Laser-enhanced Ionization Spectrometry Research Articles

Modern methods for identification of atoms, molecules, and aerosols in various objects

The study is aimed at developing highly sensitive methods of laser analytical spectroscopy. The physical mechanisms of forming useful signals (selective ionization signal and cavity ring-down signal) were identified that provided registration of parameters of atomic and aerosol systems in the intensive pulsed laser field. High-sensitive laser methods of laser resonance ionization spectroscopy in vacuum, laser-enhanced ionization spectrometry in flame, and cavity ringdown laser absorption spectroscopy (CRLAS) are used for the determination of ultra-small concentrations of atoms in different phase states of the substance. Samples of aqueous standard solutions and solid metals of s (Li, K, Na, Ca, Cs), p (Al, In), d (Cr, Mn, Fe, Co, Ni, Cu, Ag, Au, Pt, Zn, Hg), f (Yb) elements, aluminum alloys, especially pure solvents, crystals (NH4F, NaF), semi-conductor materials (GaAs, Si) and various aerosols of salts (NaCl, CsCl, NaI, NaF, KCl AgNO3), chemicals, organic dyes, alloys, soils and rocks were studied. The new mechanisms of getting free particles are revealed and new methods increasing the efficiency of atomization, selective ionization and excitation of atoms in systems «flame», «rod – flame», in atomizer «graphite – furnace» are proposed. The particle size distribution of aerosols formed under the impact of high-power laser radiation on the surface of a solid sample has been studied. The dependence of the absolute concentration of aerosol particles on their size has been determined. Aerosol extinction coefficients and extinction efficiency have been measured using intracavity laser spectroscopy. For the first time new parameters of aerosols are revealed by physical and chemical properties of aerosol plumes from solid surfaces and aerosols of salt of metals and organic aerosols. Methods of additives and calibration curve were used to examine the effects of the matrix on the analytical signal of the studied atoms.

Determination of lithium in lithium-ionic conductors by laser-enhanced ionization spectrometry with laser ablation

Laser-enhanced ionisation (LEI) with laser ablation (LA) into a methane–air flame was used for lithium determination in ferrites. Since there is a lack of certified reference materials matrix-matched to ferrites powders, a set of spinel samples were synthesised by means of aerosol spray pyrolysis (ASP) or by annealing the mixture of Li and Fe(III) nitrates. Characterisation by X-ray diffraction showed that the annealed ceramics contained two phases, such as either α-LiFe5O8 and Fe2O3 or α-LiFeO2 and Fe2O3, while ASP ferrites consisted mainly of α-LiFe5O8. Total lithium content in samples was certified by conventional flame photometry after dissolution. We explained the strong matrix effects on the LA-LEI signal by the discrepancy of surface microstructure (and hardness) due to the difference in the conditions of sample preparation. The optoacoustic signal and emission line of Fe I at 382 nm were recorded simultaneously with the LA-LEI signal in order to compensate for matrix effects. We suggest that the microstructure (ceramics or pellets from spherical microparticles) influences both mass removal (proportional to OA signal) and plasma temperature (proportional to AE signal). Correlation plots between OA and AE signals deviated strongly from sample to sample, which was the basis for the application of a multivariate correction procedure to reduce strong matrix effects. 3D correlation plots (LEI vs. OA + AE) for each sample were built to obtain a new analytical signal, which represented the slope of the correlation line to the plane of the OA + AE signals. This normalised signal was free from matrix effects within a concentration range of 0.6–7.5 Li%. The proposed approach allows the use of ferrites synthesised by the various techniques as reference materials in order to build a unified calibration curve.

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.

Reduction of the matrix influence on analytical signal in laser-enhanced ionization spectrometry with laser sampling

The novel approach using a slope of correlation line (laser-enhanced ionization of lithium versus laser-induced plasma emission of aluminum) as analytical signal was proposed for reduction of matrix interferences in laser-enhanced ionization spectrometric determination of Li with laser sampling.

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.

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.

Multielemental Laser-Enhanced Ionization Spectrometry for the Determination of Lead at the Trace Level in Pelletized Coal Using Laser Ablation and Internal Standard Signal Normalization

Laser ablation laser-enhanced ionization (LA-LEI) was used for the simultaneous measurement of lead and indium in pelletized graphite and coal samples. UV Laser ablation of the solid samples was performed in an ablation cell and the ablated material was carried by a flow of gas to a miniature LEI flame where lead was detected. The influence of parameters such as binder content of the solid pellets and dispersion of the analytes spiked in the sample material, as well as the number of ablation pulses per crater on signal repeatability and on the size and shape of ejected particles was examined. Measurement repeatability values of 2 to 5% of relative standard deviation were obtained using indium as an internal standard to correct for variations in the ablation rate. A limit of detection of 120 nanograms per gram was calculated for the determination of Pb in high-purity graphite. The Pb concentration in the NIST 1632c Bituminous Coal certified reference material was determined to within 1% of its certified value, using graphite as the matrix-matching material and In as the internal standard in pelletized solid samples.

