Derek A. Lake

Sample preparation for high throughput accurate mass analysis by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry

An automated sample preparation for high throughput accurate mass determinations by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOFMS) has been developed. Sample preparation was performed with an automated workstation and automated mass analyses were performed with a commercial MALDI-TOF mass spectrometer. The method was tested with a 41-sample library. MALDI-TOFMS was found to give the needed sensitivity, accurate mass measurement, and soft ionization necessary for structure confirmation, even of mixtures. A mass accuracy of 5 ppm or less was obtained in over 80% of known compound measurements. A mass accuracy better than 10 ppm was obtained for all measurements of known compounds. Analyses of parallel synthesis products resulted in 77% of the measurements with a mass accuracy of 5 ppm or better.

Ion Mobility Spectrometry of Gas-Phase Ions from Laser Ablation of Solids in Air at Ambient Pressure

A mobility spectrometer was used to characterize gas-phase ions produced from laser ablation of solids in air at 100 °C and at ambient pressure with a beam focused to a diameter of <0.2 mm at energy of 6 mJ/pulse and wavelength of 266 nm. Metals, organic polymers, glass, graphite, and boron nitride exhibited characteristic mobility spectra with peaks at drift times between 8.75 and 12.5 ms (reduced mobility values of 2.19 to 1.53 cm2/Vs). Ion intensities increased initially and then decreased with repeated laser shots through drilling of the solid, and persistence of signal was proportional to hardness. A single comparatively narrow peak for negative ions was observed in mobility spectra for all materials and this was mass-identified as O2−. These ions were formed in air from reactions of oxygen with electrons emitted from the ablation step. Positive ions ablated directly from the solid were masked in ion mobility spectrometry/mass spectrometry (IMS/MS) studies by ionization of moisture and impurities. Positive ions from solids were seen only in the IMS analyzer at elevated temperature and low moisture. Under such conditions, materials were classified from mobility spectra alone with principal component analysis.

Characterization of Short-Term Particulate Matter Events by Real-Time Single Particle Mass Spectrometry

Single particle measurements were made in Baltimore, Maryland from March to December 2002 using a real-time single particle mass spectrometer, RSMS-3. Particle composition classes were identified that indicated how the aerosol composition changed with time. The results were compared with collocated instruments giving particle number concentrations and size distributions, sulfate, nitrate, organic, and elemental carbon mass concentrations and total mass. Examination of these measurements revealed several particulate matter (PM) events in which the 24 h averaged PM 2.5 mass exceeded 30 μ g/m 3 . Three of these events were studied in further detail by comparing number and mass concentrations obtained by RSMS-3 with standard methods. For all three events, the number concentrations obtained with RSMS-3 and a scanning mobility particle sizer were highly correlated (R 2 ∼ 0.7). For the event characterized by a high sulfate mass concentration, the RSMS-3 provided an accurate measure of time-dependent nitrate and carbon mass concentrations, but not for sulfate and total mass. For the two events characterized by high carbon mass concentrations (one from a transcontinental wildfire and the other from stagnation during a period of high traffic), RSMS-3 provided an accurate measure of time-dependent nitrate mass, carbon mass and total mass when the aerosol was not dominated by particles outside the size limit of RSMS-3. While the time dependencies were strongly correlated, the absolute mass or number concentrations determined by RSMS-3 were sometimes off by a constant value, which permitted the relative detection efficiencies of some particle classes to be estimated. Other factors that inhibit reconciliation of mass- and number- based concentration measurements are discussed including the difficulty of detecting ammonium sulfate by laser ablation/ionization and the varying size ranges of different particle measurement methods.