Abstract: For analyzing micro-organisms and other high-molecular weight species, a sample of the substance to be analyzed is prepared, placed in a pyrolyzer where it is pyrolyzed with a selected temperature program to provide pyrolyzed product of a high-dalton mass range. The product is ionized using metastable atoms which results in efficient ionization with little fragmentation. The metastable atoms are generated using a generator that provides a beam of metastable atoms which is basically free from ions. The ionized product is then analyzed using a high acquisition rate mass analyzer, such as a time-of-flight (TOF) analyzer.