Characterization of Fluorinated Polymers by Atmospheric-Solid

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Characterization of Fluorinated Polymers by Atmospheric Solid Analysis Probe High Resolution Mass Spectrometry (ASAP/HRMS) Combined with Kendrick Mass Defect Analysis Gabriel Gaiffe, Richard B. Cole, Sabrina Lacpatia, and Maxime Cyril Bridoux Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.7b05116 • Publication Date (Web): 18 Apr 2018 Downloaded from http://pubs.acs.org on April 18, 2018

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Analytical Chemistry

Characterization of Fluorinated Polymers by Atmospheric Solid Analysis Probe High Resolution Mass Spectrometry (ASAP/HRMS) Combined with Kendrick Mass Defect Analysis Gabriel Gaiffe 1,3, Richard B. Cole1, Sabrina Lacpatia2, Maxime C. Bridoux3* 1

Sorbonne Universités, UPMC Univ Paris 06, CNRS, Institut Parisien de Chimie Moléculaire (IPCM), 4 place Jussieu 75252 Paris Cedex 05 France 2

Laboratoire Central de la Préfecture de Police, 39 bis rue de Dantzig 75015 Paris

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CEA, DAM, DIF, F-91297 Arpajon, France

Corresponding author: *Maxime C. Bridoux, CEA, DAM, DIF, F-91297 Arpajon, France Ph: +33169266743, email: [email protected]

Graphical abstract ASAP-Orbitrap

Kendrick Mass Defect

ABSTRACT

Fluorinated polymers are a diverse and important class of polymers with unique applications. However, characterization of fluorinated polymers by conventional mass spectrometric methods is challenging because: i) the high fluorine content makes them insoluble or only sparingly soluble in most common solvents; and ii) commonly used matrices employed for MALDI do not desorb/ionize them efficiently. In this work, Atmospheric Solid Analysis Probe (ASAP) high resolution OrbitrapTM mass spectrometry (HRMS) was used as a new tool for the molecular characterization of various fluorinated polymers including polyvinylidene fluoride (PVDF), and fluorinated copolymers containing PVDF and chlorotrifluoroethylene (KEL-F 800), or PVDF and hexafluoropropylene (Viton A, Tecnoflon). The major peaks of the observed distributions were assigned a composition, but the high number of species requires the use of an alternative method to treat such complex data. Kendrick mass defects (KMD) were calculated based on the “common-to-all” vinylidene difluoride repeating unit. By plotting the KMD as a function of nominal Kendrick mass (NKM), specific patterns based on homologous series emerged. Kendrick maps were

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therefore drawn to simplify the mass spectra and provide confident peak assignments through homologous series recognition. A specific fingerprint for each polymer has been identified and the ability to discern the four species present in a blend through KMD analysis was demonstrated.

INTRODUCTION The development of rapid, efficient and reliable detection methods for the characterization of explosives is of high importance to security forces concerned with terrorist threats. Typically, an explosive device contains an explosive charge embedded in a complex matrix of binders and plasticizers.1 Since the discovery of nitrocellulose in the 1850s, diverse classes of polymers have been used as binders and plasticizers in the fabrication of propellants and explosives. Polymers, e.g. fluoropolymers that are chemically inert and exhibit a good thermal stability, can be used to improve malleability and insensitivity characteristics. Polyvinylidene fluoride, chlorotrifluoroethylene, and hexafluoropropylene are examples of fluoropolymers that are used as energetic binders, providing the necessary energy for the explosion. Each class of polymer brings a particular advantage in terms of mechanical properties, weight, and hydrophobicity.2–4 Moreover, copolymers can be synthesized and incorporated to combine the properties of their components. The global characterization of an explosive sample, including both charge and polymer matrix can give significant clues to the identification of its source (geographic origin, manufacturing process, or even lot number5). Indeed, the bulk composition of an explosive sample, if thoroughly analyzed, can yield precious investigative information concerning the modus operandi of the organization which produced the explosive. Mass spectrometry is a powerful technique for the molecular characterization of polymeric samples, due to its ability to generate information on both repeating unit(s) and end groups.6–9 In addition, high resolution mass spectrometry (HRMS) has offered a great improvement in the ability to differentiate compounds having nearly the same m/z values, especially when they are present in complex mixtures such as those found in polymer blends or oils.10,11 However, two issues are still encountered: (i) increasing resolution is required to distinguish isobaric compounds as the m/z value goes higher and (ii) at higher m/z values, the number of reasonable matching molecular formula assignments also increases. To alleviate these issues, data processing methods relying on the identification of chemical families may be implemented, such as Kendrick plots.11,12

