Jhanis J. Gonzalez

Laser ablation in analytical chemistry - A review

Laser ablation is becoming a dominant technology for direct solid sampling in analytical chemistry. Laser ablation refers to the process in which an intense burst of energy delivered by a short laser pulse is used to sample (remove a portion of) a material. The advantages of laser ablation chemical analysis include direct characterization of solids, no chemical procedures for dissolution, reduced risk of contamination or sample loss, analysis of very small samples not separable for solution analysis, and determination of spatial distributions of elemental composition. This review describes recent research to understand and utilize laser ablation for direct solid sampling, with emphasis on sample introduction to an inductively coupled plasma (ICP). Current research related to contemporary experimental systems, calibration and optimization, and fractionation is discussed, with a summary of applications in several areas.

Femtosecond laser ablation ICP-MS

Femtosecond laser ablation was investigated for direct solid sample chemical analysis. The phonon relaxation time in a solid is of the order of 100 fs, which is the same as the laser pulse duration. For such excitation, there should be little time for the matrix to experience a “temperature” during the laser pulse. If the surface explodes before the photon energy is dissipated as heat in the lattice, the ablation process should produce stoichiometric vapor (elemental fractionation should be negligible). Based on this hypothesis, NIST glasses were ablated using 100 fs laser pulses at 800 nm, with subsequent elemental analysis using the ICP-MS. Pb and U intensities, and Pb/U ratios in the ICP, were measured during repetitively femtosecond-pulsed ablation. These data show that fluence (laser energy/spot area) has a significant influence on the amount of mass ablated and on the degree of fractionation. An optimal fluence was found at which the fractionation index approached unity; negligible fractionation. Infrared femtosecond laser ablation produced similar characteristics to UV nanosecond laser ablation.

Laser Ablation in Analytical Chemistry

In 2002, we wrote an Analytical Chemistry feature article describing the Physics of Laser Ablation in Microchemical Analysis. In line with the theme of the 2002 article, this manuscript discusses current issues in fundamental research, applications based on detecting photons at the ablation site (LIBS and LAMIS) and by collecting particles for excitation in a secondary source (ICP), and directions for the technology.

Laser assisted plasma spectrochemistry: laser ablation

This paper presents a brief overview of the current research issues in laser ablation for chemical analysis, discusses several fundamental studies of laser ablation using time-resolved shadowgraph and spectroscopic imaging, and describes recent data using femtosecond ablation sampling for ICP-MS and LIBS. This manuscript represents a summary of the plenary lecture presented at the 2004 Winter Conference on Plasma Spectrochemistry.

Comparison of 193, 213 and 266 nm laser ablation ICP-MS

There is a widespread discussion concerning the ‘better’ wavelength for laser ablation chemical analysis. Wavelength is believed to be an important parameter based on the sample’s optical penetration depth as well as photon energy for bond breaking. The lasers most widely employed for analytical applications are the excimer, based on an ArF mixture with a wavelength of 193 nm, and the solid state Nd:YAG, with wavelengths of 266 nm and 213 nm. NIST glasses were ablated to test the effects of these wavelengths on fractionation and transport efficiency. Crater geometry and volume were measured by using a white-light interference microscope. For all three wavelengths, linear calibration curves were obtained using NIST glasses as standards. The 208Pb/238U ratio in a tuff rock sample was measured using all three wavelengths; the value obtained using the NIST-glass calibration was compared to that measured using liquid nebulization.

Metal particles produced by laser ablation for ICP-MS measurements

Pulsed laser ablation (266nm) was used to generate glass particles from two sets of standard reference materials using femtosecond (150fs) and nanosecond (4ns) laser pulses with identical fluences of 50Jcm(-2). Scanning electron microscopy (SEM) images of the collected particles revealed that there are more and larger agglomerations of particles produced by nanosecond laser ablation. In contrast to the earlier findings for metal alloy samples, no correlation between the concentration of major elements and the median particle size was found. When the current data on glass were compared with the metal alloy data, there were clear differences in terms of particle size, crater depth, heat affected zone, and ICP-MS response. For example, glass particles were larger than metal alloy particles, the craters in glass were less deep than craters in metal alloys, and damage to the sample was less pronounced in glass compared to metal alloy samples. The femtosecond laser generated more intense ICP-MS signals compared to nanosecond laser ablation for both types of samples, although glass sample behavior was more similar between ns- and fs-laser ablation than for metal alloys.

