MALDI-TOF

MALDI-TOF is a mass-spectrometry technique built from two parts: Matrix-Assisted Laser Desorption/Ionization (MALDI), which is the method used to put the analyte into the gas phase as ions, and Time-Of-Flight (TOF), which is the method used to measure the mass-to-charge ratio of those ions. The sample is co-crystallized with a UV-absorbing small-molecule "matrix" on a metal target plate; a pulsed laser then transfers energy to the matrix, which desorbs and ionizes the analyte with minimal fragmentation. The resulting ions are accelerated through a field-free flight tube, and lighter ions arrive at the detector sooner than heavier ions. Flight time is converted to mass. The foundational technique and its application to large biomolecules is described in Karas & Hillenkamp, Anal. Chem. 1988 (NLM 3239801).

Why MALDI-TOF is common in peptide QC

MALDI-TOF is fast, tolerant of salts and common buffer components, and typically produces a dominant singly charged [M+H]+ ion for peptides below about 10 kDa. That single-peak simplicity makes it an efficient way to confirm whether a synthetic peptide has the expected intact mass and to screen for common synthesis-related mass shifts such as +16 (oxidation), -18 (dehydration), or -17 (loss of ammonia, for example from N-terminal glutamine). It is the routine mass-confirmation tool in many solid-phase peptide synthesis shops because it can process dozens of samples per hour with minimal sample preparation.

What MALDI-TOF cannot do well

The same features that make MALDI-TOF fast also limit its analytical reach. Mass accuracy on a standard linear TOF is typically in the range of tens to low-hundreds of ppm, which is sufficient to confirm a nominal mass but not sufficient to differentiate isobaric sequences or to characterize low-abundance impurities with confidence. Quantitation by MALDI is unreliable because ionization efficiency varies strongly with the co-crystallization process, and different peptides in the same mixture can ionize with very different efficiencies. For these reasons MALDI-TOF is best understood as an identity-confirmation tool that sits alongside HPLC for purity and LC-MS for impurity characterization, rather than as a replacement for either. A practical summary of the technique's strengths and limits in the context of biopharmaceutical analysis is given in Hillenkamp & Peter-Katalinic (eds.), MALDI MS, Wiley, 2nd ed., 2014 (DOI 10.1002/9783527335961).

Reading MALDI data on a COA

A MALDI-TOF result on a peptide COA should report the observed m/z (usually the [M+H]+ value), the theoretical value, the difference, and the instrument mode (linear or reflectron). Reflectron mode gives higher resolution and is preferred for peptides below about 5 kDa; linear mode is used for larger peptides and proteins where detector sensitivity matters more than resolution. Open Assay treats MALDI-TOF mass confirmation as baseline identity evidence and looks for a secondary ESI-LC-MS result when higher-accuracy identity or impurity characterization is material to the comparison.