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Surface Layer Matrix-Assisted Laser Article Cite This: Anal. Chem. 2018, 90, 13427−13433 pubs.acs.org/ac Surface Layer Matrix-Assisted Laser Desorption Ionization Mass Spectrometry Imaging: A Surface Imaging Technique for the Molecular-Level Analysis of Synthetic Material Surfaces Kevin J. Endres,†,⊥ Jacob A. Hill,†,⊥ Kuan Lu,† Mark D. Foster,*,† and Chrys Wesdemiotis*,†,‡ † ‡ Department of Polymer Science and Department of Chemistry, The University of Akron, Akron, Ohio 44325, United States *S Supporting Information ABSTRACT: Surface layer matrix-assisted laser desorption ionization mass spectrometry imaging (SL-MALDI-MSI) is a powerful new surface sensitive imaging technique to establish surface component localization of multicomponent polymer C). materials. This study demonstrates the ability of SL-MALDI- MSI to image defects from foreign materials, material absence, mechanical scribing, and solvent perturbation at the surface of low-molecular-weight poly(methyl methacrylate) and polystyrene thin films. The surface specificity of the SL-MALDI-MSI technique is validated by imaging polystyrene on poly(methyl methacrylate) bilayer films; only polystyrene ions are detected from the surface of the unperturbed polystyrene layer. A key o legitimately share published articles. process enabling SL-MALDI-MSI is the solvent-free sublimation of matrix and salt uniformly on the sample’s surface. s the first point of contact with the environment, surfaces molecular fragments, or atoms; moreover, samples must A are critical to the performance of materials in many often be labeled to generate an instrumental response. applications. For example, they are particularly important in Meeting the second challenge, obtaining laterally resolved adhesion, coating durability, surface wetting, printing, mem- information on surface composition, has been done in part brane fouling, and protein adsorption and denaturation on with scanning options of a variety of techniques, including ToF SIMS, XPS, Auger spectroscopy, and tip-enhanced plasmonic biomaterials. Optimal performance in such applications may − require consistent molecular (not just elemental) composition techniques.13 15 ToF SIMS imaging has been applied across the surface on length scales varying from nanometers to extensively to polymeric surfaces to investigate blend millimeters. This uniformity can be disrupted by various types domains,16 coatings,17 hydrogels,18,19 and nanoparticles.20 of surface defects resulting from contaminants on the surface, SIMS analyzes low-mass oligomers, fragments, and atomic damage from solvents, and alterations due to friction, abrasion, ions. In contrast, matrix-assisted laser desorption ionization 1,2 Downloaded via UNIV OF AKRON on January 1, 2019 at 19:50:47 (UT or light damage. (MALDI) produces intact molecular ions, generating less Characterization of the surface composition and lateral convoluted spectra and making analysis simpler and more uniformity of multicomponent synthetic polymer systems has straightforward. The highest spatial resolution currently faced two major challenges to be dealt with here. The first has achievable with MALDI mass spectrometry imaging (MSI) 3,4 See https://pubs.acs.org/sharingguidelines for options on how t been to characterize what is specifically at the surface (i.e., is, however, only 1−5 μm, which is inferior to the 100 nm surface specificity or depth resolution). The second has been achievable by ToF SIMS-MSI. Also, without a chromophoric to provide adequate laterally resolved compositional informa- matrix (i.e., using laser desorption ionization, LDI), fragments tion on the length scale needed. In recent decades, tremendous formed by the laser can be imaged, but these species are not progress has been made regarding the first challenge, specific to the surface. The chromophoric character of dye concerning surface specific compositional analysis. A few molecules has been exploited to image organic and inorganic among the various techniques now available to provide surface pigments in automotive coatings by LDI, via the detection of 21 compositional information include time-of-flight secondary ion intact molecules or fragments thereof. mass spectrometry (ToF SIMS),3,4 other ion beam techni- Closely related to MALDI-MSI is surface-assisted laser ques,5 X-ray photoelectron spectroscopy (XPS),6 X-ray and desorption ionization (SALDI)-MSI, in which the sample is fl 6−11 covered by sputtered platinum or silver nanoparticles that neutron re ectivity (XR, NR), and plasmonics-based 22,23 techniques such as surface enhanced Raman spectroscopy serve as the chromophoric matrix. SALDI-MSI has been (SERS) and tip-enhanced Raman spectroscopy.12 Each employed to image small molecules from inkjet ink on printed technique has advantages and