Interferences and Methods for Their Elimination in the Determination of Lanthanum as LaO by Laser-Enhanced Molecular Flame Ionization Spectrometry

Interfering effects of some elements on the determination of lanthanum as LaO in a natural gas–air flame by laser-enhanced molecular ionization spectrometry were studied. It was demonstrated that easily ionized elements (Li, K, Na, Mg) exert an ionization effect; in addition, sodium introduces spectral interference. Techniques were proposed for the elimination of these interfering effects.     

Nonlinear normalization for laser-enhanced ionization spectrometry with laser sampling into a flame

The conditions for the appearance of a significant linear correlation between laser-enhanced ionization and reference signals are found. The parameters of this correlation (polynomial coefficients) are suggested to be used as a normalized analytical signal. The use of nonlinear correlation makes it possible to extend the range of ablation laser energies.     

Discovery of an electrical post-pulse in the surroundings of a high voltage spark discharge

In the vicinity of a high voltage spark, an electrical pulse of negative polarity was observed. The pulse's behavior with respect to spark cathode composition, gas flow, spatial distribution, and related parameters was characterized. Suggestions as to the origin of the pulse are made. Attempts at observing laser-enhanced ionization in the post discharge time period are described.     

Application of Dual Laser Ionization to Trace Analysis of Sodium in A Flame

Dual laser ionization (DLI) is a technique which can be used to detect trace elements in a flame by ionizing the analyte via a dual-laser stepwise excitation. As a powerful method for flame diagnostics, DLI can detect trace amount of Na at a concentration of ppb (ng/mL), more sensitive by two orders of magnitude than laser-enhanced ionization (LEI), in which only single laser is used. To demonstrate its potential in trace analysis, we compare the detection limit of DLI for Na with other methods utilizing LEI, atomic emission and laser atomic absorption.     

Speciation of chromium by ion chromatography and laser-enhanced ionization: optimization of the excitation-ionization scheme

Ion chromatography and laser-enhanced ionization were combined for the speciation of chromium at the trace level. Several one- and two-step excitation schemes were examined, in the UV and visible regions of the spectrum. It was found that a near-resonant two-step/one-color excitation scheme, using a single strongly saturating laser for the simultaneous excitation of both steps of the (3d⁵4s) ⁷S₃→(3d⁵4p) ⁷P₃⁰→(3d⁴4s5s) ⁷D₄ sequence, provides the best analytical results. This scheme was applied to the determination of total chromium in solutions directly aspirated into the air–acetylene flame, and to the determination of Cr(III) and Cr(VI) separated by ion chromatography. A detection limit of 0.5 ng/mL was obtained by direct solution aspiration, with a dynamic range covering four orders of magnitude. Detection limits of 5 ng/mL for Cr(III) and 4 ng/mL for Cr(VI) were obtained by ion chromatography and laser-enhanced ionization.     

火焰中锂原子的激光增强电离光谱研究

报导了对火焰中锂原子的激光增强电离光谱(LEIS)行为的研究.利用锂原子的2³P_1/2,3/2→3²D_3/2,5/2激发过程,对改变激光通过火焰位置、乙炔流量、阴极电压、溶液酸度和钾、钠存在时对锂原子的激光增强电离光谱的影响,进行了实验和初步探讨.     

Signal Formation in Laser-Enhanced Atomic Ionization Spectrometry with Laser Sampling into the Flame

The results of the observation of ionization signals in the laser-ablation sampling of solid samples into the flame were reported. The signals exhibited two maxima. The first, nonselective, maximum, which appeared immediately after the pulse of the volatilization laser, is due to thermions formed in the laser plasma. The second, selective (analytical), signal appeared after the pulse of the optical excitation laser. The nonselective signal may interfere with the detection of the analytical signal. The dependence of both signals on parameters such as the height of the cathode and the height of signal observation above the burner and the time delay between the laser pulses was studied. The optimal conditions for the detection of signals in laser-enhanced atomic ionization spectrometry were found.     

Analytical applications of laser-enhanced lonization spectrometry in flames and furnaces

This paper is concerned with a critical analysis of the analytical performance of laser-enhanced ionization (LEI) spectrometry in flames and furnaces as an ultra-sensitive trace-element technique. Updated tables of LEI detection limits, both in flames and furnaces, are given. Special attention is paid to interference phenomena which are unique to the LEI technique. Ways of reducing their influence on the analytical signals are proposed. Finally, advantages and disadvantages of LEI spectrometry as a tool for ultra-sensitive trace element analysis are discussed.Presented in part at the 1989 European Winter Conference on Plasma Spectrochemistry, Reutte, Austria     

Applications of Laser-Enhanced Ionization In Analytical Chemistry

Various applications of laser-enhanced ionization (LEI) in analytical chemistry are reviewed. This technique was applied to determine some physical quantities associated with a flame through development of an appropriate model. Determinations of flame temperature and atomization efficiency of an element in the flame are examples. As trace analysis is an important application of this technique, we compare the ion yield induced in a two-step LEI with that in a one-step LEI. The factors governing the ion enhancement via two-step excitation are examined in order to make efficient use of the two-step LEI apparatus. A novel technique was designed to couple flow-injection analysis to a conventional LEI device; in this manner, electrical interference, a severe problem inherent in LEI, was successfully removed.     

