Atomic Force Microscopy Thermally-Assisted Microsampling with

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Atomic Force Microscopy Thermally-Assisted Microsampling with Atmospheric Pressure Temperature Ramped Thermal Desorption/ Ionization-Mass Spectrometry Analysis William D. Hoffmann,† Vilmos Kertesz,*,† Bernadeta R. Srijanto,‡ and Gary J. Van Berkel*,† †

Mass Spectrometry and Laser Spectroscopy Group, Chemical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States ‡ Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States S Supporting Information *

ABSTRACT: The use of atomic force microscopy controlled nanothermal analysis probes for reproducible spatially resolved thermally assisted sampling of micrometer-sized areas (ca. 11 × 17 μm wide × 2.4 μm deep) from relatively low number-average molecular weight (Mn < 3000) polydisperse thin films of poly(2vinylpyridine) (P2VP) is presented. Following sampling, the nanothermal analysis probes were moved up from the surface and the probe temperature ramped to liberate the sampled materials into the gas phase for atmospheric pressure chemical ionization and mass spectrometric analysis. The procedure and mechanism for material pickup, the sampling reproducibility and sampling size are discussed, and the oligomer distribution information available from slow temperature ramps versus ballistic temperature jumps is presented. For the Mn = 970 P2VP, the Mn and polydispersity index determined from the mass spectrometric data were in line with both the label values from the sample supplier and the value calculated from the simple infusion of a solution of polymer into the commercial atmospheric pressure chemical ionization source on this mass spectrometer. With a P2VP sample of higher Mn (Mn = 2070 and 2970), intact oligomers were still observed (as high as m/z 2793 corresponding to the 26-mer), but a significant abundance of thermolysis products were also observed. In addition, the capability for confident identification of the individual oligomers by slowly ramping the probe temperature and collecting data-dependent tandem mass spectra was also demonstrated. The material type limits to the current sampling and analysis approach as well as possible improvements in nanothermal analysis probe design to enable smaller area sampling and to enable controlled temperature ramps beyond the present upper limit of about 415 °C are also discussed.

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spectrometer (GC/MS) for separation, electron ionization (EI), and mass analysis. They also showed the ability to directly sample the vapors generated at AP into the vacuum-based EI source using a heated transfer capillary. The smallest desorption craters achieved were conical in shape, approximately 6 μm in diameter and 1.7 μm deep. With much smaller AFM-controlled nanothermal analysis (nano-TA) probes for TD, we were able to create conical desorption craters 250 nm in diameter and 100 nm deep in a caffeine thin film and record the mass spectral signal from each desorption event in near real time.3 With this nano-TA TD/I-MS combination, we have also demonstrated coregistered mass spectral chemical images and AFM topographical images from inked patterns on paper and a living bacterial colony on an agar gel;4 coregistered topographical, band excitation nanomechanical and mass spectral images of a phase separated polymer film;5 and coregistered topographical

e recently introduced atmospheric pressure (AP) proximal probe thermal desorption surface sampling/ ionization-mass spectrometry (TD/I-MS) for spatially resolved chemical profiling and imaging of surfaces.1−6 This surface sampling/ionization and analysis approach uses a heated probe tip placed in close proximity to or in actual contact with a surface to locally desorb intact molecular species or thermolysis products from a surface that are then ionized by an AP ionization source such as electrospray ionization (ESI) or atmospheric pressure chemical ionization (APCI) and analyzed using MS. Our work in this area has advanced in spatial sampling resolution using progressively smaller heated probes from a millimeter in size1 to a 50 μm diameter2 and now down to a 30 nm diameter making use of a heated atomic force microscopy (AFM) probe tip and an AFM control system.3−6 Our latter work builds on the pioneering work of Reading et al.7−11 Reading and co-workers7−11 used 5 μm tip diameter Wollaston wire heated AFM probes to perform both point thermal desorption and thermolysis, capturing the liberated vapor material and then injecting it into a gas chromatograph/mass © XXXX American Chemical Society

