Small-molecule MALDI

McCombie, G., and Knochenmuss, R. (2004). Small-molecule MALDI using the matrix suppression effect to reduce or eliminate matrix background interferences. Anal. Chem. 76 4990-4997. 

Matrix Choices for Small-Molecule MALDI 369 Table 9.1 Summary of approaches for small-molecules analysis using MALDI-MS. 

Lasers have advanced the analytical use of mass spectrometers to characterise additives in polymers, and routine application of MALDI is no longer limited to high molecular masses only. MALDI can now clearly produce isotopically resolved mass spectra of small molecules (<800 Da) in an L-ToF instrument, which can be used successfully for the characterisation of molecules of different chemical classes. High mass resolution with an improvement of mass accuracy to... 

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. 

If during the ionization the amount of energy deposited on the molecule is low, as occurs in soft techniques, i.e. Cl, ESI, DESI and MALDI, the mass spectrum is very simple. It is characterized by protonated/deprotonated molecules, and eventually few adduct ions but very few or no fragment ions. This implies that it is easy to obtain the molecular weight of the analyte under investigation, but structural information is missing. As an example, the ESI mass spectrum of a small molecule is reported in Figure 2.20. There are two main ions one at m/z 556 and another at m/z 578. As the mass spectrum has been obtained in positive... 

Cohen, L. H., and Gusev, A. I. (2002). Small molecule analysis by MALDI mass spectrometry. Anal. Bioanal. Chem. 373, 571—586. 

The use of MALDI for the analysis of small molecules was recently reported. Particularly attractive is the coupling of a MALDI source with a triple quadrupole mass analyzer for quantitative analysis in the selected reaction monitoring (SRM) mode due to very high analysis speed. 

More recently, Brennan has shown that FAC-MALDI-MS can be used to screen small molecules, relying upon MRM transitions to overcome the chemical noise generated by the matrix [16]. This is an acceptable approach for known compounds, but for ligand discovery from uncharacterized mixtures, ion selection... 

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. 

As of this writing, it remains to be seen whether the great speed potential of MALDI and DIOS for the quantitative analysis of drug molecules will be realized in practice. There are hurdles typical of any emerging application that need to be addressed. The need for providing rapid quantitative analysis of drugs is only becoming more acute, and as such, it is likely that DI techniques will become more prevalent in the area of small molecule bioanalytical MS. 

Gobey, J., Cole, M., Janiszewski, J., Covey, T., Chau, T., Kovarik, P., and Corr, J. (2005). Characterization and performance of MALDI on a triple quadrapole mass spectrometer for analysis and quantification of small molecules. Anal. Chem. 77 5643-5654. 

Kovarik, P., Corr, J. J., and Covey, T. R. (2003). Application of orthogonal MALDI for quantitation of small molecules using a triple quadrupole mass spectrometer. In Proceedings of The 51st ASMS Conference on Mass Spectrometry and Allied Topics, Montreal, Canada. 

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