Paul B. Farnsworth

A simple, non-invasive method for the measurement of gas flow velocities in the inductively coupled plasma

Abstract Gas flow velocity critically affects the stability and analytical performance of an inductively coupled plasma (ICP), however, because now velocity has heretofore been difficult to measure, it has not been routinely used as a parameter in optimization or diagnostic procedures. This paper presents a simple technique in which velocities are determined by spectroscopic tracking of inhomogeneities in the analyte atom emission of a normally operating ICP over a known spatial interval. With this method the flow velocity in the central channel of the ICP can be measured with good precision and accuracy. The effects of intermediate, outer, and nebulizer gas flows and of power on flow velocity were investigated using this method. These paraments were varied to encompass most conditions under which the ICP is usually operated. Velocities were measurable between approximately 1 mm and 42 mm above the load coil (ALC). The spatial limits of measurability by this method are dependent on the conditions under which the ICP is operated.

Characterization of Near-Infrared Atomic Emission from a Radio-Frequency Plasma for Selective Detection in Capillary Gas Chromatography

A radio-frequency plasma source has been characterized for use as an element-selective detector with capillary gas chromatography using atomic emission in the near-infrared region of the spectrum. With selected compounds introduced at rates similar to those encountered during a chromatographic run, the performance of the plasma has been characterized as a function of several interdependent variables, including plasma makeup gas flow, electrode spacing, and applied power. These settings have been optimized, and detection limits for several nonmetallic elements determined. Detection limits range from 0.2 to 50 pg/s.

Factors affecting analyte transport through the sampling orifice of an inductively coupled plasma mass spectrometer

Abstract Laser-excited ionic fluorescence has been used to study the effects of sample matrix, operating conditions, and load coil shielding on analyte ion transport efficiency through the sampling orifice of an inductively coupled plasma mass spectrometer. Significant changes in ion transport efficiency result from changes in sample composition, RF forward power, nebulizer flow and torch shield configuration. The changes in ion transport efficiency correlate well with changes in the potential recorded on a single floating probe placed 1 mm upstream from the sampling orifice.

Matrix Effect Studies in the Inductively Coupled Plasma with Monodisperse Droplets. Part I: The Influence of Matrix on the Vertical Analyte Emission Profile

The effects of different matrices on the analyte vertical emission profile in the inductively coupled plasma were evaluated with use of a monodisperse droplet sample introduction system in combination with a pneumatic nebulizer. The influence of matrix in the gas phase was separated from influences in the desolvation and volatilization steps. The results suggest that vertical shifts in the analyte emission profiles occur primarily during droplet desolvation and vaporization. The vertical shifts can be correlated to the size of the desolvated particles; larger particles produce emission profiles closer to the load coil. Possible effects of radiative heat transfer were also studied. Strongly absorbing species can increase the evaporation rate of the solvent and produce vertical shifts toward the load coil.

A differentially pumped particle inlet for sampling of atmospheric aerosols into a time-of-flight mass spectrometer : Optical characterization of the particle beam

Two methods of characterizing the particle beam generated with a differentially pumped particle inlet are presented. Both methods are based on optical scattering of a laser beam by the particle beam. The first method images a time integrated scatter signal from the entire particle beam onto a charge coupled device (CCD), and an Abel inversion is performed on the image data to arrive at the radial particle density distribution in the beam. The second method, based on counting individual (particle) scatter pulses, yields the radial particle density directly. Initial results of the performance of the particle inlet are reported for particles with diameters between 40 and 800 nm. Under optimal working conditions, particle beams were generated with a full angle divergence on the order of 1-2 mrad. The width, measured 285 mm downstream from the exit of the particle inlet, was 250mu m, half width at half maximum (HWHM).

Experimental studies of charge transfer reactions between argon and the third row metals calcium through copper in the inductively coupled plasma

Abstract The effect of charge transfer reactions on analyte excitation and ionization in the inductively coupled plasma was studied by two independent techniques. In one technique, pulsed lasers were used to either deplete the ground state of neutral analyte atoms or enhance the population of selected states of the singly charged ion. In both cases the perturbed species were collision partners with argon in potential charge transfer reactions. The effects of charge transfer collisions could be detected in the form of changes in emission from product species. In the second technique, a simple correlation method was used to detect the link via charge transfer of neutral atom ground states and highly excited ionic levels. In the presence of charge transfer collisions, the populations of such linked levels show strong positive correlations. The two techniques were used to study the effects of charge transfer reactions on the third row elements Ca–Cu. With the exception of Cr and Mn, all of the elements studied showed positive evidence of excitation and ionization by charge transfer collision with argon.

Optical measurements of ion density in the second vacuum stage of an inductively coupled plasma mass spectrometer

A portion of the ion beam in the second stage of an inductively coupled plasma mass spectrometer was mapped using laser induced fluorescence (LIF). With LIF relative density measurements are made in real time with minimal interference to the ion beam. We report axial measurements of Ba and Sc ion density from 22 to 45 mm behind the tip of the skimmer cone. Additionally, maps of radial ion density with and without Pb and Mg matrices are given for the same two analyte species. The results reveal that earlier ion deposition experiments dramatically underestimated the extent of the radial spread of the ion beam and the influence of matrix on the ion beam.

Correlation spectroscopy as a probe of excitation and ionization mechanisms in the inductively coupled plasma

Abstract Correlations between intensity fluctuations in atomic and ionic emission lines were used as indicators of ionization and excitation mechanisms in the inductively coupled plasma. Ionic lines excited by charge exchange were positively correlated with neutral atom emission, while those excited by electron impact showed a negative correlation. An analog correlator was constructed to allow rapid measurement of such correlations over a broad range of wavelengths and as a function of position in the plasma. Results obtained to date suggest that charge exchange contributes significantly to the excitation and ionization of Mg, Y, Ti and V in the inductively coupled plasma.

An investigation of the balance of charge exchange between Mg and Ar ions in the ICP

Abstract Pulsed dye lasers were used to manipulate the excited state populations of magnesium atoms and ions to examine the importance of charge exchange as an excitation and ionization mechanism in the inductively coupled plasma (ICP). Laser enhanced ionization of ground-state magnesium resulted in a decrease in emission from Mg II 4 s 2 S and 3 d 2 D levels, indicating that charge transfer is the predominant mechanism for the population of those levels. Double-resonant laser excitation of ground state magnesium ions to the Mg II 4 s 2 S level resulted in an increase in the emission from the Mg I 285.213 nm resonance line, demonstrating the importance of the reverse reaction.

Matrix Effect Studies in the Inductively Coupled Plasma with Monodisperse Droplets. Part II: The Influence of Matrix on Spatially Integrated Ion Density

Monodisperse droplet sample introduction was used to study the ionization process in the inductively coupled plasma (ICP). The influence of the matrix on the analyte ion number density was evaluated by using laser-excited atomic fluorescence spectroscopy (LEAFS). The results suggest that the presence of 0.1 M NaCl enhances the ionization of the analyte species. With the addition of the NaCl, the spatially integrated Ba(II) signal decreases, apparently due to enhanced production of Ba2+. Added NaCl accelerates the appearance of Sc+, but does not affect the spatially integrated signal once ionization is complete. The possible effects on the fluorescence signal of quenching caused by the presence of the NaCl were evaluated by comparison of time-resolved and time-integrated detection. Quenching makes a small contribution to the fluorescence intensity change produced by the matrix but does not account for large loss in signal from singly ionized Ba.