Norman N. Sesi

Fundamental studies of mixed-gas inductively coupled plasmas

Abstract The effects of adding foreign gases to the central-gas flow or the intermediate-gas flow of an argon inductively coupled plasma are presented. In particular, the influence of up to 16.7% added helium, nitrogen or hydrogen on radially-resolved electron number density, electron temperature, gas-kinetic temperature and calcium ion emission profiles is examined. It is shown that these gases affect not only the fundamental parameters and bulk properties of the plasma, but also how energy is coupled and transported through the discharge and how that energy interacts with the sample. For example, added helium causes an increase in the gas-kinetic temperature, most likely due to the higher thermal conductivity of helium compared to argon but, in general, does not appear to affect significantly either the electron temperature or electron concentration. The shift in the calcium ion emission profile towards lower regions in the discharge with added helium may be attributable to higher droplet desolvation and particle vaporization rates. In contrast, the addition of nitrogen or hydrogen to an Inductively Coupled Argon Plasma (Ar ICP) results in dramatic changes in all three fundamental plasma parameters: electron number density, electron temperature, and gas-kinetic temperature. The net effect of these molecular gases (N2 or H2) on calcium ion emission and on the fundamental plasma parameters is shown to be dependent on the amount of gas added to the plasma and whether the gas is introduced as part of the central- or intermediate-gas flow. In general, nitrogen added to the central-gas flow causes a significant reduction in the number of electrons throughout most of the discharge (over an order of magnitude in certain regions), mainly in the central and upper zones of the ICP. A drop of 3000–5000 K in the central channel electron temperature and a smaller drop in the gas-kinetic temperature are also observed when N2 is added to the central-gas flow. In contrast, the introduction of nitrogen in the intermediate flow causes about a 1 × 1015 electrons cm−3 increase in the electron concentration in the low, toroidal regions of the plasma and an increase in the gas-kinetic temperature of around 1000 K throughout most of the discharge. As seen with the addition of nitrogen to the central-gas flow, the electron temperature is found to increase in the toroidal zones of the plasma when N2 is added to the intermediate flow. These combined effects cause a 20-fold depression in the calcium ion emission intensity only a 1.7-fold depression when N2 is added to the central- or intermediate-gas flows, respectively. On the other hand, hydrogen causes a depression in the electron concentration in the upper areas of the plasma when this gas is added to the central flow but increases the number of electrons in the same region when added to the intermediate flow. Hydrogen also causes a dramatic effect on the electron and gas-kinetic temperatures, significantly increasing both of these parameters throughout the discharge. An increase in the calcium ion emission intensity, accompanied by a downward shift, elongation and broadening of the calcium ion emission profile is also observed with H2 addition.

The effect of sample matrix on electron density, electron temperature and gas temperature in the argon inductively coupled plasma examined by Thomson and Rayleigh scattering

Abstract Spatially-resolved electron temperature ( T e ), electron number density ( n e ) and gas-kinetic temperature ( T g ) maps of the inductively coupled plasma (ICP) have been obtained for two central-gas flow rates, four heights above the load coil (ALC) and in the presence and absence of interferants with a wide range of first ionization potentials. The radial profiles demonstrate how the directly measured fundamental parameters n e T e and T g can be significantly enhanced and/or depressed with added interferent, depending upon plasma operating conditions and observation region. In general, the magnitude of n e , and T e change is found to be an inverse function of interferent ionization potential; furthermore, n e enhancements in the central channel might be the result of electron redistribution from high to low electron density regions rather than from ionization of the matrix. The large measured increases in n e cannot be attributed solely to matrix ionization, especially when measurement uncertainties and the probable over-estimation in calculated n e , enhancements are taken into account. Changes in n e and T e have been correlated with axial Ca atom and ion emission profiles. A brief review of the mechanisms most likely involved in interelement matrix interferences is given within the context of the present study. This article is an electronic publication in Spectrochimica Acta Electronica (SAE), the electronic section of Spectrochimica Acta Part B (SAB). The hardcopy text is accompanied by a disk for the Macintosh computer with data files stored in ASCII format. The main article discusses the scientific aspects of the subject and gives an interpretation of the results contained in the data files.

Studies into the interelement matrix effect in inductively coupled plasma spectrometry

Abstract Radially-resolved maps of calcium atom and ion emission and calcium atom and ion number densities have been obtained in the presence and absence of several concomitant elements. These concomitants include cesium, barium, lithium, aluminum, thallium, silver, magnesium and zinc. In addition, maps of electron concentration, electron temperature and gas-kinetic temperature are given for an analyte-only (Ca) plasma. The findings suggest the interelement interference effect to be the result of at least three major processes operating simultaneously: lateral-diffusion (i.e. particle-volatilization) changes, shifts in analyte-ionization equilibrium, and differences in collisional excitation efficiency. Expanded lateral diffusion caused by enhanced particle volatility explains the greater number of analyte species in the off-axis plasma zones when an interferent is added to the sample. The combined concomitant-induced effects of expanded lateral diffusion and shift in ionization equilibrium can be used to describe a drop in analyte ion concentration that occurs in the central channel but with little or no change in the Ca atom number density, Enhanced collisional excitation caused by small increases in the plasma electron number density and electron temperature result in elevated analyte emission intensities in some plasma zones. Also, a charge-transfer mechanism is described which might result in the direct production and excitation of Ca ions from molecular analyte species.

