Laser plume

The intrinsic drawback of LIBS is a short duration (less than a few hundreds microseconds) and strongly non-stationary conditions of a laser plume. Much higher sensitivity has been realized by transport of the ablated material into secondary atomic reservoirs such as a microwave-induced plasma (MIP) or an inductively coupled plasma (ICP). Owing to the much longer residence time of ablated atoms and ions in a stationary MIP (typically several ms compared with at most a hundred microseconds in a laser plume) and because of additional excitation of the radiating upper levels in the low pressure plasma, the line intensities of atoms and ions are greatly enhanced. Because of these factors the DLs of LA-MIP have been improved by one to two orders of magnitude compared with LIBS. 

The potential of LA-based techniques for depth profiling of coated and multilayer samples have been exemplified in recent publications. The depth profiling of the zinc-coated steels by LIBS has been demonstrated [4.242]. An XeCl excimer laser with 28 ns pulse duration and variable pulse energy was used for ablation. The emission of the laser plume was monitored by use of a Czerny-Turner grating spectrometer with a CCD two-dimensional detector. The dependence of the intensities of the Zn and Fe lines on the number of laser shots applied to the same spot was measured and the depth profile of Zn coating was constructed by using the estimated ablation rate per laser shot. To obtain the true Zn-Fe profile the measured intensities of both analytes were normalized to the sum of the line intensities. The LIBS profile thus obtained correlated very well with the GD-OES profile of the same sample. Both profiles are shown in Fig. 4.40. The ablation rate of approximately 8 nm shot ... 

Figure 16. Tilt measurement from a movable atixiliary telescope looking at a foreground NGS tracked within the isoplanatic patch of the laser plume in the rnesosphere( Kagazzoni et al., 1995).

Figure 17. A frame a the Calar Alto 2.2m telescope two NGSs tire closely aligned witli tlic laser plume badcscattercd in the mesosphere from the laser beam Itujnehedby the 3.,5m telescope 300m away. The inte-...

Formation of metal-acac complexes is the basis for a method to determine the oxidation state of metal cations, by applying the MALDI-TOF-MS technique, for example, a procedure for the determination of Co(acac)2 and Co(acac)3 . Even insoluble oxides can be analyzed by sampling a fine suspension of the solid, which vaporizes in the laser plume and forms the corresponding acetyacetonate. For example, the MS of Mn(acac)3 in... 

For the case of both electrically conducting and electrically non-conducting samples, laser ablation combined with AAS may be useful. In this case AAS measurements can be performed directly at the laser plume. Measurement of the non-element specific absorption will be very important, because of the presence of particles, molecules and radicals as well as due to the emission of continuum radiation. In addition, the absorption measurements should be made in the apprppriate zones. When applying laser ablation for direct solids sampling, the atomic vapor produced can also be led into a flame for AAS work, as has previously been described by Kantor et al. [299] in their early work. 

When performing laser-excited AFS at a laser plume, it would appear to be useful to produce the laser plasma at pressures below atmospheric pressure, as then the ablation depends only slightly on the matrix [226, 669]. 

Laser ablation (LA) is a powerful tool for mass spectrometric sampling of radionuclides from solids. When sufficiently intense laser light strikes a solid, material is removed from the surface. This material can be in the form of atoms, molecules, ions, electrons, or small particles. Several approaches can be used to analyze the surface for these species. The light that usually also is emitted can be resolved by wavelength to identify atomic or molecular species associated with the sample surface. Depending upon the wavelength of the laser, the atoms and ions in the laser plume can be excited, and the resulting emission can be detected. 

Laser ablation-AAS is also useful for insulating samples, where AA analysis is performed directly in the laser plume. Due to the production of various particles in the measurement zone (solid particles, molecules, radicals) and the resulting background emission, appropriate techniques for the correction of spectral interferences must be used. 

A) Arc B) Spark C) Flame D) Plasma sources E) Low-pressure discharges F) Graphite furnace G) Laser plume ICP = Inductively coupled plasma DCP=Direct current plasmajet CMP = Capacitively-coupled microwave plasma MIP = Microwave-induced plasma GDL = Glow discharge lamp HC = Hollow cathode... 

