The list of compounds analyzed by matrix-assisted laser desorption ionization (MALDI) mass spectrometry has grown steadily, and the technique has become a viable tool for detailed characterization of synthetic polymers. These materials play very important roles in everyday life as well as in industry. Several techniques such as gel-permeation chromatography (GPC), pyrolysis gas chromatography (GC), nuclear magnetic resonance (NMR) and mass spectrometric (MS) methods have been used to characterize synthetic polymers. With the advent of MALDI-MS a new method of polymer characterization has emerged. The capability of this technique in the analysis of synthetic polymers had lagged behind, due to the unique chemical, physical, and mechanical properties of these materials. Very small variations in the structure of a given polymer formulation can significantly change its characteristics, resulting in new functions and applications. As a general trends, solubility of synthetic polymers increases as the molecular weight decrease; however, other important physical properties such as viscosity, strength, and flexibility improve with increasing molecular weight. In addition, ionization of synthetic polymers is achieved via cationization, a pathway, to which very little work has been done. Neither the mechanism of cationization, nor the general ionization process in MALDI is well understood. The most direct way to optimize mass spectral analysis of polymers has led to exploring new matrices, solvents, and ionizing agents.
Since the first reports of the application of MALDI for synthetic polymer analysis [1-2], significant advances in sample preparation protocols and instrumentation development have been taken place. MALDI-MS has been utilized to gain information on monomer reactivity ratios , end groups [4-5], and mechanisms of polymerization [6-7]. Many attempts with a variety of success have also been made to use this technique to obtain values for number averaged molecular weight, Mn, weight averaged molecular weight, Mw, polydispersity, D, repeat unit, and end group masses. The following formula describe these properties:
Where Ni and Mi are the abundance and mass of the ith oligomer, respectively. Polydispersity is then the ratio of equations 1 and 2.
Molecular weight and polydispersity data can be used to verify synthetic pathways, study degradation mechanism, look for additives and impurities, compare product formations, and provide QC data on batch-to-batch compositional variations. End-group and chemical structure data are critical to understanding structure-property relationships of polymer formulations.
One of the key advantages of MALDI-MS for synthetic polymer analysis is that the absolute molecular weights of oligomers can be determined as opposed to obtaining relative molecular weights by chromatographic techniques. In addition, it can determine molecular weight independent of polymer structure. It is important to note that for only narrow distributions where the Mnand Mw values determined by MALDI-MS are in qualitative agreement with chromatographic values .
Compared to MALDI-MS of proteins where protonated species with molecular weight up to 300,000 Da can be easily produced, the analysis of synthetic polymers can present challenges even in the 20,000 Da range. However, MALDI-MS is considered as a complementary method to GPC, a routine technique for determination of polymer mass distributions, because GPC does not always give reliable information below 20,000 Da. In addition, the operation of GPC is extremely time-consuming whereas MALDI data can be obtained in less than 20 minutes.
The solubility of matrix and polymers in the same solvent is critical for polymer analysis. Some non-aqueous polymers can simply be dissolved in acetone or methanol in order to get MALDI signals. Based on their solubility, synthetic polymers can be classified into the following four groups :
- Water-soluble polymers: poly(acrylic acid) and poly(ethylene glycol, PEG).
- Polar organic-soluble polymers: acrylics and poly(methyl methacrylate, PMMA).
- Nonpolar organic-soluble polymers: polystyrene (PS), ployvinyl chloride, and polyethlene.
- Low solubility polymers: cured polyimide.
Polymers of group 4 are soluble in solvents that are not compatible with matrix materials. Thus, they are the most difficult to analyze by MALDI because a homogeneous mixture of matrix and analyte is not easily obtained.
Sample preparation for the first three classes of polymers is fairly well established . Matrices for these polymers include 1,8,9-anthracenetriol (dithranol), DHB, trans-3-indoleacrylic acid, and 2-(4-hydroxyphenylazo)benzoic acid. However, due to the diversity of polymer materials, there is no universal approach in MALDI-MS to characterize a particular group of polymeric system. It was shown that even in the presence of acidic matrices, some of the synthetic polymers prefer cationization with a metal.
Some of the synthetic polymers lack an effective site for ionization. Therefore, laser desorption yields only neutral gas-phase species. Without the deliberate addition of a cationizing agent, the addition of Na+ and K+ from impurities present in the sample and/ or matrix is usually observed. Alternatively, a suitable salt may be added during the sample preparation. Addition of salt solutions such as NaCl, LiCl, KCl Cu(NO3)2 or AgNO3 may provide better cationization pathways. Attachments of alkali metals are particularly beneficial for the polar polymers where the heteroatoms (O, N) are the sites of alkali metal attachments.
Overall, the development of MALDI in the analysis of industrial polymers is still in the early stage. More efforts are needed to find new matrices, which are compatible with the same solvents that are used for the polymers. The optimization of instrumental parameters and the methods to enhance ionization efficiently will be the emphasis of future investigations in this field.
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