Current Research

Research in the Johnston laboratory loosely falls into three categories: instrument/method development, laboratory studies, field measurements. Graduate student research in these areas is briefly summarized below.

The Nanometer Aerosol Mass Spectrometer (NAMS) is an ion trap/time-of-flight instrument for real-time analysis of nanoparticles in the sub-50nm diameter size range. Particles are charged before entering the NAMS and then are transmitted into the mass spectrometer through a combination of aerodynamic and electrodynamic focusing lenses. The particles enter an ion trap where they are size selectively captured. The particle size is determined by the frequency applied of the RF potential applied to the ring electrode. The particles are then ablated to atomic ions with a Nd:YAG laser. The atomic ions are extracted into a time-of-flight mass spectrometer for mass analysis and the relative intensities of these ions provide a quantitative measure of the elemental composition.

Current work focuses on analysis of data collected at an urban air quality monitoring site. Particle size and composition measurements in the 10-25 nm range are combined with environmental data (e.g. temperature, rainfall, wind speed, wind direction and solar radiation fluence) to understand the mechanism of nanoparticle formation and growth in the atmosphere.

The Photo Ionization Aerosol Mass Spectrometer (PIAMS) is a relatively new technique developed in the Johnston lab for the real-time analysis of organic aerosols. Airborne particles enter the mass spectrometer and deposit on a collection probe. The collected sample is flash desorbed with an infrared laser pulse and then photoionized with a second laser pulse. This process softly ionizes both aliphatic and aromatic compounds, providing molecular ion information.

PIAMS is being used in both laboratory and field experiments to characterize the organic composition of aerosols. For example, spectrum a) below shows the distribution of organic compounds in airborne particles at an urban air quality monitoring site. A new spectrum is acquired every 3.5 minutes, allowing the relative concentration of various components. The high-time resolution data captures concentration changes that would be lost would other methods (e.g. palmitic acid, b).

Ion Formation Mechanism in Laser Desorption Ionization of Individual Nanoparticles– Melissa Reinard

Covariance mapping is used to study ion formation mechanisms in laser desorption ionization of individual 50 or 220 nm dia. particles having compositions similar to ambient aerosol. Single particle mass spectra generated from the Real-Time Single Particle Mass Spectrometer (RSMS) are found to vary substantially from particle-to-particle. This variation is systematic – the energetically preferred ions (e.g. lowest ionization energy, highest electron affinity) are positively correlated with each other and negatively correlated with less preferred ions. The results suggest that ion formation occurs by a two-stage process. In the first stage, photoionization of laser desorbed neutrals gives cations and free electrons. In the second stage, collisions in the plume cause electron capture and competitive charge transfer. When the particle ablates in a manner giving a dense plume with many collisions, the energetically preferred positive and negative ions are dominant. When the particle ablates in a manner giving a less dense plume with fewer collisions, the less preferred ions are able to survive and the energetically preferred ions constitute a lower fraction of the total ion signal.

The graphic illustrates a covariance map for particles that are composed of NaCl. Red areas indicate ions which are positively correlated (the ion signals increase and decrease together) while blue areas indicate ions which are negatively correlated (one ion signal increases while the other decreases). Shades of orange and green indicate ions that have weak positive and negative correlation, respectively, while yellow areas (though none are present) indicate ions which are not correlated. These correlations give information about ion formation mechanisms on single particles.

See: Reinard and Johnston, Journal for the American Society of Mass Spectrometry, 2007. In Press.

Understanding the chemical mechanism of particle formation in our atmosphere has become increasingly important due to particles being associated with health problems and climate changes. Currently our lab is using multiple techniques to study the formation of particles from biogenic emissions. Specifically, monoterpenes emitted into the gas phase from plants react with ozone to form products that partition into the particle phase. This process is a major source of secondary organic aerosol in the atmosphere.

In this study, ozone and monoterpene vapor are mixed in a flow tube reactor to produce nanoparticles. These particles are subsequently analyzed by two of the mass spectrometers described above, PIAMS and NAMS. PIAMS provides molecular composition information, while NAMS gives the elemental composition. Particles are also collected for off-line analysis by techniques such as size exclusion chromatography and electrospray ionization mass spectrometry.

The Soft Ionization Aerosol Mass Spectrometer (SIAMS) is the newest technique developed in the Johnston lab for the real-time analysis of aerosols. The instrumental design incorporates components from ‘older brothers’ PIAMS and NAMS, also found in the Johnston lab. Aerosols are focused through an aerodynamic lens system and are deposited onto a collection probe. Neutral species are laser desorbed and softly ionized by either chemical ionization or matrix assisted laser desorption ionization (MALDI). The analyte ions are extracted into the time-of-flight mass analyzer for detection.

SIAMS will mainly be used to address two key issues in the field of atmospheric aerosol chemistry: (1) to analyze the formation of oligomers from secondary organic aerosols (SOA) and (2) to identify and characterize particulate matter of biological origin. Controlled laboratory experiments will be conducted with SIAMS to study reaction rates and characterize high molecular mass species.