H. B. Fannin

A Comparative Study of Rotational Temperatures in a Microwave Plasma: OH Radical versus N2+

A rotational temperature comparative study of OH radical vs. N2+ was carried out on a low-power helium microwave-induced plasma. Under the prevailing conditions, N2+ was found to provide twice as many usable lines for temperature measurement than did hydroxyl radical. For the particular torch design used, both species exhibited slightly increasing rotational temperatures at lower flow rates. At fixed conditions, OH consistently indicated higher rotational temperatures than those of the molecular nitrogen ion. Positional studies revealed a slightly increasing temperature near the center of the plasma. This work suggests that N2+ may provide a number of advantages over OH radical as a thermometric probe species in the determination of plasma rotational temperature.

Novel low power/reduced pressure inductively coupled plasma ionization source for mass spectrometric detection of organotin species

A novel, low power, reduced pressure, helium inductively coupled plasma source (LP/RP-He-ICP), operated at an estimated incident rf power of 12–15 W and pressures ranging from 0.37 to 1.2 mbar, was interfaced with mass spectrometry and coupled with gas chromatography for the separation and detection of three organotin species,viz., tetraethyltin (TET), trimethyl(phenyl)tin (TMPT) and tetrabutyltin (TBT). Limits of detection (as tin) for TET, TMPT and TBT were 0.12, 0.35, and 0.56 pg, respectively. Characteristic mass spectra matching established library spectra were obtained at low nanogram levels for TET,TMPT,TBT and tetramethyltin (TMT). The potential of the source to provide tunable fragmentation was also investigated.

Fundamental Chemical Processes in a Low-Density 27-MHz Helium ICP

A variable-pressure 27-MHz helium plasma was investigated in order to study the physical chemical nature of the inductive coupling and the microscopic atomic and molecular state variations which attend regular manipulations of plasma power and of helium pressure. Two modes of helium plasma excitation, the E- and H-plasma modes, are identified by widely differing capacities to effect ionization of other plasma species such as argon atoms. The switch from the helium E- to the H-plasma is abrupt and dependent on the gas pressure. The results presented suggest that it is the production of He+ and electrons and subsequent interactions in the plasma (electron recombination, collisions, etc.) which primarily determine the microscopic atomic and molecular behavior which we observe.

Quantum-Statistical Modeling of ICPs: He(I)

Relative populations of excited states of He(I) in reduced-pressure ICPs have been shown to obey Fermi-Dirac statistical counting. A single thermodynamic temperature—2000 K—defines the distribution. The experimental relative populations and the Fermi-Dirac distributions agree within fractions of one percent.

Studies of energy transfer and ionization processes in a helium ICP

The spectral consequences of the introduction of varying concentrations of heavy rare gas atoms into a reduced-pressure helium ICP have been examined. A term-dependent quenching of He(I) emission was observed and is consistent with previous work on a kinetic model for this ICP. In addition, the ionization behavior of the added rare gases has been studied, and a simple rule seems to emerge from the results: the first ionization potential of helium represents an upper bound to the plasma energy available (probably through collisional processes) to excite and/or ionize added species. Although spectroscopic temperatures can be calculated from the state population distributions for the added neutral gas atoms, the fact that the values vary with concentration and with the chemical identity of the species betrays the fact that the plasma is not in local thermodynamic equilibrium with respect to neutral atoms. Quite differently, the results for plasma positive ions show the same statistically determined spectroscopic temperatures; thus, it appears that these ions are nearly equilibrated and also suprathermal in population distribution.

Energy Transfer and Ionization Processes in a Reduced-Pressure Argon ICP

The spectral characteristics of an argon/helium ICP operated at reduced-pressure are presented. The state population distributions deduced from quantitative intensity measurements for plasma neutral and cationic species suggest that the argon/helium plasma is essentially an argon plasma with respect to energy stratification. Comparisons are made with recently reported results for an atmospheric-pressure argon/helium ICP.

LOW-POWER INDUCTIVELY COUPLED PLASMA SOURCE FOR ELEMENT-SELECTIVE ATOMIC EMISSION DETECTION IN GAS CHROMATOGRAPHY

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A Comparative Study of Electron Density Determination Methodologies in the Helium Microwave-Induced Plasma

Electron number densities were examined in a low-power atmospheric-pressure helium microwave-induced plasma. Two hydrogen-based methods and two helium-based methods were employed to estimate electron concentration. The hydrogen 4471 Å Balmer line was examined with the use of both line shape and full width at half-maximum intensity measurements. The data suggest that half-width calculations underestimate electron densities. Half-width measurements of neutral helium lines result in number densities which appear to be overestimated. This inaccuracy is thought to be the result of apparatus broadening. Finally, the use of quasi-degenerate helium neutral lines possessing forbidden and allowed components afforded electron concentrations similar to those derived from the Balmer beta line shape analysis.

Comparison of Methodologies for the Determination of Excitation Temperatures of Plasma Support Gases

Methodologies for the determination of excitation temperatures for the argon support gas in six inductively coupled plasma systems are compared. These methods include a Boltzmann plot, partial Boltzmann plots, a polynomial fit, and a Fermi-Dirac model. The temperature(s) and fitting statistics are reported for each method. Additionally, the theoretical basis for each method is briefly reviewed. All these methods, with the exception of the first and last, yield multiple excitation temperatures; however, the Fermi-Dirac model more closely models the observed population distribution of excited electronic states in the argon atom.

Quantum-Statistical Modeling of Mixed Inert Gas ICPs

Relative populations of excited neutral and ionic states in reduced-pressure ICPs supported by a binary mixture of helium and an additional inert gas have been shown to obey Fermi-Dirac statistical counting. A single thermodynamic temperature of 2000 K is sufficient to define the distribution of both neutral and ionic state populations. High-energy states of ionic species exhibit a “blending” between Fermi-Dirac and Boltzmann behavior. Agreement between measured relative populations and the Fermi-Dirac distribution is ∼1% overall.