Scott A. Lehn

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.

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.

Computerized simulation of aerosol-droplet desolvation in an inductively coupled plasma

Abstract A mathematical model for the desolvation of solvent droplets has been used in conjunction with an existing code for simulation of ICP fundamental parameters. The combination has been used for the calculation of droplet histories and desolvation behavior along the central channel of an ICP. Calculations have been performed for droplets of various sizes and under a variety of ICP operating conditions. As central-channel gas flow rate increases, the point of complete desolvation of the droplet shifts upward in the plasma, away from the load coil. This relationship is fairly linear. As forward power increases, the point of complete desolvation moves down in the discharge, closer to the load coil. This is approximately an inverse relationship. Finally, simulation of behavior for a log-normal size distribution of a large number of droplets (10 8 ) shows that the number of surviving droplets falls sigmoidally with height above the load coil. For most nebulizer/spray chamber systems, the desolvation process is complete at a well-defined height in the plasma.

Effect of sample matrix on the fundamental properties of the inductively coupled plasma

Abstract In the inductively coupled plasma (ICP), the emission intensities of atomic and ionic spectral lines are controlled by fundamental parameters such as electron temperature, electron number density, gas-kinetic temperature, analyte atom and ion number densities, and others. Accordingly, the effect of a sample matrix on the analyte emission intensity in an ICP might be attributable to changes in these fundamental parameters caused by the matrix elements. In the present study, a plasma imaging instrument that combines Thomson scattering, Rayleigh scattering, laser-induced fluorescence and computed tomography has been employed to measure the above-mentioned parameters in the presence and absence of matrix elements. The data thus obtained were all collected on a spatially resolved basis and without the need for Abel inversion. Calcium, strontium and barium served as analytes, while lithium, copper and zinc were introduced as matrix elements. Comparing the data with and without the matrix elements allows us to determine the extent to which each fundamental parameter changes in the presence of a matrix element, and to better understand the nature of the matrix effects that occur in the ICP. As has been seen in previous studies with different matrix elements, ion emission and ion number densities follow opposite trends when matrix interferents are introduced into the plasma: ion emission is enhanced by the presence of matrix interferents while ion concentrations are lowered. These changes are consistent with a shift from collisional deactivation to radiative decay of excited-state analyte species.

Spatially resolved ground-state number densities of calcium and strontium ion in an inductively coupled plasma in contact with an inductively coupled plasma mass spectrometry sampling interface

Abstract Radial profiles of Ca and Sr ion number densities in an ICP at 6, 7 and 8 mm above the load coil (ALC) and at 1.25 kW of input rf power were measured by saturated fluorescence induced by an Nd:YAG laser-pumped dye laser at 396.85 nm and 421.55 nm, respectively. The measurements were performed in the presence and in the absence of an ICP-MS sampling interface. When in place, the orifice of the sampling cone was positioned 13 mm ALC on the axis of the plasma torch. The results show that the interface can either raise or lower the ion number densities, depending on the central-gas-flow rate, and can cause a vertical shift of their entire radial profiles with respect to the ICP axis. The introduction of Li, Cu and Zn as matrix elements reduced the ion number densities of the analytes, both in the presence and in the absence of the interface. This effect became more significant at higher central-gas-flow rates. In addition, the peak value of the radial ion number density was found to depend strongly on the central-gas-flow rate maximum occurred at 1.1 l/min for both Ca ion and Sr ion under the ICP operating conditions used in this study. This behavior is very similar to the mass spectrometric signals previously observed downstream and reported in the literature.

Experimental evaluation of analyte excitation mechanisms in the inductively coupled plasma

Abstract The inductively coupled plasma (ICP) is a justifiably popular source for atomic emission spectrometry. However, despite its popularity, the ICP is still only partially understood. Even the mechanisms of analyte excitation remain unclear; some energy levels are quite clearly populated by charge transfer while others might be populated by electron-ion recombination, by electron impact, or by Penning processes. Distinguishing among these alternatives is possible by means of a steady-state kinetics approach that examines correlations between the emission of a selected atom, ion, or level and the local number densities of species assumed to produce the excitation. In an earlier investigation, strong correlations were found between either calcium atom or ion emission and selected combinations of calcium atom or ion number densities and electron number densities in the plasma. However, all radially resolved data employed in the earlier study were produced from Abel inversion and from measurements that were crude by todays standards. Now, by means of tomographic imaging, laser-saturated atomic fluorescence, and Thomson and Rayleigh scattering, it is possible to measure the required radially resolved data without Abel inversion and with far greater fidelity. The correlations previously studied for calcium have been investigated with these more reliable data. Ion–electron recombination, either radiative or with argon as a third body, was determined to be the most likely excitation mechanism for calcium atom, while electron impact appeared to be the most important process to produce excite-state calcium ions. These results were consistent with the previous study. However, the present study suggests that collisional deactivation, rather than radiative decay, is the most likely mode of returning both calcium atoms and ions to the ground state.

A simple approach to deriving an electron energy distribution from an incoherent Thomson scattering spectrum

Abstract A clear picture is given to illustrate how the electron energy distribution in a plasma can be directly related to an incoherent Thomson scattering spectrum when the electron density distribution is spherically symmetric in velocity space. Based on this relationship, a simple approach is outlined to derive the electron energy distribution unambiguously. The method involves plotting the differences in Thomson scattering intensities between adjacent wavelength channels as a function of the square of the wavelength shift. Thomson scattering spectra obtained from microwave-induced helium plasmas sustained at atmospheric pressure at forward powers of 100 and 350 W are used to derive electron energy distributions through this procedure. Significant deviations from a Maxwellian distribution were found under both conditions over the entire wavelength span covered in the experiment, corresponding to an electron energy range of 0.1–6.6 eV. Compared with a Maxwellian energy distribution, the portion of the electrons with energies between 2 and 6 eV appear to be shifted toward both lower and higher energy values in these plasmas. The ratios of the Thomson scattering intensity at the wavelength channel farthest from the center to the corresponding Maxwellian value indicate that the total number of electrons with energies larger than 6.6 eV is at least 1.8 and 3.1 times higher than that predicted by local thermodynamic equilibrium for plasmas at 100 and 350 W, respectively. In contrast to a Maxwellian distribution, electrons in the microwave-induced plasmas are not most concentrated at the center of velocity space, but reach a maximum in a spherical layer at a distance of approximately 4×107 cm s−1 from the space center. In addition, for both plasma power levels, the bulk of the electron velocity distribution function is slightly compressed and the maximum value is shifted toward lower velocity values compared with a Maxwellian distribution.

Laser-scattering instrument for fundamental studies on a glow discharge

An instrument is described to perform laser-scattering fundamental studies on an analytical planar-cathode glow discharge. Electron properties such as electron number-density, electron temperature and the shape of the electron energy distribution function were successfully obtained via Thomson scattering. It is observed that several groups of electrons contribute to the Thomson scattering spectrum and the existence of a non-Maxwellian electron energy distribution was confirmed. The measured temperature for slow electrons is comparable to that found in previous studies performed with alternative methods. In addition, Rayleigh scattering was used to acquire argon gas-kinetic temperatures. The performance of the instrument as well as the significance of the results will be discussed.