MALDI-TOFMS

Maleamic acid, cyclization of, 293 Maleic anhydride, 59 Maleimido azine, 307 Manganese diacetate catalysts, 71 Mark-Houwink-Sakurada equation, 57 Material safety data sheets (MSDSs), 246 Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOFMS), 385, 388 McGrath, J. E., 327 MDI isomers, 210 MDIs. See Methylene diphenyl diisocyanates (MDIs)... 

The mass spectrometer is a mass-flow sensitive device, which means that the signal is proportional to the mass flow dm/dl of the analyte, i.e. the concentration times the flow-rate. It is only now possible to realise the high (theoretically unlimited) mass range and the high-sensitivity multichannel recording capabilities that were anticipated many years ago. Of considerable interest to the problem of polymer/additive deformulation are some of the latest developments in mass spectrometry, namely atmospheric pressure ionisation (API), and the revival of time-of-flight spectrometers (allowing GC-ToFMS, MALDI-ToFMS, etc.). 

In direct insertion techniques, reproducibility is the main obstacle in developing a reliable analytical technique. One of the many variables to take into account is sample shape. A compact sample with minimal surface area is ideal [64]. Direct mass-spectrometric characterisation in the direct insertion probe is not very quantitative, and, even under optimised conditions, mass discrimination in the analysis of polydisperse polymers and specific oligomer discrimination may occur. For nonvolatile additives that do not evaporate up to 350 °C, direct quantitative analysis by thermal desorption is not possible (e.g. Hostanox 03, MW 794). Good quantitation is also prevented by contamination of the ion source by pyrolysis products of the polymeric matrix. For polymer-based calibration standards, the homogeneity of the samples is of great importance. Hyphenated techniques such as LC-ESI-ToFMS and LC-MALDI-ToFMS have been developed for polymer analyses in which the reliable quantitative features of LC are combined with the identification power and structure analysis of MS. 

This chapter deals mainly with (multi)hyphenated techniques comprising wet sample preparation steps (e.g. SFE, SPE) and/or separation techniques (GC, SFC, HPLC, SEC, TLC, CE). Other hyphenated techniques involve thermal-spectroscopic and gas or heat extraction methods (TG, TD, HS, Py, LD, etc.). Also, spectroscopic couplings (e.g. LIBS-LIF) are of interest. Hyphenation of UV spectroscopy and mass spectrometry forms the family of laser mass-spectrometric (LAMS) methods, such as REMPI-ToFMS and MALDI-ToFMS. In REMPI-ToFMS the connecting element between UV spectroscopy and mass spectrometry is laser-induced REMPI ionisation. An intermediate state of the molecule of interest is selectively excited by absorption of a laser photon (the wavelength of a tuneable laser is set in resonance with the transition). The excited molecules are subsequently ionised by absorption of an additional laser photon. Therefore the ionisation selectivity is introduced by the resonance absorption of the first photon, i.e. by UV spectroscopy. However, conventional UV spectra of polyatomic molecules exhibit relatively broad and continuous spectral features, allowing only a medium selectivity. Supersonic jet cooling of the sample molecules (to 5-50 K) reduces the line width of their... 

Capillary HPLC (usually SEC or LCCC), coupled semi off-line with MALDI-ToFMS for oligomer and end-group characterisation, may be operated using collection modules precoated with matrix material. 

Many excellent reviews on the development, instrumentation and applications of LC-MS can be found in the literature [560-563]. Niessen [440] has recently reviewed interface technology and application of mass analysers in LC-MS. Column selection and operating conditions for LC-MS have been reviewed [564]. A guide to LC-MS has recently appeared [565]. Voress [535] has described electrospray instrumentation, Niessen [562] reviewed API, and others [566,567] have reviewed LC-PB-MS. For thermospray ionisation in MS, see refs [568,569]. Nielen and Buytenhuys [570] have discussed the potentials of LC-ESI-ToFMS and LC-MALDI-ToFMS. Miniaturisation (reduction of column i.d.) in LC-MS was recently critically evaluated [571]. LC-MS/MS was also reviewed [572]. Various books on LC-MS have appeared [164,433,434,573-575], some dealing specifically with selected ionisation modes, such as CF-FAB-MS [576] or API-MS [577],... 

The technique offers advantages over alternative techniques (e.g. preparative SEC fractionation followed by IR). SEC-FTIR is much more suitable to quantitation of additives than SEC-MALDI-ToFMS. Combining SEC-FTIR with SEC-MALDI offers a more powerful tool than when either is used alone. 

Principles and Characteristics Problems connected with sample preparation, ionisation and detector efficiency can lead to errors in the quantitation of mass averages and MWD in the case of ESI-MS and MALDI-MS. Coupling of SEC with MS makes it possible to overcome these difficulties. SEC-MS has developed since the early 1990s. Two methods are currently outstanding on-line SEC-ESI-MS (QMS or FTMS) and semi on-line SEC-MALDI-ToFMS [709],... 

There is no need for perfect separation in the eluting peak MALDI can improve the resolution of chromatography. Semi on-line SEC-MALDI-ToFMS makes allowance for the separation of polymer, oligomers and additives. Ion suppression has been noticed for... 

It is of particular interest that MALDI-ToFMS, which is not the most obvious choice for the analysis of additives in solution, finds practical application for rapid screening of polymer/additive dissolutions. 