Matrix assisted laser desorption ionization (MALDI) and electrospray ionization (ESI) have been widely used in the field of polymer analysis. In recent years, ambient mass spectrometry (MS) techniques, such as desorption electrospray ionization (DESI)- MS, have emerged as potentially useful tools for polymer characterization, because they require little to no sample preparation, they allow analysis directly on the condensed phase, and they are able to provide structural information even for trace level samples.13,14 Another ambient MS technique, i.e., Atmospheric Solids Analysis Probe (ASAP)15 was developed and introduced at about the same time as DESI. In ASAP, the sample is placed directly in the ionization region on a glass capillary, allowing one to carry out analysis on trace level amounts of analytes. A Corona discharge initiates the ionization of ambient gases such as N2, ionizing residual water molecules that, by proton transfer from H3O+, protonate the analyte vaporized in a hot nitrogen stream .16 Although the exact ionization mechanisms for some species are still debated,17–19 ASAP is finding increasing interest because of its speed and its solvent-free operation (thereby eliminating potential concerns over solubility limitations or flow rate optimization). ASAP analysis has shown promising results for polymer analysis, e.g., for the detection of additives in food and food packaging products.20–22 An additional advantage of ASAP over DESI or ESI is that, typically, only singly charged species are formed during ionization, leading to a simpler spectrum as compared to the case of overlapping "envelopes" of multiply charged species. Both the field of forensics23,24 and the pharmaceutical industry25,26 have benefited from the development of ambient ionization MS. Herein, we explore a new method, based on ASAP/HRMS combined with Kendrick mass defect filtering to characterize fluorinated polymers that may be employed as binders in explosive formulations.

EXPERIMENTAL Polymer samples The following polymers: polyvinylidene fluoride (PVDF, Figure S-1a)), KEL-F 800 (copolymer of chlorotrifluoroethylene and vinylidene fluoride, Figure S1b), Viton A and Tecnoflon (copolymers of vinylidene


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fluoride and hexafluoropropylene, Figure S-1c) were supplied by the CEA. A glass capillary was used to hold 50 mg portions of the solid samples that were then mounted on the probe and inserted into the ASAP source without further sample preparation. ASAP ion source conditions An ASAP ion source (M&M Mass Spec, Harbeson, De, USA) interfaced to a LTQ-Orbitrap XL mass spectrometer (Thermo Fisher Scientific, Waltham, MA) was used to acquire all mass spectra. The ASAP settings were: heated N2 auxiliary gas 400 °C; grid electrode voltage 200 V; transfer capillary 275 °C; discharge needle voltage 0.5-4 kV. Mass spectrometry High resolution mass spectrometry experiments were performed on a LTQ-Orbitrap XL mass spectrometer (Thermo Fisher Scientific, Waltham, MA). The linear ion trap mass spectrometer settings were: capillary voltage 30 V; tube lens voltage 100 V; capillary temperature 275 °C. The ion optics were as follows: multiple 1 offset voltage 4.5 V; multiple 2 offset voltage -8.0 V; lens 1 voltage -4.2 V; lens 2 voltage 15.0 V; gate lens voltage -35.0 V; front lens voltage -5.25 V. The mass range typically acquired was m/z 150-2000. The instrument m/z values were calibrated using the manufacturer’s "tune mixture" consisting of caffeine, MRFA and Ultramark 1621 for positive mode, and SDS, sodium taurocholate and Ultramark 1621 for negative mode. Resonant excitation was carried out using collision induced dissociation (CID); the LTQ was set to sum 3 microscans, activation time of 30 ms; normalized collision energies (NCE) were set between 5 and 30% and the precursor ion isolation window was set at 1.0 m/z for all investigated compounds. The ion trap collision cell was supplied with high purity (99.999 %) helium gas. Data analysis was performed using Thermo Xcalibur™ software. Accurate mass measurements were performed at high resolution (resolving power of 60,000 FWMH at m/z 400).

