Silver cluster interferences in matrix-assisted laser desorption/ionization (MALDI) mass spectrometry of nonpolar polymers

Potential difficulties associated with background silver salt clusters during matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) of nonpolar polymers are reported. Silver salt cluster ions were observed from m/z 1500 to 7000 when acidic, polar matrices, such as 2,5-dihydroxybenzoic acid (DHB), all-trans-retinoic acid (RTA) or 2-(4-hydroxyphenylazo)benzoic acid (HABA), were used for the analysis of nonpolar polymers. These background signals could be greatly reduced or eliminated by the use of nonpolar matrices such as anthracene or pyrene. Representative examples of these background interferences are demonstrated during the analysis of low molecular weight nonpolar polymers including polybutadiene and polystyrene. Nonpolar polymers analyzed with acidic, polar matrices (e.g., RTA) and silver cationization reagents can yield lower quality mass spectral results when interferences due to silver clusters are present. Replacing the polar matrices with nonpolar matrices or the silver salts with copper salts substantially improved the quality of the analytical results. In addition, it was found that silver contamination cannot be completely removed from standard stainless steel sample plates, although the presence of silver contamination was greatly reduced after thorough cleaning of the sample plate with aluminum oxide grit. Carry-over silver may cationize polymer samples and complicate the interpretation of data obtained using nonpolar matrices in the absence of added cationization reagents.     

Analysis of hydrocarbon polymers by matrix-assisted laser desorption / ionization-fourier transform mass spectrometry

A quick and effective sample preparation is demonstrated for matrix-assisted laser desorption/ionization (MALDI) analysis of nonpolar polymers. Polyisoprene, polystyrene, and polybutadiene polymers were investigated by using as matrix a 2,5-dihydroxybenzoic acid and silver nitrate combination. Silver cationized oligomers produce useful spectra that can be signal averaged to characterize polymer distributions extending up to 6000 u by using a 3-T Fourier transform mass spectrometer. Because an electrostatic ion deceleration protocol was used to extend the mass range, trapping discrimination is shown to exist for molecular weight distributions broader than about 2500 u. However, an integral procedure can be used to reconstruct the true polymer profiles through co-addition of signal transients obtained by using various gated deceleration times. For polymers with narrower mass distributions, silver cationization along with signal averaging provides rapid and accurate polymer characterization for nonpolar polymer systems by using standard MALDI Fourier transform mass spectrometry instrumentation.     

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

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. Dithranol was identified as a good matrix for polystyrene (PS), 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(methyl methacrylate), and polydimethylsiloxane] 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 _n 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 (±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 PS suggests that the oligomers are desorbed preferentially from the surface in the TOF-SIMS analysis, rather than having an increased ionization probability.     

The use of nonpolar matrices for matrix-assisted laser desorption/ionization mass spectrometric analysis of high boiling crude oil fractions

Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOFMS) with nonpolar matrices has been investigated for its applicability to the characterization of atmospheric resid crude oil fractions. The data obtained by use of nonpolar matrices was compared with that from polar matrices as well as from direct LDI-MS and field ionization mass spectrometry. Nonpolar matrices, such as anthracene or 9-cyanoanthracene, yield only a single radical molecular cation upon LDI. Thus, no interfering matrix-related ions are present during the MALDI-TOFMS analysis of the crude oil sample. Nonpolar matrices yield molecular mass distributions from linear mode MALDI-TOFMS that are comparable to distributions found with LDI-MS. An advantage of nonpolar matrices is the increased production of analyte ions, which allows reflectron mode MALDI-TOFMS to be performed. Nonpolar matrices are also shown to be less sensitive to solvent and sample preparation conditions than conventional polar matrices. These results suggest that nonpolar matrices may be favorable alternatives to more traditional polar or acidic matrices commonly used in the MALDI mass spectral characterization of crude oil related samples.     

