Polymers, analysis MALDI

A combination of NMR spectroscopy and MALDI-TOF MS is commonly employed in our laboratory for the characterisation of PPG polymers. Analysis of di-hydroxyl end-capped PPG (18) is initially described. The [H NMR spectrum [54] can be used to confirm the backbone structure of the polymer, as can be seen in Figure 22 (a and b are from the backbone of the polymer, with c from the methyl side chains). Peaks of low intensity, downfield of those from the backbone of the polymer, in the HNMR spectrum may be used to identify and quantify the allyl functionality in the polymer [55]. These resonances (d, e and f) are... 

Fig. 12. Analysis of the thymine-substituted polymers by MALDI-TOF mass spectrometry reveals that the polymers can be capped with two different groups...

By analysis of the ST HPMA polymers by MALDI-TOF MS in the (more sensitive) refectron mode it was possible to recognize macromolecules with... 

Although FAB has been used in polymer analysis, problems with fragmentation and the relatively low mass limit has made this less popular as new techniques have emerged. Plasma desorption has been used successfully but this too has waned in popularity with commercial spectrometers not really readily available. To a large extent polymer mass spectrometry equates to MALDI time-of-flight and the remainder of this article will bear this in mind. However, the use of electrospray ionisation (ESI) will be considered in conjunction with either quadrupole detectors or ion cyclotron resonance (ICR) N. B. ICR detectors can also be used with MALDI, as this is important and probably not as widely used as it could be. 

In polymer analysis by MALDI-related techniques (such as tandem MS), there are two possibilities to obtain fragment ion spectra. The first one uses the traditional MALDl... 

The 1970s and 1980s saw the advent of several new "soft" desorption/ ionization methods, many of which are now well-established in analytical mass spectrometry. The term "desorption/ionization" refers to a method in which the desorption (vaporization) and ionization steps occur essentially simultaneously. MALDI and several other techniques listed in Table 1 have important applications in polymer analysis. 

The major reason for this increase has been the use of MALDI-MS for numerous polymer applications. MALDI is by no means the only mass spectral method that is useful for polymer analysis, but it has provided the impetus to get polymer people interested in what mass spectrometry can do. 

For S5mthetic polymers, the most popular desorption/ionization method has been matrix-assisted laser desorption/ionization (MALDI-MS, Chapter 10). Several other techniques have important applications in polymer analysis. The more widely used methods are covered in this book electrospray (Chapter 4), field ionization/desorption (Chapter 6), fast atom bombardment (Chapter 7), secondary ion mass spectrometry (Chapter 8), and laser desorption (Chapters 9 and 11). 

Pastor, S. J. and Wilkins, C. L., Sustained off-resonance irradiation and collision-induced dissociation for structural analysis of polymers by MALDI-FTMS, Int J. Mass Spectrom. Ion Processes, 175, 81, 1998. 

Lasers have provided a convenient means to create gas phase ions from nonvolatile subsfances. In fhis chapter, application of lasers to FTMS for polymer analysis will be considered. The earliest combination of direct laser desorption and laser ablation techniques with FTMS will be discussed first. Moving on from that topic, the impact of matrix-assisted laser desorption/ ionization (MALDI) on FTMS will be addressed, with particular emphasis... 

Creel, H. S., Prospects for the Analysis of Rligh Molar Mass Polymers Using MALDI Mass Spectrometry Trends Polym. Sci., 1, 336,1993. 

Pasch, H. and Ghahary R., Analysis of Complex Polymers by MALDI-TOF Mass Spectrometry, Macrontol. Symp., 152, 267, 2000. 

McEwen, C. N., Peacock, P. M., and Guan, Z. Complication in the Analysis of Synthetic Polymers by MALDI, Proc. 46th ASMS Conf. Mass Spectrom., Allied Topics, Orlando, FL, 409, 1998. 

Schriemer, D. C. and Li, L., Mass Discrimination in the Analysis of Polydisperse Polymers by MALDI Time-of-Plight Mass Spectrometry. 1. Sample Preparation and Desorption/Ionization Issues, Anal Chem., 69, 4169, 1997. 

Nielen, M. W. R, Polymer Analysis by Micro-Scale Size-Exclusion Chromatography/MALDI Time-of-Flight Mass Spectrometry with a Robotic Interface, Anal. Chem., 70, 1563, 1998. 

Mass spectrometry allows performing compound characterization in a sensitive, specific, and rapid manner (high-throughput) that is particularly important in CombiChem applications.The development of ionization techniques such as EST and MALDI allows the analysis of high molecular weight compounds broadening the field of polymer analysis by mass spectrometry. ... 

Matrix-assisted iaser desorption/ionization (MALDI). This is another ionization method for the analysis of large molecules such as peptides, proteins, and nucleic acids, as well as some synthetic polymers. In MALDI, the analyte is first cocrystallized with an excess of a matrix, e.g., sinapinic acid or dihydroxybenzoic acid, that has a constituent aromatic component able to absorb photons from a UV laser beam. When the dried analyte matrix mixture is exposed (inside the vacuum chamber) to a sudden input of energy from a laser pulse the matrix evaporates, essentially instantaneously, carrying with it the analyte molecules. The matrix forms reagent ions that protonate the analytes. The selection of the matrix is critical as different compound classes exhibit substantial, matrix-dependent differences in ionization efficiency. The MALDI matrix should not be confused with the alternative use of the term matrix that is used to denote the medium in which biological and/or environmental components are presented, e.g., blood plasma, urine, sediment. 

MS compatible with sample preparation techniques used in these fields. For MALDI, the minimum amount of protein needed for a spectrum of high quality was reduced from 1 pmol in 1988 to a few femtomoles only about a year later. Today, in favorable cases, the level is now down in the low attomole range. Many other developments - both instrumental (see Chapter 2) as well as specific sample preparation recipes and assays (see other chapters of the book) - took place during the following decade, and the joint impact of aU of these together has today made MALDI-MS an indispensable tool not only in the Hfe sciences but also in polymer analysis, food sciences, pharmaceutical drug discovery, or forensic jurisprudence. 

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 example shown in Figure 8.2 illustrates that the selection of a proper matrix is important to generate a MALDI spectrum reflective of the polymer sample composihon. In this case, if only DHB were used, the MALDI spectrum produced would not reveal the low mass oligomers (m/z<10000) actually present in this sample. As only a handful of matrices are found to be practically useful for polymer analysis, it is often worthy spending the time to test these matrices on a given sample to identify the best matrix that provides good sensitivity, mass resolution, and reproducibiUty over a broad mass range. 

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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