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

Rapid communications in mass spectrometry : RCM (2012-05-05)
Caroline Barrère, Marie Hubert-Roux, Catherine M Lange, Majed Rejaibi, Nasreddine Kebir, Nicolas Désilles, Laurence Lecamp, Fabrice Burel, Corinne Loutelier-Bourhis
ABSTRACT

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. 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. 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. 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.

MATERIALS
Product Number
Brand
Product Description

Supelco
2,5-Dihydroxybenzoic acid, matrix substance for MALDI-MS, >99.0% (HPLC)
Sigma-Aldrich
Sodium iodide, AnhydroBeads, −10 mesh, 99.999% trace metals basis
Sigma-Aldrich
2,5-Dihydroxybenzoic acid, 98%
Sigma-Aldrich
Sodium iodide, ≥99.99% trace metals basis
Sigma-Aldrich
Sodium iodide, 99.999% trace metals basis
Supelco
2,5-Dihydroxybenzoic acid, matrix substance for MALDI-MS, ≥99.5% (HPLC), Ultra pure
Sigma-Aldrich
Sodium iodide, Vetec, reagent grade
Sigma-Aldrich
Sodium iodide, ReagentPlus®, ≥99%
Sigma-Aldrich
Sodium iodide, ACS reagent, ≥99.5%
Sigma-Aldrich
Sodium iodide, puriss., meets analytical specification of Ph. Eur., BP, USP, 99-100.5% (calc. to the dried substance)
Sigma-Aldrich
Sodium iodide, puriss. p.a., ≥99.0% (AT)
Sigma-Aldrich
Gentisic acid sodium salt hydrate, ≥98%