Laser-enhanced Ionization Signal Research Articles

Multivariate correction in laser-enhanced ionization with laser sampling

The opportunity of normalizing laser-enhanced ionization (LEI) signals by several reference signals (RS) measured simultaneously has been examined in view of correcting variations of laser parameters and matrix interferences. Opto-acoustic, atomic emission and non-selective ionization signals and their paired combination were used as RS for Li determination in aluminum alloys (0–6% Mg, 0–5% Cu, 0–1% Sc, 0–1% Ag). The specific normalization procedure in case of RS essential multicollinearity has been proposed. LEI and RS for each definite ablation pulse energy were plotted in Cartesian co-ordinates ( x and y axes — the RS values, z axis — LEI signal). It was found that in the three-dimensional space the slope of the correlation line to the plane of RS depends on the analyte content in the solid sample. The use of this slope has therefore been proposed as a multivariate corrected analytical signal. Multivariate correlative normalization provides analytical signal free of matrix interferences for Al–Mg–Cu–Li alloys. The application of this novel approach to the determination of Li allows plotting unified calibration curves for Al-alloys of different matrix composition.

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.

Laser-enhanced ionization detection of Pb in seawater by flow injection analysis with on-line preconcentration and separation.

The flame laser-enhanced ionization (LEI) technique is coupled with the flow injection analysis system to measure the trace lead amounts in aqueous solution and in seawater. The flow injection (FI) manifold is incorporated with a microcolumn packed with a C18 bonded silica. The chelating agent DDPA is used to form the Pb-DDPA complex, which may be sorbed in the microcolumn and then eluted with methanol. The preconcentrated Pb is then detected by the LEI technique with either single-step or two-step excitation. At 5- and 15-mL volume-fixed sample loading, the detection limits of 0.011 and 0.0033 ng/mL (11 and 3.3 ppt) and enrichment factors of 16 and 48 are achieved, respectively, using a two-step FI-LEI. The sensitivity of the current system proves to be better by at least 1 order of magnitude than that of conventional LEI method. The FI-LEI also increases the tolerance of matrix interference. The LEI signal is slightly reduced to 80% intensity as 10,000 micrograms/mL (ppm) Na and K matrixes are mixed in the lead solution. The resistance to the alkali matrixes is enhanced approximately 4 times that reported previously using a similar water-immersed probe as a LEI collector. Finally, the FI-LEI is for the first time applied to detect the Pb content in seawater, achieving a result of 0.0112 +/- 0.0006 ng/mL (ppb) consistent with the certified value of 0.013 +/- 0.005 ng/mL (ppb).

Spatially Resolved Temperature Determination of an Air/Acetylene Flame Using the Two-Step Laser-Enhanced Ionization Technique

We have demonstrated a two-step laser-enhanced ionization (LEI) technique to be potentially useful in measuring a spatially resolved temperature in an atmospheric air/acetylene flame. As a complement to the existing optical methods, LEI detection shares the advantages of extremely high sensitivity and the capability of achieving a high spatial resolution, confined to the dual-beam intersection area of 1 mm × 1 mm. A theoretical model was developed to precisely extract the temperature information from the obtained two-step LEI signal. The physical parameters required in the experiment include simply the relative values of the LEI signals, associated with the fine-structure doublets of a selected thermometric species, and their corresponding laser energies. The subsequent treatment is also convenient without the need to know the absolute energy of the incident laser beam and to accurately calibrate the detection system. Al and Ga were selected as the thermometric species. The resultant radial and axial temperature mappings are consistent with each other and are also comparable with those reported by other optical methods.

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.

Automatic determination of optimum dilution levels for laser-enhanced ionization detection of matrix-interfered sample by flow injection

A flow injection (FI) system is interfaced to laser-enhanced ionization (LEI) apparatus to handle the sample introduction. With the use of FI–LEI apparatus, the electrical interference induced by the matrix may be effectively diminished. A significant LEI signal of trace In is recovered in a solution containing an Na-matrix concentration greater than the threshold level, 4–10 ppm (µg ml–1) in our conditions, at which no conventional LEI signal is detectable. The dilution gradient, resulting from injection of a water plug into the carrier stream of sample solution, leads to the appearance of a double-peak LEI signal. By taking into account the dispersion behaviour of sample solution and the ion-sheath effect, we are able to derive an explicit formula. According to the formula, we may successfully predict the measured intensity of the FI–LEI signal and characterize the features of the double-peak observed.

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.

