Transport of ablated material

The lateral resolution obtainable during the analysis is linked to the spot size. To a smaller degree, signal generation at the ICP-MS also depends on spot size, since the dimensions of the crater diameter together with the applied power density influence the amount of ablated material. The aspect ratio of the ablation crater formed (quotient of crater depth to crater diameter) has an influence on the transport of ablated material. The higher this aspect ratio, the lower the amount of material transferred from the crater bottom to the ICP-MS. 

The influence of various gas pressure conditions within the laser ablation cell on the particle formation process in laser ablation has also been investigated.69 In LA-ICP-MS studies at low pressure (down to 2kPa) a small particle size distribution and a reduction in elemental fractionation effects was obtained. But with decreasing pressure and transport volume of ablated material, a significant decrease in the ion intensities was observed as demonstrated for uranium measurements in the glass SRM NIST 610.69 However, the laser ablation of solid materials at atmospheric pressure in LA-ICP-MS is advantageous for routine measurements due to lower experimental effort and the possibility of fast sample changing in the ablation chamber. Fractionation... 

A serious problem in LA-ICP-MS described in the literature on many occasions is the time-dependent elemental fraction (so-called ablation fractionation ) occurring during laser ablation and the transport process of ablated material, or during atomization and ionization processes in the inductively coupled plasma.20-22 Numerous papers focus on the study of fraction effects in LA-ICP-MS as a function of experimental parameters applied during laser ablation (such as laser energy, laser power density, laser pulse duration, wave length of laser beam, ablation spot size,... 

To obtain accurate, quantitative results, either the amount of material ablated per laser pulse must be similar for the standards and samples or the relative ablation rates must be experimentally measurable (through use of an internal standard, for example). The size distribution of the ablated particles must be similar enough for the standards and samples so that that transport efficiency of ablated material is similar or again can be accounted for accurately. Ideally, the ablation process should produce particles and sample vapor that have the same chemical composition as the sample. However, elemental fractionation can occur, particularly if the ablation process is predominantly thermal [61]. 

With a typical ablated particle size of about 1 -pm diameter, the efficiency of transport of the ablated material is normally about 50% most of the lost material is deposited on contact with cold surfaces or by gravitational deposition. From a practical viewpoint, this deposition may require frequent cleaning of the ablation cell, transfer lines, and plasma torch. 

In Table 4.3, the Cetac product LSX-200 is the specialized system for coupling with the ICP customer s system. It includes the laser, optical viewing system for exact positioning of the laser focus on a sample surface, and the sample cell mounted on the computer controlled XYZ translation stage. The system is also provided with the appropriate gas tuhing for transport of the ablated material into an ICP-OES/MS. 

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. 

Gonzalez et al. 2008). Laser ablation is a direct sampling technique by which a high energy laser is focused on the surface of a material and atoms, ions, and particles are ejected. The particles, which are chemically representative of the bulk sample, are then transported into an ICPMS for analysis. In LIBS, a luminous, short-lived plasma is created on the sample surface by the focused laser beam and its emission spectra are analyzed to provide both qualitative and quantitative chemical compositional analysis (Cremers... 

Analyte sensitivity as reported for different instruments is mainly influenced by the experimental crater size, energy density and repetition rate, which influence the amount of ablated and transported material. Normalizing sensitivities obtained with a similar energy density to a crater diameter of 40 pm and a repetition rate of 10 Hz leads to rather equal sensitivities of ca. 1000-3000 cps/ng for most commercially available instruments. The ensuing limits of detection are thus influenced mainly by background noise. 

The ablation rates are considerably enhanced by using the jet electrode, where the argon being used as carrier is blown through the electrode, which has a narrow bore gas channel. This results not only in a very efficient transport of the ablated material away from the arc channel, but its particle size might even be favorably... 

The main problem with in-situ chemical analysis of biogenic material is the absence of suitable solid certified standard reference materials. Fractionation of the chemical elements can occur during sample ablation (by laser or electron beam), during transport to the mass spectrometer and, in the case of LA-ICP-MS, in the plasma (e.g. Kuhn Gunther... 

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