UV-femtosecond laser ablation-ICP-MS for analysis of alloy samples

Femtosecond and nanosecond laser ablation was studied for the analysis of alloy samples by inductively coupled plasma mass spectrometry (ICP-MS). Two series of alloy standards from the National Institute of Standards and Technologies (NIST) were used to establish calibration graphs (NIST 625-629 and NIST 1737-1741). The quality of calibration (correlation coefficient, sensitivity and confidence intervals) was compared for ablation with the 266 nm wavelength from each laser. Accuracy and precision were established from these calibration graphs by determining the lead (208Pb) concentration when two of the standards were used as unknown samples (NIST 627 and 1738). Absolute quantitative measurements were obtained without using internal standards. A comparison between the calibration graphs established with the two series of standards individually and combined was performed. The use of the two series of standards combined increased the linear dynamic range of concentration as well as the accuracy and precision of analysis. Improved performance was measured using femtosecond laser pulses.

Laser plasma spectrochemistry

An overview of laser plasma spectrochemistry is presented to demonstrate its wide range of capabilities. Laser plasmas offer the ability to perform elemental, isotopic, molecular, quantitative and qualitative sample analysis with sub-micron spatial resolution, and each feature can be measured at standoff distances. Obviously, these attributes are not all achievable at the same time, but they can be optimized for specific applications. This manuscript gives a sampling (pun intended) of the research in our group that has demonstrated each of these capabilities. Although the technology is commonly referred to as LIBS (laser-induced breakdown spectroscopy), the authors prefer to use laser plasma spectrometry to represent the underlying science.

UV-femtosecond and nanosecond laser ablation-ICP-MS: internal and external repeatability

The internal (precision within an ablation spot) and external repeatability (precision between ablation spots on the sample) were evaluated during repetitive 266 nm femtosecond and nanosecond laser ablation-inductively coupled plasma spectrometry. Femtosecond laser ablation sampling into the ICP-MS improved the precision by reducing systematic errors related to the particle size distribution and resultant spikes on the signal intensity. The data are reported in terms of measured concentrations for several elements in silica glass reference materials NIST 610-612.

Assessment of the precision and accuracy of thorium (232Th) and uranium (238U) measured by quadrupole based inductively coupled plasma-mass spectrometry using liquid nebulization, nanosecond and femtosecond laser ablation

The precision and accuracy of the 238U/232Th ratio were evaluated from liquid nebulization and direct solid sampling repetitive pulsed laser ablation. Nanosecond and femtosecond pulsed lasers at 266 nm wavelength were utilized for the ablation studies. The ICP-MS and sampling parameters were optimized for each procedure; flow rates, gases, laser energy and other parameters were optimized for the particular sampling approach and therefore will not be the same. The work is not a comparison per se but represents performance metrics for three optimized sampling modalities. As expected, nanosecond pulsed ablation provided the greatest inaccuracy (>30%) from the nominal 238U/232Th bulk ratio. This deviation from bulk ratio is attributed to incomplete vaporization of large particle agglomerates produced by nanosecond laser ablation. Femtosecond pulsed ablation provided inaccuracy (∼1–3%) approaching that of liquid nebulization (∼1%). In terms of temporal relative standard deviation (TRSD) and relative standard deviation (RSD), liquid nebulization provided the best precision for the 238U/232Th ratio (TRSD ∼3–5%, RSD ∼0.2–0.6%), femtosecond laser ablation (TRSD ∼5–12%, RSD ∼1%) and nanosecond laser ablation (TRSD ∼25–48%, RSD ∼9–12%). Laser ablation requires less sample to achieve these performance metrics, in some cases less than a factor of 100-times depending on the entrainment and transport efficiency.