Novel Technique To Reduce Electrical Interference Inherent in Laser-Enhanced Ionization Detection by Using Flow Injection Analysis

The influence of electrical interference induced by the easily ionized sodium matrix upon the laser-enhanced ionization (LEI) detection of indium analyte is studied. As the LEI signal is electrically suppressed but still detectable, the original sample concentration may be effectively recovered by using the standard addition method. Nevertheless, this method fails to apply when the added Na matrix reaches a threshold level, at which the LEI signal is completely suppressed

Application of Laser-Enhanced Ionization: Atomization Efficiency Determination

We have demonstrated that the laser-enhanced ionization (LEI) technique can be used to determine the efficiency of atomization of metal elements in an atmospheric acetylene/air flame. We have derived a useful relation between the time-integrated LEI signal and the total free atom number density in a flame. We determine the efficiency of atomization of ∼0.13–0.37 for the lithium element and of ∼1.0 for the sodium element. Our results agree well with AA measurements reported previously.

Application of Laser-Enhanced Ionization to Flame Temperature Determination

We have both theoretically and experimentally demonstrated that the laser-enhanced ionization (LEI) technique can be a simple, alternative method for flame temperature determination in an atmospheric air/acetylene flame. A three-level model of rate equations is developed to determine a flame temperature based upon the Boltzmann population of the ground fine structures of the analyte selected. A Ga sample was used for the test, and the LEI signal was measured with excitation to various intermediate states. The determined flame temperatures are very consistent with each other and also with those previously reported.

Probe atomization for laser enhanced ionization in a graphite tube furnace

Reported here is the first use of laser enhanced ionization (LEI) together with modern furnace technology, for the determination of metals in samples. The experimental arrangement that was used included a pulsed excimer pumped dye laser, a graphite tube furnace, and a graphite probe that was used for both sample introduction and as an electrode. For elements with excitation transitions which promoted the atoms to energy levels within 7000 cm −1 of the ionization limit, such as thallium, indium and lithium, detection limits in the 0.7–2 pg range were obtained. Lead, magnesium and iron, which were excited to levels between 24 000 cm −1 and 31 000 cm −1 from the ionization limit, had detection limits between 10 and 60 pg. The linear dynamic range for each of the elements was between three and four orders of magnitude, and the precision for aqueous standards was between 12 and 16%. Theoretical estimates showed that the final ionization step was collisional ionization, rather than photoionization or radiative ionization by furnace black-body emission, for transitions that were less than 15 000 cm −1 from the ionization continuum. However, for transitions that were more than 25 000 cm −1 from the ionization limit, photoionization and collisional ionization both contributed significantly to the LEI signal. Interferences from easily ionized elements and acids were studied, and indium was chosen as an example analyte. Sodium severely depressed the indium LEI signal in a graphite furnace. Nitric acid and sulfuric acid had little effect upon the indium LEI signal, but the signal was suppressed at high concentrations of hydrochloric acid, probably due to the formation of diatomic indium chloride.

Determination of manganese at trace levels in water by laser-enhanced ionization spectrometry after solvent extraction

Solvent extraction was combined with one-step laser-enhanced ionization (LEI) spectrometry for the determination of trace amounts of manganese in water. The LEI signal was measured at 279.5 nm. Experimental conditions such as laser power, cathode potential, distance between cathode and burner head and laser beam height were optimized for the diisobutyl ketone extract of the manganese chelate with sodium diethyldithiocarbamate. With the solvent extraction method, the interferences from easily ionizable elements such as Na, K, Ca and Mg were completely eliminated and the detection limit (signal to noise ratio = 3) was improved to 0.09 ng ml–1 with a 10-fold concentration. The method was successfully applied to the analysis of ground, river, lake, sea, tap and waste waters.

Investigation of Background Signals from Na as a Source of Interference in Laser-Enhanced Ionization Spectrometry in Flames

Laser-Enhanced Ionization (LEI) signals from Na have been investigated by the one-step excitation scheme in flames in the UV region (220–410 nm). This knowledge is of importance for the applicability of the LEI technique since Na often exists as a matrix in a number of samples and gives rise to background signals. In the wavelength region between 220 and 240 nm, photoionization of ground-state atoms is responsible for the LEI signal. Between 240 and approximately 300 nm, the LEI signal originates mainly from wing-excitations of the ordinary 3 s-np ( n ≤ 5) transitions. Finally, in the wavelength region above the 3 s-4 p transition (i.e., approximately 360–410 nm), the LEI signal consists mainly of photoionization of the thermally excited 3 p state. A theoretical treatment of the various contributions shows good agreement with experiments. It is furthermore found that the LEI sensitivity at 408 nm (the threshold wavelength for photoionization of atoms in the first excited state, the 3 p state) is 50 times smaller than the LEI sensitivity at 240 nm (the threshold wavelength for photoionization of ground-state atoms). The selectivity between analyte atoms (Bi and Pb in the present study) and Na is also studied. It is shown that the analyte-versus-Na selectivity decreases considerably as a function of increased laser intensity. It is concluded that there exists an optimum laser intensity for the most efficient detection of a particular element in a certain Na matrix. This optimum intensity, however, is determined by factors which are specific to each experimental setup.

