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Fundamentals of the Application of Matrix-Assisted Laser Desorption

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Fundamentals of the Application of Matrix-Assisted Laser Desorption SHORT COMMUNICATION Fundamentals of the Application of Matrix­ Assisted Laser Desorption-Ionization Mass Spectrometry to Low Mass Polyfmethylmethacrylate) Polymers Barbara S. Larsen, William J. Simonsick, [r., and Charles N. McEwen E. I. du Pont de Nemours & Company, Wilmington, Delaware, USA Matrix-assisted laser desorption-ionization (MALOI) time of flight is shown to give a molar peak area response for isolated methylmethacrylate oligomers .that have 25 and ~O repeat units when run on three different instruments in reflectron or linear mode and usmg three different matrix materials. In addition, fragmentation was not observed in any of the three different matrices or at higher laser power. No spectral differences were observed for syndiotactic and isotactic methylmethacrylate olig.om.ers..These results suggest that the low most probable peak values observed for narrow distribution pol~m7th.yl~ethacrylate)stan­ dards by MALOI mass spectrometry are not the result of mass discrimination or fragmenta­ tion. (J Am Soc Mass Spectrom 1996, 7, 287-292) through mass spectrometric analysis are repeat unit mass, end group mass, cyclic structure determination, ass spectrometry is a viable technique for number average (Mn ) and weight average (Mw) analysis of synthetic polymers and has been molecular weights, polydispersity (D), random versus M used extensively for the analysis of additives, block copolymers, composition of blends, branching, impurities, and pyrolysis products [1-3]. Recent ad­ and tacticity. Accurate determination of some of these vances in ionization methods have made it possible to values requires that molecular ions of each oligomer characterize intact polymers by mass spectrometry. species be produced during ionization without frag­ These methods include field desorption (FO) [4, 5], mentation and that the ionization, instrument trans­ fast-atom bombardment (FAB) [6], secondary-ion mass mission, and detection efficiencies be known as a func­ spectrometry (SIMS)[7, 8], laser desorption (LO) [9-11], tion of mass and preferably not altered as a function of [~2, potassium ionization of desorbed species (KIOS) stereochemistry or reasonable changes in chemical 13], electrospray ionization (ESI) [14, 15], and matrix­ composition. assisted laser desorption-ionization (MALOI) [16]. In The MALDI technique has been shown to produce particular, the introduction of MALDI and ESI ioniza­ ions from several types of synthetic polymers. The tion techniques has dramatically increased the mass accuracy of the molecular weight distributions ob­ range for molecular weight analyses by mass spec­ tained by this technique previously have been assessed trometry. MALDI recently was shown to be applicable by comparison to other polymer characterization meth­ to ionization of starburst dendrimers that have molec­ ods such as SEC, light scattering, and so forth [17-21]. ular weights above 1,000,000 (P. Savickas, private com­ Montaudo et a1. [19] observed that Mp (most probable munication). Electrospray has produced ions from peak) values were consistently low for methyl­ polyethylene glycols that have molecular weights to methacrylate (MMA) polymers as measured by MALDI 5,000,000 [14]. in comparison to SEC Mp values. Burger et al. [18] Mass spectrometry has the potential to characterize used partially isolated oligomers of poly(R)-3-hy­ polymers and oligomers with greater accuracy and droxybutanoate to determine that MALOI discrimi­ precision for molecular weight and end group determi­ nates against low molecular weight 0540-u) oligomers nation than methods such as size exclusion chromatog­ for these polymers, but that for molecular weights raphy (SEC), laser light scattering, vapor phase os­ between 2900 and 8400 the response is molar. They mometry, or nuclear magnetic resonance spectroscopy. were able to calculate an ionization response factor for Polymer characteristics that are potentially available the low mass discrimination. Lehrle and Sarson [20] observed that MALOI gives low molecular weight values relative to SEC for a narrow polydispersity Address reprint requests to Dr. Barbara Larsen, The DuPont Com­ pany, r.o. Box 80228, Wilmington. DE 19880-0228. poly(methylmethacrylate) (PMMA) standard. These © 1996 American Society for Mass Speclrometry Received October 24,1995 1044-0305/96/$15.00 Revised December 11, 1995 SSDI 1044-0305(95)00706-7 Accepted December 14, 1995 288 LARSEN ET AL. JAm Soe Mass Speetrom 1996,7,287-292 authors suggested that laser-induced fragmentation or concentration of 5-mg matrix per milliliter of solvent. preferential desorption of low mass oligomers could The sample and matrix were mixed and .... 