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Evaluation of Matrix-Assisted Laser Desorption Ionization Mass Spectrometry for Polymer Characterization

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Evaluation of Matrix-Assisted Laser Desorption Ionization Mass Spectrometry for Polymer Characterization CORE Metadata, citation and similar papers at core.ac.uk Provided by Elsevier - Publisher Connector Evaluation of Matrix-Assisted Laser Desorption Ionization Mass Spectrometry for Polymer Characterization A. M. Belu, J. M. DeSimone, and R. W. Linton Department of Chemistry, University of North Carolina, Chapel Hill, North Carolina, USA G. W. Lange and R. M. Friedman Monsanto Corporation, St. Louis, Missouri, USA A protocol for the preparation of polymeric samples for time-of-flight matrix-assisted laser desorption ionization mass spectrometry (TOF-MALDI-MS) analysis was developed. Dithra- no1 was identified as a good matrix for polystyrene W’S), and the addition of silver for cationization of molecules was determined to be necessary. Based on this preparative method, low molecular weight samples of other polymers [polyisoprene, polybutadiene, poly(ethylene oxide), poly(methy1 methacrylate), and polydimethylsiloxanel were analyzed with molecular weights up to 49 ku. The effects of laser intensity were determined to influence the molecular weight distribution of intact oligomers, most significantly for low molecular weight polymers. Linear and reflectron modes of analysis were evaluated; better signal intensity and resolution were obtained in the reflectron mode. The TOF-MALDI-MS measurements are compared with time-of-flight secondary ion mass spectrometry (TOF-SIMS) and gel permeation chromatography (GPC) for the same polymers. The M, values calculated by TOF-MALDI-MS consistently are higher than values calculated by TOF-SIMS for all classes of polymers with molecular weights up to 8 ku. The molecular weights of the PS calculated from TOF-MALDI-MS are in good agreement with GPC ( f 10%). The composition of the terminal group on a polymer chain may affect the ion yields. The ion yields of intact oligomers were evaluated as a function of end group composition for both TOF-MALDI-MS and TOF-SIMS. The slight disparity of results between TOF-SIMS and TOF-MALDI-MS for the perfluoroalkyl-terminated I’S suggests that the oligomers are desorbed preferentially from the surface in the TOF-SIMS analysis, rather than having an increased ionization probability. (J Am Sot Mass Spectmom 1996, 7, 11-24) s a powerful new method for the analysis of al. [2] were the first to apply this preparation to syn- high molecular weight materials, matrix-as- thetic polymers. They analyzed poly(acrylic acid) (3 A sisted laser desorption ionization (MALDI) is ku) and poly(styrene sulfonic acid) (200 ku), which are pertinent to the characterization of polymers. To date, polar, water-soluble molecules that have good misci- the development of the technique has been directed bility with sinapinic acid. The Hillenkamp group 131 mainly toward biomolecules; few studies have focused provided the first comprehensive study of synthetic on synthetic polymers. The first examples of polymer polymers by MALDI. Molecular ions were observed characterization by MALDI were presented by Tanaka for polyfmethyl methacrylate) (I’MMA) as well as PPG et al. [II, who introduced the technique in 1988. Distri- and PEG by using a solid matrix of 2,5-dihydroxyben- butions of sodium-cationized molecular ions were zoic acid (DHB). Sometimes alkali-metal salt solutions shown for 4-ku polyfpropylene glycol) (PPG) and 20-ku (NaCl, LiCl, KC0 were added to increase the yield of polyfethylene glycol) (PEG) by using the “ultra fine cationized species. The capability of this technique to metal plus liquid matrix method,” a matrix that con- determine accurate molecular weight values was sists of cobalt powder in glycerol. By 1992, a standard demonstrated for these polymers with molecular preparation of samples for MALDI analysis of weights up to 40 ku. Also, the first MALDI analysis of biomolecules had been developed by using sinapinic a hydrophobic polymer was shown for polystyrene acid as a solid matrix with crystalline regions. Danis et (PS) with molecular weights up to 80 ku using a liquid matrix of 2nitrophenyl octyl ether (NI’OE) and silver trifluoroacetate (AgTFA) for cationization. Danis and Address reprint requests to Dr. Richard W. Linton, Department of Karr [4] further developed the approach to sample Chemistry, CB#3290, University of North Carolina, Chapel Hill, NC 27599-3290. preparation for organic-soluble polymers. The use of 0 1996 American Society for Mass Spectrometry Received January 30,1995 1044-0305/96/$15.00 Revised August 7,1995 SSDI 10440305(95)00589-7 Accepted August 9,1995 12 BELU ET AL. J Am SW Mass Spectrom 1996,7,11-24 matrices soluble in organic solvents allowed direct ducible sample preparation procedure was investi- mixing with the organic-soluble polymer and pro- gated. At the same time the role of instrumental condi- moted polymer-matrix interaction. Polymers previ- tions such .as laser power, extraction energy, and ously had been prone to precipitate out of solution analyzer mode (linear, reflectron) were explored. The (partially water-based) before the matrix crystallized. search for an ideal sample preparation method first Via the organic solvent approach, molecular weight focused on hydrocarbon polymers-specifically PS. distributions were obtained for samples of PMMA Hydrocarbons represent a large class of polymeric ma- with molecular weights up to 260 ku, prepared in a terials to which MALDI-MS has not been applied yet. matrix of DHB or in t-3-indoleacrylic acid (IAA) with The concepts learned by developing a sample prepara- acetone. This approach also was used to obtain PS tion procedure for I’S were applied toward the devel- (molecular weights of 11.6 and 120 ku) spectra with opment of MALDI-MS for other polymer classes as the silver metal salt of IAA as a matrix [5]. The sample well. Furthermore, many of the polymers studied have preparation method of Danis and Karr [4] is indicated been characterized by using time-of-flight secondary to be applicable to a range of polymers, which include ion mass spectrometry (TOF-SIMS) and gel permeation polycarbonate, poly(viny1 acetate), and poly(viny1 chromatography (GPC). The results from these tech- chloride), but a more detailed review of procedures niques allow further assessment of the information and results is needed. generated by MALDI-MS. As can be seen from the developmental emphasis of matrix-assisted laser desorption ionization mass spec- Experimental trometry (MALDI-MS) for polymer characterization thus far, the preparation of analyte in a proper matrix Materials. The polymers polystyrene (F’S), polyiso- plays a crucial role in the viability of the technique. prene (PI), polybutadiene (PBD), polyethylene PE), However, each report details a unique sample prepara- and polydimethylsiloxane (PDMS) were synthesized tion procedure. No standard protocol has been devel- by living anionic polymerization at the University of oped yet because of the diversity of polymeric materi- North Carolina at Chapel Hill [7]. Polymethyl- als available, the varying instrumental conditions methacrylate (I’MMA) was obtained from American under which the measurements are made, and the Polymer Standards Corp. (Mentor, OH) and polyteth- ambiguous role the matrix plays in desorption and ylene oxide) (PEO) was obtained from Polymer Labs ionization of the analyte. The analysis of PEG in a (Church Stretton, UK). The matrices 2-nitrophenyl octyl matrix of DHB or sinapinic acid is probably the most ether (NPOE), dithranol, t-3-indoleacrylic acid (IAA), reproduced polymer sample preparation. This sample 2,5-dihydroxybenzoic acid (DHB), sinapinic acid, and preparation is excellent for biomolecules because they 2-(4-hydroxyphenylazo)-benzoic acid (HABA) and the are similarly polar and water-soluble. Because most cationization agent silver trifluoroacetate (AgTFA) polymers are organic-soluble, the Danis and Karr ap- were purchased from Aldrich Chemical Co. (Milwau- proach to sample preparation is becoming popular due kee, WI) and were used as received. Carbostyril was to its indicated applicability to a range of polymers purchased from Kodak (Rochester, NY). The chloro and its use of a single organic solvent. form and acetone solvents were the Optima variety Few MALDI-MS results have been shown for hy- from Fisher Scientific (Fair Lawn, NJ). drocarbon polymers. To find an appropriate matrix and preparation method for these materials is espe- Standard sample preparation. A procedure for sample cially difficult because of their nonaqueous and nonpo- preparation, derived from the Danis and Karr method lar nature. Wilkins suggested that hydrocarbon poly- [4], has been developed from which reproducibly good mers lack an effective site for ionization, and laser spectra are generated. For the analysis of F’S, solutions desorption thereby yields neutral gas-phase molecules of dithranol(2 x 10-l M), AgTFA (4.5 X 10m3M), and [6]. PS may be the easiest hydrocarbon to ionize be- PS (1 x lo-” M for 10 ku) were prepared in CHCl,. cause its phenyl functionality allows for a higher ion- Fresh solutions of matrix and cationization agents were ization probability, especially compared to polyiso- prepared daily (AgTFA was observed to fall out of prene (PI) and polybutadiene (PBD) (branched poly- solution and become cloudy and grey within few hours mers with vinylic functionality) and polyethylene (PE) of preparation). The solutions were combined in a 5:l:l (a completely aliphatic polymer). Ion yields may be volume ratio of matrix to polymer to cationization increased by the presence of alkali metals, particularly agent. A fine-tipped syringe that contained 1 PL of the for polar polymers where the heteroatoms (0, N) are mixture was dragged across the sample plate (total the sites of alkali metal attachment. With nonaromatic deposition area approximately 5 mm’), which allowed hydrocarbon polymers, however, difficulties in their the solvent to evaporate slowly. The resulting total ionization remains due to their lack of energetically sample load was 1.6 x lo-’ mol of dithranol, 1.4 X favorable sites for alkali metal attachment [6]. lo-” mol of lo-ku I’S, and 6.5 x lo-” mol of AgTFA, The goal of this study is to determine optimal with a mole ratio of 11,000:1:45.
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