Mass Spectrometry

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Mass Spectrometry

The amount of material contained within a single aerosol particle is about one trillionth of a gram.  Many important reactive trace gases in the atmosphere exist as only one 1 molecule in a billion molecules of whole air.  As one of the most sensitive chemical analysis techniques, mass spectrometry is well suited to the task.

Spectrometery of Aerosols and Trace Gases

Aerosol Time-of-Flight Mass Spectrometry (ATOFMS)

The ATOFMS is a mass spectrometer developed by the Prather Research Group that is capable of measuring the size and composition of individual aerosol particles, returning the results in real time.  Particles are introduced to the instrument where they are intercepted by an ultraviolet (266 nm) laser pulse, which causes the components of the particle to vaporize and ionize.  The ATOFMS is equipped with two time-of-flight mass spectrometers: both the positive and negative ions produced by photo-ionization can be detected.  These unique instruments can help determine whether chemicals are mixed together in the same particle (‘internally mixed’) or if the various compounds detected are in different particles (‘externally mixed’).  This concept of mixing state is important in the cloud forming potential of atmospheric aerosols, for instance, and techniques that measure individual particles, like ATOFMS, are uniquely suited to resolve aerosol mixing state.

Chemical Ionization Mass Spectrometry (CIMS)

CIMS uses chemical reactions to produce molecular ions, which can then be identified by their mass-to-charge ratio. This technique is used by CAICE investigators to understand the gas-to-particle partitioning of trace gases of atmospheric importance.  Natural volatile organic compounds (VOCs), which are detected with great sensitivity by CIMS, are released from the ocean surface and are important to secondary aerosol formation. Urban pollutants (e.g., N2O5) react on sea salt particle surfaces and impact atmospheric oxidation in the morning hours; the reaction kinetics of N2O5 uptake are studied by the Bertram Research Group using flow-tube reactor coupled to CIMS.

CI-TOFMS - Bertram Group

High Resolution Mass Spectrometry

For purposes of elucidating complex organic samples, perhaps the most powerful mass spectrometry tool is the Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer.  When coupled to a linear ion trap, this type of instrument is capable of doing MS-MS experiments, in which an ion is isolated by its mass-to-charge ratio, and then fractured by bombardment with neutral molecules (usually helium atoms), permitting more detailed characterization of molecular structures.  In collaboration with the Dorrestein Research Group, CAICE researchers have been using this technique to understand the detailed nature of the organic chemical composition of aerosol and seawater samples.  Interfaced with an array of possible ionization sources, including electrospray ionization (ESI), desorption electrospray (DESI), and nanospray ionization, allowing optimal control of sample introduction and soft ionization.