José A. Olivares

Kinetic energy distributions of positive ions in an inductively coupled plasma mass spectrometer

A simple method is described for the approximate measurement of the ion energy distribution in an inductively coupled plasma-mass spectrometer (ICP-MS) with a continuum flow sampling interface. The average ion kinetic energy, kinetic energy spread, and maximum kinetic energy are evaluated from a plot of ion signal as a function of retarding voltage applied to the quadrupole mass analyzer. The effects of plasma operating parameters on ion signals and energies are described. In particular, kinetic energy is a sensitive function of aerosol gas flow rate. This behavior is attributed to a nonthermal, possibly electrical, interaction between the plasma and the sampling interface, which is induced by the presence of the axial channel in the ICP.

Development and investigation of microwave plasma techniques in analytical atomic spectrometry

Microwave plasma (MWP) sources have found extensive use in spectrochemical analysis during the past decades. As the MWP offers some attractive features, it has been widely used either as an excitation source for atomic emission spectrometry (MWP-AES) or as an ionization source for mass spectrometry (MWP-MS). The use of MWPs as an atomization source for atomic absorption spectrometry (MWP-AAS) and atomic fluorescence spectrometry (MWP-AFS) has also appeared. The historical development and recent improvements in these MWP techniques are presented in this review with emphasis on the analytical performance, characteristics, interferences and applications. Research on the fundamental properties of MWPs is also given. Both the advantages and limitations of MWPs in atomic spectrometry are discussed.

Glow discharge source interfacing to mass analyzers: theoretical and practical considerations.

The fundamental requirements for the optimum mechanical interface between a glow discharge ion source and a mass spectrometer are described in this paper. Specifically, the properties of a typical glow discharge ion source are compared and contrasted to those of a typical inductively coupled plasma ion source. The critical parameters and theoretical considerations of glow discharge and inductively coupled plasma ion source interfaces are reviewed, and the results of experiments using both quadrupole and time-of-flight mass spectrometers with a glow discharge source are presented. The experimental results clarify several important problems in the glow discharge ion sampling process. Our findings indicate that a shock wave structure does not occur in the supersonic expansion of the glow discharge ion source. Ions of different masses have similar initial kinetic energies in the glow discharge; thus, the angle of the skimmer cone is not a critical parameter for efficient ion beam extraction. Another consquence is that space charge effects in glow discharge ion sources repel heavy ions farther off axis than light ions. Thus, there are distinct and fundamental differences between glow discharge and inductively coupled plasma ion sources which are relevant to both ion sampling and ion extraction processes.

General calculations of vertically-resolved emission profiles for analyte elements in inductively coupled plasmas

Abstract A simple model is suggested to approximately describe the vertical spatial behavior of analyte emission in inductively coupled plasmas (ICPs). The degree of analyte ionization determines the relative numbers of neutral atoms and ions at each observation height. The excitation temperature ( T exc ) determines the fraction of ions in each ionic excited state and the fraction of atoms in each atomic excited state. The degree of ionization varies with observation height in a manner that is independent of T exc . Calculated vertical intensity profiles show the same general shape and element-to-element trends as the analogous experimental data.

Application of wavelet transforms and an approximate deconvolution method for the resolution of noisy overlapped peaks in DNA capillary electrophoresis

A new procedure for resolving noisy overlapped peaks in DNA separations by capillary electrophoresis (CE) is developed. The procedure combines both a wavelet-based denoising method that effectively denoises the signal and a novel approximate deconvolution technique that resolves the fragment peaks and improves the ability to separate highly overlapped peaks early in the electrophoresis process. Different kinds of overlapped peaks with and without noise simulated by computer as well as some DNA experimental electropherograms were submitted to the new procedure. A second order differential operator with variable coefficients is applied to the entire electrophoresis signal at any given time and approximate deconvolutions of the individual Gaussian peaks are performed. The operator incorporates the effect of the superposition and gives exact annihilation in the neighborhood of each peak. Overlapped peaks with a resolution higher than 0.46 can be resolved directly. Also, the method can determine the peak components of signals with a signal to noise ratio higher than 1.4