Silver nitrate chemical ionization for analysis of hydrocarbon polymers by laser desorption fourier transform mass spectrometry

Addition of silver nitrate to nonpolar hydrocarbon polymer solutions prior to their analysis by laser desorption Fourier transform mass spectrometry is shown to allow efficient silver ion chemical ionization. High-quality mass spectra are thus obtained for polymers, such as polystyrene, polyisoprene, polybutadiene, and polyethylene, that previously failed to yield useful mass spectra using conventional laser desorption. Accurate mass measurement experiments and isotopic ratios verify that the spectra are those of silver-attached oligomer ion distributions. Mass measurement errors for complete oligomer distributions average between 3 and 12 ppm for oligomers with masses between 400 and 6000 D whereas unit mass resolution is maintained throughout the entire spectral range.     

The analysis of industrial synthetic polymers by electrospray droplet impact/secondary ion mass spectrometry

Electrospray droplet impact (EDI)/secondary ion mass spectrometry (SIMS) is a new desorption/ionization technique for mass spectrometry in which highly charged water clusters produced from the atmospheric‐pressure electrospray are accelerated in vacuum by several kV and impact the sample deposited on the metal substrate. In this study, several industrial synthetic polymers, e.g. polystyrene (PS) and polyethylene glycol (PEG) were analyzed by EDI/SIMS mass spectrometry. For higher molecular weight analytes, e.g. PS4000 and PEG4600, EDI/SIMS mass spectra could be obtained when cationization salts are added. For the polymers of lower molecular weights, e.g. PEG300 and PEG600, they could be readily detected as protonated ions without the addition of cationization agents. Anionized PS was also observed in the negative ion mode of operation when acetic acid was added to the charged droplet. Compared to matrix‐assisted laser desorption/ionization (MALDI), ion signal distribution with lower background signals could be obtained particularly for the low‐molecular weight polymers. Copyright © 2009 John Wiley & Sons, Ltd.     

Anionic graft copolymerization of diene polymers with vinyl monomers

A mixture of homopolymer and graft copolymer was obtained by adding the monomer at 0°C to the polylithiodiene solution. Styrene, methyl methacrylate, and acrylonitrile were used as the monomers. Polylithiodienes were prepared by the metalation of diene polymers, i.e., polybutadiene or polyisoprene, with the use of n-butyllithium in the presence of a tertiary amine (N,N,N′,N′-tetramethylethylenediamine) in n-heptane. The graft copolymers were separated by solvent extraction and were confirmed by turbidimetric titration and elementary analysis. Oxidation of the polybutadiene–styrene grafts revealed that the molecular weight of the side chains was the same as the molecular weight of the free polystyrene formed. The grafting efficiency and grafting percentage were studied for polybutadiene–styrene graft copolymers prepared under various conditions.     

Copper(I) chloride: a simple salt for enhancement of polystyrene cationization in matrix-assisted laser desorption/ionization mass spectrometry

The possibility of using copper(I) chloride as a doping salt to enhance the cationization of polystyrene in matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) was investigated. It was shown that copper(I) chloride possesses sufficient solubility in tetrahydrofuran. The parameters of the MALDI mass spectra of different polystyrene samples, such as the number-average (M(n)) and mass-average (M(w)) molecular mass values, obtained by copper(I) cationization were compared with those obtained by means of silver(I) cationization, and good agreement was found. It was also shown that application of copper(I) chloride as a doping salt, and dithranol as a matrix, ensured good MALDI mass spectra of the sample spots even after storage for 1 month.     

Investigation of metal attachment to polystyrenes in matrix-assisted laser desorption ionization

Cationization is essential to the matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF) mass spectrometric analysis of a variety of synthetic polymers. This work studied polystyrene cationization in MALDI using the salts of eight different metals. It was found that only the salts of silver, copper, and palladium produced good metal—polystyrene cation signals. More interestingly, it was observed that MALDI could also produce metal-rich cluster cations and that the presence of polystyrenes tended to suppress formation of the metal-rich cluster cations. Based on these results and others, we propose that polystyrene cationization may proceed through gas-phase metal attachment reactions under the conditions used. With this argument, we were able to better explain a reported experimental observation that showed a strong cation concentration effect on measured molecular weight distributions of polystyrenes.     