Laser-enhanced ionization spectroscopy of sodium atoms in an air-hydrogen flame with mass spectrometric detection

Sodium atoms in an air-hydrogen flame at atmospheric pressure have been selectively ionized by laser-enhanced ionization (LEI) spectroscopy, and the resulting ions have been drawn into a vacuum and detected by quadrupole mass spectrometry. A commercial inductively coupled plasma-mass spectrometer, modified for use with a flame rather than an ICP, was used to sample and detect the LEI ions. Following double-resonance LEI using pulsed dye lasers, the detected sodium ion signal was enhanced by a factor of 350 over that induced by thermal ionization alone. Using a 5 mm laser beam diameter, the LEI signal pulse was found to last for 0.54 ms (FWHM). Spatial studies in which the position of the laser beam relative to the mass spectrometer sampler cone was varied, demonstrated that the ions produced by LEI travel with the flame velocity into the mass spectrometer, with no significant losses due to recombination from as far as 13 mm from the interface.     

Investigation of hydroxyl-mediated spectral interference from easily ionized elements in laser-enhanced ionization spectrometry in the 281–285 nm range

The spectral background from 281 to 285 nm in the laser-enhanced ionization (LEI) spectrum of aqueous samples containing easily ionized elements (EIE) at concentrations similar to those found in blood was investigated. A complex, structured spectral background was observed, which appears in the presence of Na or K, but does not match the spectral signature of either element. The same behavior was also observed for Li. It was established that this background originates from an energy transfer between laser-excited hydroxyl (OH) molecules and ground-state EIEs. The intensity of this spectral feature was found to increase with EIE concentration and applied electrode voltage. This unexpected source of spectral interference may complicate the determination of trace metals by LEI in the presence of EIEs, since it can not be prevented by simply avoiding interference from atomic lines.     

DensMat: fully time-resolved simulation of two-step pulsed laser excitation of atoms in highly collisional media

A program that simulates and displays the level populations of atomic systems exposed to dual-wavelength (i.e. two-colour) pulsed laser excitation in highly collisional media (such as flames and plasmas) has been developed. The program is based upon a previously published fully time-dependent density-matrix model that describes step-wise excitations of atoms with degenerate states under collision-dominated conditions, and which thus goes beyond the rate-equations formalism. This model can predict such phenomena as Rabi flopping and a.c.-Stark splitting, shifting and broadening. The program can be used as a prediction tool for laser-enhanced ionization, laser-induced fluorescence, fluorescence dip spectroscopy and other two-colour laser-based spectroscopic experiments. The program provides the user with a flexible four-level atomic system, configurable as a one- or two-step excitation ladder, along with an ionization continuum and non-laser-connected level(s) that may act as trap(s) or metastable level(s). Parameters such as level degeneracy, collisional rates and laser pulse widths, shapes, wavelengths, intensities and bandwidths are accessible to the user. The program can display both the time development of the level populations and also level populations versus laser wavelength. 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 containing the program DensMat, an associated on-line help file and manual, an installation program, and data files pertaining to the examples illustrated in this article. The program runs under Windows 3.1 on an IBM-compatible computer.     

Trends in optical spectrochemical trace analysis with plasma sources

The state-of-art and trends in the development of optical spectrochemical trace analysis with inductively-coupled plasmas (i.c.p.), direct current plasmas (d.c.p.) and microwave-induced plasmas (m.i.p.) are discussed. Innovation in plasma optical emission spectrometry (o.e.s.) is shown ot lie in new sources such as the low-gas-consumption i.c.p., the air and helium i.c.p. as well as the toroidal m.i.p., which is operated at medium power and possibly with moleculary gases. Sample introduction has been improved by using new pneumatic nebulizers, flow injection, electrotheraml vaporization, hydride generation, direct sample insertion and direct solid sampling. Progress in the acquisition of spectral information is attained by high-resolution spectrometry, Fourier-transform spectrometry and by the use of multichannel detectors. D.c.p./o.e.s. is a mature technique for routin work and m.i.p./o.e.s. is a powerful tool for element-specific detection is chromatography. Plasma sources are also suitable atom reservoirs for atomic fluorescence spectrometry and for laser-enhanced ionization spectrometry. Trends in the figures of merit of optical plasma spectrochemical analysis are discussed.     