Received: November 29, 2016 Accepted: February 8, 2017

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weight (Mn < 3000) polydisperse thin films of poly(2vinylpyridine) (P2VP). Following sampling, the nano-TA probes were retracted from the surface and the probe temperature ramped to liberate the sampled materials into the gas phase near a transfer capillary into the mass spectrometer for AP ionization by APCI and subsequent mass spectral analysis. The procedure and mechanism for material pickup, the sampling reproducibility and sampling size are discussed. The oligomer distribution information available from slow temperature ramps versus ballistic temperature jumps is presented. The ability to obtain sequence information from the sequentially desorbing oligomers via tandem mass spectrometry is also demonstrated.

and chemical images from both photothermal IR spectroscopy and mass spectrometry, also of a phase separated polymer film.6 In all our published nano-TA TD/I-MS examples, the sampling spatial resolution was in the very low to submicrometer range ultimately limited by the area/amount of material needed to be sampled to provide a sufficient mass spectral signal. However, we have found that when performing direct nano-TA TD or thermolysis from certain material types, particularly various synthetic polymers that have melting point temperatures much lower than their vaporization/thermolysis temperatures, it can be difficult to limit the size of the crater to low micrometer sizes. This limits the spatial sampling resolution achievable in either a chemical profiling or imaging mode. Furthermore, the direct heating of the surface destroys the physical and chemical morphologies, preventing further analysis in a relatively large area around those sampling locations. An alternative to heating the surface location to desorb material with the AFM probe is to use the nano-TA tip as a sampling and collection device for the subsequent TD/I-MS analysis away from the surface. Theoretically, the ability to sample very small areas is limited only by the size of the AFM probe tip, and the damage to the surface would be in that localized region. Mechanical, or more commonly, thermally assisted sampling with scanning probes for subsequent chemical analysis on the probe tip or gas phase by means of various optical spectroscopy 8,12−16 and mass spectrometry approaches8,17,18 has been shown. In terms of mass spectrometry, Lee, Wetzel, and co-workers17,18 used a scanning probe in vacuum to mechanically transfer material from a surface to the tip with subsequent field ionization and mass analysis revealing atomic spectra of the material that had been sampled onto the probe. Deliberately contaminating a probe tip with poly(methyl methacrylate) and heating in air, Reading et al.8 captured the evolved vapor on an absorbent tube and analyzed the material by TD/GC/MS. The only peak clearly discernible in the chromatogram was the methyl methacrylate monomer from thermolysis of the polymer on the probe tip. It is noteworthy to recall that placing a bulk sample on a heated probe for direct insertion (direct insertion probe, DIP) into a vacuum-based ionization source [EI or chemical ionization (CI)] for mass spectral analysis has been in use for at least 50 years and has been an especially important tool for polymer analysis.19,20 The popularity of the technique and its commercial availability have been muted in the last couple of decades by the availability of matrix-assisted laser desorption ionization (MALDI) and ESI, but it remains an important tool.21 More recently, DIP approaches of different types have been applied at AP with ESI and APCI, enabling their use on modern ESI/APCI instrumentation capable of tandem mass spectrometry and/or high-resolution accurate mass measurements, reviving the method for polymer analysis.21−26 One important attribute of the DIP approach is the ability to liberate into the gas phase separated in time the species on the probe through controlled temperature ramps. Components are separated as a function of their volatilities and/or thermal stabilities. This is important in direct analysis where multiple components in the gas phase can complicate the resulting mass spectra and compete for charge, skewing the distribution of material ionized and observed in the gas phase. Herein, we report on the use of the nano-TA probes for spatially resolved thermally assisted sampling of micrometersized areas from relatively low number-average molecular

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EXPERIMENTAL SECTION Chemicals and Polymer Thin Films. P2VP polymers (number-average molecular weight (Mn) = 970, polydispersity index (PDI) = weight-average molecular weight (Mw)/Mn = 1.05, Mn = 2070, PDI = 1.03 and Mn = 2970, PDI = 1.02) were purchased from Scientific Polymer Products, Inc. (Ontario, NY). The general polymer structure is shown in Scheme 1a. Scheme 1. (a) Basic Structure of P2VP Polymers Used in This Study and (b) Series of Dissociation Products Which Can Be Identified as b″, a, z″, and y Ions Arising from Cleavage of the P2VP n-mer Shown in (a)