Identification of low molecular weight carbohydrates employing new binary mixtures for matrix-assisted laser desorption/ionisation mass spectrometry

Various energy-absorbing substances, aminopyrazine (AP), 4,4'-azodianiline (ADA), and 1-chloro-4-hydroxyisoquinoline (CHIQ), together with their binary mixtures with existing acidic MALDI matrices were subjected to matrix-assisted laser desorption/ionisation mass spectrometry (MALDI-MS) and evaluated for the analysis of low molecular weight carbohydrates. The newly introduced systems, especially AP and the combination of 2,5-dihydroxybenzoic acid and aminopyrazine (DHB-AP), have solved almost all the existing problems of the generally low sensitivity of carbohydrate analysis and of the strong background noise produced from single acidic matrices. In fact, especially at a mixing ratio of 3:1 (DHB/AP), outstanding results could be achieved, enabling the detection of analytes down to a concentration of 4 fmol/microL with mass accuracy of 37 ppm. The performance of the system was finally proven by analysing dextrins and biological samples each of which showed excellent signal intensity and signal-to-noise ratio.     

Synthesis of well‐defined comb‐like graft (co)polymers by nucleophilic substitution reaction between living polymers and polyhalohydrocarbon

A series of graft (co)polymers were synthesized by nucleophilic substitution reaction between iodinated 1,2‐polybutadiene (PB‐I, backbone) and living polymer lithium (side chains). The coupling reaction between PB‐I and living polymers can finish within minutes at room temperature, and high conversion (up to 92%) could be obtained by effectively avoiding side reaction of dimerization when living polymers were capped with 1,1‐diphenylethylene. By virtue of living anionic polymerization, backbone length, side chain length, and side chain composition, as well as graft density, were well controlled. Tunable molecular weight of graft (co)polymers with narrow molecular weight distribution can be obtained by changing either the lengths of side chain and backbone, or the graft density. Graft copolymers could also be synthesized with side chains of multicomponent polymers, such as block polymer (polystyrene‐b‐polybutadiene) and even mixed polymers (polystyrene and polybutadiene) as hetero chains. Thus, based on living anionic polymerization, this work provides a facile way for modular synthesis of graft (co)polymers via nucleophilic substitution reaction between living polymers and polyhalohydrocarbon (PB‐I). © 2013 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2013     

The metathetic degradation of polyisoprene and polybutadiene in block copolymers using Grubbs second generation catalyst

A degradation study of polystyrene-polybutadiene-polystyrene and polyisoprene-polystyrene-polyisoprene in both dichloromethane and hexane solvents is presented. Alternative solvents for metathetic degradation provide the potential for greener chemistry, better selectivity, and control over the products. The catalyst concentration and solvent selection both determine the products formed. The degradation of polyisoprene and polybutadiene in a particular solvent was controlled by the solubility of polyisoprene/polybutadiene, and by its solubility relative to polystyrene. A large difference in solubility between the polymers in the selected solvent provides an additional driving force for block separation, encouraging reaction close to the interface between different blocks. Furthermore, solubility of the block copolymer speeds the degradation reaction. This tailoring of the reaction mechanism yields a new control over the products of polymer degradation.     

Anionic polymerization of styrenic macromonomers of polyisoprene,polybutadiene, and polystyrene