Analytical signal normalization in laser-enhanced ionization spectrometry with laser ablation of solid samples into a flame

The characteristics of analytical and reference signals (RS) were studied in laser-enhanced ionization (LEI) spectrometry combined with laser ablation of solid samples into a flame. Optoacoustic (OA) and atomic emission (AE) signals have been considered as possible RS. An algorithm of RS selection has been developed. Correlations between LEI signals and RS, which carry information on sample evaporation processes, have been explored. OA (the 1st minimum) and AE (AlI, λ = 396.1 nm) signals were found to be useful as RS (the correlation coefficients were 0.992 and 0.993, respectively). Both RS did not depend on the analyte (Li) content in the sample. It has been shown that the slope of the correlation plots (LEI vs. RS) depends on the Li content in the solid sample. This slope may be used as a novel type of normalized analytical signal, which does not depend on the ablation pulse energy. The influence of both ablating laser radiation characteristics and sample matrix composition on the analytical signal is eliminated (or at least diminished) when using this normalization.     

Analysis of Slightly Volatile Samples by Atomic-Ionization Spectrometry with Laser Ablation into Flame

The potentialities of laser-enhanced atomic-ionization spectrometry with laser ablation for the analysis of slightly volatile and difficultly ionized samples were studied using lithium polyvanadate as an example. The effect of the cathode voltage and the power of the ablation beam on the signal in the analysis of such samples was studied. The optimum measurement range was determined as a function of the number of subsequent ablation pulses acting upon the same surface point. This minimized the effect of the surface contaminants and the change in ablation conditions as the crater deepened.     

Anomalous saturation curves in laser-induced fluorescence

This is partly a tutorial and partly a review paper (with a few original additions) on saturation curves (SC) which describe the dependence of fluorescence intensity on laser intensity in atomic spectroscopy. The interest in SCs stems from applications in analytical chemistry, plasma diagnostics, physical and chemical kinetics, etc., as well as from their fundamental implications. After a brief introduction, some general facts and basic assumptions regarding atom-laser interactions are critically examined (Section 2.1) and the concepts of the “ideal” SC and saturation parameter are defined (Section 2.2). In the following Sections 3–7 various effects are discussed that can distort the SC and shift the (apparent) saturation parameter. The effects of a spatially, a temporally and a spectrally inhomogeneous laser beam, of laser-enhanced chemical reactions and ionization processes, of an optically thick atomic vapour and of various non-steady-state processes are successively reviewed. Atom trapping and polarization effects on the SCs measured recently with an atomic Na beam in vacuo are reported and discussed in some detail. Also, some new observations at high resolution on the spectrum of pulsed and cw multimode dye lasers are reported. In Section 8 some general conclusions are drawn and warnings given, and the possible extension of the concept of SC to multiphoton and multistep excitation processes, as well as to optogalvanic spectroscopy, is suggested.     

Molecular Beam Studies on the Laser-Enhanced Surface Reaction of GaAs(100) with Chlorine

A cw supersonic Cl₂ molecular beam coupled with an angle-resolved time-of-flight (TOF) technique has been used to investigate the laser-enhanced surface reaction of GaAs(100) with chlorine. The mass and velocity distributions of the major reaction products under 1064 nm laser irradiation have been measured as a function of laser fluence, detection angle, surface temperature and normal component of the translational energy of the incident chlorine molecules. It has been found that increasing both laser fluence and the translational energy of incident chlorine molecules markedly enhance mis surface reaction. The measured flux angular distributions of major reaction products can be fit satisfactorily with a bi-cosine function. Measurements of the mass and angular distributions of reaction products by a modulated molecular beam mass spectrometry show that the surface temperature effect is obvious for the Cl₂/GaAs(100) “dark” thermal reaction. A direct activated dissociative chemisorption is proposed for the mechanism of Cl₂ chemisorption on the GaAs(100) surface.     

An overview of some recent developments in ionization methods for mass spectrometry

An overview of some recent advances in ionization sources for mass spectrometry is presented. Limitations were set so that the overview covers ionization techniques relevant to organic and biological analysis that have appeared in the literature since the year 2000. No effort is made to be comprehensive. Rather, a broad sweep overview of author-subjective highlights among a wide variety of sources is presented. These ionization sources include electron ionization, chemical ionization, various atmospheric plasma ionization sources, laser desorption sources, sonic spray and electrospray ionization sources.     

介质阻挡放电离子源质谱分析

敞开式离子化质谱可对表面样品进行直接快速分析而受到关注,成为质谱分析的热点研究方向.介质阻挡放电离子源是一种基于等离子体放电机理的敞开式离子源,近年来得到了较快的发展.本文着重介绍该离子源的基本原理、性能特征以及应用进展,并对其发展趋势进行了展望.