disadvantages. Notably, none of these techniques is able to provide direct information on the Received: July 19, 2018 actual composition of specific molecules at the surface, but Accepted: October 22, 2018 rather, the techniques probe compositions of segments, Published: October 22, 2018 © 2018 American Chemical Society 13427 DOI: 10.1021/acs.analchem.8b03238 Anal. Chem. 2018, 90, 13427−13433 Analytical Chemistry Article paper22 and the distribution of the antioxidant Irganox 1010 in minutes to hours without expensive and perturbing labeling on polystyrene (PS) films.23 Significant fragmentation was methods. observed in these experiments, pointing out that SALDI is a Most MALDI imaging sample preparation procedures relatively hard ionization method in comparison to MALDI. employ solvent-based matrix application that perturbs the The majority of MALDI-MSI studies has been performed on surface due to solvent penetration into the sample. This biological samples. MALDI, especially when interfaced with problem is bypassed in the SL-MALDI-MSI technique time-of-flight (ToF) mass analysis, is however a well- presented here using solvent-free matrix application. The established technique for characterizing synthetic polymers simultaneous deposition of matrix and ionizing agent is also − and additives as well,24 26 which is due to its ability to form key. MALDI imaging experiments have primarily been singly charged ions over a virtually infinite molecular weight performed on biological tissues, which promote ionization by range. MSI is widely used to determine the localization of protonation and do not require the addition of salts to facilitate peptides, proteins, drugs, and metabolites within biological ion formation. Such salts are, however, needed for the tissues. Analogous information on the spatial distribution of ionization of most classes of synthetic materials. Here we oligomers, (co)polymers, additives, and degradation products demonstrate the successful sublimation of MALDI matrices within synthetic materials is highly desirable. Unlike biological blended with cationization salts onto the surface of synthetic materials. We also document the utility of SL-MALDI-MSI as a tissues, which are mechanically sliced prior to MSI, both depth fi resolution (i.e., surface sensitivity) as well as lateral resolution surface-speci c analytical technique that images surfaces based on intact molecular ion detection and with the depth are critical for understanding surface segregation, defects, and 40 degradation on synthetic material surfaces. Applications of resolution characteristic of SL-MALDI-MS (<2 nm). These capabilities are illustrated by the acquisition of images of MALDI-MSI to address these issues are scarce. MALDI-MSI fi has been used to image changes on PS films as they were UV- various types of surface defects on PS and PMMA lms caused irradiated,27 demonstrate photolithographic structuring for by surface contamination (modeled by stamping), masking, 28 scratching, and solvation. printing, determine the chemical composition and physical characteristics of polysulfone and polyvinylpyrrolidone dialyzer 29 ■ EXPERIMENTAL SECTION membranes, analyze lipid adsorption and degradation in polyethylene joint implants,30 and confirm tocopherol acetate Film Fabrication. PMMA (7 kDa) with the connectivity 31 absorption inside laminate films. In all of these studies, H-[CH2C(CH3)COOCH3]n-H was purchased from Polymer solutions of the matrix and cationizing salt were deposited onto Source Incorporated. Analysis of this sample by MALDI-MS yielded a bulk number-average molecular weight (Mn) of 6850 the solid sample by spin-casting or spraying. This process can ± cause extraction, influence dynamics within the bulk of the 50 Da. PS with the connectivity s-C4H9-[CH2CH(C6H5)]n- H, synthesized by living anionic polymerization at The material, and change the surface composition. Such changes ± make MALDI-MSI unsuitable for probing material surfaces University of Akron, was found to have a bulk Mn of 6030 ff 130 Da. The corresponding polydispersities (PDIs) were 1.01 which may di er in composition from the bulk. fi Solvent-free sample preparation, developed for polymer for the 6 kDa PS and 1.03 for the 7 kDa PMMA. Thin PS lms − − analysis,32 36 can be applied to surface characterization to were prepared by spin-casting from toluene solutions of 1 3 avoid perturbing the surface. Solvent-free matrix deposition has wt % at a rate of 2000 rpm to obtain smooth and laterally uniform films. Films were spun cast onto silicon wafers cleaned been used for MSI of biological samples to control the sizes of in piranha solution. Film thicknesses were determined with a the matrix crystals and produce more uniform crystal layers, − spectroscopic ellipsometer (VASE, M-200 UV−visible-NIR improving
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