Selective ionization for elemental analysis with an ion trap mass spectrometer

The ion storage capacity (<10⁶) of ion trap mass spectrometers (ITMS) can sometimes limit the ability to analyze trace components in complex mixtures. We demonstrate here that resonant laser ablation (RLA) can offer a degree of selectivity in the ionization process, thus allowing the preferential accumulation of analyte ions in the trap. Selectivities of 75 and 50, for chromium and iron, respectively, are reported here for RLA of stainless steel in an ITMS. We offer suggestions to improve both the selectivity and the ionization efficiency, relative to the results reported here.     

Studies on laser defocusing effects on laser ablation inductively coupled plasma-atomic emission spectrometry using emission signals from a laser-induced plasma

The effect of laser defocusing on analytical performance of laser ablation inductively coupled plasma atomic emission spectrometry (LA-ICP-AES) was studied by varying laser focus conditions with respect to the surface of a low-alloy steel and a powdered sediment pellet. Laser-induced plasma (LIP) and LA-ICP-AES emission signals and LIP excitation temperatures (LIP T_ex) were determined and compared for different laser defocus conditions. LIP Fe and LA-ICP-AES Fe emission signals and LIP T_ex decreased when the laser was defocused for the low-alloy steel. On the other hand, when the sediment pellet was ablated, LIP T_ex decreased when the laser was defocused. However, LA-ICP-AES Fe emission signals increased at first, then decreased when the laser was defocused more. It was concluded that LIP T_ex and LIP and LA-ICP-AES Fe emission signals are dependent on laser shot conditions (focus–defocus), and are also dependent on sample type (texture, mineralogy, hardness, conductivity and heat capacity).     

The influence of laser-particle interaction in laser induced breakdown spectroscopy and laser ablation inductively coupled plasma spectrometry

Particles produced by previous laser shots may have significant influence on the analytical signal in laser-induced breakdown spectroscopy (LIBS) and laser ablation inductively coupled plasma (LA-ICP) spectrometry if they remain close to the position of laser sampling. The effects of these particles on the laser-induced breakdown event are demonstrated in several ways. LIBS-experiments were conducted in an ablation cell at atmospheric conditions in argon or air applying a dual-pulse arrangement with orthogonal pre-pulse, i.e., plasma breakdown in a gas generated by a focussed laser beam parallel and close to the sample surface followed by a delayed crossing laser pulse in orthogonal direction which actually ablates material from the sample and produces the LIBS plasma. The optical emission of the LIBS plasma as well as the absorption of the pre-pulse laser was measured. In the presence of particles in the focus of the pre-pulse laser, the plasma breakdown is affected and more energy of the pre-pulse laser is absorbed than without particles. As a result, the analyte line emission from the LIBS plasma of the second laser is enhanced. It is assumed that the enhancement is not only due to an increase of mass ablated by the second laser but also to better atomization and excitation conditions favored by a reduced gas density in the pre-pulse plasma. Higher laser pulse frequencies increase the probability of particle-laser interaction and, therefore, reduce the shot-to-shot line intensity variation as compared to lower particle loadings in the cell. Additional experiments using an aerosol chamber were performed to further quantify the laser absorption by the plasma in dependence on time both with and without the presence of particles. The overall implication of laser-particle interactions for LIBS and LA-ICP-MS/OES are discussed.     

Characterization of acoustic signals produced by ultraviolet laser ablation inductively coupled plasma atomic emission spectrometry

A simple device was designed to measure the acoustic signal accompanying laser ablation. The potential use of this signal for laser ablation-inductively coupled plasma atomic emission was examined. A frequency quadrupled pulsed Nd:YAG laser radiation was used for the ablation of glass, steel and ceramic samples. The relation between the acoustic signal, the laser energy, the analyte signal and the amount of ablated material was studied and evidence of the use of the acoustic signal for the exact focusing of the laser beam onto the sample surface was given. A more intense acoustic signal was observed for the exact focusing with a formation of larger ablation craters in glass and ceramics.     

Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS): comparison of different internal standardization methods using laser-induced plasma (LIP) emission and LA-ICP-MS signals.