An imaging-based instrument for fundamental plasma studies

A description is given of an imaging-based instrument capable of mapping several fundamental properties of analytical plasmas, especially the inductively coupled plasma. The plasma fundamental parameters include electron temperature and number density, heavy-particle gas-kinetic temperature, analyte and argon atom and ion concentrations, and analyte emission intensities. These parameters can be measured on the same plasma running under identical operation conditions. The techniques to probe the basic plasma properties include Thomson scattering (for electron concentration and temperature), Rayleigh scattering (for heavy-particle temperature), computer-aided optical tomography (for three-dimensional emission maps), and laser-induced saturated fluorescence (for analyte and argon atom and ion number densities). A brief description of these techniques is presented. Also, a new approach to determining absolute number densities from fluorescence measurements is introduced. This novel method is based on normalizing a measured fluorescence intensity by the room-temperature Rayleigh-scattering signal.

A LabVIEW® program for determining electron number density from Stark broadening measurements of the hydrogen-beta line

A useful plasma diagnostic is the measurement of electron number density. One way to accomplish such measurements is to determine the contribution to the broadening of a spectral line due to the Stark effect. To simplify and extend such electron density measurements across computer platforms, a program that calculates electron number density from the Stark-broadened hydrogen-beta line has been written for the LabVIEW® environment. This program calculates electron number densities from the field strength that would be exerted on a hydrogen atom immersed in a plasma. Using the new program, the electron number density in a glow discharge is calculated for two different operating conditions. Not surprisingly, the results indicate that to increase the current density in the discharge, the source electrodes must be reduced in surface area. This article is an electronic publication in Spectrochimica Acta Electronica (SAE), the electronic section of Spectrochimica Acta Part B (SAB). The hardcopy text is accompanied by one disk with an executable program (written for Apple Macintosh), data and text files including a manual.

Comparison of simulated and experimental fundamental ICP parameters

Abstract False-color spatial maps of experimentally determined and simulated values of electron number density ( n e ), electron temperature ( T e ) and heavy-particle temperature ( T g ) for an argon inductively coupled plasma (ICP) in the plasma decay region (tail flame) are compared in detail. Experimental and theoretical values are in general very consistent; the difference between experiment and computation is approximately 10% for n e and T e and 20% for T g in the plasma region examined. The errors in n e and T e are larger at the edge of the plasma, most likely because air entrainment becomes significant. This comparison provides a link between measurements and the current mathematical model and serves to partially validate both methods. Sources of error in both experiment and theory are considered and discussed.

Evaluation of an automated on-line method for generating calibration curves in inductively coupled plasma atomic emission spectrometry

SummaryA high-performance liquid chromatography pump (HPLC) is used for the computer-controlled on-line generation of calibration standards in inductively coupled plasma-atomic emission spectrometry. Such a pump, employed as a solution-delivery device, is well suited for feeding a nebulizer, since it is capable of delivering a range of constant flows and of mixing selected stock solutions or solvents automatically; this capability translates into the capacity to dilute stock or sample solutions and to introduce the resulting mixtures at constant flow into any of a number of nebulizers. Further, the range of useful nebulizers is broadened, since the mixture is supplied under high pressure. This feature makes the method especially useful for devices such as the jet-impact nebulizer (JIN), which requires a pressurized, constant feed. At present, the principal limitation of the new approach is the flow resolution of available HPLC pumps. Although this limitation does not ordinarily cause difficulties in schemes for automatic dilution or calibration, it can be a serious shortcoming if the apparatus is to be used for automated standard-additions analysis.

Evaluation of a linear-flow torch for inductively coupled plasma atomic emission spectrometry

Simplex optimized operating conditions for a linear-flow torch (LiFT) were evaluated with respect to those for a conventional tangential-flow torch (TFT). The LiFT was shown to out-perform an 18 mm i.d. TFT in terms of gas consumption, precision, long-term stability, noise amplitude and level of background molecular-band emission. However, the two types of torches offered comparable detection limits and similar responses to the addition of an easily ionizable element.

A LabVIEW®2 program for data collection and calculation of figures of merit

Abstract This article is an electronic publication in Spectrochimica Acta Electronica , the electronic section of Spectrochimica Acta Part B . The hardcopy text is accompanied by a diskette containing the program discussed in this publication. The main article discusses the scientific aspects of the subject and the Appendix describes the operation and organization of the program. A glossary of unfamiliar terms is also included. The program presented here allows baseline, blank background, and signal waveforms to be collected from an optical detection system associated with a d.c. signal source. From these data, the following figures of merit are calculated: net signal; background; signal-to-background ratio; signal-to-background noise ratio; relative standard deviation of the background; and relative standard deviation of the signal. Because the figures of merit are calculated directly from the collected waveforms and stored to disk, data processing time is greatly reduced and the likelihood of error is minimized, making optimization studies much easier and informative. An example of figures of merit collected with this program is included. Another feature that makes this program unique is that it is written in an object-oriented programming language, making it much easier to evaluate and modify than character-based languages. Additionally, the user interface is graphical, making it easy to understand and manipulate.