RESULTS AND DISCUSSION ASAP HRMS of PVDF In order to obtain characteristic mass spectral fingerprints, the fluorinated polymers PVDF, Tecnoflon, Viton A and KEL-F 800 were first analyzed individually using the ASAP ion source. This allowed the observation of characteristic spectral features for each class of fluorinated polymer. The positive mode ASAP HRMS full scan spectra of these four fluorinated polymers are

displayed in Figure 1. Each mass spectrum was acquired in about 5 minutes upon introduction of the glass capillary in the ASAP ion source. The ASAP mass spectrum obtained from PVDF is displayed in Figure 1.a. Most ions observed in the m/z 300-2000 range correspond to singly charged protonated molecules. The spectrum is characterized by at least two interlaced distributions, centered on an average m/z of approximately 1200. Pairs of positively charged ions within each distribution are separated by a 128.0249 unit mass difference, corresponding to two PVDF repeat units (C4H4F4). One distribution is centered on m/z 1007.1600, while the other one, less intense, is centered on m/z 1071.1724. This 64.0125 mass unit gap corresponds to one repeat unit of the [C2H2F2] monomer. It should be noted that, while 64 is the most frequent mass gap observed between two peaks, mass shifts corresponding to others series are observed that are separated from the most intense peaks by 14.0157 (CH2), 44.0062 (C2HF), 49.9968 (CF2) or 149.9904 (C3F6) m/z units. The Xcalibur software was used to predict the most likely chemical formula of the most intense ions in the ASAP Orbitrap mass spectrum. Filter parameters were applied on the basis of prior observations regarding the possible repeat units, and on the assumption that the molecules will not contain many unexpected elements. Thus, the formula search criteria included the following: C0-50 H0-100 F0-100 O0-50. Oxygen was included in the atom list because even if none of the studied polymers contain this element, hydroxyl radicals can be produced by secondary source reactions and may react with the desorbed species. The ring and double bond equivalence (DBE) was set to be between -0.5 and 5. The upper limit of the degree of unsaturation was set to a low value based on the supplied information that the identified repeating units were saturated. The most likely chemical formula of the ion of m/z 1007.1600 is C31H27F32+ with an error of 0.397 ppm. Although PVDF is supposed to be saturated, this species has a 2.5 double bond equivalency, as supported by accurate mass measurements (Table S1). Previous studies conducted on PVDF have shown that elimination of hydrogen fluoride occurs readily during thermal degradation, along with chain cleavage; hence the predominance of unsaturated species.27,28 The predominance of unsaturated species, in addition to elimination of HF, may also be the result of ionization efficiency greatly favoring unsaturated species, double bonds being more likely to capture protons. To obtain complementary information, sequential MS 2 experiments were carried out for the protonated oligomers. MS/MS CID experiments are displayed featuring the fragmentation of the peaks at m/z 1007.1610


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(Figure S-3a), 1021.1772 (Figure S-3b) and 1157.1907 (Figure S-3c). The CID MS spectra of these three species produce a series of 20 Da neutral losses from the precursor ions, corresponding to consecutive eliminations of HF. In the fragmentation of both m/z 1007 and 1021 (separated by a CH2 unit) a loss of 182 Da is also observed, producing ions at m/z 825.1641 and m/z 839.1799, respectively. This neutral is assigned as C4HF7, as supported by accurate mass measurements (accurate mass values for CID fragment ions are listed in Table S2). The hypothesis that this C4HF7 loss might result from a HF loss, followed by the loss of C4F6 was tested by performing MS3 on m/z 1007 and 987. The resulting MS3 spectrum is displayed in Fig. S3d, showing the absence of a peak at m/z 825 and discredits this hypothesis. This 182 Da neutral loss was not observed during the fragmentation of m/z 1157 (separated from m/z 1007 by C6H5F3O). Taking into consideration the CID MS/MS experiments, and the fact that the bond dissociation energy of CF2-CF2 is higher than that of CH2-CF2 and CH2-CH2,29 we are able to propose a structure for the ion at m/z 1007 and a fragmentation mechanism in Figure S2. This assumption is based on the observed C4HF7 neutral loss and on the need for an adjacent CH2 moiety for the fragmentation to be energetically favored. The fact that this neutral loss was not observed from the degradation of the m/z 1157 precursor ion (Figure S-3c) leads us to believe that the additional C3F6 group on m/z 1157 is adjacent to the potential leaving group C4HF7; this makes it impossible for the fragmentation to occur following the same pathway as for m/z 1007, due to the high bond dissociation energy required to break a CF2-CF2 bond.

