Attributes and Limitations of MALDI-MS

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. 

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). 

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 

While MALDl-MS is widely used for polymer characterization, it does have certain limitations. The first limitation is that not all narrow-polydispersity polymers can be ionized by MALDl. For example, polyethylene, perfluoropolymers, and polycationic polymers are difficult to analyze by MALDl-MS. Direct laser desorption ionization with the assistance of metal powder can be used to ionize low-molecular mass polyethylene ( 5000 Da) [112], but high-mass polyethylene is currently not amenable to MALDl analysis. 

The analysis of a broad-polydispersity polymer (PD 1.2) can be carried out by combining GPC or size-exclusion chromatography (SEC) with MALDI-MS [161-172]. In this approach, a wide-polydispersity polymer is first separated by GPC and fractions at a defined time interval are collected. The time interval is properly chosen so that the individual fraction would contain only a narrow-polydispersity (PD 1.2) polymer, which can then be analyzed by MALDI-MS for accurate molecular mass determination. The molecular mass information generated from the MALDl analysis of aU individual fractions can be used to convert the time domain in the GPC chromatogram into a mass domain. The polymer distribution can be determined from this chromatogram. 

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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. 

The hnished product will be subjected to inspection and rigorous testing for identity, uniformity, residual water content, stability, sterility and potency. In addition, all analytical techniques employed in testing these attributes will themselves have been subjected for reliability, reproducibility, experimental uncertainty limits. The biotechnological revolution has resulted in the appearance of ever more rehned and sensitive analytical techniques, mainly novel types of spectroscopy and coupled techniques, based on mass spectrometry, known usually by complex acronyms, e.g. MALDI-TOE-MS (Matrix-Assisted-Laser-Z)esorption-71me-of-Tlight-Mass Spectrometry). Some of the available analytical procedures are treated in more detail in the next chapter. 

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