Mass spectrometer analyzes fine aerosols

A laser mass spectrometer for ultrafine particle analysis, developed by Murray Johnston and Anthony Wexler of the University of Delaware (Newark, DE), performs on-line analysis of individual aerosol particles with diameters between 10 and 150 nm by laser ablation. This is roughly one order of magnitude smaller than existing transportable instruments, says Johnson.

Aerosol particles are size-selected with a differential mobility analyzer and then drawn into a mass spectrometer where they are ablated with a high-energy laser pulse (see photo). The ions produced are detected in a time-of-flight mass analyzer.

In most aerosol mass spectrometers, particles are initially detected by scattering radiation from a continuous-wave (CW) laser beam. Once detected, the particles are ablated with a high-energy pulsed laser. Scattering of the CW laser beam can be used to size the particle, and it provides a method of synchronizing the firing of the pulsed laser with the arrival of the particle. This approach has a difficult time analyzing individual particles with diam eters less than 100 nm because the scattering efficiency is too low.

In the method developed by Johnston and Wexler, the pulsed laser is free-fired at a high repetition rate and the laser and particle beams are aligned collinearly. When the laser fires, a particle can be at any location in the mass spectrometer. A special ion focusing arrangement allows ions produced from particles over a 4-cm region to be collected and analyzed. This combination of a high repetition rate, collinear beam alignment, and elongated ion source region significantly increases the probability of hitting a particle as it passes through the instrument.

Atmospheric aerosols are produced by human activities or by natural events such as volcanic eruptions. They may have direct or indirect effects on global climate. Aerosols may counter global warming directly by scattering light, or they may indirectly cool various regions of the Earth by accumulating in clouds that act like mirrors, bouncing sunlight back into space. The laser mass spectrometer analyzes aerosol particles at the critical early stages of their growth.

The initial version of the instrument uses a miniature excimer laser from MPB Technologies (Quebec, Canada) for ablation and ionization of particles. The laser provides 1.5- to 3-mJ pulses at 193 nm with a repetition rate up to 100 Hz. With this instrument, Johnston and Wexler have studied the analytical capabilities of laser ablation. Ultrafine particles yield primarily positive ions, electrons, and neutrals when irradiated with an ultraviolet laser beam. Because of the small particle size, the density of the plume generated by laser ablation is low, and the probabilities of both electron capture by neutrals and positive-negative charge recombination in the plume are also low. Thus, the positive ion yield is high and the negative ion (as opposed to electron) yield is low. Relative to the total mass of the particle, a small particle exhibits a much higher ion yield than a large particle.

Currently, the researchers are developing a new inlet that provides greater size selection capability and improved transmission of small particles into the mass spectrometer. The mass spectrometer is also being redesigned to permit electrodynamic trapping of particles to increase the probability of laser ablation. At present, the instrument is small enough to be transported in a van or small truck. Both laboratory studies of aerosol reactions and field measurements of ambient aerosols are planned.

A patent disclosure has been filed, but there are no immediate plans for commercialization.