MALDI

Among the various mass spectrometry techniques, MALDI is probably the most important as it provides an absolute method for molar mass determination and molar mass distribution, as well as information on end groups and copolymer composition. The MALDI process consists of the ablation of the polymer molecules dispersed in a matrix typically made up of aromatic organic acids. The matrix needs to be able to absorb at the wavelength of a laser (usually 337 nm). This process excites the matrix molecules, which vaporize at the same time, the polymer molecules desorb into the gas phase, where they are ionized. Thus, the role of the matrix is that of transferring the laser energy to the polymer molecnles. 

Ions are formed by either removal or addition of an electron, giving a radical cation M+ or radical anion M , or by the addition of otho- charged species. Synthetic polymers are not easily charged, unless they contain labile protons. However, as the matrix is usually an organic add, it can readily furnish a proton or, alternatively, salts can be added to the polymer-matrix mixture for effective ion formation. 

Once the polymer molecules have been transferred to the gas phase as ions, they are separated on the basis of their mass-to-charge ratio. Mass spectrometers nsed for MALDI analysis may differ, but for those that are commercially available, separation is effected by TOE Other methods such as quadrupole filter and Fourier transform mass spectrometry (FTMS) may also be used. 

Sample preparation is crucial in MALDI analysis. Intimate mixing between matrix and analyte is necessary because only the soluble portion of the analyte can be detected by MALDI. The simplest procedure involves dissolving the analyte and the matrix in a common solvent, and this is best achieved when the polarity of the analyte and matrix are similar. 

FIGURE 9.12 MALDI-TOF mass spectra (DHB matrix) of (a) poly(tetrahydrofuran) and (b) a low-monol PPG, both with an of 2000 g moPf Inset a minor peak series originates from [M + K]+ adducts. (From Luftmann, H. et al.. Macromolecules, 36, 6316, 2003. With permission of ACS.) 

Laser desorption mass spectrometry and matrix-assisted laser desorption/ ionization (MALDI) 

An outstanding quaHty of polymer MALDI is that it offers the possibiUty of measuring molar masses. Very accurate values can be obtained for oligomers with molar masses up to several thousand gmoh, but the determination of much higher molar masses is difficult. Nevertheless, the successful analysis of a polystyrene sample of molar mass 1.5x 10 g moT has been claimed [78]. Typical MALDI mass spectra of high molar mass polystyrene samples are shown in Fig. 9.9. 

For more detailed information concerning this interesting field, the reader is referred to relevant Hterature reviews [79-82]. 

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A connnon feature of all mass spectrometers is the need to generate ions. Over the years a variety of ion sources have been developed. The physical chemistry and chemical physics communities have generally worked on gaseous and/or relatively volatile samples and thus have relied extensively on the two traditional ionization methods, electron ionization (El) and photoionization (PI). Other ionization sources, developed principally for analytical work, have recently started to be used in physical chemistry research. These include fast-atom bombardment (FAB), matrix-assisted laser desorption ionization (MALDI) and electrospray ionization (ES). 

Other methods of sample introduction that are commonly coupled to TOP mass spectrometers are MALDI, SIMS/PAB and molecular beams (see section (Bl.7.2)). In many ways, the ablation of sample from a surface simplifies the TOP mass spectrometer since all ions originate in a narrow space above the sample surface. 

Lasers can be used in either pulsed or continuous mode to desorb material from a sample, which can then be examined as such or mixed or dissolved in a matrix. The desorbed (ablated) material contains few or sometimes even no ions, and a second ionization step is frequently needed to improve the yield of ions. The most common methods of providing the second ionization use MALDI to give protonated molecular ions or a plasma torch to give atomic ions for isotope ratio measurement. By adjusting the laser s focus and power, laser desorption can be used for either depth or surface profiling. 

Until about the 1990s, visible light played little intrinsic part in the development of mainstream mass spectrometry for analysis, but, more recently, lasers have become very important as ionization and ablation sources, particularly for polar organic substances (matrix-assisted laser desorption ionization, MALDI) and intractable solids (isotope analysis), respectively. 

Modern commercial lasers can produce intense beams of monochromatic, coherent radiation. The whole of the UV/visible/IR spectral range is accessible by suitable choice of laser. In mass spectrometry, this light can be used to cause ablation, direct ionization, and indirect ionization (MALDI). Ablation (often together with a secondary ionization mode) and MALDI are particularly important for examining complex, intractable solids and large polar biomolecules, respectively. 

The hybrid can be used with El, Cl, FI, FD, LSIMS, APCI, ES, and MALDI ionization/inlet systems. The nature of the hybrid leads to high sensitivity in both MS and MS/MS modes, and there is rapid switching between the two. The combination is particularly useful for biochemical and environmental analyses because of its high sensitivity and the ease of obtaining MS/MS structural information from very small amounts of material. The structural information can be controlled by operating the gas cell at high or low collision energies. 