Anhydrous chloroform (≥99%) was purchased from SigmaAldrich (St. Louis, MO) and used without further purification. Preparation of the ∼6−7 μm thick polymer thin films used here and the analysis by Fourier transform infrared (FTIR) spectroscopy are detailed in the Supporting Information. Nano-TA TD/I-MS Experimental Setup and Operation. The basic AFM controlled nano-TA TD/I-MS experimental setup used in this work has been described and illustrated in detail elsewhere.5 A Veeco Multimode AFM (Bruker AXS, Santa Barbara, CA) equipped with a closed loop N-Point stage (N-Point, Madison, WI) and a Nanonis system controller (SPECS Zurich GmbH, Zurich, Switzerland) were used to obtain the nanometer scale topographical images and to control the sampling and desorption processes. The heated tips were VITA-MM-NANOTA-300 nano-TA AFM probes (Bruker AXS, Camarillo, CA). An LTQ XL mass spectrometer (Thermo Fisher Scientific, Waltham, MA) was used in this work and operated using “Turbo” scan rate in the high mass range mode allowing unit resolution mass accuracy. Material thermally desorbed from the nano-TA probe was drawn by the vacuum drag from the mass spectrometer (∼1.0 L/min) into an in-line, cross geometry vapor extractor/corona discharge APCI source via a 4.5 cm long extractor capillary (0.817 mm o.d.; 0.508 mm i.d.) placed next to the AFM probe. Ions created in this region were transferred into the mass spectrometer for analysis by the same vacuum draw. Desorbed and ionized species were monitored with full scan mass spectra and MS/MS product ion spectra (normalized collision energy 30%). The thermal desorption and mass spectral analysis with the nano-TA TD/IMS system was completely automated by means of a custom B

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lifted 120 μm above the surface (Figure 1c). By turning off the heating while the probe tip was still in the surface, a thermal gradient between the probe (cold) and the molten material on the surface (hot) was created. Thus, we believe that surface material that had been liquified was transferred from the surface up the cantilever probe tip through a thermocapillary-driven mass transfer process27 and accumulated (and solidified) on the cantilever support around the base of the probe tip. Actual material accumulated around the tip base following sampling of the P2VP polymer with Mn = 970 can be seen in the SEM image shown in Figure 2c. The quantity of material sampled

software (AFM Assistant) written in-house. Details of the automation are described in the Supporting Information.

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RESULTS AND DISCUSSION AFM-Based Thermally-Assisted Microsampling, Temperature Ramped Thermal Desorption, Mass Analysis Concept. Shown in Figure 1 is a schematic representation of

Figure 2. (a) 3D AFM topographical color map of nine sampling spots from a P2VP thin film with Mn = 970 when the target sampling penetration was 2 μm. The average depth of the sampled spots was (2.1 ± 0.1) μm. Spot diameters across the center of the hole were measured as (17.4 ± 0.7) μm and (11.1 ± 0.4) μm along the X and Y axes at an average full width at 10% of maximum hole depth. (b) 2D heat map of the sampled spot indicated by the black arrow in (a). (c) SEM image of the probe tip area with sampled polymer present. Image taken at a 59o tilt angle and 10000× magnification.

Figure 1. Schematic illustration of the sequential steps in the thermally assisted microsampling, temperature ramped thermal desorption and mass spectral data acquisition: (a) nano-TA probe tip engaged the surface; (b) nano-TA probe tip heated to soften the material and penetrate about 2 μm into the surface; (c) nano-TA heating element turned off when 2 μm penetration reached and after about 5 s the probe now carrying sampled material withdrawn 120 μm from the surface; (d) nano-TA probe temperature ramp initiated to thermally drive material sampled from the probe tip area into the gas phase for ionization and mass spectrometric analysis; (e) nano-TA heating element voltage as a function of time showing ballistic (red) and slow (blue) temperature ramp conditions. Common part of the two voltage programs prior to the temperature ramps is shown in purple. (f) Schematic ion maps (m/z plot as a function of time) corresponding to ballistic (red) and slow (blue) temperature ramp conditions shown in (e), indicating the major ionic species (monomer and intact oligomers, respectively) that were observed by the mass spectrometer.

