–1220 range and (b) spectrum after baking on 3_Selection. The additional peaks because of PDMS ions are well separated from TAGs and marked with arrows.

related to one defined oligomer. Here, this difference in appearance helps to quickly identify PDMS signals among TAGs. Mini pizzas In order to include a different type of food sample, deep-frozen mini pizzas (Wagner Piccolinis) were also examined. As may be deduced from the previous sections, the relative intensity of PDMS ion signals was found between that of the shortbread and that of the freshly baked biscuits (Fig. 4). The lower PDMS uptake as compared with the freshly baked dough can be attributed to the reduced contact surface because of the crusty nature of this prebaked type of food. Nonetheless, there is a substantial transfer of PDMS from the silicone-coated parchment paper onto the food. As stated before, the corresponding peaks are clearly separated from the TAG matrix and identified by the accurate mass data that is within a 2 ppm error between experimental and calculated masses. The full spectrum of the mini pizza baked on 3_Selection parchment paper also shows the entire series of silicone ions (Supporting information, Fig. S7).

Conclusions

Acknowledgments E. A. Crawford would like to thank Prof. Dr. D. A. Volmer for his permission to cooperate in this project and Bruker Daltonik GmbH for the generous travel support. A. Jakob thanks his mother C. Jakob for the recipe of the spritz biscuit dough.

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It has been demonstrated in this work that the transfer of PDMS oligomers is not just a phenomenon unique to silicone rubber baking molds but appears to represent a general characteristic of this compound class. Leaching of silicone from coated parchment paper becomes more notable when baking is performed at higher temperature and for longer duration. Also, the fat content of the baked food and the intensity of the contact between the food and parchment paper play a role. While PDMS oligomers are extracted from the parchment paper causing its depletion in

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silicone, the baked goods are concomitantly enriched. Provided that omission of parchment paper is not an option, from the consumer point of view, multiple uses of the parchment paper as suggested by some manufacturers appear in fact the best approach to reduce the overall uptake of PDMS from this source. We are well aware of the limitations of this simple analytical approach, in particular in terms of quantitative aspects; however, at the same time, we are convinced that the main findings of this work are solid and that this method would be suitable as a rapid screening technique. It is suggested that officially recognized food safety and consumer product laboratories undertake more detailed analyses of the amount and potential health risk of this PDMS leaching into food.

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Supporting Information Additional supporting information may be found in the online version of this article at the publisher’s web site.

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