MASS DETERMINATION OF HUMAN-IMMUNOGLOBULIN IGM USING MATRIX-ASSISTED LASER-DESORPTION IONIZATION TIME-OF-FLIGHT MASS-SPECTROMETRY

Monoclonal human immunoglobulin IgM has been analyzed using matrix-assisted laser desorption/ionization time-of-flight mass spectrometry. Using a matrix of sinapinic acid with a laser wavelength of 355 nm, positive ions with 2 to 6 charges were observed in the mass spectra. Two major molecular species were present. The molecular weights estimated from the multiply-charged signals were found to decrease systematically with decrease in the charge state of the ion. This effect was determined to stem from the non-linear influence of the initial kinetic energy of desorption on the final kinetic energy of the multiply-charged species. A method of successive approximations, with the molecular weights and desorption velocities of the analytes as variables, was used to correct for these non-linearities. Mean molecular weights for the two IgM species were found to be 939 000 +/- 2000 and 982 000 +/- 2000 Da. An initial desorption velocity of 565 +/- 10 ms/s was determined for both.