|
|
|
Chemical Analysis by Laser Ablation Mass SpectroscopyThese research efforts support a wide range of applications for laser ablation analysis including such diverse areas as nuclear waste and dating of Martian rocks. The work in the EMSL in this area is focused on understanding the basic physical and chemical processes involved in laser ablation. The research described below is generated by interaction of the EMSL personnel with external users for specific applications. Current results are for elemental analysis of the sample of interest. The investigation of methods to determine molecular composition is also under way. 1. Mechanisms of Macro-Particle Formation in Laser Ablation Sampling for Chemical Analysis of Nuclear WasteM. L. Alexander, A. L. Hedges,(a,b)S. C. Langford,(b,c) and J. T. Dickinson(b,c) Supported by the DOE Environmental Management Science Program.
(a) Eastern Oregon University. Recent efforts by researchers at PNNL have resulted in the installation of a laser-ablation, inductively coupled mass spectrometer (LA/ICP-MS) system in a hot cell at the Hanford Site. The hot cell system has been used to analyze actual waste samples from the Hanford waste tanks. Precision and accuracy range from 5-10% and 10-20%, respectively. Empirical studies showed that the best results are obtained when the ablation laser produces particulates in the size range from 0.1-1 microns. These studies motivated research into the production mechanisms for particles in laser ablation to allow the performance of this system to be improved still further. We are engaged in research on 1) ablation mechanisms and 2) the effect of the physical and chemical state of the sample on the character of the particles produced by laser ablation. Areas of investigation include formation mechanisms, morphology, and composition of laser-produced particulates that are delivered to a distant ICP unit for mass analysis. Particles were collected from laser ablation of materials with a variety of physical properties, including thermal conductivity, melting point, and optical absorption. The sample matrix types included amorphous glasses, single crystals, metals and aggregated particulates. The laser ablation wavelength was varied from 1900 to 266 nm; pulse lengths were from 10 ns to 20 ps; and pulse energies ranged from 10 m J/pulse to 1 mJ/pulse in a spot size of 50 microns. Particles were collected on a filter and analyzed by scanning electron microscopy (SEM). Three classes of particulates were observed for ablation from all types of samples: 1) large, irregular, sharp-edged particles >5 microns in diameter, 2) smooth spherical or ovoid particles from 1-2 microns, and 3) aggregates of many small (10-50 nm), ultra-fine particles. An SEM picture of particles produced by laser ablation at 1064 nm of a dark-colored glass sample with optical properties similar to those for vitrified waste glass is shown in Figure 8.1.
The mechanisms proposed for the formation of these particle types are, respectively, 1) fracture due to transient thermal gradients, 2) spallation of melted material, and 3) condensation and agglomeration of vaporized material. The type and composition of particles was found to be a strong function of laser wavelength and sample absorption characteristics. Figure 8.2 is an SEM photograph of particulates produced by laser ablation at 266 nm of the same glass sample used to generate the particles at 1064 nm. Only melted and ultra-fine particles are observed. The majority of the mass is present in the form of particles smaller than 100 nm in sharp contrast to the results at 1064 nm. These results suggest a relation between the type of particles formed by laser ablation and the absorption coefficient at that laser wavelength. The absorption coefficient of the vitrified waste simulant is smaller at 1064 nm than at 266 nm.
An SEM photograph of particles produced by ablation of MgO at 266 nm is shown in Figure 8.3. MgO has a low absorption coefficient at this wavelength. The particle types are similar to those produced by ablation of the wasteform material at 1064 nm. These results reinforce the importance of absorption coefficient in the mechanism of particle production in laser ablation.
The composition of the three types of particulates was determined by energy dispersive x-ray spectroscopy (EDX). Figure 8.4 compares the relative composition of major elements for fracture, spallation, and ultra-fine aggregated particles as well as for the target material.