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of MS spectra from various samples including the polymeric species presented here. Kendrick mass defect analysis34 converts m/z values to Kendrick mass values by multiplying the m/z value by the ratio of the nominal mass (14.00000) to the IUPAC exact mass of a CH2 group (14.01565). Then, KMD is determined by subtracting the nominal Kendrick mass (NKM) from KM. However, the Kendrick mass is not limited to the CH2 base, and in fact, any other repeat unit in the data set can be used to generate a Kendrick plot displaying the KMD values against their NKM values. One advantage of this treatment is that oligomeric sequences with the same end-groups will all cluster on a single horizontal line, while oligomers with different endgroups will display different KMD values, thereby facilitating visual distinction of species with differing terminal groups. In the case of polymer blends, ions carrying an oligomeric repeat unit that differs from the base unit will also depart from a horizontal alignment and will align along an oblique direction. In the case of copolymers, the KMD plot displays a scatter plot with an elliptical shape, extending in both the horizontal direction (co-monomer used as the base unit) and the oblique direction (the other co-monomer).35 Because the monomer VDF is included in all the analyzed polymers presented herein, the Kendrick mass (Kmass) was defined here as the product of the measured m/z multiplied by the ratio of the nominal mass of a VDF unit over the exact mass of a VDF unit (i).

𝐾𝑀(𝑉𝐷𝐹) = 𝑀𝑒𝑥𝑝𝑒𝑟𝑖𝑚𝑒𝑛𝑡𝑎𝑙 ∗

𝑀𝑛𝑜𝑚𝑖𝑛𝑎𝑙 (𝑉𝐷𝐹) 𝑀𝑒𝑥𝑎𝑐𝑡 (𝑉𝐷𝐹)

(i)

Kendrick Mass Defect analysis The use of high resolution mass spectrometry significantly increases the size of generated data files, and routinely obtained spectra may sometimes contain thousands of peaks. As a means of simplifying data treatment and interpretation for complex mixtures run directly by high resolution mass spectrometry, Kendrick mass defect (KMD) plots have been found to be useful for allowing improved visualization of structural features. In particular, for complex polymers, KMD plots rely upon use of the repeating unit of a polymeric backbone as the base unit to allow enhanced visualization of structural trends and subtle variations between structurally-related oligomers.24,30,31 This method has been successfully used to represent complex data such as those obtained during petroleomics, or in polymer or environmental studies.12,32,33 A KMD plot constitutes a user-friendly data treatment and visualization tool to aid in the interpretation

The Kendrick mass defect is then calculated as the difference between the nominal mass of the measured m/z and it’s KM (ii).

𝐾𝑀𝐷(𝑉𝐷𝐹) = 𝑁𝐾𝑀 − 𝐾𝑀(𝑉𝐷𝐹) (ii) Figure 2a. displays the Kendrick mass defect plot of the polymer PVDF using a VDF base unit (64 amu). Each point or "dot" corresponds to a measured peak found in the mass spectrum on Figure 1a. On the x axis are featured the experimental masses and on the y axis are featured their corresponding mass defects, KMD(VDF). The diameter of a point is proportional to the relative intensity of the corresponding peak found in the mass spectrum. Figure 2a reveals two distinct areas: the first one is an oblique ellipse going from ca. m/z 200 to 1300. These data points, which depart from a horizontal alignment, all


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align along a straight line in the oblique direction; they correspond to cationic polymeric species whose oligomeric repeat units differ from the VDF base unit used to calculate the KMD (64 amu). Consecutive points on this line are separated by 14.01 mass units, i.e., there is addition of -CH2- into the structure. Even more interesting is the 2nd area of the map, in the region from m/z 500 to 1800. Here, the points are scattered on horizontal parallel lines, grouped in a horizontal ellipse. The biggest dot (representing the first distribution’s apex) corresponds to m/z 1007.1587. The points along this same line all share a KMD of 0.037 units, which indicates that the [C2H2F2]repeating-unit structure has undergone almost no substantive variations. In fact, the composition of the molecules sharing this KMD have a general formula of [(C2H2F2)n+1C -5H +2F]. The other lines represent minor distributions that are rather difficult to discern on the mass spectrum, but are clearly visible on the Kendrick map. On these lines are plotted compounds only differing from the two main distributions by the number of unsaturations or functional groups present. Addition of a double bond entails the subtraction of a group such as H2, F2, or HF. It should also be noted that the contribution of 13 C atoms results in a different m/z and therefore a different KMD compared to the corresponding 12C compound. These points can also be seen on the graph as juxtaposed parallel lines. This graphical representation method enables easier distinction of points belonging to previously overlaid distributions, as well as easier visualization of characteristic points exhibiting low intensities. The m/z 1250-1500 area is magnified in the dashed line inset and displays the different end groups, eliminations and substitutions occurring. Because CID tandem mass spectra of polymeric ions can provide information on: (i) the number of repeat units and (ii) the nature of the end group(s) in each oligomeric backbone, computing the KMD from accurately measured m/z values of CID fragment ions should help to identify similar or different degradation pathways based on their positioning on a Kendrick plot.36 Figure 3b displays the results of CID MS/MS experiments (Figure S3 a, b and c), plotted on the previous Kendrick map with the fragmentation of selected precursors at m/z 1007 (red dots), 1021 (green dots) and 1157 (purple dots). As previously stated, the dots corresponding to the product ions of the three precursors are set on horizontal lines corresponding to “MS peaks”, which means that they only differ from these peaks by the number of repeating units [C2H2F2]. Therefore, their elemental composition can be assigned in an unambiguous way from one point to neighboring aligned points. Indeed, despite being a

