Laser Power Dependence of Mass Spectral Signatures from Individual Bacterial Spores in Bioaerosol Mass Spectrometry

Bioaerosol mass spectrometry is being developed to analyze and identify biological aerosols in real time. Characteristic mass spectra from individual bacterial endospores of Bacillus subtilis var. niger were obtained in a bipolar aerosol time-of-flight mass spectrometer using a pulsed 266-nm laser for molecular desorption and ionization. Spectra from single spores collected at an average fluence of ~0.1 J/cm2 frequently contain prominent peaks attributed to arginine, dipicolinic acid, and glutamic acid, but the shot-to-shot (spore-to-spore) variability in the data may make it difficult to consistently distinguish closely related Bacillus species with an automated routine. Fortunately, a study of the laser power dependence of the mass spectra reveals clear trends and a finite number of "spectral types" that span most of the variability. This, we will show, indicates that a significant fraction of the variability must be attributed to fluence variations in the profile of the laser beam.