onto the probe tip correlated well with the volume of the sampling holes created on the surface (see below). In experiments where the sampling heat was not turned off prior to moving the probe up from the surface, little or no accumulation of material on the probe was observed nor was an obvious sampling hole created nor was mass spectral signal observed (data not shown). For the final step in the process, a probe temperature ramp was initiated to thermally desorb the material sampled from the probe tip area into the gas phase for ionization and mass analysis (Figure 1d). The temperature ramp was enabled via a linear ramping of heating voltage spanning the range from a ballistic voltage jump or ramp (>100 kV/s, red line in Figure 1e) to significantly slower 0.054−2.19 V/s heating voltage ramp rates (blue line in Figure 1e). The components that were sampled onto the probe were observed by the mass spectrometer as a function of their relative volatilities and/or thermal stabilities (Figure 1f). Note that by positioning the probe well above the surface after the sampling process, thermal damage to the surface during this analysis step was avoided.

the thermally assisted surface sampling process (Figure 1, a−d), a plot of the corresponding nano-TA heating voltages/ temperatures and timing of each stage of the process (Figure 1e), and a representation of the types of mass spectral data acquired during the analysis step (Figure 1f). To sample from a surface, the nano-TA probe tip was first brought to the surface (Figure 1a) then heated just enough to soften the material so the probe tip could penetrate into the surface (Figure 1b). When the probe tip reached a depth of 2 μm, the heating element was turned off. After 5 s, the probe was C

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Analytical Chemistry SEM examination of the nano-TA probe tip following the temperature ramps showed that all material was removed from this point on the cantilever. It is important to note that the nano-TA tip heating temperature is not a simple linear function of applied voltage (see Figure S1).28 The cantilever electrical resistance (and the resulting temperature) increases monotonically with applied voltage and then dramatically decreases, at which point the temperature uncontrollably increases. This behavior is typical of the temperature dependence of doped silicon,29 and the point at which the electrical resistivity of the cantilever is at its maximum is commonly called the resistance turnaround point. The maximum controllable temperature at this turnaround point using a heating voltage of ∼6.8 V was ∼415 °C in the present setup. Sampling Spot Size and Reproducibility. The AFM topography image of an array of nine automated repetitive samplings shown in Figure 2a demonstrates the shape, size, and reproducibility of the thermally assisted samplings for a thin film of P2VP with Mn = 970. The shape of the area sampled was that of an elliptical cone (Figure 2b). The depth of the cones was (2.1 ± 0.1) μm with an average full width at 10% of maximum of (17.4 ± 0.7) μm and (11.1 ± 0.4) μm along directions perpendicular and parallel to the length of the AFM cantilever, respectively. The automated sampling system provided consistent sampling with the average volume of these nine-sampled regions calculated as (150 ± 18) μm3 (12% RSD). This average sampled volume number agreed well with the 140 μm3 volume of sampled material estimated using the SEM image of the nano-TA cantilever tip postsampling of the spot in the middle of the array (see Figure 2c and the Supporting Information for details on the volume calculation). For the Mn = 970, the 140 μm3 volume equates to ∼165 fmol of total P2VP polymer sampled onto the cantilever. Temperature-Ramped Thermal Desorption of Sampled Material and Mass Analysis. Shown in Figure 3a is the ion map obtained from the thermally assisted sampling, temperature ramped thermal desorption, and mass analysis of the same P2VP polymer used to obtain the data shown above. Thermal sampling was accomplished as illustrated in Figure 1 with a subsequent ramp of the nanoTA heating voltage (temperature) from 0 (roughly ambient temperature) to 6.8 V (∼415 °C) at a rate of 0.16 V/s. Noteworthy in the mass spectral data was the sequential appearance of protonated, intact P2VP oligomers from the 4mer (m/z 480) up to at least the 18-mer (m/z 1952). The chemical structure of this particular P2VP polymer has a terminating sec-butyl group as shown in Scheme 1a. Individual oligomers, or n-mers, in the polydisperse polymer differ in mass by 105 Da, the mass of the poly(2-vinylpyridine) repeat unit. The abundance of the individual oligomers observed rose and fell roughly in order of increasing molecular mass as the heating voltage and probe temperature increased. The molecular weight of the oligomers as a function of the temperature at which their maximum mass spectral signal was observed is shown in Figure 3b. The mass spectrum obtained by averaging over the time of the desorption of all oligomers [i.e., from the appearance of the first oligomer (4-mer at m/z 480) at about 2.9 V heating voltage (∼100 °C) until the resistance turnaround point (∼415 °C)] is shown in Figure 3c. This average mass spectrum was found to be comparable to the mass spectrum obtained by simple infusion of a 25 μM solution of the polymer into the commercial APCI source on this mass