The source of signal variations that governs the analytical performance of laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) was investigated in this study. In order to specify the source of signal variations of LA-ICP-MS, laser-induced plasma (LIP) Fe emission, LA-ICP-MS Fe+ and LA-ICP-MS Ni+ signals were used as internal standards for the determination of trace elements in low-alloy steel certified reference materials (BS 50D and JSS 1005-1008). Fe 1373.5 nm emission signals from LIP were measured, while trace element LA-ICP-MS signals were collected. After that, the LIP emission signals, LA-ICP-MS Fe+ and LA-ICP-MS Ni+ signals were used as internal standards, and the analytical performance was evaluated by the RSDs and the correlation coefficients (r) of the calibration curves. The improvement factors were dependent on the internal standardization methods. Analytical precisions (RSDs) of trace element LA-ICP-MS signals were improved by factors of 1.5-3.3 using LIP Fe emission signals as an internal standard. The improvement factors of 2.5 - 5.9 and 4.1 - 17 were obtained by using LA-ICP-MS Fe+ and LA-ICP-MS Ni+ signals as internal standards, respectively. Better correlation coefficients (r) were also obtained using the LA-ICP-MS signal compensation (0.9985 by LA-ICP-MS Fe+ and 0.9996 by LA-ICP-MS Ni+) rather than the LIP Fe emission compensation (0.9932). In this paper we compare and discuss the analytical performance achieved by LA-ICP-MS using LIP Fe emission, LA-ICP-MS Fe+ and LA-ICP-MS Ni+ signals as internal standards.     

Characterization of acoustic signals produced by ultraviolet laser ablation inductively coupled plasma atomic emission spectrometry

A simple device was designed to measure the acoustic signal accompanying laser ablation. The potential use of this signal for laser ablation-inductively coupled plasma atomic emission was examined. A frequency quadrupled pulsed Nd:YAG laser radiation was used for the ablation of glass, steel and ceramic samples. The relation between the acoustic signal, the laser energy, the analyte signal and the amount of ablated material was studied and evidence of the use of the acoustic signal for the exact focusing of the laser beam onto the sample surface was given. A more intense acoustic signal was observed for the exact focusing with a formation of larger ablation craters in glass and ceramics. Received: 25 June 1998 / Revised: 25 September 1998 / Accepted: 30 September 1998     

Resonant laser ablation as a selective metal ion source for gas-phase ion molecule reactions

Resonant laser ablation (RLA) is used as a source to selectively generate multiple metal ion species from the same sample. The capability of rapidly changing metal ions for gas-phase ion chemistry studies is a significant advantage in ion-molecule chemistry. The simple experimental arrangement uses relatively modest laser pulse energies (≤ 25 µJ/pulse) from a tunable dye laser to desorb and selectively ionize different metal atoms from a multicomponent sample. In turn, this allows the chemistry of several components to be investigated without breaking vacuum or altering the experimental geometry. This work demonstrates the use of RLA as a selective source of several reagent metal ions for gas-phase ion chemistry investigations. In particular, the reactivity of acetone with Cr⁺, Fe⁺, Ni⁺, and Cu⁺ was examined for metal ions selectively created by RLA from a standard steel sample.     

The role of continuum radiation in laser induced plasma spectroscopy

This paper focuses on the interpretation of the origin of the continuum radiation in Laser Induced Plasma (LIP) emission spectra, a subject that has received little consideration in the literature when compared to the analysis of the line emission spectrum. The understanding of the spectral peculiarities observed immediately after the laser pulse, when the continuum radiation prevails on discrete emission lines, can be extremely important to retrieve the initial conditions of LIP and to correlate the produced plasma to the ablation mechanism. In this work, in addition to a qualitative interpretation of the LIP continuum in the initial stage of expansion, a methodology is proposed for a better measurement of the atomic temperature in the expansion stage of the LIP. Such methodology is based on the analysis of the combined Boltzmann and Planck plots. The results obtained stress once again the importance of considering non equilibrium effects in the initial stage of LIP expansion.     

Dual-pulse laser induced breakdown spectroscopy: Time-resolved transmission and spectral measurements

Spectral analysis of laser-induced plasmas for surface ablation has demonstrated the possibility of analyte signal enhancement with dual-pulse configurations as compared with traditional single-pulse LIBS. Using an orthogonal dual-pulse arrangement, measurements were performed using glass microscope slides to allow both spectral analysis as well as optical transmission measurements. Order of magnitude enhancements in Mg and Si atomic emission signal peak intensities were recorded along with similar enhancements of the continuum emission for dual-pulse LIBS as compared to single-pulse. Peak-to-base measurements showed a roughly 50% increase, while signal-to-noise ratios were enhanced by a factor of 2–3. Temporal analysis of the measured transmitted laser pulse waveforms showed no significant differences between dual-pulse and single-pulse LIBS configurations, providing additional insight into the possible laser coupling processes for the dual-pulse configuration.     