0.5 /-LL account for the low MALOI values. were spotted under ambient conditions on the stainless Here we report on the utility of MALOI as a means steel target. to characterize low molecular weight methacrylate polymers. Isolated syndiotactic and isotactic Results and Discussion poly(methylmethacrylate) oligomers that contain 25 and 50 repeat units were used to assess propensity for Figure 1 is the MALOI mass spectrum of a molecular MMA oligomers to fragment under MALDI conditions weight standard PMMA with Mp = 5270 (SEC). Table and to determine if observed ion currents are indepen­ 1 shows the comparison of Mp' M n , Mw, and 0 dent of oligomer mass. We previously reported that if values measured by MALDI and those supplied by the peak area compression is taken into account in peak manufacturer (SEC). With the exception of polydisper­ area calculation, the response by ESI mass spectrome­ sity (D), all molecular weight values measured by try for these PMMA oligomers is molar and fragmen­ MALOI are low in comparison to the SEC values. tation is not observed [15]. It is not apparent that a PMMA standards (MW < 50,000) have been shown to molar response will be obtained for oligomers in result in MALOI spectra that are lower in mass than MALDI because a molar response usually is not ob­ expected in comparison to the manufacturers' values served for peptide or protein mixtures [22]. of Mp [19]. Even after correction for the expected differences in the Mp values measured by SEC and Experimental MALOI, the MALOI values are lower than expected. The number assigned to the most probable peak (Mp) Mass spectra were obtained on the following time-of­ in a polymer distribution is dependent on how the flight (TOF) mass spectrometers: a Fisons TOFSpec data are plotted [25]. Low Mw values by mass spec­ (Manchester, England), a Finnigan VISION-MAT 2000 trometry may be the result of fragmentation, or mass (Brernen, Germany), and a Bruker Reflex (Bremen, discrimination in the ionization, transmission, or detec­ Germany). Data were obtained in both the linear and tion of ions, or possibly a matrix effect. reflectron modes on the Vision 2000 and the Bruker Reflex instruments. The deflector ion impact energy Fragmentation of Methylmethacrylate Oligomers (the combined source acceleration voltage and detector postaccelerating voltage) was either 25 or 30 kV, unless MALDI mass spectra of peptides and small proteins otherwise noted. The Vision 2000 was used for those generally show no significant fragment ions under experiments in which 6-kV ion impact voltage was MALDI-TOF conditions. Therefore, fragmentation of indicated by operating in the reflectron mode with MMA oligomers was not expected and is not observed 1-kV postacceleration. Nitrogen lasers (337 nm) were in the MALDI-TOF spectra of polymer distributions. used with beam attenuation set to be near ion appear­ However, fragmentation in which monomer (repeat) ance threshold. units are lost would not be observed in a spectrum of a PMMA samples were obtained from Polymer Labo­ distribution of oligomers because the fragment ions ratories (Church Stretton, UK). The individual MMA and molecular ions would overlap. oligomers were supplied by Professor K. Hatada (Osaka University, Japan). The samples were isolated from isotactic and syndiotactic MMA oligomers by CH CH using preparative supercritical fluid chromatography II3 3 [23,24]. The amounts of each oligomer collected ranged CH -C-CH-C from 0.1 to 0.7 mg. Equimolar solutions were obtained 2 I 2 I from weighed quantities of each isolated oligomer. C=O C=O Matrix materials, trans-3-indole acrylic acid (IAA; ICN II Biochemicals, Cleveland, OH), 2,5-dihydroxybenzoic OCH 3 OCH3 IJ acid (OHB; Sigma, St. Louis, MO), and [2-(4-hydroxy­ (it) phenylazol-benzoic acid] (HABA; Aldrich, Milwaukee, WI) were used as received from the manufacturer without purification. The PMMA samples were run by OCH 3 using IAA. The individual MMA oligomers were run I CH 3 C=O in IAA, OHB, and HABA. II Samples were prepared by dissolving the PMMA or CH -C-CH-C 2 I 2 I MMA oligomer samples in either chloroform or ace­ C=O CH tone to make a solution concentration of - 20 pm/J.LL. I 3 Two microliters of the polymer solutions were mixed OCH3 with 8 J.LL of the matrix solution. The matrix solution IJ was prepared in either chloroform or acetone at a (st) JAm Soc Mass Spectrom 1996, 7, 287-292 2200,---------------- --. 2150 2100 2050 2000 1950 IMIl J.I Jll. v ......" 3000 4000 5000 6000 Figure 1. MALDI-TOF mass spectrum of PMMA 5270 that used indole acrylic acid OAA) as the matrix and laser irradiation at 337 nm acquired in the reflectron mode. Table 1. A comparison of MALDl and SEC molecular weight values for PMMA 5270 Ion Current Detection as a Function of Oligomer MALO I SEC Mass 4600 5270 Weighed quantities of the isolated isotactic and syn­ 4625 4977 diotactic MMA 25-mers and 50-mers each were dis­ 4890 5256 solved in chloroform to make known molar solutions. 1.06 1.06 Equimolar (±5%) chloroform solutions of the 25-mers and 50-mers were prepared and premixed with three different matrix solutions. MALDI mass spectra of MALOI mass spectra of isolated isotactic and syn­ these solutions were obtained on three different diotactic MMA oligomers with 25 and 50 repeat units MALDI-TOF instruments to determine the molar re­ were obtained with three different matrix materials.
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