John W. Olesik

Incompletely desolvated droplets in argon inductively coupled plasmas: their number, original size and effect on emission intensities

Abstract The presence of incompletely desolvated droplets in the Ar ICP results in changes in Ca I emission intensity as large as a factor of 20 and variations in the Ca II emission intensity as great as a factor of 2.5. Most of the signal fluctuations are due to the effect of vaporizing droplets or their vaporization products on analyte atoms and ions produced from previously vaporized and atomized droplets. Above the normal analytical zone some peaks in the time-resolved analyte emission intensity result from vaporizing, desolvated particles. The number of incompletely desolvated droplets in the plasma varies from more than 15000 per second low (5 mm above the load coil) in the plasma to less than 100 per second high (above the normal analytical zone) in the plasma. The number of incompletely desolvated droplets in the plasma is a strong function of the center (nebulizer) gas flow rate as well as the applied power. Emission intensities from locations within about 1.4 mm of a vaporizing droplet are dramatically affected. Ca I emission intensities are enhanced near vaporizing droplets while Ca II emission intensities are depressed. The size of droplets at the exit of the spray chamber that produce detectable laser light scattering signals and emission intensity fluctuations are greater than about 12 μm in diameter. The size of droplets that are incompletely desolvated in the plasma depends on the applied power and center (nebulizer) gas flow rate. The results have far reaching implications on fundamental understanding of processes controlling emission intensities in Ar ICPs. Potential biases introduced into time-integrated measurements due to the presence of incompletely vaporized droplets must be considered.

Vertical and radial emission profiles and ion-atom intensity ratios in inductively coupled plasmas: the connection to vaporizing droplets

Abstract Vertical Ca I and Ca II emission intensity profiles appear to be directly related to the number of incompletely desolvated droplets at each height in the plasma. The number of desolvated aerosol droplets at the height of peak time-integrated Ca I 422 nm emission intensity is constant (about 10.5 thousand counts/s) regardless of where the peak: occurs as the power and center (nebulizer) gas flow rate are varied. Similarly, the number of incompletely desolvated droplets near the height of peak time-integrated Ca II emission intensity is constant (about 550 counts/s) even when the power and center gas flow rate are changed, resulting in variations in the location of the maximum Ca II 393 nm emission intensity. Therefore, incompletely desolvated aerosol droplets appear to be a key variable controlling vertical emission profiles. Radial Ca I and Ca II emission intensity profiles are different in the presence and absence of a nearby incompletely desolvated droplet. Time-resolved Ca II to Ca I emission intensity ratios acquired from a fixed height in the plasma vary by up to a factor of 70. Between the height of maximum time-integrated Ca I emission intensity and maximum time-integrated Ca II emission intensity the time-integrated ion to atom emission intensity ratio is controlled by the fraction of time an incompletely desolvated droplet is near the observation zone.

Easily and Noneasily Ionizable Element Matrix Effects in Inductively Coupled Plasma Optical Spectrometry

Changes in analyte emission intensities occur when either easily or non-easily ionizable elements are present as concomitant species at a concentration of 0.05 M. The direction (enhancement or depression of emission signals) and magnitude of the matrix effect are strongly dependent on radial and vertical location in the plasma. At some heights in the ICP, matrix-induced depressions of the emission intensity in the center are equal to enhancements off-center. As a result, no change in the line-of-sight emission intensity is observed. Initial fluorescence measurements suggest that the number of analyte ions in the normal analytical zone decreases in the presence of each of the concomitant species studied. However, it appears that the presence of concomitant species enhances the fraction of ions that are excited and that therefore emit light. The presence of Na and K resulted in larger enhancements in the fraction of ions excited than did the presence of Fe, Ni, or Ba.

Laser-Excited Fluorescence Studies of Matrix-Induced Errors in Inductively Coupled Plasma Spectrometry: Implications for ICP-Mass Spectrometry

Laser-induced Sr ion fluorescence measurements were made as a function of nebulizer gas flow rate and applied power in the presence and absence of concomitant species for comparison to previously published ICP-MS data. The fluorescence data are used to assess the contribution of matrix effects in the plasma itself to errors observed in ICP-MS due to the presence of concomitant species. Significant matrix effects do appear to originate in the plasma. Reconsideration of ambipolar diffusion shows that the matrix effects in the plasma should not be dependent on the mass of the analyte or concomitant species. Experimental, radially resolved measurements confirm the lack of mass dependence.

Effect of central gas flow rate and water on an argon inductively coupled plasma: spatially resolved emission, ion-atom intensity ratios and electron number densities

Abstract The effect of aqueous aerosol on plasma properties and emission signals was investigated. Plasma behavior was compared when using spark-generated dry sample introduction to that when conventional aqueous aerosol sample introduction was used. The shape of vertical and radial Mg II and Mg I emission profiles were strongly influenced by aqueous aerosol introduction. The Mg II to Mg I emission intensity ratios varied little with radial position or central gas flow rate when dry sample was introduced. However, the emission intensity ratios were strongly dependent on radial position and central gas flow rate when aqueous aerosol was introduced. The Mg II to Mg I emission intensity ratio varied widely with radial position even when measured electron number densities were constant over the same locations. The Mg II to Mg I emission intensity ratios were less than local therrnodynamic equilibrium (LTE) predictions when no water was present. The intensity ratio was close to LTE values when small aqueous aerosol mass transport rates and low central gas flow rates were used but was far below LTE values when typical aerosol mass transport rates were used. Desolvating droplets may be responsible for the observed behavior. Time averaging of droplet-induced signal fluctuations may introduce bias into measurements of plasma properties.