Analysis of aluminum alloys by resonance-enhanced laser-induced breakdown spectroscopy: How the beam profile of the ablation laser and the energy of the dye laser affect analytical performance

In resonance-enhanced laser-induced breakdown spectroscopy, the sample was ablated by a laser pulse and the expanding plume was photoresonantly rekindled by a dye laser pulse. By sampling aluminum alloys for Mg, Pb, Si, and Cu, we showed that for the ablation step, Gaussian beams gave 2 to 3× better signal-to-noise ratio (SNR) than non-uniform beams. For the rekindling step, if no further sample destruction was allowed, dye laser pulses that intercepted the plume transversely gave 6 to 12× higher SNR than the longitudinal case. By combining Gaussian beams and transverse rekindling, the mass limit-of-detection for Mg was about 100 amol while non-resonant analysis was 10× more destructive. Sub-monolayer of oxides grown on laser-cleaned aluminum surfaces was detected by monitoring the AlO emissions of rekindled plumes; without resonant enhancements, they were not detectable no matter how destructive was the analysis. Time resolved studies showed that the Gaussian beam produced less dispersed plumes and that a stronger dye laser beam directed transversely heated up a bigger plume mass without over-heating the plume core. The analyte emissions were sustained while the continuum background remained low.     

氯原子光诱导能级移动和增宽

应用半经典的微扰论方法计算出了氯原子光诱导能级移动和增宽,其大小与光强成正比光频率为41018 cm-1时,氯原子初态3p52P03/2和3p52P01/2在非共振情况下,光移动分别为67.6和26.9 MHz/Wμm-2.当激光频率v接近3p52P03/2→4s2P3/2的跃迁频率时,氯原子基态3p52P03/2的|MJ,MI〉=|-1/2,3/2〉→|1/2,3/2〉超精细ESR谱线发生近共振频率光移动,大小量级为0.1 MHz/Wμm-2,能级增宽一般远小于光移动     

Dynamics of a laser-irradiated adatom

The dynamics of an adsorbed atom irradiated by an I.R. laser in resonance with a single pair of states of the vibrational adbond is studied. Using a non-perturbative treatment for the laser-adbond interaction, a master equation is derived, which governs the time evolution of the populations of the laser-dressed states of the adbond. The effect of resonant heating and laser-induced desorption, as an example of a possible laser-induced surface process, is discussed.     

Spatial Taming and Trapping of Molecules

Molecular beam techniques for study of collisional and spectroscopic processes have recently been enhanced by use of static electric or magnetic fields to orient or align molecules with permanent dipole moments. A more general method is now in prospect, applicable both to alignment and to spatial trapping of molecules. This exploits the anisotropic interaction of the electric field vector of intense laser radiation with the dipole moment induced in a polarizable molecule by the laser field. The interaction creates directional superpositions of field-free states that correspond to oblate spheroidal wavefunctions, with eigenenergies that decrease with increasing field strength. We suggest that this polarizability interaction produces the marked alignment found in laser-induced dissociative ionization of CO by the Saclay group. We also present calculations illustrating the feasibility of spattal trapping. In combination with supermirror focussing and buffer-gas cooling, an intense infrared laser can typically confine molecules for long-times (-hours) within a small (-picoliter) and cold (?1°K) “pocket of light.”     

Determination of lead in water using laser ablation–laser induced fluorescence

The detection sensitivity of laser-induced breakdown spectroscopy (LIBS) is improved by coupling it with a laser-induced fluorescence method. A waterjet sample containing 500 ppm of Pb as an analyte was ablated by a 266 nm, frequency-quadrupled Q-switchedNd:YAG laser at an energy of ~ 260 μJ. After a short delay the resulting plume was re-excited with a 283.306 nm, nanosecond pulse dye laser at energies ranging from 45 to 100 nJ. The limit of detection (LOD) of lead in water was determined both by the single-pulse LIBS technique and Laser Ablation coupled with Laser-Induced Fluorecence (LA–LIF) method. It was found to be 75 ppm in the case of single-pulse LIBS and 4.3 ppm for LA–LIF. When the resonant pulse was detuned from the transition wavelength the LA–LIF signal disappeared demonstrating the resonant selectivity of this technique.     

Open-path laser-induced plasma spectrometry for remote analytical measurements on solid surfaces

Open-path laser-induced plasma spectrometry has been studied for elemental analysis at a distance of 45 m from the target. The 230-mJ pulsed radiation of a Q-switched Nd:YAG laser at 1064 nm has been used to produce a plasma on the sample and light emission has been collected under an off-axis open-path scheme. Under such conditions, the main variables influencing the signal response such as beam focal conditions, laser incidence angle and laser penetration depth have been identified and diagnosed on the basis of spectral signal-to-noise ratio considerations. The incidence angle is critical beyond 60°. Crater morphology and ablation rates have been studied also. A semi-quantitative analysis of several stainless steel grades has been implemented using a pattern recognition algorithm, which allowed to discriminate successfully the samples on the basis of their variable content in alloying elements.     

