Fragmentation mechanisms of polyamides containing N-methylpyrrole and N-methylimidazole by electrospray ionization tandem mass spectrometry

Solvent-based and solvent-free characterization of low solubility and low molecular weight polyamides by mass spectrometry: a complementary approach

RATIONALE

Polyamides (PA) belong to the most used classes of polymers because of their attractive chemical and mechanical properties. In order to monitor original PA design, it is essential to develop analytical methods for the characterization of these compounds that are mostly insoluble in usual solvents.

METHODS

A low molecular weight polyamide (PA11), synthesized with a chain limiter, has been used as a model compound and characterized by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF-MS). In the solvent-based approach, specific solvents for PA, i.e. trifluoroacetic acid (TFA) and hexafluoroisopropanol (HFIP), were tested. Solvent-based sample preparation methods, dried-droplet and thin layer, were optimized through the choice of matrix and salt. Solvent-based (thin layer) and solvent-free methods were then compared for this low solubility polymer. Ultra-high-performance liquid chromatography/electrospray ionization (UHPLC/ESI)-TOF-MS analyses were then used to confirm elemental compositions through accurate mass measurement.

RESULTS

Sodium iodide (NaI) and 2,5-dihydroxybenzoic acid (2,5-DHB) are, respectively, the best cationizing agent and matrix. The dried-droplet sample preparation method led to inhomogeneous deposits, but the thin-layer method could overcome this problem. Moreover, the solvent-free approach was the easiest and safest sample preparation method giving equivalent results to solvent-based methods. Linear as well as cyclic oligomers were observed. Although the PA molecular weights obtained by MALDI-TOF-MS were lower than those obtained by (1)H NMR and acido-basic titration, this technique allowed us to determine the presence of cyclic and linear species, not differentiated by the other techniques. TFA was shown to induce modification of linear oligomers that permitted cyclic and linear oligomers to be clearly highlighted in spectra.

CONCLUSIONS

Optimal sample preparation conditions were determined for the MALDI-TOF-MS analysis of PA11, a model of polyamide analogues. The advantages of the solvent-free and solvent-based approaches were shown. Molecular weight determination using MALDI was discussed.

Amino-catalyzed fragmentation characteristics of polyamides containing N-methylpyrrole and N-methylimidazole by electrospray negative ionization mass spectrometry

Different negative fragmentations of synthesized polyamides containing N-methylpyrrole and N-methylimidazole were analyzed by electrospray ionization tandem mass spectrometry with low-energy collision-induced dissociation. An interesting rearrangement derived from amino catalysis of N-methylimidazole was observed. The observation is useful for the study of the correlations between the sequences of polyamides and their mass fragmentation pathways.

Studies on electrochemical oxidation mechanisms of polyamides containing N-methylpyrrole and N-methylimidazole by electrospray ionization tandem mass spectrometry

Electrochemical oxidization mechanism of polyamides containing N-methylpyrrole and N-methylimidazole was investigated by electrospray ionization tandem mass spectrometry (ESI-MS(n)). The spectra of the oxidization products unambiguously revealed that one or two oxygen atoms are added to the polyamide after the bulk electrolysis. It was found that electrochemical oxidation preferably takes place on the imidazole ring of the polyamide with both imidazole and pyrrole to produce a carbonyl group on the rings. ESI tandem mass spectrometry has proven to be an excellent method for the structural identification of electrochemical oxidation products of DNA-recognizing polyamides.