Mao Huang

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.

Non-thermal features of atmospheric-pressure argon and helium microwave-induced plasmas observed by laser-light Thomson scattering and Rayleigh scattering

Abstract Laser-light Thomson scattering and Rayleigh scattering have been measured from a microwave-induced plasma sustained at atmospheric pressure, using both argon and helium as a support gas. The measurements were performed at several spatial positions in each plasma, and at forward microwave power levels of 350 W for argon, and at 350 W and 100 W for helium. It was found from these measurements that both argon and helium plasmas deviate substantially from local thermodynamic equilibrium (LTE), Measured electron temperatures range from 13 000–21 500 K, whereas gas temperatures are generally lower by a factor of 2 to 10, depending on the support gas and the spatial position in the discharge. At the same forward microwave power, the electron temperature of the helium plasma is about 3500–7000 K higher than that of the argon plasma. Yet, the argon plasma has a higher electron number density than the helium plasma. Electron number densities in both argon and helium plasmas are roughly two to three orders of magnitude lower than what LTE would predict, based on the measured electron temperatures and the Saha Equation. Even more interestingly, signals in the far-wing portion of the Thomson-scattering spectrum were found to be significantly higher than are predicted by a fitted Maxwellian curve, indicating that there exists an over-population of high-energy electrons. It is concluded that, compared to the inductively coupled plasma, the microwave-induced plasma is highly non-thermal and remains in an ionizing mode in the analytical zone.

Isocontour maps of electron temperature, electron number density and gas kinetic temperature in the Ar inductively coupled plasma obtained by laser-light Thomson and Rayleigh scattering

Abstract Isocontour maps of electron temperature (Tc), electron number density (nc, and gas-kinetic temperture (Tg) in an argon inductively-coupled plasma (ICP), have been constructed for five rf power settings and in the presence and absence of an aqueous aerosol, based on the simultaneous measurement of spatially resolved Thomson and Rayleigh scattering. Each map was assembled from 21 observation heights and 31 radial positions, the location of each point being controlled by a computer-driven three-axis translation stage. The maps demonstrate how Te,ne and Tg all increase throughout the plasma as rf power is raised, with the electron number density increasing most rapidly. At greater rf power levels (1.75 kW), the introduction of an aqueous aerosol into the central channel was found to increase ne over the entire plasma. In contrast, at lower applied power (0.75 kW), nc is depressed throughout the discharge upon the introduction of water aerosol. At intermediate rf powers (1.0, 1.25 and 1.5 kW), aerosol introduction raises nc only in the regions near the hot torus, while reducing nc at all other spatial positions, particularly lower in the central channel. Aerosol introduction was found also to thermalize the plasma at all rf power settings by increasing Tg in the higher regions and decreasing Te lower in the central channel. In general, Te is substantially higher than Tg in the lower regions of the discharge, but the two approach each other in the upper zones.

Thomson scattering from an ICP

Abstract Fundamental parameters related to laser light scattering by electrons in the ICP are explained and calculated. The theory concerning how electrons in the plasma scatter incident laser light and how the scattered-light spectra are related to electron-density fluctuations is briefly described. Special problems and detailed experimental considerations for Thomson scattering from an ICP are discussed.

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.

Comparison of electron concentrations, electron temperatures, gas kinetic temperatures, and excitation temperatures in argon ICPs operated at 27 and 40 MHz

Abstract Spatially resolved electron temperatures (Te), electron number densities (ne) and gas kinetic temperatures (Tg) were measured for 27 and 40 MHz argon inductively coupled plasmas (ICPs) by means of Thomson and Rayleigh scattering. The study used the same r.f. generator, impedance-matching network, load coil, torch, and operating conditions for both frequencies. The experiments were carried out at three r.f. forward power settings (1.00, 1.25 and 1.50 kW) and three central gas flow rates (0.80, 1.00 and 1.201 min−1). The results show that all three fundamental parameters drop when the r.f. frequency is increased from 27 to 40 MHz under all operating conditions used. The change in ne was the most significant. The relative change in each of the fundamental parameters depends mainly on the observation position in the plasma; the largest drop is generally found in the central channel. Under the same operating conditions, the 40 MHz ICP shows a larger, clearer central channel than the 27 MHz ICP, offering ease of sample introduction. This beneficial plasma environment cannot be created in a 27 MHz ICP simply by lowering the r.f. power, but could be produced by raising the central gas flow at the expense of shortening the sample residence time. The measured excitation temperature (Texc) also declines with increasing r.f. frequency. The change in Texc is comparable with those in Te and Tg. The argon ionization temperature (Tion) obtained from measured ne values and the Saha equation is higher than Te at both r.f. frequencies, indicating that the recombining mode is a common feature in the region above the load coil in an ICP.