Laser-induced ablation of a steel sample in different ambient gases by use of collinear multiple laser pulses

The sensitivity of laser-induced breakdown spectroscopy of solid samples depends on the number of ablated and excited analytes. Laser ablation of solid samples can be enhanced by using collinear multiple laser pulses, for example double or triple pulses, rather than single laser pulses with the same total laser pulse energy. The ablation rates and the plasma conditions are affected by the ambient gas. In this study laser ablation was examined by varying the interpulse separation of the multiple pulses, within double and triple-pulse bursts, and the gas mass density at constant gas pressure. Different ambient gases and gas mixtures consisting of argon, oxygen, and nitrogen were used to study their effect on ablation rates. In a pure argon atmosphere (99.999% v/v Ar) the ablation burst number required to penetrate a steel plate of thickness 100 μm is reduced by a factor of approximately six by use of triple-pulse bursts with a symmetric interpulse separation of 15 μs rather than single pulses with the same total burst energy of 105 mJ. For double and single pulses the factors are 1.6 for Ar and 2.8 for synthetic air. Analyte lines are 4 to 8 times more intense if an argon atmosphere, rather than air, is used.     

Use of sol–gels as solid matrixes for laser-induced breakdown spectroscopy

Silicon, zirconium and aluminum sol–gels were investigated as suitable starting materials for tunable matrix calibration standards for laser-induced breakdown spectroscopy. A fast and simple preparation method was developed, using aluminum i-propoxide as the precursor in the sol–gel synthesis, which allows one to quickly prepare solid calibration standards offering very homogenous analyte distribution in the matrix, low optical spectral background, as well as reproducible behavior towards laser ablation and vaporization. The surface of the calibration targets and the morphology of the ablation craters were examined by optical and scanning electron microscopy, and the material ejection process was observed by shadowgraph imaging. Low μg/g detection limits and 4–15% relative standard deviation were measured by laser induced breakdown spectroscopy for Pb, Cr and Be used as internal standards.     

Laser ablation inductively coupled plasma atomic emission spectrometry of a uranium-zirconium alloy: ablation properties and analytical behavior

The ablation properties and analytical behavior of a uranium-zirconium alloy have been examined using tandem laser ablation/pneumatic nebulization sample introduction in conjunction with inductively coupled atomic emission spectrometry (LA-ICP-AES). An apparent change in composition of the laser ablation aerosol (1–15 GW cm⁻² Zr deficient, 40–250 GW cm⁻² Zr rich) is observed. This phenomenon is independent of laser wavelength. After collection and bulk chemical analysis of the ablation product, this phenomenon is attributed to an atomization interference in the ICP.

Two distinct modes of laser ablation have been observed which depend upon the wavelength of the ablating laser (visible or near infrared). These two modes result in characteristic ablation crater types and analyte emission behavior. Ablation yields at 1064 nm are dependent upon laser power density only, whilst yields at 532 nm are dependent upon both laser power density and illumination area. The latter is considered to be symptomatic of direct interaction of the laser light with the surface, and the former, of indirect coupling of laser energy, via a micro-plasma, into the surface.     

Inter-pulse delay optimization in dual-pulse laser induced breakdown vacuum ultraviolet spectroscopy of a steel sample in ambient gases at low pressure

Time-integrated spatially-resolved Laser Induced Breakdown Spectroscopy (LIBS) has been used to investigate spectral emissions from laser-induced plasmas generated on steel targets. Instead of detecting spectral lines in the visible/near ultraviolet (UV), as investigated in conventional LIBS, this work explored the use of spectral lines emitted by ions in the shorter wavelength vacuum ultraviolet (VUV) spectral region. Single-pulse (SP) and dual-pulse LIBS (DP-LIBS) experiments were performed on standardized steel samples. In the case of the double-pulse scheme, two synchronized lasers were used, an ablation laser (200 mJ/15 ns), and a reheating laser (665 mJ/6 ns) in a collinear beam geometry. Spatially resolved and temporally integrated laser induced plasma VUV emission in the DP scheme and its dependence on inter-pulse delay time were studied. The VUV spectral line intensities were found to be enhanced in the DP mode and were significantly affected by the inter-pulse delay time. Additionally, the influence of ambient conditions was investigated by employing low pressure nitrogen, argon or helium as buffer gases in the ablation chamber. The results clearly demonstrate the existence of a sharp ubiquitous emission intensity peak at 100 ns and a wider peak, in the multi-microsecond range of inter-pulse time delay, dependent on the ambient gas conditions.