Mass Spectrum

Fig 3. MALDI-TOF & ES MS of a 42-aa peptide (theoretical molar mass 4497).

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Matrix-Assisted Laser Desorption Ionization-Time Of Fly (MALDI-TOF) is one of the latest and most gentle ionization approach in Mass Spectrometry. This approach uses UV laser pulses to generate and sputter ions into the gas phase from high molecular weight species such as peptides that are embedded within an organic matrix compound. The resulting ions are then accelerated in an electric field and down a flight tube and collected in a detector. The lag time between ionization and detector impact (i.e. Time Of Fly) is measured and correlates directly to mass/charge (m/z) ratio. 

Some phenomenon should be kept in mind when using and interpreting MALDI-TOF MS. The extend of ionization and ejection in the gas phase of a given peptide will determine peak intensity. The latter is sequence-, mass- and sample-dependent (Fig. 4). Some higher MW peptides may give small intensity Molecular Ion peak with this approach (Fig. 3). An alternative approach is a higher-energy ionization techniques such as Electrospray Ionization (Fig. 3). Another inherent phenomenon is the slight broadening of MH+ peaks resulting from very small local differences of absorbed energy during the initial UV-light laser pulses between similar peptide ions which in turn results in slight differences in Times of Flight and hence a <> spectrum. Routine calibration procedures afford average mass error of � 0.1%.

Fig 4. (a) Two independent MALDI-TOF MS analysis. (a) 21-aa peptide (theoretical molar mass 2404).

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Fig 4. (b) (b) 8-aa peptide (theoretical molar mass 854).

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Data acquired in a MALDI TOF spectrophotometer provides <> mass for the singly protonated peptide MH+. The detailed mechanism for ionization is still uncertain and the process can also generate cationized species, e.g. [MNa+] (Fig #4).