MALDI-MS for Polymer Characterization

With the development of new polymerization chemistry, catalysis and formulation processes, a great number of polymeric materials with diverse properties can be produced. A detailed characterization of these materials is important to relate their chemical structure and composition to their functions. For example, modification of the end groups of a polymer can significantly alter its characteristics, such as chemical reactivity, solubility, and miscibility with other chemicals. Polymer characterization is not a simple task and often involves the use of multiple analytical techniques, with each generating a piece of useful information that is necessary to provide a comprehensive interrogation of the polymer. A number of analytical techniques, including chromatographic methods, spectroscopy, and mass spectrometry (MS), have been developed and applied to study areas such as polymer structure, polymer composition, molecular mass and molecular mass distribution, bulk and surface properhes and impurity content [1-3]. 

During recent years, MS has become an indispensable tool for polymer charac-terizahon [5, 6, 7], and today can produce rich chemical information that is highly specific for polymer structural analysis. MS is also very sensitive, allowing the detection and identification of minor polymer components or impurities in a composed polymeric material, and any byproducts of polymerization reactions of a desired polymer. In addihon, MS can potentially provide quantitative information required to determine the average molecular mass and molecular mass distribution of a polymer, or to characterize the relative amounts of the different components of a polymer mixture. Some forms of MS, such as secondary ion mass spectrometry (SIMS), can also be used to characterize polymer surfaces [7]. 

Many different types of MS technique have been used for polymer characterization [4]. Before the introduction of matrix-assisted laser desorption ionization (MALDI) and electrospray ionization (ESI) during the 1980s, MS was limited to the analysis of relatively low mass polymers of less than 3000 Da. Desorption techniques such as SIMS, fast atom bombardment (FAB), and laser desorption/ ionization (LDI) could ionize a polymer with mass of up to 10000 Da. However, 

MALDI MS A Practical Guide to Instrumentation, Methods, and Applications, Second Edition. 

Edited by Franz Hillenkamp and Jasna Peter-Katalinic. 

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MALDI-MS transforms the practice of polymer characterization [8-12], and today has become a widely used technique for the analysis of a huge variety of polymers [6]. There are several unique attributes of MALDI-MS which, together, make it a powerful technique for polymer characterization. In this chapter, these attributes will be discussed, along with many technical issues related to the use of MALDI-MS for polymer characterization. A few selected applications of MALDI-MS and MS/MS will also be outlined in order to illustrate the power of the technique in solving practical problems. This chapter does not aim to survey all published studies in the area of MALDI polymer characterization rather, it attempts to provide an overview on the technique, attributes, and current limitations of MALDI-MS for polymer analysis. 

A number of pubUcations have demonstrated the applications of MALDI-MS for polymer characterization. Of particular interest, Hanton provided a Ust of pubUcations summarized according to the type of polymers analyzed by MALDI-MS [11]. In addition, an updated web-based resource for polymer/matrix preparation protocols is available from the Polymers Division of the National Institute of Standards and Technology (NIST) homepage (http //polymers.msel.nist.gov/ maldirecipes). 

Weidner S, Falkenhagen J. LC-MALDl MS for polymer characterization. In Li L, editor. Maldi mass spectrometry for synthetic polymer analysis. Hoboken, NJ John Wiley Sons 2010. p. 247-65. 

MALDI technique for polymer characterization fails to detect the high-mass tail of PEG sample 2. SEC verifies the presence of the high mass component of sample 2. The IMS-MS plot shows charge-state families that are highly specific of polymer size, type, and cation used and are thus unique to each polymer composition as one can see for poly(methyl methacrylate) (PMMA) 6 kDa (Figure 10.4a) and PEG 6 kDa, both doped with CsCl (Figure 10.4b). 

