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

Abstract

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 approximately 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.

Desorption/Ionization Fluence Thresholds and Improved Mass Spectral Consistency Measured Using a Flattop Laser Profile in the Bioaerosol Mass Spectrometry of Single Bacillus Endospores

“…The basic instrumental layout is shown in Figure 1 and was described previously 13 . Samples were aerosolized using a Collison nebulizer 22,23 .…”

“…The fluence distribution plots (or modified fluence histograms) introduced previously 13 are a useful tool to quantify the flatness of the "flattop" profiles. The distributions are not commonly presented so their generation will be briefly described.…”

“…As a result, it is critical that the properties of the laser pulses used for desorption and ionization be carefully chosen. In one of our previous papers 13 , it was shown that the laser fluence has a significant effect upon the mass spectra produced by Bacillus spores.…”

Toward understanding the ionization of biomarkers from micrometer particles by bio-aerosol mass spectrometry

“…A significant factor is the diameter and the profile of the Nd:YAG laser beam. In these experiments, the diameter is approximately 400 m [24]. Because the profile is Gaussian, the ionization of the particle will depend on where it interacts with the beam.…”

“…All spectra were acquired at a laser pulse energy of 0.21 mJ per pulse, which was optimized for Bacillus spore spectra. The Nd:YAG laser diameter has been previously measured at 400 um [24], thus 0.21 mJ corresponds to an average fluence of ϳ1.7 nJ/ m 2 [24]. All spectra shown are the sum averages of single particle spectra for each sample.…”