Polymer analysis MALDI matrices

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). 

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... 

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. 

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. 

Basile, F. Kassalainen, G.E. Williams, S.K.R. Interface for direct and continuous sample-matrix deposition onto a MALDI probe for polymer analysis by thermal field flow fractionation and off-line MALDI-MS. Anal. Chem. 2005, 77, 3008-3012. 

The major reason for this increase has been the use of matrix-assisted laser desorption/ionisation-MS (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. 

Agapito et al. ° applied fuzzy logic to an array of semiconductor gas sensors to analyze different atmospheres for the different gases and Otto et al. developed a scheme based on the principles of fuzzy logic that makes use of various pieces of information available either from spectroscopic knowledge or from the particular spectrum that requires evaluation. A fuzzy expert system has also been successfully developed for the automated qualitative and semiquantitative interpretation of X-ray diffraction spectra/ automation of matrix-assisted laser desorption-ionization mass spectrometry (MALDI), and for polymer analysis. 

The structural and compositional analysis of synthetic polymers by MALDI MS has developed rapidly. The MALDI technique involves embedding the analyte in a matrix that absorbs at the wavelength of the laser. The energy is then transferred from the matrix to the analyte, which is desorbed and subsequently ionized in the gas phase. The ions produced can be analyzed in a time-of-flight (TOP) mass analyzer, a natural choice for pulsed-ionization techniques. 

MALDI polymer analysis consists of three steps, namely sample preparation, mass spectral recording, and data analysis. Matrices and sample preparation are crucial points for the applicability of MALDI-MS. The analyte molecules, often a polymer, are dispersed in a matrix of small organic molecules with strong optical absorption at the desorption laser wavelength. In this way desorption/ionisation of the analyte molecules can be achieved regardless of their absorption properties. The sample is prepared by... 

Mass analyzers. MALDI is ideally coupled to time-of-flight (TOF) for polymer analysis (17). This combination is ideal due to the pulsed nature of the laser and the theoretically unlimited mass range and high transmission efficiency of TOF. Ions are produced by laser ablation of the dried matrix/analyte mixture and are then accelerated by a fixed potential into a drift tube that does not contain an external electric field. The ions continue moving toward a detector. Since all the ions are accelerated at the same potential, the kinetic energy (KE) of a given ion can be given by eq. 1 ... 

Modem TOF-SIMS applications are often in the field of synthetic polymer analysis, but rarely for the determination of molecular weight distributions which is the domain of MALDI. More frequently, low-molecular-weight additives or surface modifications are analyzed as there is no interference with a matrix as would... 

MALDI - Pyrolyzates with much higher molecular weight can be detected. - Observation of the thermally induced structural changes in the polymer samples. - Fast time of analysis. - MWD determination by SEC-MALDI. - Matrix interference - no low-molecular-weight detection (below miz 500). - Difficulties to analyse unsoluble or apolar polymers. - Indirect method - only the degradation products most thermally stable will survive. - Suppression of degradation products present in trace. - Only quahtative analysis. 

Improvements in consistency of sample preparation have been achieved with a recent innovation involving the use of a robotic interface for polymer analysis using microscale SEC/MALDI-ToF MS [33, 71]. The MALDI matrix solution is coaxially added to the column effluent and spotted onto MALDI targets in a robotic interface. Each spot corresponded to a 10 second elution window from the SEC. 

Applications employing laser ablation of polymers include film deposition and the synthesis of certain organic compounds. Laser beam ablation in conjunction with mass spectrometry is an important tool for polymer analysis, which is referred to as laser desorption mass spectrometry (LDMS). One particular type of LDMS, termed matrix-assisted laser desorption/ionization (MALDI), has contributed essentially to the analysis of proteins (Nobel prize for chemistry to K. Tanaka in 2002) [126,127]. Further information on this subject is available in Ref [4]. 

An review is given of the use of matrix-assisted laser desorption/ionisation time-of flight (MALDI TOF) mass spectroscopy to determine MWDs and structures of synthetic organic polymers, including the ion desorption mechanism, synthetic polymer analysis, molecular weight analysis, and its use in calibration of GPC profiles. 30 refs. 

Two relatively new techniques, matrix assisted laser desorption ionization-lime of flight mass spectrometry (MALDI-TOF) and electrospray ionization (FS1), offer new possibilities for analysis of polymers with molecular weights in the tens of thousands. PS molecular weights as high as 1.5 million have been determined by MALDI-TOF. Recent reviews on the application of these techniques to synthetic polymers include those by Ilantoif54 and Nielen.555 The methods have been much used to provide evidence for initiation and termination mechanisms in various forms of living and controlled radical polymerization.550 Some examples of the application of MALDI-TOF and ESI in end group determination are provided in Table 3.12. The table is not intended to be a comprehensive survey. 

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. 

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