R. Kenneth Marcus

An atmospheric pressure glow discharge optical emission source for the direct sampling of liquid media.

A glow discharge optical emission spectroscopy (GD-OES) source that operates at atmospheric pressure is described. This device utilizes an electrolytic solution containing the analyte specimen as one of the discharge electrodes. The passage of electrical current (either electrons or positive ions) across the solution/gas phase interface causes local heating and the volatilization of the analyte species. Collisions in the discharge region immediately above the solution surface result in optical emission that is characteristic of the analyte elements. Operation of this device with the analyte solution acting as either the cathode or anode is demonstrated. Current-voltage (i-V) plots reveal abnormal glow discharge characteristics, with operating parameters being dependent on the electrolyte concentration (i.e., solution conductivity) and the gap between the solution surface and the counterelectrode. Typical conditions include discharge currents of 30-60 mA, and potentials of 500-900 V. Electrolyte solutions having pH, pNa, or pLi values of 0.5-2 and interelectrode gaps of 0.5-3 mm produce stable plasmas in which the analyte solutions flow at rates of up to 3.0 mL/min. Preliminary limits of detection are determined for the elements Na, Fe, and Pb to be in the range of 11-14 ppm (approximately 60 ng) for 5-microL sample volumes.

Characterization of a radio frequency glow discharge emission source

Abstract A radio frequency glow discharge atomic emission source is described. This source incorporates two optical windows, one positioned such that both the glow region of the discharge and the sample surface may be viewed simultaneously, and the other positioned such that only the glow may be viewed. Preliminary signal-to-noise ratios obtained with the two optical geometries are compared and found to be similar. However, optical monitoring of both the glow and sample surface results in significantly higher emission intensities, owing to the more complete viewing of the energetic regions of the discharge. As a result, this geometry may be considered to be analytically more advantageous. Additionally, data are presented to illustrate the dependences of the dc self-bias potential, excitation temperature, sputter rate, and emission intensities of sputtered species on discharge pressure and generator output power. Scanning electron micrographs of the surface of a machinable glass ceramic (MACOR) sample indicate that appreciable sputtering of electrically nonconductive solid samples occurs in the rf glow discharge. Comparison of data acquired with conductive and nonconductive samples is made to show the similar behavior of the discharge in the presence of the two sample types. Exemplary spectra are presented to illustrate the potential usefulness of the source for the emission spectrometric analysis of both electrically conductive and nonconductive solids.

Capillary-channeled polymer fibers as stationary phases in liquid chromatography separations

A method utilizing capillary-channeled polymer (C-CP) fibers as stationary phases in high-performance liquid chromatographic separations has been investigated. Polymeric fibers of differing backbones (polypropylene and polyester) having nominal diameters of approximately 50 and approximately 35 microm and a channeled structure on their periphery were packed into stainless steel tubing (305 x 4.6 mm I.D.) for use in reversed-phase separations of various mixtures. The fibers have eight channels running continuously along the axis which exhibit very high surface activity. As such, solvent transport is affected through the channels through wicking action. Bundles of 1000-3000 fibers are loaded co-linearly into the tubing, providing flow channels extending the entire length of the columns. As a result, backing pressures are significantly lowered (approximately 50% reduction) in comparison to packed-sphere columns. In addition, the capital costs of the fiber material (< US

Sampling an RF-Powered Glow Discharge Source with a Double Quadrupole Mass Spectrometer

0.25 per column) are very attractive. Flow-rates of up to 5 ml/min can be used to achieve near baseline separation of related compounds in reasonable run times, indicating very fast mobile phase mass transfer (C-terms). The polymer stationary phases demonstrate high selectivity for a wide variety of analytes with gradient elution employed successfully in many instances. Specifically, separations of three polyaromatic hydrocarbons (benzo[a]pyrene, chrysene, pyrene), mixtures of both organic and inorganic lead compounds [chlorotriethyllead, chlorotriphenyllead, lead nitrate, lead(II) phthalocyanine], and a lipid standard of triglycerides were accomplished on the polymeric stationary phases. Other species of biological interest, including groups of aliphatic and aromatic amino acids have also been effectively separated. The reversed-phase nature of the fiber surfaces is supported through atomic force microscopy measurements using hydrophilic and hydrophobic functionalized polystyrene beads as the probe tips. Separations of the various analytes demonstrate the feasibility of utilizing C-CP fibers as stationary phases in reversed-phase LC. It is envisioned that columns of this nature would be particularly useful in prep-scale separations as well as for immobilization matrices for organic constituents in aqueous environments.

Liquid Sampling-Atmospheric Pressure Glow Discharge Ionization Source for Elemental Mass Spectrometry

A double quadrupole mass spectrometer has been assembled to perform collision-induced dissociation (CID) of molecular ions sampled from a radio-frequency (rf) powered glow discharge atomization/ionization source. Low-energy (Elab = 20 and 40 eV) collisions with argon target atoms efficiently dissociate species of the form M2+, MAr+, MO+, and Ar2+, and residual gas species. Exemplary CID mass spectra illustrate the enhanced reduction of potential molecular isobars relative to scattering losses of analyte (atomic) ions.