Figure 3. (a) Ion map showing the ion current recorded in positive ion mode APCI as a function of nano-TA heating element voltage (0.16 V/s heating voltage ramp rate) from the sampling and analysis of a P2VP (Mn = 970) thin film. (b) m/z of the different oligomers as a function of the nano-TA probe temperature at which their most intense signal was observed. Background-corrected positive ion mode APCI mass spectrum (c) averaged for the displayed heating voltage range shown in (a) and (d) recorded during infusion of a 25 μM solution of the same polymer in 50/50 (v/v) chloroform/methanol at 10 μL/min into the commercial APCI source on this mass spectrometer. APCI heated nebulizer = 450 °C, corona discharge current = 1 μA.

spectrometer (Figure 3d). The Mn of this polymer calculated from these two sets of mass spectral data were Mn = 908 (nanoTA/APCI-MS) and Mn = 1105 (infusion APCI-MS), both with a PDI of 1.06. These values matched closely (difference 100 kV/s) or constant elevated temperatures applied to the probe tip to desorb material directly from a surface location with the tip being in contact with the surface. With the lower molecular weight polymers used here, and high molecular weight polymers examined in other work,5,6 only the monomer unit of the respective polymers was observed in the gas phase when using the ballistic temperature ramp. Moreover, slower temperature ramps with the tip contacting the surface were found to provide little polymer chemical information in the mass spectrum. In addition, the prolonged heating caused damage to a very large area around the sampling site (100 μm or more in diameter, see Figure S3). This extended area surface damage in effect limited the spatial resolution of the sampling process and prevented additional analyses in the same general location. Ballistic temperature ramps resulted in the appearance of only the monomeric species even when the polymer sample was first picked up onto the nano-TA probe. This is illustrated by the data in Figure S4, panels a−c, which shows that only protonated 2-vinylpyridine at m/z 106 was observed following the sampling of the same P2VP polymer but with the ballistic heating voltage ramp. When the temperature ramps were slowed, the appearance of the monomer at the beginning of the temperature ramp disappeared and the sequential appearance of the progressively larger mass oligomers with increasing temperature began. It thus appears that when the ballistic temperature ramp was used to remove material from the nanoTA tips, the time needed for material to physically desorb from the probe was longer that the time necessary to decompose the

Figure 5. (a) MS/MS product ion current chronogram from the sampling and analysis of P2VP (Mn = 970) thin film using a 0.16 V/s heating voltage ramp rate. MS/MS product ion mass spectra of the protonated (b) 6-mer (m/z 690), (c) 9-mer (m/z 1005), and the (d) 12-mer (m/z 1321). Numbers and letters in (a) indicate the n-mer targeted for dissociation and the particular time in the chronogram that corresponds to the MS/MS product ion mass spectra actually shown in (b−d), respectively. Dissociation products are identified as b″, a, z″, and y ions arising from cleavage of the oligomers as schematically shown on Scheme 1b.

