Laser ablation sample cells

The volume of the laser ablation sample cell, the geometry, the type of carrier gas and its flow pattern, and the tubing properties of the transfer line to transport the aerosol into the ICP are all together important factors that contribute to the total transport process and the transient signal structure. The transport efficiency of laser-induced particles therefore restricts the detection ca-pabihties for laser ablation microanalysis. Any enhancement in the transport process will make it possible to decrease the laser spot size and still detect a signal from the ablated mass, and therefore wih lead to an increase of the spatial resolution that can be successfully used for laser ablation. 

Hgure 6.4 Schematic of a standard laser ablation cell. (Source Simon Nelms, Thermo Electron Corporation.) 

Inductively Coupled Plasma Mass Spectrometry Handbook 

To analyze trace elements using laser ablation ICP-MS in materials such as biological samples or frozen samples, one has to ensure that these samples are either freeze dried or kept frozen, even during the laser ablation. Therefore, different groups have developed cryogenically cooled ablation cells that enable direct analysis of thin biological sample sections and ice core samples 

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Calibration and quantification procedures are easier in LA-ICP-MS compared to other solid-state mass spectrometric techniques because the laser ablation and the ICP ion source operate at normal pressure and the laser ablation of solid samples and ionization of analytes are separated in space and time. Therefore the advantage of solution calibration in ICP-MS can be applied in this solid-state analytical technique. The introduction of solution based calibration, which is only possible in LA-ICP-MS, was an innovative step in the development of this sensitive mass spectrometric technique. A number of different calibration approaches using aqueous standard solutions in the dual gas flow technique have been discussed by various authors.74 75 In the dual gas flow injection technique , the nebulized standard solution and the laser ablated sample material are mixed in the -piece and the two gas flows from the nebulizer (e.g. ultrasonic nebulizer) and laser ablation chamber are added. Using solution based calibration with the addition of a standard solution, Leach et alP determined minor elements in steel reference materials with a relative accuracy of a few %. In comparison to the so-called dual gas flow technique proposed in the literature, where the argon flow rates through the nebulizer and ablation cell add up to 11 min-1 (e.g. 0.451 min-1 and... 

Although laser-ablation sample preparation and analysis are conducted with relative ease, quantification of data can prove challenging. With liquid samples, the amount of material introduced into the ICP-MS remains relatively constant, and instrument drift is usually corrected through the use of internal standards. However, in LA-ICP-MS, conditions such as the texture of the sample, ablation time, the location of the sample within the laser cell, surface topography, laser... 

Several other alternative calibration methods using aqueous standard solutions have been applied in the case that appropriate standard reference materials are not available. " Online solution-based calibration utilizes the introduction of mixed laser-ablated sample material together with nebulized standard solutions through desolvation units to the ICR Two setups are in use, based on adding the solutions either before (the so-called single gas flow system) or after (known as the dual gas flow system) the laser cell (see Figure 6.13). ° Using such a procedure, one can use either external standardization, analyte standard addition, or even isotope dilution techniques for small sample sizes. ... 

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. 

There are several sample introduction methods that are used in conjunction with ICP, including nebulization, electrothermal evaporation, gas chromatography, hydride generation, and laser ablation [30]. Laser ablation combined with ICP (LA-ICP) is useful for analysis of solids. In such a source the sample is positioned in a chamber prior to the ICP source, the ablation cell. Argon gas at atmosperic pressure flows through the cell towards the ICP source. The sample is irradiated by a laser beam and... 

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... 

In inductively coupled plasma-mass spectrometry, isobaric interference occurs between species with the same mass and charge. Interference can be eliminated if the mass spectral resolution is sufficiently great or by dissociating an interfering polyatomic species with a collision cell. When laser ablation is used to sample a solid, matrix-matched standards are often necessary for quantitative analysis. 

The laser ablation system consists of a high-power pulsed laser, optics to focus the laser at or near the surface of the sample, and an ablation cell. Small ablated particles are swept out of the ablation cell and carried into the ICP in a flowing gas. Often a microscope lens and video camera are positioned to allow the operator to view the sample surface before and after ablation. A high-quality microscope and precise positioning of the sample relative to the laser beam are essential for good spatially resolved sampling. 

Analyzer Q = quadrupole, CC = collision cell, DRC = dynamic reaction cell, MC = multicollector, SF = sector field. Analytical details CV = cold vapor, ETV = electro-thermal vaporization, FI = flow injection, HG = hydride generation, ID = isotope dilution, LA = laser ablation, UN = ultrasonic nebulization. Sample introduction in liquid or slurry (si) form. 

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