Use of a cylindrical Langmuir probe for the characterization of charged particle populations in a planar, diode glow discharge device

A new, low power ionization source for elemental MS analysis of aqueous solutions is described. The liquid sampling-atmospheric pressure glow discharge (LS-APGD) operates by a process wherein the surface of the liquid emanating from a 75 μm i.d. glass capillary acts as the cathode of the direct current glow discharge. Analyte-containing solutions at a flow rate of 100 μL min(-1) are vaporized by the passage of current, yielding gas phase solutes that are subsequently ionized in the <5 W (maximum of 60 mA and 500 V), ~1 mm(3) volume, plasma. The LS-APGD is mounted in place of the normal electrospray ionization source of a Thermo Scientific Exactive Orbitrap mass spectrometer system without any other modifications. Basic operating characteristics are described, including the role of discharge power on mass spectral composition, the ability to obtain ultrahigh resolution elemental isotopic patterns, and demonstration of potential limits of detection based on the injection of aliquots of multielement standards (S/N > 1000 for 5 ng mL(-1) Cs). While much optimization remains, it is believed that the LS-APGD ion source may present a practical alternative to high-powered (>1 kW) plasma sources typically employed in elemental mass spectrometry, particularly for those cases where costs, operational overhead, simplicity, or integrated elemental/molecular analysis considerations are important.

Glow Discharge Sputter Atomization for Atomic Absorption Analysis of Nonconducting Powder Samples

Abstract The application of the single Langmuir probe techniques to the measurement of electron temperature, electron number density, positive ion number density, average electron energy and electron energy distribution functions in the negative glow region of a planar diode argon glow discharge device is described. These charged particle properties were determined as a function of axial distance from a copper cathode surface at various discharge conditions. The effect of source pressure was determined in both constant current and constant voltage modes. The general range of the charged particle properties are electron temperature 0.20–0.35 eV; electron/ion densities on the order of 1011 cm−3; and average electron energy 0.7–1.0 eV. The electron energy distribution functions show non-Maxwellian distributions, with depletion of high energy electrons.

Role of powering geometries and sheath gas composition on operation characteristics and the optical emission in the liquid sampling-atmospheric pressure glow discharge

A methodology has been developed for the analysis of nonconducting (oxide) powder samples by glow discharge atomization-atomic absorption spectrometry (GDA-AAS). The mixing of an oxide powder with a copper host matrix (1:9) allows pressing of a disk sample for glow discharge sputtering. Sample-to-sample precisions are on the order of 3–4% for iron in a geological specimen. The ability to generate analytical working curves is demonstrated for the analysis of iron by mixing Fe (III) and Al (III) oxides in the copper matrix material. The possible utility of the methodology is illustrated by the analysis of iron in an NBS geological reference material. The ability to perform analyses of these sample types suggests its applicability to such matrices as ceramics, glasses, and refractory-based catalysts.

Electrical and optical characteristics of a radio frequency glow discharge atomic emission source with dielectric sample atomization

Characterization of the liquid sampling-atmospheric pressure glow discharge optical emission spectroscopy (LSAPGD-OES) source is described with regards to applications in low-flow separations such as capillary liquid chromatography and electrophoresis. Four powering modes are investigated, including the effects of the individual modes on current–voltage characteristics, analyte emission response, and temporal broadening of flow injection profiles. A concentric sheath gas is employed to stabilize the solution delivery at low liquid flow rates. Sheath gas composition ( No r He) effects analyte emission responses as well as gas phase rotational and excitation temperatures. 2 The respective powering modes both measures of temperature, with the OH rotationalgas temperatures ranging from ;2100 to 3000 K and the Fe (I) excitation temperatures ranging from ;2400 to 3600 K. Rotationaltemperature values increase slightly when helium is employed as a sheath gas as opposed to nitrogen, with the corresponding excitation temperatures increasing somewhat as well. Analytical response curves for Na and Hg in the various powering modes demonstrate good linearity, with the limits of detection for the analytes found to be on the order of ;4–10 ppm for 5 ml injections; equating to absolute detection limits of between 20 and 45 ng. It is believed that the approach demonstrated here suggests further improvements that will permit applications in a wide variety of aqueous solution analyses where low-flow rates and limited volumes are encountered. 2002 Elsevier Science B.V. All rights reserved.

Emission characteristics of a pulsed, radiofrequency glow discharge atomic emission device

Abstract A parametric study has been conducted on a radio frequency powered glow discharge atomic emission spectrometry (rf-GD-AES) source to evaluate its performance in the direct analysis of non-conducting solid materials. These experiments include both the emission and electrical characterization of this system with respect to discharge power, pressure, limiting anode orifice diameter, and sample size. The rf-GD-AES source has been demonstrated to operate interchangeably between conducting and non-conducting sample materials; however, the energy dissipated within the plasma appears to be reduced with the dielectric samples, resulting in lower emission intensities and sputtering rates. The power losses have also been found to be a function of the size, or thickness, of the sample materials. Despite these limitations of the system, preliminary emission data demonstrate that the rf-GD-AES system can be successfully employed in the direct, trace analysis of non-conducting sample materials.