visualization tool, KMD can also be used to help assign molecular formulas, by establishing homologous series that can be expanded from low m/z to high m/z. In a high resolution mass spectrometric analysis, peaks at low m/z can more easily be assigned a molecular formula because fewer formulas exist within the selected error limit (usually 0.5 ppm). Often, only 1 molecular formula exists within this error limit for peaks below m/z 500. As one moves up in m/z value, however, the number of reasonable formulas increases exponentially. However, if one is able to unambiguously assign molecular formulas to peaks of low m/z using Kendrick maps, peaks at higher m/z can be directly related to formulas assigned at lower mass by reasonably presuming that they belong to the same homologous series. If a high m/z value peak can be assigned to a homologous series, then it is very likely to represent the correct formula. This approach to formula assignment is called ‘formula extension’ and can be performed manually or written into software designed to assist in formula assignment.37,38 While CH2 is the most commonly used group, other functional groups can be utilized (i.e., OCH2, COO, O, H2O, H2, etc.) depending upon the makeup of the sample.39 ASAP HRMS of KEL-F 800 and Kendrick Mass Defect analysis Figure 1b. displays the acquired mass spectrum for polymer KEL-F 800. The spectrum here is more complex than the previous one. As a matter of fact, the distributions corresponding to the constituting repeat monomers VDF [C2H2F2] (Mw 64.01245) and CTFE [C2ClF3] (Mw 115.9641) are centered at about 1400 Da and extend from ca. m/z 600-2000. The presence of an additional peak at [M+2] due to the contribution of the 37 Cl isotope brings an additional level of complexity to the spectrum. This renders further analysis of this spectrum even more difficult. The most intense peaks in the distribution do not correspond to the first, but rather to the third, peak of the isotopic pattern. The peak at m/z 744.8822 was assigned the following composition C20H16O35Cl7F12+ (DBE = 3.5) with an error of 0.3 ppm. From this assigned composition, a formula can be proposed for the other peaks by adding either [C2H2F2] or [C2ClF3] monomers. As for PVDF, other mass shifts are also observed such as: 14.01(CH2), 44.01 (C2HF), 49.99 (CF2) or 149.99 (C3F6). CID MS/MS experiments were attempted to observe the fragmentation of selected precursor ions, but the high number of species observed in a 1 Da window made it impossible to precisely isolate a unique precursor. Figure 3c displays the Kendrick map of


