Polystyrene MALDI mass spectrum

The picture is rather different with synthetic polymers, as samples generally contain molecules with a wide range of molecular weights. Here, metal ion attachment, rather than protonation, is common and the matrix is thus prepared by addition of a suitable metal salt. Figure 28.32 shows a MALDI spectrum of polystyrene, with repeat units of 104 Da, produced by attachment of Ag ions. Clearly, the molecular weight distribution of the polymer can be readily determined from the MALDI mass spectrum. 

Figure 28.32 A MALDI mass spectrum of polystyrene (molecular weight distribution about 800-2400 Da). Note that each of the polymer (M) peaks is formed by attachment of Ag to yield M Ag+, and that the ions below m/z = 600 are produced from the matrix...

FIGURE 33.1 (A) MALDI-mass spectrum of polystyrene standard of Mp 5050 u and polydispersity 1.04, with dithranol matrix and silver salt reflector TOP mode. (B) MALDI-mass spectrum of polystyrene standard of Mp 31,500 u and polydispersity 1.04, with dithranol matrix and silver salt linear TOP mode. 

The MALDI-TOF spectrum of [G-3] poly(benzyl ether) dendrimer-po-ly(ethylene glycol) triblock copolymer shows a broad band of peaks between 4300 and 6100 D with resolution of the individual ethyleneoxide (44 D) units. The MALDI-TOF spectrum of a [G-3] dendrimer with two polystyrene blocks (molecular peak=8073 D) shows material with 6000-11,000 D and a broad band corresponding to material with 2 M+Ag+. SEC can be used to prove that the latter species is indeed an artifact of the mass spectroscopic method. The authors claim almost exact agreement between the polydispersities derived from MALDI-TOF and SEC [40]. This does, however, not leave any room for the unavoidable column spreading in the latter method. Furthermore, anionically prepared low MW polymers have a minimum polydispersity given by (1 + 1/DP) [41]. 

HPLC can be used also to separate macromolecules of different sizes. Spickermarm et al. used HPLC to separate the high mass fraction of a polystyrene sample. The MALDI-TOF spectrum of the high mass fraction turned out to be more informative than the MALDI-TOF spectrum of the whole sample, since the former displayed some peaks which were absent (or below the detection limit) in the latter spectrum. ... 

Figure 17 Copolymerfingerprint of polystyrene-Woc/r-polyisoprene (50/ 50 mol/mol), obtained from a MALDI-ToF mass spectrum. Reprinted from Wiiiemse, R. X. E. New Insights into Free-Radical (Co)Poiymerization Kinetics, Eindhoven University of Technoiogy Eindhoven, The Netheriands, 2005. ...

In principle, a determination of the mass allows for a measurement of the unique composition of block copolymers. It has been possible to determine the molecular weights and molecular weight distributions of both components of a block copolymer of polystyrene-WocA -poly(a-methylstyrene) by using MALDI-TOF [58]. The MALDI/TOF mass spectrum of a block copolymer sample containing 69% styrene and 31% a-methylstyrene is shown in Fig. 10.15. 

The MALDI-TOF spectrum with a matrix of dithranol of the cyclized polystyrene obtained from molar mass linear precursor (Mw = 1860). (Source Ref. [59, fig. 2].)... 

MALDI is the method of choice for the analysis of synthetic polymers because it usually provides solely intact and singly charged [62] quasimolecular ions over an essentially unlimited mass range. [22,23] While polar polymers such as poly(methylmethacrylate) (PMMA), [83,120] polyethylene glycol (PEG), [120,121] and others [79,122,123] readily form [M+H] or [M+alkali] ions, nonpolar polymers like polystyrene (PS) [99,100,105,106] or non-functionalized polymers like polyethylene (PE) [102,103] can only be cationized by transition metal ions in their l-t oxidation state. [99,100] The formation of evenly spaced oligomer ion series can also be employed to establish an internal mass calibration of a spectrum. [122]... 

Fig. 1.3 Spectrum obtained using MALDI-TOF of a sample of polystyrene using a dithranol matrix with silver trifluoroacetate added. (The peak masses are from the polymer chains combined with a silver ion.)...

The need for a cationization reagent in MALDI analysis of polymers can also create some complications in mass spectral interpretation [42, 56]. For example, the spectrum in Figure 8.3b shows a secondary distribution of lower intensity in addition to the principal distribution. This secondary distribution could be due to cation adduction with different ionic species and/or the presence of other polymeric species with different end-group structures. In this case, the secondary distribution has oligomer mass shifts of -1-22.4 Da from the nearest oligomer of lower mass in the principal distribution. This is consistent with the generation of salt cluster complexes, similar to what has been observed in the ESI of polystyrene [57]. For polyisoprene, it takes the form of [polyisoprene-i-Cu(copper retinoate)]L This amounts to an actual mass shift of -1-363.0 Da with respect to the principal distribution of [polyisoprene-tCujL This is consistent with the observed mass shift of 22.4 Da plus six repeat units of 64.2 Da. 

Fig. 3. MALDI-tof-ms of polyst5rrene around 7000 molecular mass. This MALDI spectrum was generated using dithranol as the matrix and AgTFA as the salt. The polystyrene, dithranol, and salt were dissolved in tetrahydrofuran. The solution was then electrosprayed onto the sample plate. The pol5rmer analyzed in reflectron mode with an extraction voltage of about 25 keV.

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