Laser system, beam delivery, and analyzer configuration for fs-LA-ICP-MS at 800 nm and 400 nm (components include BBO: beta barium borate crystal, DM: dichroic mirror, CCD: charge-coupled device camera, ICP: inductively coupled plasma, and QMS: quadrupole mass spectrometer). The ablation chamber is mounted on an XYZ translation stage.
ICP-MS signal intensity vs. time at various energies for fs laser ablation of NIST 610 glass standard using (a) 400 nm and (b) 800 nm laser excitation. The laser was triggered at 10 s and fired at a single spot on the target at 10 Hz for 60 s, with 40 s washout time. The lowest energies in both graphs exhibit near ablation threshold characteristics.
Transient RSD as a function of laser energy for all four NIST samples with 400 nm and 800 nm LA. Lower energies show a consistently higher TRSD, and the TRSD is also higher for 800 nm LA than for 400 nm LA, indicating that 400 nm LA is preferable when very precise data are required.
Integral ICP-MS signal vs. energy for 400 nm and 800 nm LA of NIST 610 SRM. The intensity values were computed from 30 s after the signal peaked. The 400 nm signal is higher at all energies but saturates at ∼400 μJ, whereas the 800 nm signal continues to increase toward the 400 nm saturation level.
Calibration plots for (a) Fe, (b)Pb, (c) Th, and (d) U, for laser ablation at 200 μJ, 400 nm wavelength. Calibration was attained by ablating four NIST glass standards with different concentrations of the selected elements: NIST 610, 613, 615, and 616. The data points were fitted with a linear least squares fit, which was used to determine the detection limit of each element.
Elemental ratios of (a) U/Th and (b) U/Pb are given as a function of energy for 400 nm and 800 nm laser excitations. The sample used for this study was NIST 610. The expected ratios based on elemental concentrations in the standard are 1.009 for U/Th and 1.083 for U/Pb as indicated by the dashed lines; the experimental U/Th ratio is closer to the expected value than the U/Pb ratio. Both ratios are more consistent for 400 nm ablation than 800 nm at lower laser energy levels.
Elemental ratios of U/Th at (a)400 nm and (b) 800 nm, and U/Pb at (c) 400 nm and (d) 800 nm are given as a function of time at 300 μJ laser energy. The sample used for this study was NIST SRM 610. The expected ratios based on elemental concentrations in the standard are 1.009 for U/Th and 1.083 for U/Pb as indicated by the dashed lines. Both ratios are more consistent for 400 nm ablation than 800 nm.
Particle counts is plotted as a function of diameter for both wavelengths. For LA, NIST 610 sample was used with laser energy of 300 μJ. The results indicate that the peak particle diameter falls in the range of 100 nm to 150 nm for both wavelengths. Wavelength does not appear to have an effect on peak particle size. However, the particle counts are found to be significantly higher for shorter wavelength.
Peak number of particle counts obtained for 400 nm and 800 nm laser ablation at various energy levels. ICP-MS signal intensity has been added to the figure for comparison; the open square and circle represent the peak particle counts for 400 nm and 800 nm, respectively, and the filled square and circle correspond to the ICP-MS signal for 400 nm and 800 nm, respectively. The lines here represent the data trend, demonstrating that the peak particle counts follow approximately the same trend as ICP-MS signal.
ICP-MS signal intensity for various particle diameters for 400 nm and 800 nm LA. For obtaining data, LA was performed on NIST 610 sample with energy of 250 μJ. The maximum signal is seen for 125 nm-150 nm, with steady signal for particle sizes 200 nm-350 nm. The trend is similar for both 400 nm and 800 nm, indicating that the relative contribution of each particle size is not wavelength dependent.
To study the role of particle size in elemental fractionation, (a) U/Th ratio and (b) U/Pb ratio vs. particle diameter were obtained for 400 nm and 800 nm LA. NIST 610 was ablated using 250 μJ laser energy. The U/Th ratio data are consistent at all particle diameters. The U/Pb ratio data show that particle diameter plays a major role in the obtained ratio, with the data closest to the expected values for particles with 125–150 nm diameters.
Concentrations (in ppm) of elements analyzed in this study for NIST SRMs 610–616.
Detection limits, given in ppb, at various energies for LA using 400 nm and 800 nm laser wavelengths. Detection limits were computed as three times the standard deviation of the background for each element divided by the slope of the calibration curve. The detection limits are lower at lower energies for 400 nm ablation but at higher energies both 400 nm LA and 800 nm LA approach the same detection limit. Possible error is introduced due to extrapolation of calibration plots to low elemental concentrations.
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