Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) has enabled the analysis of large biomolecules that were previously only amenable to study using conventional techniques (SDS-PAGE, etc.). The technique is routinely used to study molecules in excess of 100,000 Da. We have recently observed singly- charged ions in excess of 1,000,000 Da (1 MDa) from a sample of human immunoglobulin M (IgM). The spectrum is shown below.
A practical consideration in the analysis of such large molecules is the effect of the desorption velocity on the determined mass. The initial desorption velocity and kinetic energy lead to a deviation between calculated and observed flight times. This is demonstrated for a different sample of IgM in the following figure.
The upper curve represents the deviation of the observed flight times from the calculated flight times for the +2 to the +6 charged species of IgM. The lower curve represents the convergence of the observed flight times to the calculated values. A series of successive approximations was used to converge the flight times. Without the correction algorithm, calculated masses may be in error by ~5%, due to an initial energy of ~1500eV.
A compiled QBasic version of the algorithm EnergyCorrect can now be downloaded.
Details of the algorithm may be found in the following publications:R. W. Nelson, D. Dogruel, P. Williams, Rapid. Commun. Mass Spectrom. 8, 627, 1994
Research is continuing on methods to successfully generate time-of-flight mass spectra from extremely large biomolecules and other high-mass materials.