William B. Whitten

Micro ion trap mass spectrometry

Experiments to perform mass spectrometry with ion traps of submillimeter dimension are described. Several trap geometries amenable to microfabrication have been explored. In these experiments, vapor-phase molecules were ionized by a pulse from a frequency-quadrupled Nd:YAG laser. The ions were trapped and mass analyzed by manipulating the radiofrequency voltages on the trap electrodes. Effects of trapping parameters on mass resolution and sensitivity are discussed.

Room-temperature microparticle-based persistent spectral hole burning memory

We show both theoretically and experimentally that a random distribution of spherical microparticles may be used as a spectral hole burning memory. This microparticle hole burning memory, which can be both written and read at room temperature, is a direct consequence of the properties of morphology-dependent resonances of microparticles.

The Elucidation of Charge-Transfer-Induced Matrix Effects in Environmental Aerosols Via Real-Time Aerosol Mass Spectral Analysis of Individual Airborne Particles

Matrix effects in real-time aerosol mass spectrometry (RTAMS) were investigated using Standard Reference Materials (SRMs) obtained from the National Institute of Standards and Technology (NIST). Suppression of major component ions by much less concentrated species was observed. Attempts were made to mimic the ion suppression using binary systems but were unsuccessful. Data are presented that suggest the origin of the matrix effect is charge transfer induced neutralization in the ablation plume.

Self-scanning of a dye laser due to feedback from a BaTiO 3 phase-conjugate reflector

Feedback from a self-pumped BaTiO(3) phase-conjugate reflector into an untuned cw dye laser produces substantial line narrowing (4-6 GHz) and generates a recurrent wavelength sweep from the rhodamine 6G peak at 618 nm to as far as 644 nm.

Homogeneous linewidths of Rhodamine 6G at room temperature from cavity-enhanced spontaneous emission rates

Fluorescence lifetimes of Rhodamine 6G in levitated micron‐sized droplets have been measured using a time‐correlated photon counting technique. The coupling of emission into spherical cavity modes of the droplet results in significant emission rate enhancements which allow estimation of the homogeneous linewidth at room temperature.

Direct Observation of the Evolution of the Soot Carbonization Process in an Acetylene Diffusion Flame via Real-Time Aerosol Mass Spectrometry

Abstract Real-time aerosol mass spectrometry was used to measure the size and composition of individual soot particles in an acetylene diffusion flame. This on-line data analysis technique permits for the first time direct observation of the evolution of the carbonization process, determination of the degree of carbonization of each measured particle, and measurement of the size distribution of both polycyclic aromatic hydrocarbon (PAH)-containing and mature soot particles. The carbonization process is characterized by rapid exchange of hydrogen between the PAH and pyrolytic addition of small hydrocarbons to form larger PAH molecules. The hydrogen exchange rate builds until carbon–carbon bond rearrangement becomes facile. The structural rearrangement/dehydrogenation process is very rapid once started. Rapid carbonization permits unambiguous size measurement of both PAH-containing and mature soot particles. Rapid hydrogen exchange yields a low activation energy path for making radicals in the particle phase and permits the PAH-containing media to act as a hydrogen sink. The presence of substantial amounts of labile hydrogen in the PAH-containing particle is demonstrated by the presence of preferentially hydrogenated PAHs. The absence of ethynylated PAHs and the presence of the hydrogenated PAHs are the result of flame pyrolysis of the PAH-containing particles. Optical images of flame particles collected by an independent sampling method conclusively confirm the presence of the micron sized PAH-containing particles in the flame and strongly suggest that mature soot aggregates are formed directly from the micron-sized PAH-containing particles. The optical images and the size distribution data cast doubt on the currently accepted mechanisms for mature soot aggregate formation. A new mature soot aggregate formation mechanism found in the aerosol literature is presented. This mechanism readily explains our results and many of the universal characteristics of soot. The implications of these measurements are discussed.

Real-time detection of individual airborne bacteria

Airborne bacteria and bacterial spores were directly sampled by an ion trap mass spectrometer with an atmospheric pressure inlet system. Samples were aerosolized from suspensions of single species. The organisms were individually characterized in real time by laser ablation mass spectrometry. Either positive or negative ions could be studied. Ions of a particular value of m/z (mass to charge ratio) could be further characterized by tandem mass spectrometry in the ion trap.

SINGLE-MOLECULE ANALYSIS OF ULTRADILUTE SOLUTIONS WITH GUIDED STREAMS OF 1-MU M WATER DROPLETS

We describe instrumentation for real-time detection of single-molecule fluorescence in guided streams of 1-µm (nominal) water droplets. In this technique, target molecules were confined to droplets whose volumes were comparable with illumination volumes in diffraction-limited fluorescence microscopy and guided to the waist of a cw probe laser with an electrostatic potential. Concentration detection limits for Rhodamine 6G in water were determined to be ∼1 fM, roughly 3 orders of magnitude lower than corresponding limits determined recently with diffraction-limited microscopy techniques for a chemical separation of similar dyes. In addition to its utility as a vehicle for probing single molecules, instrumentation for producing and focusing stable streams of 1–2-µm-diameter droplets may have other important analytical applications as well.

Electron impact ionization in a microion trap mass spectrometer

A microscale ion trap mass spectrometer (r0=0.50 mm, z0=0.55 mm) with an electron gun for electron impact ionization of gaseous samples is described. Operated in the mass-instability mode, the trap had a m/z range from 40 to 400 Da. For single scans, peak widths of less than 0.2 Da were obtained.

Confinement and manipulation of individual molecules in attoliter volumes.

We report observation of fluorescence from individual rhodamine 6G molecules in streams of charged 1-μm-diameter water droplets. With this approach, illumination volumes comparable to diffraction-limited fluorescence microscopy techniques (≤500 aL) are achieved, resulting in similarly high contrast between single-molecule fluorescence signals and nonfluorescent background. However, since the fluorescent molecules are confined to electrically charged droplets, in situ electrodynamic manipulation (e.g., focusing, switching, or merging) can be accomplished in a straightforward manner, allowing experimental control over both the delivery of molecules of interest to the observation region and the laser-molecule interaction time. As illustrated by photocount statistics that are independent of molecular diffusion and spatial characteristics of the excitation field, individual rhodamine 6G molecules in 1-μm droplets are reproducibly delivered to a target a few micrometers in diameter at a rate of between 10 and 100 Hz, with laser beam transit times more than 1 order of magnitude longer than diffusion-limited laser-molecule interaction times in equivalent volumes of free solution.