Laser-Enhanced Ionization Signals Obtained with Microwave Resonant Cavities

A systematic study, both experimental and theoretical, is described for an analysis of the laser-enhanced ionization (LEI) detection scheme with a microwave resonant cavity. Signal intensity for various resonant modes is consistent with the result of a numerical calculation based on a plasma-microwave interaction. The LEI signals of sodium atoms aspirated into a C3H8/O2 flame have been observed down to the concentration of 0.1 ng/mL. Broad band LEI signals have also been observed in a fuel-rich flame and are considered to be due to laser-induced heating and subsequent thermal ionization of large molecules.

Reduction of Spectral Interferences from Na in Laser-Enhanced Ionization Spectrometry by Laser Pre-Ionization

Pre-ionization of Na atoms prior to the analysis of other analyte atoms using the Laser-Enhanced Ionization (LEI) technique in flames has been investigated in order to reduce the influence of spectral interferences (in this case of Na) to the measured LEI signal. A large fraction of the Na atoms present in the flame was ionized with the use of one laser system. A short time later (400 µs) another laser system was aimed at a position 4 mm above the first laser in the flame, thus probing the part of the flame in which the concentration of (neutral) Na atoms had been significantly reduced. Different excitation schemes were used for the preionization process. It was found that the degree of depletion of Na was 13% when the atoms were first excited to the 3 p state by 589-nm light and then photoionized by focused pump laser light (308 nm). When the atoms were excited in two resonant steps (3 s-3 p-7 d), 80% of all the Na atoms could be depleted. With all three lasers illuminating the atoms, finally, 83% of the Na atoms were depleted. (If laser-enhanced chemical processes can be neglected within the duration of the laser pulse, which is most probable in the case of ns pulse duration, the degree of depletion is equivalent to the degree of ionization.) The second laser system was then used for studies of the LEI signal from other elements in the same manner as above. It was then found that this Na pre-ionization did not interfere in any way with the detection of other elements which were detected 400 μs later (4 mm higher in the flame). This was illustrated by a measurement of Mg in a Na matrix in which the Mg signal recovery was 100% while the Na signal was decreased by a factor of 5 (resulting in a corresponding increase in selectivity).

Anomalous Contributions to the Signal in Laser-Enhanced Ionization Spectrometry from “Scattered” Laser Light

Anomalous contributions to the measured signals in Laser-Enhanced Ionization (LEI) spectrometry in flames is investigated. It was found that a significant part of the one-step LEI signal (»90% in certain cases) can be attributed to ionization of atoms which are positioned outside of the “true” interaction region (the region out-traced by the laser beam in the flame) due to excitation of atoms by “scattered” light (the word “scattered” is here used in its widest meaning, i.e., it refers to scattered, diffracted as well as fluorescence light). This is especially significant when small beam diameters and high light intensities are being used. The anomalous contributions to the LEI signals from “scattered” light are investigated both in a study of the shape (the widths) of one-step LEI signals and in experiments with two nonoverlapping beams in the flame. Several origins for the “scattered” laser light are proposed and investigated: (1) diffraction of laser light from apertures, (2) emitted fluorescence from analyte atoms in the interaction region, and (3) scattering from flame related particles or molecules in the interaction region. The exact mechanism has not yet been determined, but the findings are indicative of some sort of saturable scattering process from a flame constituent which exists in higher concentrations in fuel-rich flames than in lean flames. The existence of “scattered” light outside of the interaction region implies, furthermore, that erroneous conclusions may be drawn when, for example, number densities, ionization yields, signal enhancements, and saturation effects are studied by LEI.

Simultaneous detection of laser-enhanced ionization and laser-induced fluorescence in flames: noise correlation studies

Random signal fluctuations of simultaneously detected laser-enhanced ionization (LEI) and laser-induced fluorescence (LIF) were investigated and analyzed for correlation with each other and with laser power fluctuations. A greater degree of noise correlation was found between LEI and LIF than between either technique and laser power fluctuations. A background correction method is proposed which subtracts the LIF signal of the background-inducing interfering element from the combined LEI signals from analyte and interferant. Noise correlation between LEI and LIF can be utilized to cancel the noise in the interfering background signal. Broadband spectral interference due to wing excitation of sodium LEI was used as an example to demonstrate the method. Excitation spectra and laser power dependencies of sodium LEI and LIF were also studied, and found to be identical.