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the polymer KEL-F 800 with the same VDF base unit as previously used for the polymer PVDF. Due to the isotopic pattern, the dots are gathered in clusters. These clusters form horizontal parallel segments, themselves forming an ellipse with a positive slope. The slope can be calculated and was logically found to correspond to the CTFE monomer. For illustration purposes, a 2-base-units Kendrick map was drawn by calculating the Kendrick mass defect of the MS data using both a VDF and a CTFE base unit, and then plotting the CTFE mass defect as a function of the VDF mass defect; the result is displayed on Figure 3d. On this figure, each cluster of points has a ± VDF (vertical axis) and a ± CTFE (horizontal axis) analog cluster. On this map, the previously mentioned ion at m/z 744.8822 is displayed with its two-dimensional coordinates as a red dot. Using its composition C20H16O35Cl7F12+ as a reference point, a molecular formula can be attributed to every other point of the map following the principles that: (a) increasing the KMD(VDF), i.e. moving to the right on a horizontal line to the next cluster of points corresponds to adding one CTFE monomer, and (b) decreasing the KMD(CTFE), i.e. moving down on a vertical line to the next cluster of points corresponds to the addition of one VDF monomer. The minor remaining points are attributed to species derived from the addition of small functional groups such as -CF2-, -CH2-, or HF. ASAP HRMS of Tecnoflon and Viton A and Kendrick Mass Defect analysis Figure 1.c. displays the acquired spectrum of the copolymer Tecnoflon, composed of the monomers, [C2H2F2] (Mw 64.01245) and [C3F6] (Mw 149.9904). Two distributions can be observed, one with an apex at m/z 743 (assigned as C24H24F21O2+) and mass shifts of 64.0125 mass units. The other distribution is centered at m/z 961 and has peaks separated by 64.0125 or 149.9904 mass units. CID MS/MS experiments were conducted to observe the fragmentation of two precursor ions chosen among the lower intensity peaks, namely m/z 923.1065 (Figure S-4 a) and 557.0772 (Figure S4b). Results of these CID MS/MS experiments are summarized in Table S1. The precursor 923.1065 undergoes 5 consecutive HF losses, producing the ion at m/z 823.0751. Another transition can be noticed, producing the ion at m/z 895.0752 through a loss of ethylene, followed by the loss of HF to obtain m/z 875.0680. Finally, m/z 821.0766 is thought to result from a C2H5F3O loss. The second precursor chosen at m/z 557.0772 undergoes 2 HF losses resulting in the product ion at m/z 517.0647 (Figure S3.b). The m/z 467.0489 product ion likely originates from a

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loss of C4H4F2, i.e. the VDF monomer plus C2H2. The product ion at m/z 437.0382 results from a consecutive loss of CH2O; subsequently HF can be lost to produce m/z 417.0319. In an alternative pathway, the product ion at m/z 403.0362 can arise from the loss of one VDF monomer from m/z 467.0489. Finally, a peak is observed at m/z 189.0330 which is assigned as C6H6F5O+. The KMD(VDF) Kendrick map corresponding to the polymer Tecnoflon is displayed in Figure 3a. Similar to what was observed for PVDF in Figure 2a, two areas can be observed in Figure 4: the first one is a diagonal ellipse with a negative slope (similar to Figure 3), ranging from ca. m/z 500 - 1000 and with KMDs going from -0.4 to 0.8. Consecutive points on this line are separated by 14.01 mass units, i.e. addition of -CH2- in the structure. The 2 nd area of the map, going from m/z 650-1950 features points scattered on horizontal parallel lines, gathered in an oblique ellipse. The biggest dots are almost on the 0 KMD line (9.10-4 KMD(VDF)) for m/z 743.1436 assigned as C24H24F21O2+) and they belong to the first distribution observed in the mass spectrum (Figure 1c). The positive slope of this ellipse is due to the presence of hexafluoropropylene, C3F6. The KMD(VDF) of the CID MS/MS product ions of the precursors at m/z 923 (red dots), and 557 (green dots) are overlaid on the Kendrick map of Tecnoflon (Figure 3a.). The dots corresponding to these MS2 experiments: i) don’t overlap on dots corresponding to the MS spectrum, and ii) are gathered, for the most part, on two parallel lines. The slope of these lines is calculated to correspond to successive HF neutral losses. The fluorinated polymer Viton A is comprised of the same monomers, i.e. [C2H2F2] (Mw 64.01245) and [C3F6] (Mw 149.9904), as the "Tecnoflon" polymer, but with a different repetition number for each monomer. Its mass spectrum is displayed in Figure 1d. The key features of the mass spectra of the two compounds are similar, but some differences can still be observed. The 64.0125 and 149.9904 mass unit gaps corresponding to the VDF and HFP monomers are observed for each. However, while the two Tecnoflon’s distributions are centered on two different m/z values, Viton A’s distributions are centered on a single peak at m/z 871.1689. This latter ion is common to both polymers and was assigned as C28H28F25O2+. CID MS/MS experiments were conducted to fragment two precursor ions, namely m/z 871.1680 and 555.1154 (Figure S-5 a and S-5 b). The m/z 871.1680 ion undergoes 7 consecutive HF losses to produce m/z 731.1254. Another peak is observed at m/z 658.1355 and is thought to correspond to a C3F3 loss from m/z 751.1306


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(Table S-2 in Supporting information). The precursor at m/z 555.1154 undergoes 3 consecutive HF losses to produce m/z 495.0975. Another product is observed at m/z 395.0650 (resulting from the loss of C8H7F3), as well as another at m/z 317 resulting from the loss of C8H9F7. Finally, a product ion peak is observed at m/z 189.0325, which is assigned as C6H6F5O+, similar to what was observed for the fragmentation of Tecnoflon’s m/z 557.0772. The mass difference between ions at m/z 317.0570 and 189.0325 corresponds exactly to the mass of two VDF monomers.

