Oleg V. Borisov

Laser ablation sampling

Abstract Laser ablation sampling provides significant benefits and capabilities for chemical analysis. It represents one of the most promising technologies for direct solid sample introduction. Despite the advantages, there are a number of issues that should be addressed to better understand and utilize this technology. Laser ablation itself is a complex process and is poorly understood, fundamentally. In this paper, we describe the current achievements and limitations in order to better understand and utilize laser ablation sampling for chemical analysis. Several current issues related to laser ablation sampling are discussed, including calibration and optimization, fractionation, sensitivity enhancements, mass loading, and particle transport.

Effects of crater development on fractionation and signal intensity during laser ablation inductively coupled plasma mass spectrometry

Abstract The effects of crater development on ICP-MS signal intensities and elemental fractionation have been presented in this work. Craters formed after repetitive 266-nm Nd/YAG laser ablation with 1.0-mJ pulses had a cone-like shape. The laser ablation rate (ng/s) depended on the laser irradiance (laser pulse energy per unit time and unit area), decreasing as irradiance increased. In contrast, the particle entrainment/transport efficiency did not significantly change with irradiance. As the crater-aspect ratio (depth/diameter) increased above some threshold value of six, the Pb/U elemental ratio departed from the stoichiometric value. However, good stoichiometry of ablated mass could be achieved when experimental conditions were carefully selected. The exact mechanism of how crater development affects fractionation is not well understood. In this work, actual irradiance was introduced instead of a nominal value. Actual irradiance decreased as the crater deepened due to changes of the effective area, sampled by the laser beam.

Inductively coupled plasma mass spectrometric study of non-linear calibration behavior during laser ablation of binary Cu)Zn Alloys

Abstract Laser ablation behavior of a suite of 10 Cu–Zn binary alloys was studied using inductively coupled plasma mass spectrometry. Three laser systems (20 ns KrF excimer, 6 ns and 35 ps Nd:YAG) were used for ablation. Non-linear calibration plots for both Cu and Zn were observed using all three lasers, despite significant differences in laser ablation mechanisms and good stoichiometry of ablated mass. Crater volume measurements were used to determine the amount of mass removed during repetitive laser ablation from each sample. A change in mass ablation rate for samples with different composition explains observed phenomena. Despite the differences in ablation behavior of these alloys, linear calibration curves were obtained when Zn signal intensity was normalized to signal intensity of Cu or to crater volume.

Pb/U Fractionation during Nd:YAG 213 nm and 266 nm Laser Ablation Sampling with Inductively Coupled Plasma Mass Spectrometry

Elemental fractionation during laser ablation sampling was investigated by measuring Pb/U ratios in NIST 610 synthetic glass. Two Nd:YAG lasers with wavelengths of 213 and 266 nm were used to ablate the sample into an inductively coupled plasma mass spectrometer. Pb/U fractionation was observed to be similar for both laser wavelengths, and dependent on the irradiance. For representative Pb/U measurements, the necessary laser irradiance should be > 0.6 GW/cm2. However, if the laser beam is initially focused close to the sample surface, fractionation increases and is influenced by the formation of a crater during repetitive pulsing at a single sample location. As the ratio of crater depth to radius increases, plasma sampling and/or an effective irradiance decrease could cause additional fractionation. A good correlation was found between the fractionation of 14 elements in NIST 610 glass and the logarithms of their oxide melting temperatures.

Enhancements in laser ablation inductively coupled plasma-atomic emission spectrometry based on laser properties and ambient environment

Analytical performance of laser ablation inductively coupled plasma-atomic emission spectrometry (ICP-AES) depends critically on the interaction between the laser light and the sample. The analyte emission line intensity in ICP-AES depends on the quantity of mass ablated. The effect of laser parameters (wavelength, pulse duration, and power density) was investigated for increasing the quantity of ablated mass. For fixed laser beam energy, the ablated mass can change 2 to 3 orders of magnitude by changing the laser beam spot size on the sample. The ablated mass quantity also depends on laser pulse duration and wavelength; and on ambient gas in the sample chamber. The shorter the pulse duration and wavelength, the higher the quantity of ablated mass. By using He in the chamber, the amount of mass increases by a factor of 2 for 30 ns excimer laser ablation and by an order of magnitude for ps-laser ablation.