Characterization of an improved thermal ionization cavity source for mass spectrometry

A new thermal ionization source for use with a quadrupole mass spectrometer has been designed and characterized. The new source provides significant advantages over the previously reported prototype source and traditional filament-type thermal ionization sources. The operational interface between the source and the quadrupole mass spectrometer has been redesigned. A vacuum interlock, a translational stage, and an adjustable insertion probe are added to improve the source performance. With these modifications, the source is easier to operate while maximizing sample throughput. In this work, the performance of the newly developed source is examined. The ionization efficiencies are measured with a quadrupole mass spectrometer. The efficiency values obtained with this system are comparable to those obtained from a large scale isotope separator. The relationships among the ionization potential, vapor pressure, and measured ionization efficiency results are discussed. The crucible lifetime has been quantitatively estimated by measuring the crucible sputtering rate. Diagnostic studies of the new source show that the crucible position is a crucial parameter for sensitivity and performance. Stability tests demonstrate that the source can be run several weeks at a fixed emission current without significant degradation.

Atmospheric analysis using a microwave plasma ionization source and ion trap mass spectrometry

Abstract The on-line detection of both positive and negative ions has been demonstrated using a microwave plasma ionization source coupled with an ion trap mass spectrometer. Real-time xenon and krypton isotope measurements in air were made using either supplemental helium or atmospheric argon for plasma generation. The halogen signature from an organic molecule was detected as a negative ion using a helium plasma. Two instrument configurations have been assembled, one combining a microwave plasma source, an injection lens, and the ion trap; in the second assembly, a radiofrequency-only quadrupole mass filter was inserted between the lens and the ion trap. Most of the data reported here were acquired with the first configuration, although preliminary data with the second configuration has been taken. General operating parameters are discussed. The precision of xenon isotope ratio measurements ranged from 0.8% to 5% relative standard deviation, depending on the magnitude of the ratio.

Instrumentation and Fundamental Studies on Glow Discharge—Microwave-Induced Plasma (GD-MIP) Tandem Source for Optical Emission Spectrometry

Instrumentation and fundamental studies on glow discharge–microwave-induced plasma (GD-MIP) tandem source are described in this paper. Mutual effects among parameters, vacuum pressure, discharge current, voltage, and microwave power are examined with different cathode materials. Sputtering rates with and without microwave boost are measured under various experimental conditions. The experimental results demonstrate that the introduction of a microwave plasma will significantly decrease the sampling rate. A possible mechanism for the more uniform erosion obtained with microwave plasma boosting is suggested and discussed. Excitation temperatures are measured with the line-pair method and Boltzmann plot. A considerable increase in the excitation temperature (from about 3500 to 4200 K by using the line-pair method) is found with the addition of microwave plasma boosting. A comparison of the behavior of glow discharge alone and the GD-MIP tandem source is made. Significant enhancements in signal intensities are observed. The experimental results suggest that the excitation temperature plays an important role in signal enhancement.

Two-Color Capillary Electrophoresis with On-Column Excitation and Wave-Guide Based Fluorescent Detection

An optical wave-guide based two-color capillary electrophoresis laser induced fluorescence (CE-LIF) instrument is described. The wave-guide based approach allows for on column excitation and detect...

Analysis of solid uranium samples using a small mass spectrometer

Abstract A mass spectrometer for isotopic analysis of solid uranium samples has been constructed and evaluated. This system employs the fluorinating agent chlorine trifluoride (ClF 3 ) to convert solid uranium samples into their volatile uranium hexafluorides (UF 6 ). The majority of unwanted gaseous byproducts and remaining ClF 3 are removed from the sample vessel by condensing the UF 6 and then pumping away the unwanted gases. The UF 6 gas is then introduced into a quadrupole mass spectrometer and ionized by electron impact ionization. The doubly charged bare metal uranium ion (U 2+ ) is used to determine the U 235 /U 238 isotopic ratio. Precision and accuracy for several isotopic standards were found to be better than 12%, without further calibration of the system. The analysis can be completed in 25 min from sample loading, to UF 6 reaction, to mass spectral analysis. The method is amenable to uranium solid matrices, and other actinides.