Similar to Tecnoflon and Viton A, repetitions of the [C2H2F2] monomer are also present, causing a horizontal “spreading” of the ellipse. Area D gathers points belonging to the alkane chains of the PVDF and Tecnoflon polymers. By tracing straight lines through each ellipse, an “average” structure can be proposed for every cloud of points. In this way, we can assign the average formula [(C2H2F2)n+C3F6]x for the PVDF polymer, [(C2ClF3)3n(C2H2F2)n]x for KEL-F 800, [(C2H2F2)7n(C3F6)2n]x for Tecnoflon and (C2H2F2+C3F6)x for the Viton A polymer.

The KMD(VDF) as a function of m/z was also plotted for the polymer Viton A. The corresponding Kendrick map is displayed in Figure 3b. An area spreading from m/z 550 to 1500 features points scattered on horizontal parallel lines, gathered in an oblique ellipse. The biggest dots are almost exactly on the 0 KMD line (1.10-3 KMD(VDF) for m/z 871). This ellipse’s positive slope is due to the presence of the hexafluoropropylene, C3F6 moiety. Data corresponding to the CID MS/MS experiments on m/z 871 and 555 precursors, respectively appear as red dots and green dots. Similar to the case of Tecnoflon, the dots corresponding to these MS2 experiments don’t overlap with dots from the MS spectrum, and most of them are gathered on two parallel lines. The slope of these lines can be calculated and is found to correspond to successive neutral HF losses.

CONCLUSION

Kendrick Mass Defect Analysis of a fluorinated polymer blend In order to evaluate the ability to discern MS experimental data originating from each polymer, all the obtained patterns were plotted on the same graph (without the intensity dependent size of the dots), resulting in what is displayed in Figure 4. Four distinct areas appear on the map, labeled as A, B, C and D. Area A is the horizontal ellipse containing the points from the polymer PVDF. Area B contains two ellipses, both in diagonal, with the same slope. These ellipses contain points originating from the Tecnoflon and Viton A polymers that share the same constituting monomers [C2H2F2] and [C3F6]. The presence of the latter hexafluoropropylene monomer causes the positively sloped direction of the ellipses’ major axes. As repetitions of the [C2H2F2] monomer are also present, the ellipses of Tecnoflon and Viton A also have a horizontal component. Area C contains points belonging exclusively to the chlorinated fluoropolymer KEL-F 800. These points have strictly negative KMDs and form a diagonal ellipse. Indeed, the presence of the chlorotrifluoroethylene monomer [C2ClF3] and especially of chlorine atoms, causes these very negative KMDs.

We report here the novel application of ASAP HRMS coupled with Kendrick mass defect (KMD) analysis for the rapid characterization of the fluorinated polymers PVDF, KEL-F 800, Tecnoflon and Viton A. Ambient ionization mass spectrometry has proven to be well-suited for the analysis of vinylidene difluoride (VDF) polymers and copolymers. The acquired mass spectra, however, displayed a few thousand peaks in some cases, making them difficult to interpret and, a fortiori, complicating the comparison of one polymer’s spectral fingerprint with that of another. As the common repeating unit, VDF is known to be present in all the samples; it was used as a renormalization unit for the establishment of Kendrick maps based on a VDF KMD. The renormalized Kendrick maps illustrated the predominance of peak families comprised of homologous compounds, differing only by the number of unsaturations or functional groups. Knowledge of one reference point’s composition on these maps can readily permit assignment of an elemental composition to any other point. Furthermore, the overlay of the four individual Kendrick maps on one VDF-based map allowed the identification of four distinct areas. The separated zones represent a type of specific fingerprint for each fluoropolymer thus enabling the discrimination of the components in the blend. The combination of the three elements –the ambient ionization source that offers rapid acquisitions without sample preparation requirements, high resolution mass spectrometry, and efficient data processing by Kendrick mass defect analysis – shows much promise for the characterization of trace level unknown energetic materials in a non-targeted approach.

