Single Particle Aerosol Mass Spectrometry (SPAMS) is a method for detecting and analyzing almost anything that can be introduced into the system as an aerosol particle, whether the sample is already airborne, is found as particles on a surface, is found suspended in a liquid (or is a liquid itself), or is found as a powder. Livermore Instruments’ flagship product, the SPAMS 3.0 can be viewed from two different perspectives: as a sensor, it is unique in its amazingly rapid and broad spectrum sensing abilities; as a mass spectrometer, it is unique in its simplicity of operation and its ability to collect mass spectra very rapidly from real-world samples. The operating principles of a SPAMS 3.0 are dramatically simplified versus prior instruments to minimize the difficulty of their operation and maintenance.
A SPAMS 3.0 functions by collecting a laser mass spectrum of the small molecules (up to 350 Daltons in both positive and negative polarities simultaneously) of aerosol particles individually. The system is maintained under vacuum. Aerosol particles are introduced from the top through a series of aerodynamic focusing lenses. The lenses focus the particles into a tight beam and also accelerate them to a final velocity as a function of their aerodynamic diameters. Each particle continues across a continuous wave visible light laser, labeled as "GREEN YAG" in the schematic, but which can actually be any continuous laser. The top and bottom of the laser are parallel and the height of the beam is known. As the particle passes through the beam, it scatters light which is detected by a photomultiplier tube. The duration of the light scattering event corresponds to its velocity which is used to compute its aerodynamic diameter. The aerodynamic diameter is useful in that it determines how far the particle will be transported, either in the environment or in the human respiratory system.
As the particles emerge from the continuous wave laser, they cease to scatter light. When the end of the light scattering is observed, a pulsed laser, oriented just below the first laser, is fired, generating ions from the particle. That laser is oriented across the center of the source region of a dual polarity time-of-flight mass spectrometer so a dual polarity mass spectrum is collected. An example of such a mass spectrum appears below. The two polarities are plotted back-to-back with the positive ion mass spectrum plotted to the right of the negative ion mass spectrum. The spectrum is of a single particle of aluminum oxide with a mass of roughly two picograms which has been doped with ammonium perchlorate. Notice that the laser has fragmented the perchlorate ion and chlorate, chlorite, hypochlorite and chloride ions are also visible in the spectrum. Up to 1000 of these mass spectra can be acquired per second, each from an individual aerosol particle.
The mass spectra are analyzed in real-time by a two stage data analysis algorithm known as the Palisade algorithm. First, the mass spectra are compared to a library of known mass spectra from previous training experiments to determine the general nature of the particle. Subsequently, the presence and absence of specific mass peaks are confirmed according to a decision tree to categorize the particles more precisely. Surprisingly, the small molecule composition of, for example, Bacillus spores is sufficient to distinguish some species from one another, as in the figure below where the red ovals encircle peaks that are present in Bacillus atrophaeus but are absent from Bacillus thuringiensis. These peaks are robustly present and absent across a wide variety of growth conditions.
Furthermore, because each particle is analyzed individually, the concentrations of the different microorganisms can be determined in a mixture. Bacillus spores have been detected and classified when aerosolized directly from their own growth medium with no preparation other than their aerosolization. Mycobacterium tuberculosis has also been distinguished from other mycobacteria. In each case, the identification is made in real-time. An accelerated article published in Analytical Chemistry last year demonstrated the ability of a SPAMS sensor to instantaneously detect and identify biological agents, drugs of abuse, chemical agents, explosives and enriched metals, all using the same instrument and within minutes of one another.