Laser ablation processes investigated using inductively coupled plasma–atomic emission spectroscopy (ICP–AES)

Abstract The symbiotic relationship between laser ablation mechanisms and analytical performance using inductively coupled plasma–atomic emission spectroscopy are addressed in this work. For both cases, it is important to ensure that the ICP conditions (temperature and electron number density) are not effected by the ablated mass. By ensuring that the ICP conditions are constant, changes in spectral emission intensity will be directly related to changes in laser ablation behavior. Mg ionic line to atomic line ratios and excitation temperature were measured to monitor the ICP conditions during laser-ablation sample introduction. The quantity of ablated mass depends on the laser pulse duration and wavelength. The quantity of mass removed per unit energy is larger when ablating with shorter laser wavelengths and pulses. Preferential ablation of constituents from a multicomponent sample was found to depend on the laser beam properties (wavelength and pulse duration). For nanosecond-pulsed lasers, thermal vaporization dominates the ablation process. For picosecond-pulsed lasers, a non-thermal mechanism appears to dominate the ablation process. This work will describe the mass ablation behavior during nanosecond and picosecond laser sampling into the ICP. The behavior of the ICP under mass loading conditions is first established, followed by studies of the ablation behavior at various power densities. A thermal vaporization model is used to explain nanosecond ablation, and a possible non-thermal mechanism is proposed to explain preferential ablation of Zn and Cu from brass samples during picosecond ablation.

Laser ablation inductively coupled plasma mass spectrometry of pressed pellet surrogates for Pu materials disposition

Successful Pu disposition by immobilization in glass or ceramic form requires accurate and precise knowledge of impurity amounts. Analysis of Pu material by conventional liquid nebulization requires dissolution, which is difficult due to the refractory nature of the samples. Laser ablation is a suitable sampling technique for direct analysis of solids. This paper demonstrates the procedures that were established for PuO2 analysis using laser ablation inductively coupled plasma mass spectrometer (ICP-MS). Pressed pellets prepared from CeO2 were used to simulate PuO2. Effects of laser conditions, sample preparation, and matrix composition, specifically mass of a matrix element and color, on the analyses of CeO2, Bi2O3, and PtO2 based pressed pellets were examined. Influence of mixing/grinding time on particle sizes, sample homogeneity, and ablation efficiency were investigated. Laser conditions that produce stoichiometric sampling were examined.

Optical emission spectroscopy studies of the influence of laser ablated mass on dry inductively coupled plasma conditions

The amount of ablated mass can influence the temperature and excitation characteristics of the inductively coupled plasma (ICP) and must be taken into account to ensure accurate chemical analysis. The ICP electron number density was investigated by using measurements of the Mg ionic to atomic resonant-line ratios during laser ablation of an aluminum matrix. The ICP excitation temperature was measured by using selected Fe lines during laser ablation of an iron matrix. A Nd:YAG laser (3 ns pulse duration) at 266 nm was used for these ablation-sampling studies. Laser energy, power density, and repetition rate were varied in order to change the quantity of ablated mass into the ICP. Over the range of laser operating conditions studied herein, the ICP was not significantly influenced by the quantity of solid sample. Therefore, analytical measurements can be performed accurately and fundamental studies of laser ablation processes (such as ablation mass roll-off, fractional vaporization) can be investigated using inductively coupled plasma-atomic emission spectroscopy (ICP-AES).

Time-resolved parametric studies of laser ablation using inductively coupled plasma atomic emission spectroscopy

Abstract The quantity of ablated mass and its composition strongly depend on the number of laser pulses and laser fluence at the sample target surface. For chemical analysis, thin-film deposition, cutting, and other laser-ablation applications, the quantity of mass removed vs. number of laser pulses is important. In addition, the composition of the vapor can be critical, for example, in providing accurate chemical analysis or well-defined thin film structures. In this work, mass ablation rate and ablated mass composition were studied by monitoring the time dependence of emission intensity using Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP) during repetitive laser ablation at a single location on the sample target. Spectral emission intensity in the ICP is directly related to the quantity of mass ablated by the laser. The ratio of spectral emission lines in the ICP gives an indication of the relative composition of ablated constituents. In this work, a brass sample was ablated using several lasers with various properties. Emission intensities of Cu and Zn ionic lines, after the occurrence of an initial signal spike, increase with increasing number of laser pulses at high fluence, whereas at low fluence no significant changes were observed in the mass ablation rate. The zinc-to-copper ratio was used to monitor fractionation processes during repetitive laser ablation. The ratio increased with increasing ablation time at low fluence. In contrast, the ratio was almost constant, and close to the accurate level at high laser fluences. The effect of various laser parameters on the mass ablation rate and mass composition are discussed in this paper.