SUPPORTING INFORMATION AVAILABLE Structures of the four studied fluorinated polymers (PVDF;, KEL-F 800, Tecnoflon and Viton A), accurate mass measurement of ions detected in the PVDF ASAP-


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MS mass spectrum, accurate mass measurements of the selected parent ions and their daughter ions in CID MS² experiments, positive ASAP HRMS CID MS/MS spectra of three ions from PVDF, namely a) m/z 1007; b) m/z 1021 and c) m/z 1157, proposed structure for the ion observed at m/z 1007 and degradation pathway illustrating a neutral loss of 182 mass units, positive ASAP HRMS CID MS/MS spectra of two ions from the fluoropolymer Tecnoflon, namely a) m/z 923 and b) m/z 557, positive ASAP HRMS CID MS/MS spectra of two ions from the fluoropolymer Viton A, namely a) m/z 871 and b) m/z 555.

CORRESPONDING AUTHOR INFORMATION Maxime C. Bridoux CEA, DAM, DIF, F-91297 Arpajon, France [email protected]

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(10) Marshall, A. G.; Rodgers, R. P. Acc. Chem. Res. 2004, 37, 53-59 (11) Qi, Y.; Hempelmann, R.; Volmer, D. A. Anal. Bioanal. Chem. 2016, 408, 4835-4343 (12) Carré, V.; Schramm, S.; Aubriet, F. AIMS Environ. Sci. 2015, 2 (3), 547-564. (13) Jackson, A. T.; Williams, J. P.; Scrivens, J. H. Rapid Commun. Mass Spectrom. 2006, 20, 2717-2727 (14) Nefliu, M.; Venter, A.; Cooks, R. G. Chem. Commun., 2006, 8, 888-890 (15) McEwen, C. N.; McKay, R. G.; Larsen, B. S. Anal. Chem. 2005, 77, 7826-7831 (16) McEwen. C.N. Atmospheric-Pressure Solid Analysis Probe (ASAP); John Wiley & Sons, Hoboken; 2010 (17) Chernetsova, E. S.; Morlock, G. E.; Revelsky, I. A. Russ. Chem. Rev. 2011, 80 (3), 235–255.

ACKNOWLEDGMENT Financial support from the CEA, CNRS, Laboratoire Central de la Préfecture de Police de Paris (LCPP), and Université Pierre et Marie Curie is kindly acknowledged.

(18) Shelley, J. T.; Wiley, J. S.; Chan, G. C. Y.; Schilling, G. D.; Ray, S. J.; Hieftje, G. M. J. Am. Soc. Mass Spectrom. 2009, 20, 837-844 (19) Song, L.; Dykstra, A. B.; Yao, H.; Bartmess, J. E. J. Am. Soc. Mass Spectrom. 2009, 20, 42-50

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Figure 1: ASAP HRMS spectra of the four fluorinated polymers: (a) PVDF, (b) KEL-F 800, (c) Tecnoflon, (d) Viton A.


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Figure 2: Kendrick map of the polymer PVDF calculated with a VDF base unit a) and daughter ions of the precursors at m/z 1007 (red dots), 1021 (green dots) and 1157 (purple dots) plotted on the KMD(VDF ) Kendrick map of PVDF (b).


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Figure 2: Kendrick map of (a) the polymer Tecnoflon calculated with a VDF base unit (blue dots) and daughter ions of the precursors at m/z 923 (red dots) and 557 (green dots), (b) the polymer Viton A calculated with a VDF base unit (blue dots) and daughter ions of the precursors at m/z 871 (red dots) and 555 (green dots), (c) the polymer KEL-F 800 calculated with a VDF base unit plotted on the KMD(VDF ) Kendrick map and (d) Kendrick map of the polymer KEL-F 800 calculated against a VDF base unit (x axis) and a CTFE base unit (y axis)


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Figure 4: Kendrick map of the polymers PVDF (blue dots, ellipse A), KEL-F 800 (red dots, ellipse C), Tecnoflon (green dots, ellipse B) and Viton A (purple dots, ellipse B) calculated with a VDF base unit. Ellipse D contains data points belonging to the alkane chains of PVDF and Tecnoflon.


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Characterization of Fluorinated Polymers by Atmospheric-Solid

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