The method has evoked renewed interest, mainly due to the availability of improved Nd-YAG laser systems. Different types of detector, such as microchannel plates coupled to photodiodes and CCDs in combination with multichannel analyzers, make it possible to record an analytical line and an internal standard line simultaneously, so that analytical precision is considerably improved. By optimizing the ablation conditions and the spectral observations, detection limits obtained with the laser plume as source for AES are in the range 50-100 pg/g with standard deviations ca. 1 % [142],... 

Graphite electrothermal atomizers (GETA), flames, glow discharges, and laser plumes are the most widely used types of atomizers in modem LEAF analytical practice. 

The sensitivity, accuracy, and precision of solid sample analysis were greatly improved by coupling of LA with ICP-OES/MS. The ablated species are transported with a carrier gas (usually argon) into the plasma torch. Additional atomization. excitation and ionization of the ablated species in a stationary hot plasma provide a dramatic increase in the sensitivity of emission detection (LA-ICP-OES) or detection of ions (LA-ICP-MS). The efficiency of the transport of ablated species into an ICP strongly depends on the size of the particles. The optimal conditions for ablation in the ca.se of LA-ICP differ significantly from the optimal conditions for LIBS because the efficient transport of the ablated matter to an ICP requires a fine aerosol (with solid particle diameters less than a few micrometers), whereas direct optical emission spectroscopy of the laser plume needs excited atoms and ions. 

So far the pulsed lasers with a pulse duration of some nanoseconds have been most widely used for LA-Nd YAG (1064, 532, 354.7. 266 nm), ex-cimer lasers XeCI (308 nm), KrF (248 nm). For these lasers the mechanisms responsible for the material removal are thermal melting and evaporation or some kind of explosive evaporation. As a result, so called fractional evaporation could occur—the elements with different melting and boiling temperatures evaporate from a melt at different rates. Because of this composition of a laser plume does not match the bulk composition. This was a serious problem as inadequate probing of a sample could not be corrected for by any sensitive detection scheme. The optimal conditions for laser ablation and advanced methods of data processing were found to avoid the problem of fractionation for different classes of samples in the case of nanosecond pulses. 

Optimize doR FP see Fig. 9.3a) while refocusing the lens to retain the pixel size obtained in Section 3.2. The aim of this step is to optimize vertically the overlap between the laser plume and the electrospray, thereby improving the LAESI ion yield. Please note that the laser beam, the emitter, and the orifice axis should remain in the same plane see Note 4.2.2). 

An analysis of the above pnbUcations indicates a wide range of experimental conditions (laser parameters, target, and type of Uqnid) tested to determine the combination favorable for formation of diamond structures by PLA. It is believed that this process evolves formation of the so-called laser plume or a cloud of reaction products consisting of the evaporated substrate material and, partially, the surrounding liquid. These evaporated substances form bubbles inside the liquid. As the amount of the evaporated material increases, the bubbles expand and, as the pressure and temperature reach a certain critical combination, they collapse. At the collapse of the bubbles, the temperature and pressure may reach in the range of thermodynamical stability of diamond. 

Rabek [51] and others [52] have described laser-induced decomposition of polymers. Comprehensive reviews have appeared on the interaction of laser radiation with solid materials and its significance in analytical chemistry [53,53a]. Various reviews cover the subjects of optical and mass spectrometry performed directly on the laser plume [54,55]. Moenke-Blankenburg [38] has described laser ablation for sample introduction. Advances in laser ablation of materials were recently reported [56,57]. 

High-powered lasers have proved to be useful sources for the direct ablation of solids. In atomic emission spectrometry, ruby and Nd YAG lasers have been used since the 1970s for solids ablation. When laser radiation interacts with a solid, a laser plume is formed. This is a dense plasma containing both atomized material and small solid particles that have evaporated and or have been ejected from the sample due to atom and ion bombardment. The processes occurring and the figures of merit in terms of ablation rate, crater diameter (around 10 pm), and depth... 

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