Evaluation of matrix-assisted laser desorption ionization mass spectrometry for polymer characterization

Mass spectra of atactic polystyrene were collected into the mega-dalton mass range with a matrix-assisted laser desorption ionization time of flight (MALDI TOF) mass spectrometer, which incorporates a cryodetector comprised of an array of 16 superconducting tunnel junctions (STJ). The STJ cryodetector, theoretically, has no loss in signal response at any mass compared with the reduced signal found at high mass when using a conventional secondaryionization detector. Since ion detection at high m/z is one of the fundamental limitations of mass spectrometry (MS), the cryodetector was used to explore the high m/z limit of the MALDI TOF technique for the analysis of two polymer types. Mass spectra were collected for polystyrene at M n 170, 400, 900, and 2000 kDa and polymethyl methacrylate (PMMA) at M n 62.6 kDa and 153.7 kDa. For polystyrene, the data showed a trend toward increased aggregation and charge state with mass. The M n 2 MDa polystyrene data revealed a peak at m/z 2.2 MegaTh and a charge state analysis revealed that these ions were primarily polystyrene aggregates with a mass of ϳ4 MDa. This aggregate assignment was possible because the cryodetector response allows for the determination of a charge state up to about four. The contribution of each charge state for a selected peak can be determined in this fashion. This analysis revealed the preferential formation of doubly charged even-numbered aggregates over oddnumbered aggregates for high molecular mass polystyrene. A potential mechanism for the aggregation process for doubly charged species is discussed. Karas and Hillenkamp [3]. The MALDI technique was a major breakthrough in mass spectrometry and it has seen widespread use in chemical and biological research because it offers several key advantages. First, it is a relatively "soft" ionization technique that can produce intact low charge state molecular ions at low laser power. Second, like electrospray ionization (ESI), MALDI is suitable for the ionization of large nonvolatile molecules including synthetic polymers and biomacromolecules, which cannot be brought into the gas phase and ionized intact by most other more energetic introduction and ionization techniques. Third, the MALDI technique is more robust than ESI for samples that contain high concentrations of salts and buffers. And finally, because MALDI can be operated in a pulsed ionization mode, it can be conveniently coupled to a time-of-flight (TOF) mass spectrometer. The theory and applications of the MALDI method are reviewed elsewhere [4 -6].It is reasonable to assume that the MALDI technique may have an upper m/z limitation. For example, the laser desorption ionization (LDI) technique, is limited to ϳ1000 Da for intact biological molecules and a few kDa for synthetic polymers [7]. Compared with LDI, MALDI shows reduced fragmentation of macromolecules [8] and the mass range has been extended to several 100 kDa. This extension of the mass range is attributed to the abundant matrix, usually in a ratio of at least 1000:1 with the analyte th...

Section: Sample Preparationmentioning

confidence: 99%

“…Before this study, others have collected mass spectra of high mass polystyrene and PMMA [15,28,29]. The highest reported MALDI TOF distribution of polystyrene ions were observed at m/z 750 kTh for the ϩ2 charge state of the 1.5 MDa polystyrene [15].…”

mentioning

confidence: 99%

The analysis of polystyrene and polystyrene aggregates into the mega dalton mass range by cryodetection MALDI TOF MS

Aksenov

Bier

2008

“…The assumption was valid in both cases, i.e., quantification of oligomers with different end groups by MALDI showed no obvious bias towards a particular end group, and as such no detailed investigation of an end group bias was done. In one case where the influence of end functional groups was quantified, Belu and coworkers explored the relative ionization yields of dimethylphenylsilyl-and perfluoroalkylsilyl-terminated PS as compared to that terminated with a proton but found little difference in the aggregate values [20]. In another study, Weaver and coworkers explored the quantification of hydroxyl-and hydrogen-terminated polystyrene, finding equal yields of the polymers for equal molar amounts.…”

mentioning

confidence: 99%

Characterization and relative ionization efficiencies of end-functionalized polystyrenes by matrix-assisted laser desorption/ionization mass spectrometry