Anionic polymerization and high‐vacuum techniques were used to prepare a series of well‐defined polyisoprene, polybutadiene, and polystyrene polymacromonomers. The procedure involved (1) the synthesis of styrenic macromonomers in benzene by the selective reaction of the corresponding macroanion with the chlorine of 4‐(chlorodimethylsilyl)styrene (CDMSS) and (2) the in situ anionic polymerization of the macromonomer without previous isolation. The synthesis of the macromonomers [polyisoprene macromonomer: 11 samples, weight‐average molecular weight (M_w) = 1000–18,000; polybutadiene macromonomer: 5 samples, M_w = 2000–4000; and polystyrene macromonomer: 2 samples, M_w = 1300 and 3600] was monitored by size exclusion chromatography with refractive index/ultraviolet detectors. Selectivity studies with CDMSS indicated that polybutadienyllithum had the highest selectivity, and polystryryllithium the lowest. From kinetic studies it was concluded that the polymerization half‐life times were longer but comparable to those of styrene, and they appeared to only slightly depend on the molecular weight of the macromonomer chain (at least for low degrees of polymerization of the polymacromonomer and for M_w < 7000 for the macromonomer side chain). Dependence on the polymerization degree of the polymacromonomer product was also observed. All the prepared polymacromonomers were characterized by size exclusion chromatography with refractive index, ultraviolet and two‐angle laser light scattering detectors, and NMR spectroscopy. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 1038–1048, 2005     

Hydrodynamic behavior of anionically prepared linear polyisoprenes and polystyrenes in carbon tetrachloride

Intrinsic viscosity, [η], weight-average molecular weight, M_w, relationships are reported for narrow molecular weight distribution linear polyisoprene and polystyrene samples in CCl₄ at 25°C. Molecular weight values cover a range nearly two orders in magnitude, extending as low as 3 × 10³. In the case of polystyrene there exists a molecular weight range (around M_w = 16,700) corresponding to a change in the Mark-Houwink-Sakurada (MHS) exponent from α = 0.71 to α = 0.54. Comparisons between the viscometric and hydrodynamic radii, from literature data, are made. For polyisoprene the MHS relationship is reported in CCl₄, for the first time. For this case α = 0.713 for the whole range of molecular weights studied. Values for the second virial coefficient from low-angle light-scattering measurements support the conclusions drawn from viscometry that CCl₄ is a good solvent for both polymers studied. The different behavior of the MHS exponent may be attributed to the difference in chain flexibility. © 1995 John Wiley & Sons, Inc.     

Carbon disulfide reagent allows the characterization of nonpolar analytes by atmospheric pressure chemical ionization mass spectrometry

While atmospheric pressure ionization methodologies have revolutionized the mass spectrometric analysis of nonvolatile analytes, limitations native to the chemistry of these methodologies hinder or entirely inhibit the analysis of certain analytes, specifically, many nonpolar compounds. Examination of various analytes, including asphaltene and lignin model compounds as well as saturated hydrocarbons, demonstrates that atmospheric pressure chemical ionization (APCI) using CS₂ as the reagent produces an abundant and stable molecular ion (M^+?) for all model compounds studied, with the exception of completely saturated aliphatic hydrocarbons and the two amino acids tested, arginine and phenylalanine. This reagent substantially broadens the applicability of mass spectrometry to nonvolatile nonpolar analytes and also facilitates the examination of radical cation chemistry by mass spectrometry. Copyright © 2011 John Wiley & Sons, Ltd.     

Electrospray droplet impact/secondary ion mass spectrometry (EDI/SIMS) using NaF and AgF as cationization matrices

In our previous paper, it was suggested that metal fluorides may be useful as cationization matrices in Electrospray droplet impact/SIMS. In this work, NaF and AgF were used as the cationization matrices for cyclodextrin (CD), polyethylene glycol (PEG), polystyrene (PS), garlic juice, and sliced raw rice. EDI mass spectra were measured without and with the use of matrices. Enhancement of ion abundances of [M + Na]⁺ for CD and PEG with NaF matrix and that of [M + Ag]⁺ for PS with AgF matrix were observed. However, the addition of matrices was not effective for the cationization of garlic juice and sliced raw rice samples. This may be due to the Coulombic repulsion of the reagent ions of Na⁺ or Ag⁺ with the preformed K⁺ adducts of oligosaccharides already present in the samples. Copyright © 2015 John Wiley & Sons, Ltd.     