(TIC) chronogram (2.8−6.8 V heating voltage shown) from the sampling and thermal desorption of the Mn = 970 thin film of P2VP. Shown in Figure 5 (panels b−d) are the product ion spectra from the protonated 6-mer, 9-mer, and 12-mer observed at m/z 690, 1005, and 1321, respectively. The time in the TIC chronogram at which each of these three MS/MS spectra were acquired is annotated in Figure 5a. The specific product ions are identified using the Wesdemiotis et al.30 nomenclature for MS/MS dissociation of synthetic polymers of this basic structural type and protonated P2VP oligomers, in particular, as b″, a, z″, and y ions (see Scheme 1b). The successful detection of these specific fragments proved that slowly ramping the nano-TA probe temperature coupled to data-dependent MS/MS spectra collection provides a method E

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surface sampling overcame problems we have encountered previously with the nano-TA/I-MS approach in minimizing the sampling location size or area of surface damaged when attempting to directly TD relatively low melting materials with the nano-TA probes. In addition, the temperature-ramped TD was found to provide oligomer information that was previously lost when using constant high temperatures or ballistic temperature ramps to liberate material to the gas phase. For the Mn = 970 P2VP, the Mn and PDI determined from the nano-TA/I-MS data were in line with both the label values from the sample supplier and the value calculated from the infusion APCI mass spectrum, indicating that little or no thermolysis of the sampled oligomers had occurred. With P2VP samples of higher Mn (Mn = 2070 and 2970), intact oligomers were still observed, but a significant abundance of thermolysis products were also detected. Even so, intact oligomer ions from the Mn = 2070 P2VP sample as high as m/z 2793 (26-mer) were observed, which are the largest mass species observed to date using AFM controlled nano-TA/I-MS analysis. Of note is that this oligomer information is not directly available from physical measurements with AFM or even IR spectroscopy (see IR spectra of P2VP polymers with Mn = 970 and 2070 in Figure S6). An additional advantage demonstrated for the present sampling and analysis protocol was the sequential desorption of the oligomers and an extended time period (several seconds) of their appearance allowing for more detailed interrogations using data-dependent MS/MS. Future work with this sampling and analysis approach should include an investigation of chemically heterogeneous thin polymer films where such spatially resolved sampling would prove particularly analytically important and a study of the range of applicable sample types. At present, it appears that if the sample material can be controllably melted at a relatively low temperature and desorbed intact at the present upper temperature limit of ∼415 °C, this sampling and analysis protocol should be successful. Included within this temperature window are a larger range of lower molecular weight polymers.31 Preliminary data in hand shows that other vinyl linked polymers like poly(methyl methacrylate) and polystyrene behave in a similar fashion to the P2VP polymers reported on here. To expand further the range of materials for which the technique is applicable, the use of nano-TA probes that can achieve and operate long-term at significantly higher controlled temperatures than those presently used would be beneficial.32 The use of ESI might also provide a benefit by allowing for softer ionization and the possibility for multiple charging. Finally, to reduce the sampling spot size, thus improving the spatial sampling resolution, alternative nano-TA tip designs might be investigated. Signal levels achieved with P2VP for the ∼165 fmol of material sampled indicate that sample sizes might be reduced by at least 100 times and still provide a detectable signal. A sampled spot of 100 times less material, with the same basic shape and aspect ratio, would be an elliptical cone approximately 3.6 μm × 2.4 μm wide × 0.5 μm deep in size (i.e., dimensions of the original sample spots decrease by the cubic root of 100). Improvement in sample gas phase transport and ionization and the use of a more sensitive mass spectrometer might be employed to realize even smaller area sampling and mass spectral detection.