MALDI-MS is a powerful tool for polymer characterization. Compared with analytical techniques currently used for polymer analysis, it provides several unique features. In MALDI-MS, molecular mass and molecular mass distribution information can be obtained for polymers of narrow polydispersity with both precision and speed. The accuracy, though difficult to determine due to the lack of well-characterized standards, also appears to be good [150]. The MALDI analysis of polymers does not require the use of polymer standards for mass caUbration. Furthermore, this technique uses a minimum amount of solvents and other consumables, which translates into low operational costs. MALDI-MS can also provide structural information, if the instrumental resolution is sufficient to resolve oligomers. In this case, monomer and end-group masses can be deduced from the accurate measurement of the mass of individual oligomers. This is particularly true when a FT-ICR MS is used for polymer analysis. With the use of MALDI-MS/ MS, stmctural characterization can be facilitated. Finally, impurities, byproducts, and subtle changes in polymer distributions can often be detected even for relatively complex polymeric systems such as copolymers. 

In many laboratories, MALDI-MS has become a routine tool for polymer characterization. This is evident from an increasing number of pubUcations in polymer Uterature (i.e., Macromolecules) which indicate the use of MALDI-MS as a tool for characterizing newly grafted or synthesized polymers. In an industry deaUng with polymeric materials, MALDI-MS is often combined with other analytical techniques to provide detailed analyzes of a polymeric system. In some cases, MALDI-MS is the only technique that can provide the information required to solve a practical problem. One example is in the area of product failure analysis... 

Polymer characterization usually requires a combination of several analytical tools such as NMR and GPC. Today, a number of analytical techniques exist that can provide molecular mass, structure, and composition information, and MALDI-MS is now emerging as a powerful method for polymer characterization. Some demonstrated advantages of the technique include the ability to determine average molecular mass and distributions without the need of polymer standards and with high speed, precision, and accuracy, to analyze polymer mixtures with minimiun sample work-up, and to provide structural and compositional information via... 

Because of the diversity associated with polymer chemistry, there is no universal approach in MALDI-MS for the analysis of polymers. The major challenge in applying MALDI-MS to characterize a particular polymeric system lies in developing a suitable sample preparation protocol tailored to this polymer. This usually involves screening and selecting a suitable matrix from a list of known matrices used for the analysis of similar polymers, and in some cases this requires new matrices to be identified. Once an appropriate matrix is found, however, attention must still be paid to many details in the MALDI analysis procedure to ensure that the final results reflect the true chemical nature of the polymeric system. 

In summary, MALDI-MS is an important tool for polymer characterization, with many attributes that complement those of other analytical techniques. Future advances in analytical method development, as well as of our understanding of... 

Classical MALDI-MS requires that the material should be soluble in a suitable solvent. A suitable solvent means a solvent that is sufficiently volatile to allow it to be evaporated prior to the procedure. Further, such a solvent should dissolve both the polymer and the matrix material. Finally, an ideal solvent will allow a decent level of polymer solubility, preferably a solubility of several percentage and greater. For most synthetic polymers, these qualifications are only approximately attained. Thus, traditional MALDI-MS has not achieved its possible position as a general use modern characterization tool for synthetic polymers. By comparison, MALDI-MS is extremely useful for many biopolymers where the polymers are soluble in water. It is also useful in the identification of synthetic polymers, such as PEO where the solubility requirements are fulfilled. Thus, for PEO we have determined the molecular weight distribution of a series of compounds with the separations in ion fragment mass 44 Da corresponding to CH2-CH2 units. 

Although the technique is useful for nonmetal and metal-containing polymers, this chapter emphasizes its use with metal-containing polymers. Identification of polymer structure is an essential characterization imperative. For metal-containing polymers, this is more difficult for metal-containing polymers. The difficulty involves both increased complexity of possible structures and decreased (or lack of) solubility of the polymers. F MALDI MS allows for ready identification of the repeat unit and can allow determination of some low-molecular-weight range chains. This chapter describes our overall approach and presents data that justifies the use of this technique for the identification of polymer structure. 

Because the amount of polymer samples available is usually not limited, it is possible to underestimate the sensitivity issue in MALDI polymer characterization. In reality, the use of a MS instrument that provides high sensitivity and a wide dynamic range of ion detection is pivotal to the success of polymer analysis. This is true not only for the measurement of polymer average mass, but also for the determination of polymer composition [110, 113-121]. With limited detection... 

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