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.

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.

A comparison of continuous flow hydride generation laser-induced fluorescence and laser-enhanced ionization spectrometry approaches for parts per trillion level measurements of arsenic, selenium and antimony

Hydride generation (HG) is a sample introduction technique that provides enhanced sensitivity for metalloids and allows chemical speciation between selected arsenic (As) and selenium (Se) anion species. The present studies have demonstrated high sensitivity for As, Se, and antimony (Sb) using continuous flow HG-laser induced fluorescence (LIF) and HG-laser enhanced ionization (LEI) spectrometric techniques. The limits of detection (LODs) are 200, 90 and 300 fg ml−1 for As, Se and Sb, respectively, using HG-LIF and 50 and 2 pg ml−1 for As and Se, respectively, using HG-LEI approaches. Measurements performed using HG combined with LIF detection in an electrothermal atomizer resulted in LODs of 3 pg ml−1 (6 pg absolute mass) and 20 pg ml−1 (40 pg) for As and Se, respectively. All of the techniques are linear in their responses to at least 10 ng ml−1 for the corresponding elements. Reliable chemical speciation of As(III/V) and Se(IV/VI) species has been obtained using the flame HG-LIF approaches. Selective determinations of Se(IV) and Se(VI) have been carried out using an on-line pre-reduction procedure, where selective HG of Se(IV) is performed in 31% HBr at room temperature, while HG of [Se(IV) + Se(VI)] is performed following pre-reduction in 31% HBr at 100°C. Selective HG of As species is accomplished by control of the solution pH, where selective HG of As(III) is performed at pH 6 and HG of [As(III) + As(V)] is performed at pH 0. The results obtained demonstrate that the HG approaches are selective and provide quantitative recoveries of the individual Se and As species at sub-ppb levels.

Far-Ultraviolet Excited Laser-Enhanced Ionization Spectrometry of As, Se, Cu, and Sb in Air/Acetylene and Ar/O2/Acetylene Flames

Laser-enhanced ionization (LEI) measurements of As, Se, Sb, and Cu in air/acetylene and Ar/O2/acetylene flames are reported. The use of far-UV excitation wavelengths to perform one- and two-step LEI excitation from the ground states of As, Se, and Sb is reported for the first time. Significant ionization background signals are observed for far-UV excitations in the air/acetylene flame, but are reduced by substituting an (80:20) Ar/O2 mixture for air as the flame oxidant. The sensitivities of the individual LEI techniques are also increased by the Ar/O2 gas substitution. The limits of detection for As, Se, Sb, and Cu are 30, 0.5, 9 and 2 ng/mL, respectively, and compare favorably with previous LEI reports. Due to the poor atomization characteristics of As and Se, the utility of hydride generation (HG) sample introduction with LEI detection has been explored. Although high sensitivity has been observed for the HG-LEI approach, limits of detection were not evaluated due to nonlinear responses of the signals with respect to analyte concentration.

Comparison of Atomization Sources for a Field-Deployable Laser Optogalvanic Spectrometry System

A field-deployable laser optogalvanic spectrometry (LOGS) system has been developed to measure the concentration of metal species present in the near-atmospheric pressure off-gases of mixed-waste thermal-treatment systems, where metal species occur primarily as airborne metal oxide and/or metallic particles rather than as free, single atoms. Atomization is required to simplify the analyte and facilitate the detection of low concentrations. A comparative study of different LOGS atomization sources (electric glow discharge, radio frequency discharge, flames) was performed with respect to the feasibility of deploying each source. The advantages and disadvantages of the various atomization sources are discussed in terms of real-time monitoring, ease of operation, and ruggedness for field deployment. Although the various atomization sources permit a wide variety of laboratory-based LOGS applications, it is concluded that a flame-based atomization source is best suited for a field-deployable LOGS system to monitor airborne metal species in near-atmospheric pressure off-gas systems. Flame-based LOGS systems are usually referred to as laser-enhanced ionization spectrometry systems.

Laser-enhanced ionization spectroscopy of mercury Rydberg states

The spectral characteristics of mercury Rydberg states ( n = 10–42) were observed and studied. Each principal quantum number was observed as a triplet, and after observing a quartet of lines for n = 10, 11, and 12, these lines were assigned, from red to blue, as belonging to upper levels 1P 1 0, 3P 2 0, 3P 1 0, and 3P 0 0. These levels were studied as a function of variable applied high voltage (between the electrodes) and variable buffer gas pressure. In this way, the broadening and splitting caused by the influence of Stark effects and of increasing buffer gas pressure were observed for different n values. These observations will allow one to choose the optimal level for excitation, as well as the optimal operating conditions in terms of pressure and applied high voltage, for obtaining the best sensitivity and limit of detection by analytical laser-enhanced ionization spectroscopy.

