Analytical Chemistry Seminar: Harald Stark and Doug Day
Analytical & Environmental Chemistry Division and Atmospheric Chemistry Program Seminar
Jointly sponsored by the Department of Chemistry and Biochemistry, CIRES, and the Environmental Program
High-resolution mass spectrometry: peak fitting and aerosol partitioning
by Harald Stark - Department of Chemistry and Biochemistry, University of Colorado Boulder
When measuring oxidized organic molecules in the atmosphere, often thousands of compounds can be present and provide a challenge for conclusive assignment in high-resolution mass spectra, particularly when the goal is to retrieve data series over long time periods of weeks or months, as typical in field campaigns. We have developed algorithms to identify and assign peaks in complex mass spectra to produce reliable peak lists for analyzing large data sets. Simulation of mass spectra confirmed the validity of the assignment algorithms for calculation of bulk chemical properties such as carbon number and carbon oxidation state. A new method of deriving such chemical properties without the need for any peak assignment will also be presented.
Application of a peak list from the new algorithms to measurements from a chemical-ionization mass spectrometer deployed in a Pine forest in Colorado allowed measurement of gas-particle partitioning of hundreds of organic compounds. Comparing two methods of deriving saturation concentration distributions (“volatility basis sets”) from the same dataset highlights the importance of fragmentation in thermal desorption techniques..
Particle Size Resolution of the Aerodyne Aerosol Mass Spectrometer
by Doug Day - Department of Chemistry and Biochemistry, University of Colorado Boulder
Real-time determination of chemically-resolved particles size distributions and their evolution in laboratory and ambient studies can be used to investigate changing source contributions, growth rates, secondary aerosol yields, cloud condensation nuclei potential and other physicochemical processes. The Aerodyne Aerosol Mass Spectrometers (AMSs) provides fast size and chemical composition measurements of particles from ~50 nanometers to ~1 micrometer. This work addresses the resolution of the size measurement via particle time-of-flight (PToF) in this instrument. An algebraic model is used to estimate size resolution for different configurations of the instrument as well as particle diameters and vaporization rates. Under typical AMS configurations, for a given particle size, resolution is primarily controlled by the uncertainties in particle time-of-flight due to the finite opening time of the particle gating chopper and particle vaporization. The model was tested with a range of chopper gate widths, particle sizes, and particle compositions, and agreements and disagreements are investigated. The methodology to estimate the size resolution can be applied to all instruments that use particle time-of-flight to infer particle size. Instrument and data acquisition configurations can be optimized for a range of different particle sampling conditions including ambient sampling under low or high loadings, for aircraft platforms, for laboratory applications where loadings may be controllable or narrow particle size ranges are present, and when single-particle chemical or light-scattering detection is possible.