Modern mass spectrometry of synthetic polymers involves soft ionization techniques. Whereas matrix-assisted laser desorption/ionization (MALDI) and electrospray (ESI) are employed routinely, atmospheric pressure chemical ionization (APCI) and more recently atmospheric pressure photoionization (APPI) are used to a lesser extent. However, these latter ionization methods coupled to liquid-phase separation techniques create new opportunities for the characterization of polymers, especially for low molecular weight compounds or for the polymers that are poorly ionizable by the usual methods. After a part devoted to the description of classical MS methods employed for polymer analysis (MALDI, ESI, and their use with chromatography), APCI and APPI techniques will be described, discussed, and selected examples will present the interest of these ionization sources (or interfaces for LC/MS) in the field of polymer analysis.

Triblock copolymers of ethylene oxide (EO) and propylene oxide (PO) are widely used in the chemical industry as nonionic surfactants. Triblock copolymers can be arranged in a EO-PO-EO or PO-EO-PO sequence. This arrangement results in an amphiphilic copolymer, in which the block sequence and block length determine the properties of the copolymer. MALDI-TOF MS was used to analyze various triblock copolyethers: EO-PO-EO (Mn =2000 g.mol(-1)), PO-EO-PO (Mn = 2000 g.mol(-1)), and a random copolymer EO/PO (Mn = 2500 g.mol(-1)). Data treatment was assisted by using a homemade software allowing a picture of monomer composition of oligomers from the mass spectra. MALDI-TOF mass spectra of EO/PO copolymers were shown to depend strongly on the number of laser shots, relative proportions of polymer/salt, and the nature of the matrix. An unsaturated byproduct was detected. Its presence was demonstrated by prefractionation of copolymers by SEC before MALDI-TOF analysis, and its content was estimated by 1H NMR. The formation of layers inside the MALDI deposit was evidenced by varying the number of laser shots. Lighter oligomers of the copolymer, unsaturated byproduct, or both would be in the core of the deposit, coated with heavier oligomer. The layer formation depends on the nature of the matrix and the quantity of added salt. DHB matrix with a relative high sodium salt content induces layer formation inside the deposit, whereas dithranol matrix or low salt content does not. Consequently, an optimization of experimental parameters in order to estimate the lighter oligomers or unsaturated byproduct content or to obtain the actual representation of the monomer contribution in the copolymers from the MS data only seems obviously critical. MALDI-TOF mass spectrometry is obviously a powerful technique to analyze copolymers, but a careful survey of the experimental parameters is required. The combination of MALDI-TOF MS with separations techniques and NMR brings precious complementary information.

Chemical properties of ethylene oxide (EO) and propylene oxide (PO) block copolymers are strongly dependent on their sequence. Useful information about copolymer sequence can be obtained by tandem mass spectrometry (MS/MS). In this work, collision-induced dissociation (CID) of ammonium adducts of various linear triblock and glycerol derivative diblock copolyethers produced by electrospray ionization was studied under low-energy conditions. At first, homopolymers MS/MS spectra enabled us to identify the nature of the product ions and to suggest decomposition pathways. Then, it was shown that copolyethers with the same composition in each repeat unit but with inversed block sequences (i.e., PEO-b-PPO-b-PEO vs PPO-b-PEO-b-PPO and gPEO-b-PPO vs gPPO-b-PEO) can be easily distinguished with characteristic fragment ions. In the case of linear copolymers, CID spectra gave pertinent information about block lengths.