Cox

Johnston

Dasgupta

2003

The properties and relative ionization efficiencies of a series of polystyrenes (PS) with hydroxyl, hydrogen, tertiary amine, and quaternary amine end-functionalities were examined by matrix-assisted laser desorption/ionization (MALDI) time-of-flight (TOF) mass spectrometry (MS). The hydrogen and hydroxyl functionalized PS ionized via attachment of a single Ag ϩ cation, as expected. However, tertiary amine PS oligomers were found in (M Ϫ H) ϩ and (M ϩ H) ϩ forms, in addition to M ϩ Ag ϩ , while the quaternary amine PS oligomers only ionized to the M ϩ form. Analysis of tertiary amine PS spectra revealed varying ratios of the three ionic forms depending on the oligomer length, pointing to a change in ionization efficiency. When the bulk samples were compared, the quaternary amine end-functionalized PS gave a ten-fold higher ionization efficiency over all others studied, likely because of the preexisting charge on the functionality. Samples with hydroxyl and hydrogen functionalities had similar ionization efficiencies, with the tertiary amine slightly higher, depending on the molecular weight. Changes in molecular weight affected the relative ionization efficiencies in varying fashion depending on the end functionality, though average molecular weight measurements were largely unaffected by end-functionality. Quantification of end-functionalized polystyrenes with different ionization efficiencies was found to be possible if due care was taken. (J Am Soc Mass Spectrom 2003, 14, 648 -657)

“…This change in peak intensities was attributed to side reactions such as chain redistribution and backbiting. Other studies have also used mass spectrometry to determine structural and mechanistic information on polymeric systems that would not be possible with other techniques [17][18][19][20].…”

Section: Inear Polydimethylsiloxane (Pdms) Is Primarilymentioning

confidence: 99%

Evaluation of matrix-assisted laser desorption ionization mass spectrometry for studying the sec-butyllithium and n-butyllithium initiated ring-opening polymerization of hexamethylcyclotrisiloxane (D₃)

Hawkridge

Gardella

2003

Matrix assisted laser desorption ionization (MALDI) was used to study the organolithium initiated ring-opening polymerization of hexamethylcyclotrisiloxane (D 3 ) in a mixed solvent system. The mass spectral peak intensities were monitored to determine the effects of polymerization time, initiator concentration, and reaction temperature on the formation of the mono, di, and trisiloxanolate initiator species and the extent of chain redistribution. The three initiator species were formed by reacting n-butyllithium and sec-butyllithium with D 3 in nonpolar solvent. The mass spectral results showed that sec-butyllithium and n-butyllithium form different populations of initiator species under the same conditions and that the measured mass spectral peak intensities do not accurately represent the population of siloxanolate initiator species prior to propagation. The changes in peak intensities were attributed to chain redistribution. . The molecular weight and product purity in these systems are difficult to control because both methods are susceptible to side reactions that broaden the polydispersity (M w /M n ). The most common side reactions are chain redistribution and cyclization shown in Scheme 1. Chain redistribution occurs when a propagating chain reacts with another chain to remove a segment of polymer without terminating the reaction. Cyclization occurs when a propagating chain reacts with itself to form cyclic species (D 3 , D 4 , D 5 , etc.). This type of side reaction is commonly referred to as chain backbiting. Lee et al. [3] first reported a new method for producing narrow molecular weight PDMS that was free of cyclic impurities. It was shown that n-butyllithium opened the D 3 ring in nonpolar solvents to form lithium siloxanolate initiators but did not propagate without the addition of a small amount (1-3%) of base promoting solvent (in this case these include polar aprotic solvents such as DMSO and THF). The control of the initiator basicity coupled with the high reactivity of the D 3 monomer significantly reduced chain redistribution and eliminated the formation of cyclic impurities.The reaction between organolithium initiators and D 3 is not well understood due in large part to the unique behavior of the initiators and the lithium counterions in solution. Organolithium compounds associate in nonpolar solvents to form aggregated species of various degrees due to the electron deficient nature of the lithium atom [4]. N-butyllithium, for example, forms a hexameric aggregate [5] whereas sec-butyllithium forms a tetrameric aggregate [6] in hydrocarbon solvents. Studies involving the reaction of organolithium species with styrene [7,8] and dienes [8,9] have shown that the degree of association directly affects the reactivity. The less associated organolithium species are found to be generally more reactive. Upon reacting with D 3 , it has been proposed that the lithium counter ions associate to form lithium siloxanolate aggregates. The degree to which these proposed structures are associated and the mech...