Determination of opitmum condiitons for laser-enhanced ionizaiton spectrometry in flames—I: One-step signal strength versus excitaiton transiiton

A general theory for determination of the most sensitive one-step laser-enhanced ionization (LEI) transitions for atoms in flames is presented. A simplified expression for the one-step LEI signal strength as a function of excitation wavelength (or signal strength vs the energy of the uppermost laser-connected level) for atoms in flames is derived. From that expression a general element-independent prediction of the most sensitive unsaturated one-step LEI transition is given (i.e. the largest number of charges produced for given amount of laser exposure). The optimum unsaturated one-step LEI transitions in atoms in flames are predicted to be those which excite the atoms to states which are positioned approximately (1.5 + 4.5 k T/ E ion kT below the ionization limit ( E ion is the ionization limit of the atom, k the Boltzmann constant and T the temperature). In an acetylene/air flame this corresponds to approximately 3000 cm −1. This prediction is verified experimentally by measurements in two simple atomic systems (Li and Na). The agreement between the general element-independent prediction of most sensitive one-step LEI transition and the experimental findings for these two elements is good. Furthermore, in order to obtain more accurate predictions of the most sensitive transitions and to handle elements with a perturbed series of oscillator strengths within the principal series the derived expression for signal strength versus excitation wavelength is used also for element-specific predictions of the most sensitive one-step transitions for a number of elements (Li, Na, K, Rb, Cs, Mg, Ca, Sr, Ba, Al, Ga, In and Tl). The most sensitive unsaturated one-step LEI transiitons in an acetylene/air flame are predicted to be: Li 2 s-5 p at 256.3 nm; Na 3 s-6 p at 268.2 nm; K 4 s-5 p at 404.5 nm or 4 s-6 p at 344.7 nm; Rb 5 s-7 p at 358.8 nm; Cs 6 s-8 p at 387.7 nm, Mg 3 s 2-3 s4 p at 202.7 nm; Ca 4 s 2-4 s6 p at 227.6 nm; Sr 5 s 2-5 s9 p at 235.5 nm; Ba 6 s 2-6 s7 p at 307.2 nm, Al 3 p 2P 3 2 -6 d 2 D 5 2 at 221.1 nm; Ga 4 p 2 P 5 2 at 233.9 nm; In 5 p 2 P 1 2 -6 d 2 D 3 2 at 256.1 nm; and Tl 6 p 2 P 1 2 -8 d 2 D 3 2 at 223.9 nm. For some of these elements there are, however, also some additional transitions which have only slightly lower sensitivities as compared to those above. Finally, the influence of opitcal saturation upon the determination of the most sensiitve transition is also discussed. It is illustrated that the more transitions are saturated, the closer to the ionization limit the atoms are to be excited in order to find the strongest one-step LEI signal.

Laser-Enhanced Ionization in Microwave-Induced Plasmas

Laser-Enhanced Ionization (LEI) signals have been detected for Na, Li, and Ba analytes in a microwave-induced plasma (MIP). A 300-mW continuous-wave (cw) dye laser pumped by a 5-W argon-ion laser was used to promote measurably increased ionization rates for these elements. A low-power, high-efficiency microwave plasma at 1 atmosphere with nitrogen and nitrogen-containing support gas was employed as the atom reservoir. The effects of varying applied microwave power, support gas composition, electrode voltage, and geometry were studied and results are given. The experimental variables that most significantly affect LEI signal intensity are: (1) electrode geometry, spacing, voltage, and distance above the cavity; (2) applied microwave power; (3) gas composition in an argon and nitrogen mixture; and (4) laser intensity. Experimental results are presented from the studies of Na LEI signals as a function of each one of these variables. Preliminary analytical studies yield Na detection limits in the low ng/mL range, showing this method to be competitive with other laser-based ionization methods.

Simultaneous measurements of laser-induced fluorescence (LIF dip) and laser-enhanced ionization (LEI enhancement) in flame—investigation of physical properties

An investigation of the physical properties of Laser-Enhanced Ionization (LEI) and Laser-Induced Fluorescence (LIF) signals in a flame has been performed on Sr atoms. A theoretical model based on a rate-equation formalism is presented in order to evaluate expressions for signal strengths for both one- and two-step LEI and LIF signals as well as for LEI enhancement and LIF dip (the increase of the ionization and the decrease of the fluorescence light when a second exciting step is applied, respectively). The theory is compared with experimental results and the agreement is good. The model is based on a three level system and an ionization level and also includes the effects of atomic depletion, trapping and photoionization. A special emphasis is given to the LEI enhancement and the LIF dip which are studied with respect to various laser intensities in both the first and second excitation step and compared with experimental findings. The relation between the LEI enhancement and the LIF dip is also investigated. Experimental results (fully explained by the theory) are, for example, a decrease of the ionization enhancement with respect to the intensity of the exciting light of the first transition and large fluorescence dips (of up to 85%). A theoretical treatment shows that the latter value can be used for a determination of the degree of depletion (ionization) or the degree of trapping of the atomic system.