Radiative properties of an electrically-vaporized thin film plasma in an external magnetic field

An external magnetic field of a few kilogauss is used to alter significantly the radiative properties of the plasmas generated by the capacitive discharge vaporization ofthin Ag films. The magnetic field is generated by the plasma current in a large inductor surrounding the plasma. The magnetic field is oriented normal to the electric field in the plasma. This generates a drift motion of the plasma normal to the plane established by the electric and magnetic field vectors. The drift motion can direct the plasma toward the plastic substrate on which the Ag film and a powder or solution residue sample were originally located. This increases the plasma-sample interaction. Time integrated, spatially resolved photographic spectra and spatially resolved photoelectric radiation profiles show that with the magnetic field, the continuum background radiation is confined to a relatively small region near the substrate surface. This results in significantly larger analyte line-to-background intensity ratios in the region 1-2 cm above the substrate surface.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/26412/1/0000499.pd

Dielectric breakdown characteristics of a high current metal vapor plasma

Time- and spatially-resolved spectroscopy is used to study the early-time spectral features of the plasmas produced by high-current, capacitive discharges through thin silver films. Spectra are compared for several support gases including CO2, He, and an Ar/O2 mixture. All measurements were made during the first 40 [mu]s of the discharge. At atmospheric pressure for all three gases, spectra from support gas species show intense lines for only a brief interval between 10 and 30 [mu]s after the start of the discharge. Greatest intensity from silver lines always occurs at the film surface; while greatest intensity from support gas species occurs about 2.0 mm from the film surface. A magnetic field of a few kG normal to the electric field in the plasma and parallel to the thin film surface almost completely eliminates spectral lines from the support gas species.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/26994/1/0000561.pd

Improved Thermal Modulator for Gas Chromatography

An improved thermal modulator has been invented for use in a variant of gas chromatography (GC). The variant in question denoted as two-dimensional gas chromatography (2DGC) or GC-GC involves the use of three series-connected chromatographic columns, in the form of capillary tubes coated interiorly with suitable stationary phases (compounds for which different analytes exhibit different degrees of affinity). The two end columns are relatively long and are used as standard GC columns. The thermal modulator includes the middle column, which is relatively short and is not used as a standard GC column: instead, its temperature is modulated to affect timed adsorption and desorption of analyte gases between the two end columns in accordance with a 2DGC protocol

Exploding wire excitation for trace analysis of Hg, Cd, Pb and Ni using electrodeposition for preconcentration

The use of exploding silver wires for spectrochemical excitation is described. The apparatus used in these studies is discussed along with the electrical and radiative properties of exploding silver wires. The use of controlled-potential electrodeposition onto silver wires for sample introduction is considered. Experimental parameters for electrodeposition and exploding-wire excitation are presented. Cd, Ni, Hg and Pb are considered for analysis and show absolute detection limits of 10 ng, 10 ng, 30 ng and 15 ng, respectively. These amounts of material on the wire represent relative detection limits of 0.001 ppm, 0.002 ppm, 0.003 ppm and 0.001 ppm respectively in the analytical solutions. The exploding wire excitation method exhibits minimal matrix dependency and good reproducibility. Percent relative standard deviations for Cd, Ni, Hg and Pb are +/-17.3%, +/-16.6%, +/-26.5% and +/-12.4%, respectively. Sample introduction by controlled-potential electrodeposition offers not only a convenient means of preconcentrating trace metals from solution but also the possibility of selectively plating one element in the presence of a large excess of other elements in solution.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/33764/1/0000016.pd

Characterization of a hybrid flame/arc excitation source using monodisperse aerosol introduction

A hybrid flame/arc excitation source together with a monodisperse droplet generator is used to inject solute particles into selected regions of the arc plasma. With astigmatic image transfer and a 0.5 [mu]s time constant detection system, individual solute vapor cloud emission profiles are detected in the arc tail flame. These emission profiles are always asymmetric with sharper leading edges. Emission profiles are presented for Ca at several positions in the arc tail flame. The (zinc) excitation temperature was measured at 6600 K in the region 1-2 mm above the arc conduction channel. A double aperture technique was used to estimate the arc convective velocity at about 50 m s-1. When the emission plume origin from a monodisperse droplet stream is positioned in the tail flame region, analytical line-to-background ratios typically are one-two orders of magnitude greater than when the plume origin is located below the conduction channel. Analytical curves from the tail flame region are linear over at least two-three orders of concentration, and detection limits are significantly lower than literature values for the region just under the conduction channel of an Ar-stabilized bent arc.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/25428/1/0000877.pd