Simultaneous measurement of spatially resolved electron temperatures, electron number densities and gas temperatures by laser light scattering from the ICP

Abstract Spatially resolved electron temperatures, electron number densities and gas-kinetic temperatures have been determined at five observation heights, nine radial positions and two settings of rf input power in an aerosolfree argon ICP sustained with a commercial MAK. torch. The measurements were obtained by means of simultaneous ruby-laser Thomson scattering and Rayleigh scattering. In the central channel of the ICP the electron temperature ranges from 7000 to 8500 K, the gas kinetic temperature is about 1000–2000 K lower than the electron temperature, and the electron number density varies from 1.9 to 7.0 × 10 15 cm −3 . Among the measured parameters, the electron number density is most sensitive to the input rf power and is noticeably higher than the LTE value determined from the measured electron temperature and the Saha equation. The overpopulation of electrons can probably be attributed to fast ambipolar diffusion and slow ion-electron recombination processes, resulting in a recombining plasma in the analytical zone. This non-LTE feature might be significant for excitation and ionization mechanisms in the ICP.

Effect of an inductively coupled plasma mass spectrometry sampler interface on electron temperature, electron number density, gas-kinetic temperature and analyte emission intensity upstream in the plasma

Abstract Thomson scattering, Rayleigh scattering and line-of-sight emission intensities of Ca ion and Sr ion from the inductively coupled plasma were measured in the presence and in the absence of an inductively coupled plasma mass spectrometry sampler interface. When present, the sampler interface was located 13 mm above the load coil (ALC); optical measurements were made 6, 7 and 8 mm ALC. The experimental results suggest that both the electron temperature (Te) and gas-kinetic temperature (Tg) dropped in the presence of the sampler interface, with the change in Tg seemingly greater than that in Te, suggesting a faster cooling process for the heavy particles. In contrast, electron number density (ne) seemed to be generally increased in the outer regions of the discharge but went down in the central channel, a reflection that ne is possibly dominated by ambipolar diffusion which becomes less efficient as Te drops. Assuming these results, the plasma decays more gradually ALC and deviates from local thermodynamic equilibrium even more significantly in the presence of the sampler interface. Analyte line emission intensity was either depressed or enhanced in the presence of the interface, depending on the element being observed and the operating conditions. In addition, the change in emission intensity caused by the sampler interface became much more dramatic when a matrix element, such as Li or Zn, was introduced.

Verification of a Maxwellian electron-energy distribution in the ICP

Abstract A 20-Hz frequency-doubled Q-switched Nd : YAG laser has been used to measure Thomson-scattering spectra from an inductively coupled argon plasma (ICP). The experimental data were treated with a non-linear regression curve-fitting program to determine electron temperatures and electron number densities under different plasma operating conditions. A comparison of the measured Thomson-scattering spectra with the corresponding fitted curves indicates that the electron energy distributions in the ICP are very close to Maxwellian under a wide range of sample-introduction conditions and at locations above, below, and at the tip of the “bullet”. As a result, electron temperature can be used as a meaningful fundamental parameter. The theory used in this study is briefly reviewed and the detailed experimental considerations are described.

A new procedure for determination of electron temperatures and electron concentrations by Thomson scattering from analytical plasmas

Abstract A new data-treatment procedure allows for more accurate determination of electron temperatures and electron concentrations in analytical plasmas. A Thomson-scattering spectrum, useful for these determinations, is often not purely Gaussian in shape, even when the probed electrons possess a Maxwellian velocity distribution. Nonetheless, an unambiguous relationship exists between electron temperatures and concentrations that truly exist in a plasma and those calculated from a distorted Thomson-scattering spectrum. Understanding this relationship permits a look-up table to be constructed, from which observed values can be corrected. Theory concerning this procedure is described and details for using it with both a ruby laser and frequency-doubled Nd:YAG laser are discussed. Examples of electron temperature and electron concentration determined with this procedure in an ICP are given. The possibility of using the new procedure to study electron-energy distributions is also assessed.