The particles with compositions closest to the target material are the fracture and melt particles although the laser parameters that result in the most reliable chemical analysis, UV irradiation near 3 J/cm2, produce a particle distribution with the majority of the material present as ultra-fine aggregates (Alexander et al. 1999). This supports previous work (Alexander et al. 1998) that suggested the lower levels of precision and accuracy for IR laser ablation versus UV ablation were due to the incomplete transport and digestion of larger particles in the ICP-MS. The result from the EDX analysis indicating that the smallest particles are the least representative, suggests that significant improvements in LA-ICP/MS analysis can be achieved by generating small fracture or melt particles. Recent work (Dickinson et al. 1999) has demonstrated the generation of small fracture particles from the laser ablation of sodium nitrate, one of the major components in the Hanford tank waste. Current efforts are focused on evaluating the effect of this type of particulate production of the quality of analysis and understanding the formation mechanism in order to optimize the process. 2. Laser Ablation for In Situ Geochronology Measurements in Martian LanderM. Alexander, G. Cardell,(a,b) and M. Taylor(a,b) Supported by EMSL Operations and NASA. Large differences in the dates of the epochs arise from the use of different assumptions about the crater formation rates on Mars. In-situ geochronology measurements could be used to calibrate the Martian cratering record by providing an age measurement of a Martian rock selected from an age boundary where conflicting models in the predicted ages by more than the uncertainty of the age measurement. Terrestrial geochronology uses laboratory-based mass spectrometric methods to determine 87Sr/86Sr versus 87Rb/86Sr to determine the age at which rock formations solidified. The time since crystallization—the "age" of the rock—is extracted from the isochron using a line fit to the data points and the known decay rates of the parent isotope.
Laser ablation has been proposed as a method that can meet the need for these in-situ measurements on Mars. The analytical problems are very similar to those encountered in developing laser ablation as a remote sampling and analysis method for DOE nuclear waste. Representative material must be removed from the Martian surface and transported to a light, portable mass spectrometer for analysis. Work in this collaboration has two main focus areas: 1) establishing laser parameters for faithful sampling and transport of material, and 2) design of a mass spectrometer system with low mass and power consumption to be carried on a Martian Lander. Figure 8.5 shows an SEM photograph of particles resulting from ablation of the mineral albite at 355 nm.
Albite has a composition similar to minerals found on the Martian surface. Albite is relatively transparent at 355 nm. The observed large fracture particles are consistent with the model for laser ablation based on absorption coefficient described earlier. No round melt particles are observed, but aggregated ultra-fine particles are produced. Ablation at 266 nm produces similar particles, but the fracture particles are somewhat smaller, reflecting the higher absorption coefficient and smaller penetration depth at the shorter wavelength. An SEM photograph of particles from ablation at 266 nm is shown in Figure 8.6.
EDX analysis of the particles from ablation of these minerals indicated that the composition of the fracture particles most closely matched the target material. Initial studies were also done this year to investigate the feasibility of using laser desorption inside a quadrupole mass spectrometer to make the isotope measurements. Ion trap mass spectrometers are inherently small and lightweight and therefore ideally suited for mounting in a space vehicle. The laser desorption mass spectrometer (LD-ITMS) that is part of the EMSL user facility was used for initial isotope measurements. An initial mass spectrum is shown in Figure 8.7.
The relative elemental/isotope ratios observed in this mass spectrum are in excellent agreement with those determined by independent means, including LA-ICP/MS. ReferencesAlexander, M. L., A. L. Hedges, S. C. Langford, and J. T. Dickinson, "Mechanisms of Macro-Particle Formation in Laser Ablation Sampling for Chemical Analysis," invited talk at 1999 FACSS Conference, Vancouver, B.C. Alexander, M. L., M. R. Smith, J. S. Hartman, A. Mendoza, and D. W. Koppenaal, "Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA/ICP-MS)," Applied Surface Science 127-129, 255-261 (1998). Dickinson, J. T., S. C. Langford, and M. L. Alexander, "Production of Submicron Particles by Laser Ablation of Sodium Nitrate" to be published in App. Phys. A, 1999.
William R. Wiley Environmental Molecular Sciences Laboratory Feedback: webmaster@emsl.pnl.gov Revised: April 17, 2000 Security & Privacy PNNL-13147 |
Figure 8.1. SEM photograph of particles produced by laser ablation of vitrified waste simulant at 1064 nm. The width of the picture is 60 microns.
Figure 8.2. SEM photograph of particles produced by laser ablation of vitrified waste simulant at 266 nm. The width of the picture is 30 microns.
Figure 8.3. SEM photograph of particles produced by laser ablation of MgO at 266 nm. The width of the picture is 15 microns.
Figure 8.4. Relative composition of different particle types and target material, determined by EDX.
Figure 8.5. SEM photograph of particles from laser ablation of mineral albite.
Figure 8.6. SEM photograph of particles from ablation of albite at 266 nm.
Figure 8.7. Mass spectrum from albite showing Sr and Rb isotopes.