Some solid materials are very intractable to analysis by standard methods and cannot be easily vaporized or dissolved in common solvents. Glass, bone, dried paint, and archaeological samples are common examples. These materials would now be examined by laser ablation, a technique that produces an aerosol of particulate matter. The laser can be used in its defocused mode for surface profiling or in its focused mode for depth profiling. Interestingly, lasers can be used to vaporize even thermally labile materials through use of the matrix-assisted laser desorption ionization (MALDI) method variant. 

For solids, there is now a very wide range of inlet and ionization opportunities, so most types of solids can be examined, either neat or in solution. However, the inlet/ionization methods are often not simply interchangeable, even if they use the same mass analyzer. Thus a direct-insertion probe will normally be used with El or Cl (and desorption chemical ionization, DCl) methods of ionization. An LC is used with ES or APCI for solutions, and nebulizers can be used with plasma torches for other solutions. MALDI or laser ablation are used for direct analysis of solids. 

El = electron ionization Cl = chemical ionization ES = electrospray APCI = atmospheric-pressure chemical ionization MALDI = matrix-assisted laser desorption ionization PT = plasma torch (isotope ratios) TI = thermal (surface) ionization (isotope ratios). 

When mass spectrometry was first used as a routine analytical tool, El was the only commercial ion source. As needs have increased, more ionization methods have appeared. Many different types of ionization source have been described, and several of these have been produced commercially. The present situation is such that there is now only a limited range of ion sources. For vacuum ion sources, El is still widely used, frequently in conjunction with Cl. For atmospheric pressure ion sources, the most frequently used are ES, APCI, MALDI (lasers), and plasma torches. 

Mass-Analyzed Laser Desorption Ionization (MALDI)... 

Ionization can be improved in many cases by placing the sample in a matrix formed from sinapic acid, nicotinic acid, or other materials. This variant of laser desorption is known as matrix-assisted laser desorption ionization (MALDI). The vaporized acids transfer protons to sample molecules (M) to produce protonated ions [M + H]+. 

The ablated vapors constitute an aerosol that can be examined using a secondary ionization source. Thus, passing the aerosol into a plasma torch provides an excellent means of ionization, and by such methods isotope patterns or ratios are readily measurable from otherwise intractable materials such as bone or ceramics. If the sample examined is dissolved as a solid solution in a matrix, the rapid expansion of the matrix, often an organic acid, covolatilizes the entrained sample. Proton transfer from the matrix occurs to give protonated molecular ions of the sample. Normally thermally unstable, polar biomolecules such as proteins give good yields of protonated ions. This is the basis of matrix-assisted laser desorption ionization (MALDI). 

Laser-desorption mass spectrometry (LDMS) or matrix-assisted laser desorption ionization (MALDI) coupled to a time-of-flight analyzer produces protonated or deprotonated molecular ion clusters for peptides and proteins up to masses of several thousand. 

Peptides and proteins can be analyzed by mass spectrometry. Molecular mass information can be obtained particularly well by MALDI and ESI. 

MALDI. matrix-assisted laser desorption ionization... 

MALDI = matrix assisted laser desorption, ftms = Fourier transform mass spectrometry TOF = time of flight. 

Matrix-assisted laser desorption/ionization (MALDI) is widely used for the detection of organic molecules. One of the limitations of the method is a strong matrix background in low-mass (up to 500-700 Da) range. In present work an alternative approach based on the application of rough matrix-less surfaces and known as surface-assisted laser desoi ption/ionization (SALDI), has been applied. 

MALDI is a LIMS method capable of vaporizing and ionizing large biological molecules such as proteins or DNA. The biological molecules are dispersed in a solid matrix that serves as a carrier. 

Electron impact (El), or Efectrospray ionization (ESI), or Matrix-assisted laser desorption ionization (MALDI)... 

Figure 12.9 MALDI-TOF mass spectrum of chicken egg-white lysozyme. The peak at 14,307.7578 daltons (amu) is due to the monoprotonated protein, M+H+, and that at 28,614.2188 daltons is due to an impurity formed by dimerization of the protein. Other peaks are various protonated species, M+H rH ...

With the identities and amounts of amino acids known, the peptide is sequenced to find out in what order the amino acids are linked together. Much peptide sequencing is now done by mass spectrometry, using either electrospray ionization (ESI) or matrix-assisted laser desorption ionization (MALDI) linked to a time-of-flight (TOF) mass analyzer, as described in Section 12.4. Also in common use is a chemical method of peptide sequencing called the Edman degradation. 

MALDI (Section 12.4) Matrix-assisted laser desorption ionization a mild method for ionizing a molecule so that fragmentation is minimized during mass spectrometry. 

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