Analytical time-resolved laser enhanced ionization spectroscopy I. Collisional ionization and photoionization of the Hg Rydberg states in a low pressure gas

The temporal behavior of the laser enhanced ionization signal of mercury was studied in a quartz cell under low buffer gas pressure. Using fast electronics and a short (34 ns) laser pulse, it was possible to distinguish, in one single time-resolved ionization waveform, the non-selective photoionization component of the signal from that which was due to collisional ionization from selected levels. Experimental results were shown to agree with those obtained by computer simulation, and optimal conditions for deconvolution of the two components were studied.

The determination of lead in whole blood by laser enhanced ionization using a combination of electrothermal vaporizer and flame

A system for electrothermal vaporization — laser enhanced ionization spectrometry (ETV-LEIS), developed in this laboratory, was used for the determination of lead concentrations in whole blood. Blood standards from the Centers for Disease Control (CDC) and the National Institute of Standards and Technology (NIST) were diluted 21:1 with ultrapure water and analyzed. Good agreement was found between the CDC and NIST standards. A linear analytical curve was obtained with a detection limit (3σ) of 8.9 × 10 −3 μg dl −1 (890 fg absolute) for lead in whole blood. This compares favorably with other current methods for blood-lead determinations including isotope dilution inductively coupled mass spectrometry (ID-ICP-MS) and graphite furnace atomic absorption spectrometry (GFAAS).

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.

Liquid chromatography–laser-enhanced ionization spectrometry for the speciation of organolead compounds

Liquid chromatography (LC) and laser-enhanced ionization spectrometry (LEI) have been combined to provide a very sensitive method for the measurement of organolead species. Measurement is possible in environmental and biological matrices, which often contain high levels of easily ionized elements that normally interfere with LEI measurements. However, LC is able to resolve the interferents from the analytes, and LEI is sensitive enough to counterbalance the increase in detection limits that results from the use of LC. The method has been applied to the determination of organolead compounds in National Institute of Standards and Technology Standard Reference Material 1566a, Oyster Tissue. Trace levels of trimethyllead were observed in the Oyster Tissue, but concentrations were variable among the samples tested.

Recent advances in the field of laser atomic spectroscopy

An overview of recent advances in the field of analytical laser atomic spectroscopy will be discussed, in particular the laser's use as a light source in atomic absorption spectroscopy, as an excitation source in laser atomic fluorescence and laser enhanced ionization spectroscopy, as a method of solid sample introduction, and as a diagnostic tool. Selected current research in these fields will be highlighted together with their results.

Imaging the active flame volume for pulsed laser-enhanced ionization spectroscopy

A technique has been developed which can determine where within a flame laser-enhanced ionization (LEI) actually takes place, thus forming an image of the LEI active volume. The method is based upon spatial control of the flame volume sampled by the ionization detection electrodes by variation of the applied voltage. As and example, this procedure has been used to observe anomalous contributions to LEI caused by scattering or fluorescence from the laser beam to other regions in the flame

Lifetime measurements of metastable states of Au, Bi, Cd, Mg, Pb, and Sr in an acetylene/air flame by laser-enhanced ionization spectrometry

The collisional dominated lifetimes of 9 metastable states of Au, Bi, Cd, Mg. Pb, and Sr in an acetylene/air flame have been measured by the step-wise delayed laser-enhanced ionization spectrometry technique. First, the dependency of the lifetime of the metastable 6p23P1 state in Pb upon both flame composition and height in the flame were studied. It was found that the lifetime was significantly shorter, both for lean as well as for rich flames, as compared to a stoichiometric composition. The lifetime was furthermore found to be dependent of the height in the flame with a maximum around 10 mm. The lifetimes were not significantly affected by the electron concentration in the flame. The lifetimes for the other metastable states were determined for one given flame composition (φ ≈ 0.9 φstoich) and one given height (20 mm) only. The measured lifetimes were found to be between 5 ns and 3.1 μs. The results show clearly that the lifetimes of metastable states are significantly shortened in the flame (as compared to natural lifetimes). The results also indicate that the metastable states which are of the same parity as the ground configuration (Au, Bi and Pb) in general have longer (collisional) lifetimes (85 ns to 3.1 μs) than those which are forbidden to decay to the ground configuration only by violation of spin conservation (Cd, Mg and Sr) (5–33 ns).