Sample small-molecule MALDI

Electron ionization (El) was the primary ionization source for mass analysis until the 1980s, limiting the chemist to the analysis of small molecules well below the mass range of common bioorganic compounds. This limitation motivated the development of the techniques commonly known as ESI, 1 MALDI, 2 and fast atom bombardment (FAB) 3,4 (Table 1). These ion sources allow for rapid and easy peptide analyses that previously required laborious sample preparation or were not possible with electron ionization. The mechanism of ionization these ion sources employ, which is somewhat responsible for their ability to generate stable molecular ions, is protonation and/or deprotonation. 

This desorption ionisation technique leads to weak fragmentation. The analyte is incorporated into a solid organic matrix (such as hydroxybenzoic acid) and the mixture is placed on a sample holder that is irradiated with UV laser pulses (e.g. N2 laser, A = 337 nm, pulse width = 5 ns). The laser energy is absorbed by the matrix and transferred to the analyte, which becomes desorbed and ionised (Fig. 16.18c). Although MALDI is considered to be a soft ionisation technique, a substantial amount of energy is involved. Because the technique involves pulsed ionisation, it is well suited for time-of-flight mass analysis of biomolecules. The analysis of small molecules (M < 500 Da) is limited because the matrix decomposes upon absorption of the laser radiation. However, solid supports such as silicone can be used as the matrix to overcome this disadvantage. 

Donegan, M., Krishnan S., Hattan, S., Juhasz, P., and Martin, S. (2003). Sample preparation methods for MALDI analysis of small molecule metabolites. In Proceedings of the 51st ASMS Conference on Mass Spectrometry and Allied Topics, Montreal, Canada. 

Yang, M., James, A., Covey, T., and Kovarik, P. (2005). High-speed automated deposition of matrix onto tissue samples for small molecule imaging application using MALDI MS/MS. In Proceedings of the 53 rd ASMS Conference on Mass Spectrometry and Allied Topics, San Antonio, TX. 

The sample preparation procedures for the direct analysis of small molecules in tissue have been described by several papers [120-124], Tissues (brain, heart, lung, kidney, liver, etc.), were immediately frozen and stored at -80 °C after harvest. The frozen tissues were subsequently cut into serial 10-20 pm thick section which was typically prepared by cryosectioning on a microtome at a temperature of -20 °C. The adjacent sections were gently mounted onto a conductive surface, MALDI imaging target plate or glass slides. These plates were desiccated under low vacuum for a short period of time until dry, then robotically or manually coated with the... 

In addition to MALDI, matrix-free LDI techniques have been proved to be useful in IMS studies of small molecules (62). Thus, NIMS has also been proved to be a highly sensitive matrix-free method (in which functionalized surfaces are used to absorb the laser, eliminating the need for matrices) for tissue imaging in metabolomics (75,76). It has been described that NIMS surface can be easily treated or modified with different chemical initiators. As a result, distinct metabolite profiles from the same biological sample can be obtained. For instance, coating NIMS surface with cationization agents... 

One drawback for the use of MALDI with small molecules is that sample preparation is more intensive than that for electrospray because of the need for matrix addition. To some extent this sample preparation can, however, be automated. A more significant drawback is the presence of a low-mass-noise background arising from the matrix. This noise background can interfere with the direct assessment of the spectrum for the compound of interest. Considerable research efforts have been directed at approaches that eliminate the low-mass... 

More success was obtained using on-line solid phase extraction before offline analysis by capillary electrophoresis with matrix-assisted laser-desorption ionization (MALDI) MS [18]. Because microdialysis is a method used for analysis of small molecules and peptides, MALDI is not used frequently with microdia lysis sampling. Another disadvantage is that MALDI cannot be used on-line with the separation because it is a vacuum ionization technique. However, if the pep-tide is large enough (0 1000 Da) MALDI can be useful. For the analysis of peptides in dialysate an appropriate separation is important before mass spectrometric detection. In a comparison with direct sampling of dialysate in MALDI, capillary electrophoresis provides the high efficiency separations necessary to resolve all... 

Because ESI is less tolerant of sample conditions, sample preparation is even more critical than in MALDI. In addition, the solvent used for the ESI process is extremely important, unlike in MALDI, where the solvent evaporates away before the analysis. Nonvolatile solvents like water or a 1% concentration of sodium-dodecyl-sulfate (SDS) tend to produce poor sprays and a poor or noisy ion current. A sample in 99% water will not spray as well as a sample prepared in 50% methanol. A peptide sample prepared using acetic acid will also produce better results than one using trifluoroacetic acid (TEA) probably due to a counter ion effect. Nonvolatile salts or small molecules will eventually plug the entrance hole to the mass spectrometer and so they should be avoided. 

Zhao, I.Y. et al., Application of MALDI-MS/MS molecular imaging software for small molecule profiling in tissue samples, Proceedings of the 51st ASMS Conference, Montreal, Quebec, Canada, 2003. 

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