Simultaneous Detection of One-Dimensional Laser-Induced Fluorescence or Laser Light Scattering Images in Plasmas

An instrumental system to simultaneously detect one-dimensional atomic or ionic fluorescence images is described. The instrument can also be used to acquire laser light scattering images or laterally resolved emission images. The fluorescence, scattering, or emission image passes through a monochromator and is re-imaged on an intensified diode array detector. Measurement of spatially resolved ground-state populations in inductively coupled plasmas is discussed. Interpretation of the fluorescence data obtained under different plasma operating conditions is considered. Results with the use of laser-induced fluorescence imaging to study the effect of sample transport rate, concomitant species-induced matrix effects, and modulated power plasmas are discussed. Comparison of fluorescence and emission images shows the complementary nature of the information provided by each. Laser light scattering off intact droplets or particles in a 1.0-kW inductively coupled plasma is discussed. Means to minimize the scattering signal are evaluated. Detection of laser light scattering images is discussed.

Analyte excitation in the inductively coupled plasma studied by power modulation

Abstract Energy coupling, propagation, excitation and relaxation processes in the inductively coupled plasma have been studied by modulating the power delivered to the plasma. Spatially and temporally resolved emission from Ar I, Ca II, Ca I and Li I in response to abrupt changes in power from 450 to 1000 W was measured. The emission intensities begin to fall within 100–300 μs of the high to low power transition. However, a spatially and species dependent delay of up to 3 ms is observed between the sudden change from low to high power and an increase in the emission intensities. At 20 mm above the load coil, Ca I emission intensity increases about 1 ms before an increase in the Ar I or Ca II intensities. Changes in the outer and central flow rates were found to have little effect on relaxation processes responsible for decay in emission after the power is abruptly decreased. However, an increase in the outer flow rate decreases the delay time between the low to high power transition and the increase in emission intensity. The data show that two ingredients seem to be necessary for analyte excitation: species (most likely electrons) created low and away from the radial center of the plasma must be transported upward and inward, and a direct coupling of energy into the analytical zone must occur. The excitation mechanisms or energy requirements for production of Ca I and Ca II emission are found to be different.

Spatially and temporally resolved emission and fluorescence in a power modulated inductively coupled plasma

Abstract Energy coupling, vertical transport and radial transport processes were studied via time and space resolved Ba II emission and fluorescence measurements. The Ba II emission to fluorescence intensity ratios in the first 2 ms following an abrupt decrease in power were consistent with some coupling of energy more than 10–15 mm above the load coil. During this time changes in excitation were dominant over variations in Ba ion number density. There was no evidence of radially dependent cooling of the plasma. At times later than 3 ms from the decrease in power, large changes in Ba + number density were observed. The Ba + number density changes were consistent with transport of cooler gas from below. The Ba II emission and fluorescence behavior following an abrupt increase in power was consistent with production of energetic species low in the plasma followed by transport upward and inward. At 9 mm above the load coil and higher, vertical transport processes were more important than radial transport. Emission response times for species with a wide range of excitation energies were similar to those expected from equilibrium models except for two Mg II lines. The sum of excitation and ionization energies for the two anomalously behaving Mg II lines was greater than 16 eV, precluding excitation via charge transfer from ground state Ar ions.

An Instrumental System for Simultaneous Measurement of Spatially Resolved Electron Number Densities in Plasmas

An instrumental system based on a two-dimensional detector to measure electron number densities in plasmas is described. High-resolution H spectra are detected simultaneously at up to 200 spatial locations. Radially resolved electron number densities are determined, following Abel inversion and comparison of H line profiles to theoretical Stark-broadened spectra. The particular interpolation procedure used to calculate H spectra from discrete literature data can affect the calculated electron number density values. Radially and vertically resolved electron number maps for inductively coupled plasmas supported in Fassel-type and low-flow torches are presented. Electron number densities in the center of the 1.0-kW ICPs are less than 2.5 × 1014 cm−3. Short-term precision is better than 1% relative standard deviation except low in the center of the plasma, where the H intensity is low on-axis and larger off-axis.

Consideration of norm temperature and local thermodynamic equilibrium models for emission and ionization in inductively coupled plasmas

Abstract Norm temperature and local thermodynamic equilibrium (LTE) models of emission intensity and ionization in ICPs are discussed. Based on each of the two models, detailed predictions of emission intensities and total atom, singly charged ion and doubly charged ion fractions are presented. The predicted behavior is compared to previously published experimental data. The norm temperature model results appear to be consistent with the experimentally observed behavior of the peak in emission intensity as a function of height in the plasma. However, assumptions made in the norm temperature model are inconsistent with LTE. LTE model predictions do not match experimentally observed data as a function of temperature. The LTE model results do not show a peak in emission intensity or ion to atom intensity ratio as a function of temperature from 3000 to 10000 K. The LTE model predicts that analytes in He ICPs should be more highly ionized than in LTE Ar plasmas. However, experimentally, He plasmas should be further from LTE. Possible causes of the disagreement between model predictions and experimental data include radiative deexcitation, non-LTE electron temperatures, mass transport effects, slow vaporization or ionization kinetics and non steady-state conditions.