Excitation temperature, degree of ionization of added iron species, and electron density in an exploding thin film plasma

Time and spacially resolved spectra of a cylindrically symmetric exploding thin film plasma were obtained with a rotating mirror camera and astigmatic imaging. These spectra were decouvolved to obtain relative spectral emissivity profiles for nine Fe(II) and two Fe(I) lines. The effective (electronic) excitation temperature at various positions in the plasma and at various times during the first current halfcycle was computed from the Fe(II) emissivity values using the Boltzmann graphical method. The Fe(II)/Fe(I) emissivity ratios together with the temperature were used to determine the degree of ionization of Fe. Finally, the electron density was estimated from the Saha equilibrium. Electronic excitation temperatures range from 10,000-15,000 K near the electrode surface at peak discharge current to 7000-10,000 K at 6-10 mm above the electrode surface at the first current zero. Corresponding electron densities range from 1017-1018 cm-3 at peak current to 1015-1016cm-3 near zero current. Error propagation and criteria for thermodynamic equilibrium are discussed.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/24579/1/0000862.pd

Characterization of a circular thin film plasma source for atomic emission spectroscopy

A plasma source for analytical atomic emission spectroscopy is described based on the electrical vaporization by capacitive discharge of a thin Ag film deposited on a polycarbonate membrane filter. The source is designed for the rapid, direct analysis of solid powder samples collected by filtration from fluid media. A concentric electrode system consisting of a ring-shaped graphite electrode placed on the thin film surface and a pointed graphite rod located under the membrane substrate results in a plasma with cylindrical symmetry and a radial current path. Discharge current vs time and intensity vs time profiles are compared for the concentric electrode geometry and the linear geometry used in previous studies. Two values of tank circuit inductance also are compared. Both neutral-atom and ion line radiation from an Mn sample are more intense when the center electrode is initially cathodic. The inside diameter of the ring-shaped electrode and thus the surface area of the Ag film exposed to the plasma have relatively little effect on the intensity of continuum background and line radiation from a sample deposited near the center of the film. Particle size effects, while significant, are smaller than with the linear electrode geometry. Analytical curves are presented for several lines using both a low-inductance and a high-inductance discharge. Log-log slopes range from about 0.85 to 1.05 for ion lines. Detection limits are somewhat poorer than with the linear geometry.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/25931/1/0000494.pd

Magnetic-field modulation of a two-electrode direct-current plasma

The magnetic field produced by a pair of electromagnets is used to alter the shape and location of the current channel in a two-electrode direct-current plasma (DCP) designed for solution aerosol analysis. The magnetic field B is oriented normal to the electric field E in the plasma. This results in an E-cross-B (E x B) drift motion of plasma electrons. Experiments are described using both cw and oscillating magnetic fields. With continuous wave (cw) fields of 10-20 G, the current channel can be shaped to improve sample aerosol penetration. Oscillating fields in the frequency range 25-500 Hz cause the current channel to oscillate. At the higher frequencies, the current channel loses its discrete character and the plasma becomes more diffuse.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/28944/1/0000781.pd

The Disunity of Consciousness

It is commonplace for both philosophers and cognitive scientists to express their allegiance to the "unity of consciousness". This is the claim that a subject’s phenomenal consciousness, at any one moment in time, is a single thing. This view has had a major influence on computational theories of consciousness. In particular, what we call single-track theories dominate the literature, theories which contend that our conscious experience is the result of a single consciousness-making process or mechanism in the brain. We argue that the orthodox view is quite wrong: phenomenal experience is not a unity, in the sense of being a single thing at each instant. It is a multiplicity, an aggregate of phenomenal elements, each of which is the product of a distinct consciousness-making mechanism in the brain. Consequently, cognitive science is in need of a multi-track theory of consciousness; a computational model that acknowledges both the manifold nature of experience, and its distributed neural basis

Thermal modulation for gas chromatography

A thermal modulator device for gas chromatography and associated methods. The thermal modulator device includes a recirculating fluid cooling member, an electrically conductive capillary in direct thermal contact with the cooling member, and a power supply electrically coupled to the capillary and operable for controlled resistive heating of the capillary. The capillary can include more than one separate thermally modulated sections