Principles and Characteristics Although it might appear that MALDI-ToFMS should perform particularly well only for the polymer part of polymer/additive systems, the technique also yields useful information about additives contained in UV-insensitive polymers, such as polyolefins. The latter materials are hardly an insignificant part of the total polymer market ... 

Table 9.7 Features of MALDI-ToFMS analysis of poly-...

Da of the monoisotopic masses, high sensitivity (down to 20fmolp,L 1), and the absence of any fragmentation, are important advantages for a L-ToF system. The main characteristics of MALDI-ToFMS, as applied directly to polymer/additive dissolutions, are summarised in Table 9.7. 

Applications MALDI-ToFMS is at its best as a rapid screening technique for quick identification of known additives. However, this screening is rendered slightly more complicated by the fact that MALDI-ToFMS spectra of pure additives and of additives in the presence of excess macromolecules are not always identical (matrix effect) [55]. For unknown additives, the relation MALDI-ToFMS spectrum-chemical structure is not easily established, and the use of FD or MALDI-MS/MS is then needed. As MALDI-MS shows a sensitivity difference for the various additives, it cannot easily quantify them unless the analytes are very similar. For differentiation of additives with the same mass number (e.g. Tinuvin 315 and Cyasorb UV3638 with m/z = 368) high resolution is required, as provided by delayed extraction MALDI-ToFMS. 

Meyer-Dulheuer [55] has analysed the pure additives (phenolic antioxidants, benzotriazole UV stabilisers and HALS compounds) of Table 9.8 in THF solutions by means of MALDI-ToFMS. As it turns out, polar molecules in the mass range of below 800 Da, which have a high absorption coefficient at the laser wavelength used, can often be measured without any matrix [55,56]. In this case, there is no matrix-assisted laser desorption and ionisation (MALDI) process any more. It is a simple laser desorption/ionisation (LDI) process. The advantage of this method is a matrix-free mass spectrum with the same mass resolution as in the MALDI case,... 

Table 9.8 Characterisation of additives in polymers by means of MALDI-ToFMS...

Even HALS compounds which absorb weakly at 337 nm can be analysed directly without matrix assistance, with the exception of the high-MW Hostavin N 30 (ca. 1500 Da), which fragments by direct laser desorption ionisation of intact molecules occurs only in the presence of a (dithranol) matrix. Direct laser desorption leads only to noncharacteristic, low-MW fragments. Hostavin N 20 leads to [M + H]+, [M + Na]+, [M + K]+ and some fragmentation peaks. MALDI-ToFMS of Tinuvin 765, which consists of a mono- and bifunctional sterically hindered amine, only shows the adduct peaks of the bifunctional amine apparently, the monofunctional amine is not ionisable. 

Figure 9.4 MALDI-ToFMS spectrum of Tinuvin 350. After Meyer-Dulheuer [55], Reproduced by permission of H. Pasch...

Table 9.8 shows the results of MALDI-ToFMS experiments of the selected additives in PP, HDPE, PA6 and PMMA. The observed detection limits are in the technical range of interest. Identification of the additives... 

For PMMA/additive dissolutions, it was not possible to identify any additive characteristic mass peaks, either by direct laser desorption or with matrix-assistance (dithranol, DHBA or sinapinic acid, 4-hydroxy-3,5-dimethoxy-cinnamic acid). This has again been ascribed to very strong interaction between PMMA and additives, which suppresses desorption of additive molecules. Also, partial depolymerisation of pho-tolytically labile PMMA by laser irradiation may play a role, which leads to saturation of the detector by PMMA fragment-ions and disappearance of additive mass peaks below noise level. Meyer-Dulheuer [55] has also reported MALDI-TOFMS analysis of a coating/2-ethylhexyldiphenylphosphate sample. Quantitative determination of the additives by means of MALDI-ToFMS proved impossible. Possibly the development of reproducible (automated) sample handling procedures or thin films might overcome this problem. 

It is concluded that MALDI-ToFMS is a suitable method for direct analysis of low-MW additives in complex polymeric materials (in dissolution), in particular as a rapid screening technique (within 0.5 h). However, in order to turn this method into a general tool for identification and quantitation, considerably more work needs to be done. Identification of additives in polymeric matrices by means of MALDI-ToFMS would greatly benefit from reference libraries of additives contained in such matrices. This is not unlike the situation observed for ToF-SIMS. 

Advances in size-exclusion chromatography, coupled with refractive index, absorption, viscosity, and lightscattering detectors, and MALDI-ToFMS, have made it possible to accurately determine molecular weight distribution (oligomer profiling), even at the relatively low values of polymeric additives (up to about 5000 Da). Advances in column design, e.g. high-resolution PS/DVB columns (> 105 plates m-1) mean that SEC can provide a valuable alternative to conventional HPLC techniques for the separation of small molecules. 

Dickinson, D. N. La Due, M. T. Satomi, M. Winefordner, J. D. Powell, D. H. Venkateswaran, K. MALDI-TOFMS compared with other polyphasic taxonomy approaches for the identification and classification of Bacillus pumilus spores. J. Microbiol. Meth. 2004,58,1-12. 

Welham, K. J. Domin, M. A. Scanned, D. E. Cohen, E. Ashton, D. S. The characterization of micro-organisms by MALDI-TOFMS. Rapid Comm. Mass Spectrom. 1998,12,176-180. 

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