Using matrix-assisted laser desorption/ionization time-of-flight mass spectrometry for the characterization of functionalized carbon nanotubes.

RATIONALE: Functionalization of carbon nanotubes (CNTs) generates complex systems that require the development of suitable characterization protocols. New techniques have been explored, and existing analytical and spectroscopic methods to characterize functionalized CNTs have been adapted. Presently, chemical characterization of functionalized CNTs (f-CNTs) remains a difficult task. METHODS: Matrix-assisted laser desorption/ionization (MALDI) analysis is performed on f-MWCNT samples prepared via grafting or absorption of anti-oxidant (AO) molecules on both MWCNT-COOH and MWCNT-OH. Covalently functionalized MWCNTs were subjected to thermal degradation and/or hydrolysis reaction before analysis, whereas MWCNTs with a physical adsorption of the functionalizing molecules were directly spotted in the target sample. Noteworthy, in our approach f-MWCNTs constitute at the same time analyte and MALDI matrix. RESULTS: The identification of functionalizing AO molecules is ascertained after degradation or hydrolysis reactions in both MWCNT-COOH and MWCNT-OH grafted samples. Absorbed AO molecules, as well as organic impurities derived from grafting reactions, are also revealed by MALDI MS without any preliminary cleavage reaction. CONCLUSIONS: A simple MALDI-TOF mass spectrometry method permits to obtain the unambiguous discrimination between grafted or adsorbed functionalized molecules onto the surface of MWCNTs.

Matrix-assisted laser desorption/ionisation time-of-flight characterisation of biodegradable aliphatic copolyesters

Matrix-assisted laser desorption/ionisation time-of-flight (MALDI-TOF) and the off-line size exclusion chromatography matrix-assisted laser desorption/ionisation (SEC/MALDI) method has been applied to the structural characterisation and the molar mass (MM) determination of a series of biodegradable copolyesters synthesised by high temperature melt polycondensation reaction, and of two commercial copolyesters with the trade name Bionolle. The MALDI-TOF spectra of these copolymers showed the presence of cyclic oligomers in the lower mass region, in accord with expectations from polycondensation kinetics, and the presence of all linear species expected from their method of synthesis. The presence of unexpected linear species with olefin and carboxyl as end groups suggested the occurrence of undesirable thermal degradation processes during the melt polycondensation reaction. The absolute average molar masses obtained by the SEC-MALDI method turned out to be lower, by a factor of about two for succinate/adipate copolymers, and by a factor of three for succinate/sebacate copolymers, with respect to those computed by using polystyrene standards in SEC. Furthermore, the MALDI-TOF spectra of SEC fractions allowed not only the detection of linear and cyclic oligomers contained in these samples, but also the simultaneous determination of the average molar mass of both cyclic and linear oligomers. Due to the smaller hydrodynamic volume of cyclic chains with respect to linear ones, the ratio (M( cycle)/M( linear))( Ve) at a fixed elution volume was found to be 1.25, in good agreement with the theoretical value of 1.24. Copyright 2000 John Wiley & Sons, Ltd.

Analysis of poly(bisphenol A carbonate) by size exclusion chromatography/matrix-assisted laser desorption/ionization. 1. End group and molar mass determination

The determination of molar mass (MM) data for polydisperse polymers by SEC/MALDI involves the fractionation of samples through analytical size exclusion chromatography (SEC). Selected SEC fractions are then analyzed by matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) and the mass spectra of these nearly monodisperse samples allow the determination of the average molar masses. The SEC/MALDI procedure has now been applied to two polycarbonate samples, PC1 and PC2. The results show that the MALDI spectra of the SEC fractions allow not only the detection of linear and cyclic oligomers contained in these samples, but also the simultaneous determination of their average molar masses. Two slightly differing SEC calibration plots were obtained, due to the smaller hydrodynamic volume of the polycarbonate cyclic chains with respect to the linear ones. In agreement with theory, the ratio (M(cycle)/M(linear))(Ve) at a fixed elution volume was found to be 1.22, independent of the molar mass values. Copyright 1999 John Wiley & Sons, Ltd.

Analysis of poly(bisphenol A carbonate) by size exclusion chromatography/matrix-assisted laser desorption/ionization. 2. self-association due to phenol end groups

We report here a case of apparent failure of the size exclusion chromatography/matrix-assisted laser desorption/ionization (SEC/MALDI) method to provide polymer fractions with narrow molar mass distribution, showing that intermolecular chain association is responsible for this phenomenon. Poly(bisphenol A carbonate) (PC) chains terminated with hydroxyl groups undergo self-association by hydrogen bonding, providing macromolecular aggregates with higher hydrodynamic volume. These aggregates are eluted through SEC columns at the same volume as higher molar mass chains, which remain non-associated. Thus, self-association affects negatively the SEC fractionation experiments, and even the sharpest SEC fractions contain a heterogeneous mixture of PC chains of different size. When the off-line SEC/MALDI procedure is applied, the SEC fraction is diluted in the matrix which, being a dissociating medium (carboxylic acid) for hydrogen-bonded aggregates, suppresses the chains' self-association. Therefore, the MALDI spectra of these PC fractions indicate a polydisperse character, with irregular bimodal distributions of peaks. As a consequence, in the presence of chain association, the SEC/MALDI method for the calculation of molar masses of polymers cannot be directly applied. In the present case we have found that, under opportune experimental conditions, self-association in polycarbonates can be avoided, so that nearly monodisperse SEC fractions can be obtained and the SEC/MALDI method can be applied. Our results also show that MALDI is a very sensitive technique for the detection of association of polymers in dilute solutions. Copyright 1999 John Wiley & Sons, Ltd.