for confident identification of the individual oligomers via their specific dissociation patterns. Sampling and Analysis of Higher Molecular Weight P2VP Polymers. Similar thermally assisted sampling, temperature ramped TD, and mass analysis experiments were done using P2VP thin films with Mn = 2070 (PDI = 1.03) and Mn = 2970 (PDI = 1.02). Sampling and analysis conditions were identical to those described above. As with the lower molecular weight versions of this polymer, intact oligomers desorbed with increasing temperature in order of increasing molecular mass when using a 0.16 V/s heating voltage ramp rate. However, thermolysis products were also detected with the product abundance and complexity greatest for the highest molecular weight P2VP polymer. These phenomena are illustrated by the data in Figure S5 for the P2VP polymer with Mn = 2070. Figure S5a is the mass spectrum obtained for thermally assisted sampling, temperature-ramped TD of this polymer by averaging the spectra acquired over the time from the appearance of the first oligomer (6-mer at m/z 690) at a heating voltage of 4.8 V (∼235 °C) until the resistance turnaround point (∼415 °C). Comparing this average mass spectrum with the direct infusion APCI mass spectrum of the same polymer (Figure S5b) makes it clear that some thermolysis of the polymer occurred. The value of Mn calculated from the spectrum in Figure S5a (Mn = 1655) was significantly lower than the specified value (Mn = 2070) or the value calculated from the infusion APCI spectrum in Figure S5b (Mn = 1830). Also apparent were several abundant series of ions with 105 Da spacing that were not observed in the infusion APCI spectrum. These presumably arise from thermal degradation of the oligomers. Figure S5c is an ion map from this data set created by plotting ion abundance in ±5 m/z windows using the m/z values of the intact of protonated P2VP n-mers as center points. Note that the series of ions from the intact oligomers appear at the same m/z values as the b″ type fragments. Similar ion maps were created for two of the abundant ion series in the average mass spectrum which corresponded in m/z to a type (Figure S 5d) and y type (Figure S5e) fragments of the P2VP n-mers. These ion maps show the sequential appearance of the progressively larger mass oligomers with increasing temperature (indicated by the dashed white arrow in Figure S5c). They also show that as the heating voltage (and temperature) increased, the abundance of the thermolysis products also increased (Figure S5d and S5e). For the larger mass oligomers in particular there was clearly a competition between intact desorption and thermolysis. The degree of thermolysis was even more significant for the Mn = 2970 P2VP sample (data not shown). In sum, these results indicate that the ability to desorb a polymer (or other material) intact will be compound (chemical composition) specific and in general the probability of doing so will decrease with increasing molecular mass.

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CONCLUSIONS In this report, we demonstrated that spatially resolved micrometer-sized locations (ca. 11 μm × 17 μm wide × 2.4 μm deep) could be reproducibly sampled from relatively low molecular weight (Mn < 3000) P2VP thin films onto an AFMcontrolled nano-TA probe using a thermally assisted sampling process. The sampled material could then be liberated from the probe intact into the gas phase via a controlled temperatureramped TD process for subsequent ionization at atmospheric pressure and mass spectrometric analysis. This approach to F

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge via the Internet at The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/ acs.analchem.6b04733. Material and procedure details; volume calculation schematics; calculation of peak overlap percentage schematics; calibrated nano-TA probe temperature as a function of heating voltage; normalized maximum signal, wpeak, and Opeak percentage; optical images; extracted ion current chronograms; mass spectra; and FTIR spectra (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Tel: +1-865-574-3469. *E-mail: [email protected]. Tel: +1-865-574-1922. ORCID

Gary J. Van Berkel: 0000-0001-5224-3969 Notes

This manuscript has been authored by UT-Battelle, LLC under Contract no. DE-AC05-00OR22725 with the U.S. Department of Energy. The Department of Energy will provide public access to these results of federally sponsored research in accordance with the DOE Public Access Plan (http://energy. gov/downloads/doe-public-access-plan). The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The work of V.K., W.D.H., and G.J.V.B. on the AFM/MS system fundamentals, optimization, and application described here was supported by the United States Department of Energy, Office of Science, Basic Energy Sciences, Chemical Sciences, Geosciences, and Biosciences Division. The SEM analyses (B.R.S.) were performed at the Center for Nanophase Materials Sciences, which is a DOE Office of Science User Facility and sponsored at Oak Ridge National Laboratory by the Scientific User Facilities Division, Office of Basic Energy Sciences, U.S. Department of Energy.

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REFERENCES

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DOI: 10.1021/acs.analchem.6b04733 Anal. Chem. XXXX, XXX, XXX−XXX