Andrew J. Schwartz

Visual observations of an atmospheric-pressure solution-cathode glow discharge

The solution-cathode glow discharge (SCGD) is an optical emission source for atomic spectrometry comprised of a moderate-power atmospheric-pressure DC glow discharge sustained directly upon the surface of an electrically conductive solution. The SCGD boasts a simple, inexpensive design and has demonstrated detection limits similar to those of more conventional excitation sources used in atomic spectrometry. Although the analytical performance of the SCGD as an optical emission source is well characterized, the mechanism through which the discharge atomizes and excites analyte from the sample solution remains a point of debate. The current paper presents visual observations of the SCGD from a variety of imaging techniques. The implications of the images regarding the mechanism of analyte solution-to-plasma transport and excitation in the SCGD are discussed.

Determination of trace sodium, lithium, magnesium, and potassium impurities in colloidal silica by slurry introduction into an atmospheric-pressure solution-cathode glow discharge and atomic emission spectrometry

Trace impurities of sodium, lithium, magnesium, and potassium in colloidal silica were determined by slurry introduction into an atmospheric-pressure solution-cathode glow discharge (SCGD). The applied voltage, solution flow rate, and distance between the metal anode and surface of the solution were optimized. Emission from K (766.5 nm), Na (589.0 nm), Mg (285.2 nm) and Li (670.8 nm) demonstrated a linear range of nearly 4 orders of magnitude (R2 ≥ 0.998) and steady-state sample introduction yielded limits of detection of 0.7, 0.4, 0.5 and 0.2 ng mL−1, respectively. For an integration time of 0.3 s, relative standard deviations (RSDs) from 1000 ng mL−1 standard solutions introduced continuously were found to be better than 3% for all four elements. Transient sample introduction into the SCGD was also optimized and provided limits of detection for K, Na, Mg and Li of 3, 2, 2 and 0.8 ng mL−1, respectively, and RSDs for 1000 ng mL−1 standard solutions of better than 3%. Determined concentrations of trace impurities in colloidal silica agreed satisfactorily (accuracy from 1.3 to 7.7% and precision from 4 to 14%) with those obtained from inductively coupled plasma atomic emission spectrometry.

Evaluation of interference filters for spectral discrimination in solution-cathode glow discharge optical emission spectrometry

An inexpensive, computer-controlled wheel outfitted with several interference filters is evaluated as a spectral discrimination device for solution-cathode glow discharge (SCGD) optical emission spectrometry. Analytical performance of the instrument with wide-bandpass (10 nm) filters was critically compared to its performance with a traditional, narrow-bandpass monochromator (0.08 nm). With optimized spatial filtering, the SCGD-filter wheel (FW) instrument provided good precision (0.5–4.6% relative standard deviation), 1.5–4.9 orders of linear range, and limits of detection (LODs) that ranged from 0.5–450 ppb for the alkali and alkaline-earth metals, with both continuous sample introduction and 100 μL injections. Although the precision and upper limits of linearity of the instrument were comparable to those obtained with a traditional spectrometer, the LODs were 2–43 times higher (worse) and were strongly dependent upon the background emission accepted over the spectral bandpass of each filter. Despite the promising performance obtained during the initial analytical evaluation, application of the SCGD-FW instrument to complex samples was critically hindered by changes in background emission induced by the sample matrix, which could not be easily corrected. As a result of these interferences, the filter wheel, in its present form, seems unlikely to serve as a suitable alternative to a traditional spectrometer.

New inductively coupled plasma for atomic spectrometry: the microwave-sustained, inductively coupled, atmospheric-pressure plasma (MICAP)

A novel inductively coupled plasma (ICP), termed the microwave-sustained, inductively coupled, atmospheric-pressure plasma (MICAP), has been developed that operates at microwave frequency (2.45 GHz). To sustain the new plasma, a dielectric resonator ring (fabricated from an advanced technical ceramic) is coupled with a 2.45 GHz microwave field generated from a microwave-oven magnetron. The microwave field induces polarization currents (small shifts in the equilibrium positions of bound electrons) in the resonator that generate an orthogonal magnetic field, analogous to that produced by electrical current within a traditional ICP load coil. This magnetic field is capable of sustaining an annular plasma in either air or nitrogen that can readily accept solution samples in the form of a wet aerosol produced from a conventional nebulizer and a spray chamber. An initial analytical evaluation of the MICAP with radially viewed optical emission spectrometry (OES) revealed that limits of detection ranged from 0.03–70 ppb with relative standard deviations from 0.7–2.0%. In addition, the new plasma exhibited good tolerance to solvent loading, and was found to be capable of accepting a wide variety of organic solvents directly and salt solutions up to 3% w/w concentration. Combined, the results suggest that the MICAP could be a competitive, simpler alternative to traditional, radiofrequency argon ICP-OES.

Use of Gradient Dilution to Detect and Correct Matrix Interferences in Inductively Coupled Plasma–Time-of-Flight Mass Spectrometry (ICP-TOFMS)

Our research group earlier used dispersion that occurs during flow injection to detect and reduce matrix interference in inductively coupled plasma–time-of-flight mass spectrometry (ICP-TOFMS). In the absence of a matrix interference, the ratio of signals from any two sample constituents should remain constant, independent of the dilution, over the course of a flow-injection transient. However, when an interferent is present, the signal ratio from different analytes will change with dilution, owing to the difference in severity of the interference on specific analytes. As a result, matrix interference can be recognized (flagged) by monitoring the signal ratios of two analytes over the course of a flow-injection transient; a ratio that changes over time indicates the presence of an interferent. The drawback of this earlier method was that dispersion, and therefore dilution, was somewhat element-specific, causing the ratios to wander even when no interference existed. Here, a gradient HPLC pump is used to overcome this drawback by creating a longer, better-controlled dilution. Under these conditions, variation in dispersion between elements is negligible and difficulties associated with it are reduced or eliminated. Further, when an interference exists, the optimal dilution factor to reduce the interference to an acceptable level can be found from the gradient-dilution curve as the point where the signal ratio between two elements becomes constant.

Preliminary survey of matrix effects in the Microwave-sustained, Inductively Coupled Atmospheric-pressure Plasma (MICAP)

Matrix effects caused by Na and Al in the nitrogen Microwave-sustained, Inductively Coupled, Atmospheric-pressure Plasma (MICAP) were investigated. Easily ionizable elements, such as Na, can suppress or enhance the analyte signal; Al is shown here to produce a similar effect. The influence of these matrices was examined for 18 emission lines of 8 analyte atoms and ions having a wide range of excitation and ionization energies. The plasma operating conditions were fixed during all experiments at a total nitrogen flow of 19.4Lmin-1 and a microwave power of 1.5kW. An Fe solution was used to determine the excitation temperature of the plasma by the Boltzmann plot method at selected matrix concentrations. In addition, vertical emission profiles of the plasma were measured. The matrix effect becomes worse at higher concentrations of an easily ionizable element. The effect is caused not only by a shift in ionization equilibrium but also by a possible change in plasma ionization temperature. Correction methods to reduce the matrix effects were tested and are discussed.