Matrix-assisted laser desorption ionization time-of-flight mass spectrometry for the analysis of polydienes

An analytical method based on matrix-assisted laser desorption ionization (MALDI) time-of-flight mass spectrometry has been developed to provide information on oligomer structure, average molecular weight, and molecular weight distributions of polydienes (e.g., polybutadiene and polyisoprene), an important class of industrial polymers. This MALDI method involves the use of all-trans-retinoic acid as the matrix, copper (II) nitrate as the cationization reagent, and tetrahydrofuran as the solvent. The incorporation of this copper salt generates Cu⁺ adducts with the polymer chains. It also improves the signal strength and extends the upper mass range when used with all-trans-retinoic acid, as compared to silver nitrate. With this formulation, it is demonstrated that polybutadienes of narrow polydispersity with masses up to 300,000 u and polyisoprenes of narrow polydispersity with masses up to 150,000 u can be analyzed. The upper molecular weight limit is set by the requirement of using higher matrix-to-polymer ratios with increasing polymer molecular weight, to the point where the instrument can no longer detect the small quantity of polymer present in the matrix host. It is also shown that this sample preparation generates previously unreported adduction behavior. The practical implications of this adduction behavior on polymer structural analysis, accuracy of molecular weight determination, and the upper molecular weight limit of oligomer resolution are discussed. It is illustrated that, in a linear time-lag focusing MALDI instrument, oligomer resolution can be obtained for polydienes with molecular weights up to 24,000, providing structural confirmation of the end-groups and the repeat unit. The average molecular weights of a number of polydienes of narrow polydispersity determined by MALDI are compared to those obtained by gel permeation chromatography, and discrepancies are noted.     

Synthesis of well‐defined polymers end‐functionalized with crosslinkable aziridine groups by living anionic polymerization

Well‐defined end‐functionalized polystyrene, poly(α‐methylstyrene), and polyisoprene with polymerizable aziridine groups were synthesized by the termination reactions of the anionic living polymers of styrene, α‐methylstyrene, and isoprene with 1‐[2‐(4‐chlorobutoxy)ethyl]aziridine in tetrahydrofuran at ?78 °C. The resulting polymers possessed the predicted molecular weights and narrow molecular weight distributions (weight‐average molecular weight/number‐average molecular weight < 1.1) as well as aziridine terminal moieties. The cationic ring‐opening polymerization of the ω‐monofunctionalized polystyrene having an aziridinyl group with Et₃OBF₄ gave the polymacromonomer, whereas the α,ω‐difunctional polystyrene underwent crosslinking reactions to afford an insoluble gel. Crosslinking products were similarly obtained by the reaction of the α,ω‐diaziridinyl polystyrene with poly(acrylic acid)‐co‐poly(butyl acrylate). © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 4126–4135, 2005     

On-line viscometry combined with gel permeation chromatography. I. Instrumental calibration and testing with linear polymers

Gel permeation chromatography (GPC) was combined with flow time measurements on the eluent to provide both the distribution of hydrodynamic volumes and the distribution of intrinsic viscosities in linear polymers. Standard polystyrene samples were used to establish a universal hydrodynamic volume calibration as well as the zone spreading and viscometer transfer line tailing parameters. Viscometry data are particularly helpful in establishing the zone spreading parameters and the calibration curve at very high molecular weights. The results were applied to measurements on samples of linear polybutadiene and polyvinyl acetate. Agreement between values of M _w from GPC with those obtained by light scattering confirmed the universal calibration principle.     

Synthèse de polyalcadiènes à extrémités fonctionnelles chlorure d'acide—étude de la réactivité de ces fonctions terminales

Two types of chemical modification of polymers are considered: attachment of functional groups as end-groups for diene polymers and also reaction of these end-groups with diamines. The deactivation, by means of the di-acid chloride of ortho-phthalic acid, of living polybutadiene or living polyisoprene leads to polydienes with molecules having one or two acid chloride end-groups. Chemical modification of these end-groups by an aliphatic diamine leads either to a new polymer having amine end-groups or to a polycondensation causing a substantial increase in molecular weight, depending upon the conditions.