It's challenging to describe antonyms for technical terms like Matrix Assisted Laser Desorption Ionization Mass Spectrometry. Nevertheless, let's try to provide some comparative words that may help:
Instead of "Matrix Assisted," one may use "Matrix-free," "Matrix-independent," or "Matricless." The Antonym of "Laser" could be "non-laser," "LED Irradiation," or "conventional light-based irradiation." The opposite of "Desorption" could be "adsorption" or "absorption." "Ionization" antonyms may include "Deionization," "Neutralization," or "Depolarization." Lastly, "Mass Spectrometry" antonyms could be "Photon Spectrometry," "Infrared Spectrometry," or "Ultraviolet Spectrometry.