Martha Schulz Hendrick

Excitation of analytes and enhancement of emission intensities in a d.c. plasma jet: a critical review leading to proposed mechanistic models

Abstract Experimental data and theoretical criteria are used to critically review existing models for analyte emission enhancement in the 3-electrode d.c. plasma (DCP). The analytical zone is characterized as a non-optically thin recombining plasma in partial thermodynamic equilibrium (PTE). Spectrochemical excitation the authors ascribe largely to: (1) argon resonance line radiative transport; (2) inversion of optically pumped argon states; (3) inversion of analyte populations by Franck-Condon collisions with argon; (4) energy cascading in analytes via a multitude of channels. Adding an easily ionized element (EIE): (1) induces additional resonance line radiative transfer; (2) raises electron densities in cooler, analyte-rich plasma margins; (3) locally increases argon optical absorption cross sections via Stark broadening; (4) redistributes ohmic heating. Coupling between the proposed mechanisms is non-linear. Relationships between radiative transfer and collisional redistribution and (1) background suppression by EIE and (2) analyte emission enhancement by helium are also examined. Similarities between DCP and inductively coupled plasma (ICP) excitation mechanisms are noted and practical implications are addressed.

Spectral Efficiency of the SpectraSpan III Echelle Grating Spectrometer

The efficiency of an echelle grating spectrometer incorporating prism cross-dispersion is empirically evaluated over a broad wavelength range incorporating a large number of orders. A derived parameter, the spectrometer efficiency, proportional to the spectrometer luminosity, conveniently gauges dynamic features of the echelle efficiency. Characteristics of efficiency over broad wavelengths suggested by earlier work are clearly observed. The shift of blaze wavelength with decreasing order of diffraction, predicted in a recent model of echelle grating efficiency, is experimentally documented.

Wavelength-dependent enhancement of DCP metal lines by saline matrices

Abstract Sodium-induced emission enhancement of transition metal resonance lines are measured in a d.c. plasma (DCP) for wavelengths from 210 to 395 nm. Systematic differences in enhancement are observed within individual spectra (Fe I, Ni I, Sc II), and the enhancement and the excitation potential of a line are found to be linearly related. Electron density and apparent temperature data lead to an interpretation of thiis energy dependence within the context of a recombining plasma in partial thermodynamic equilibrium.

Applications of d.c. argon plasma emission spectroscopy to saline waters: a study of enhancement effects

Abstract A commercially available d.c. argon plasma emission spectrometer was used to determine transition metals (3d and 4d) and also Be in salt and brackish water. The effects of salinity on the enhancement of emission intensities of the analyte lines were studied using an empirical approach combined with statistical analysis. One set of experiments deals with the effects of trace metal concentration and salinity on the relative emission intensities of 14 elements using a completely randomized experimental design, i.e. the sequence in which the 48 treatment combinations (12 levels of salinity and 4 levels of metal concentration) were measured was determined by randomization, with the results evaluated by an analysis of variance program. The second set deals with the effect of relative enhancement to salinity on selected ion and atom emission line pairs for various elements at 100 and 50% salinity relative to fresh water (0% salinity). These results are analyzed by a stepwise linear multiple regression analysis program using selected parameters of theoretical interest. It was discovered that the differences in relative enhancement for ion—atom emission line pairs for 100% salinity could be predicted basically with two variables. A coefficient of determination of 95% was achieved by employing the energy of transition for the atomic line and the number of unpaired d-electrons in the lower atomic state.

Laser excited atomic fluorescence for studies of enhancement effects in the direct current plasma

Abstract This paper reports a preliminary use of laser excited atomic fluorescence spectrometry (LEAFS) to study analyte population enhancement caused by easily ionized elements (EIEs) in the direct current plasma (DCP). Spatial atom density profiles in the DCP were obtained using resonance fluorescence at the calcium atom line at 422.7 nm, with and without the addition of an EIE. Variations in atom density caused by an EIE were found to be far too small to account for the marked enhancements of atomic emission signals which are caused by EIEs. Direct line fluorescence of the barium ion, excited at 614 nm and detected at 455 nm, was used to probe the effect of an EIE on excited state populations. Measurements in the analytical region of the plasma close to the core revealed that enhancements of fluorescence signals at low laser powers disappeared at laser powers which were sufficient to saturate the atomic transitions. While this result does not clarify any of the mechanisms of excitation in the DCP, it does lend support to two of the fundamental postulates of a recent model of the spectrochemical excitation processes in the DCP. These are first, that the analytical region of the DCP is not in local thermodynamic equilibrium (LTE) and second, that EIE enhancement proceeds by modulating the rates of power distribution among various plasma zones. In the outer zone of the analytical region of the DCP, depressive interferences occurred. These did not disappear upon saturation which indicates that they were not rate effects but effects that resulted from atom density changes.

Optimization of a direct-current plasma emission echelle spectrometer

Abstract Simplex optimization and Box-Behnken partial factorial experimental design techniques were used to optimize the quantitative performance of a direct-current plasma system. Argon pressure and plasma position were selected as variables. The simplex optimization located the region of optimum sensitivity, and the partial factorial design generated quadratic equations describing the relationship between the variables in this area. Results compared well with the results of a univariate search done in the region of optimum sensitivity. Canonical analysis of the response-surface equations showed that there was no unique optimum, but rather a stationary ridge, with combinations of conditions which give similar sensitivities. Studies of response surfaces for imprecision and drift failed identify one optimal region. Comparison of the imprecision and drift contours with the sensitivity results showed that a significant improvement in precision or drift cannot be made by changing the instrumental conditions.

Laser Excited Atomic Fluorescence in the Direct Current Plasma

Abstract Detection limits of between 3 and 72 ng mL−1 (barium, calcium, sodium, vanadium and iron) and linear dynamic ranges of just over four orders of magnitude (barium and calcium) are reported for laser excited atomic fluorescence spectrometry (LEAFS) in the three electrode direct current plasma. The detection limits are not as good as those that have been reported for LEAFS in both the inductively coupled plasma and flames. The limiting factors appear to be the high plasma background, scatter off sample droplets and the inefficiency of the sample introduction process.

Direct current plasma as an excitation source for flame atomic fluorescence spectrometry—some applications

The concentrations of manganese, chromium, copper and cobalt were accurately determined in various standard steel samples, as were manganese, copper, zinc and iron in tomato leaves and bovine liver (National Bureau of Standards reference materials). Scatter signals were evaluated by means of the two-line method. Various potential spectral interferences were studied including the effect of 5000 µg ml–1 iron solutions on manganese and chromium determinations. The plasma behaved as would be expected of a narrow line source. The two-electrode version of the direct current plasma that was used, limited the precision of the measurements to 10% relative standard deviation.