Comparison of the ablation behaviour of 266 nm Nd:YAG and 193 nm ArF excimer lasers for LA-ICP-MS analysis

The ablation characteristics of a 266 nm Nd:YAG laser and a 193 nm excimer laser were compared by successive experiments by inductively coupled plasma mass spectrometry (ICP-MS), using the same ablation cell without changing the carrier-gas flows within comparative experiments. Both laser-optical systems have a fairly flat-topped lateral energy distribution yielding pan-shaped ablation pits on the sample. Comparative experiments with the two optical systems were carried out with argon and with helium+argon as carrier gases. For both lasers and both gas set-ups, signals of 40 s duration were recorded with a pulse rate of 10 Hz, with similar fluence adjusted to give comparable rates of material ablation. ICP-MS signal intensities were normalised to the total ablated volume, to compare the effects of lasers and gases on transport efficiency, ionisation efficiency and response and time-dependent element fractionation. The accuracy of trace element results was tested using two materials that allow stable ablation with both lasers but have significantly different matrix compositions (SRM 612 silicate glass from NIST and AGV-1 geological reference material in lithium tetraborate fusion). The time-averaged rate of material ablation is similar for both lasers and independent of the carrier gas in the sample chamber, but decays more rapidly with the 266 nm system. In argon, the signal unit response per volume of ablated material is similar with both lasers. In helium, the signal intensity with the 266 nm laser is enhanced slightly (maximum two-fold) compared with argon, but with the 193 nm system a consistent 2-3-fold signal enhancement is achieved. The use of helium reduces the amount of visible (>1 µm size) particle deposition in the ablation cell, irrespective of laser wavelength, and tests with successive ablations of chemically contrasting samples indicate memory effects with the 266 nm system that are absent in the same experiment with the 193 nm system. The limits of detection for both lasers were further improved by the use of He owing to the decrease in background intensities. Time-dependent element fractionation during a 40 s single-spot ablation is almost eliminated with the 193 nm system (<10%), but with the 266 nm laser inter-element intensity ratios comparing the first and the second halves of the ablation period varied by up to 60% for some elements. Results for the cross-calibration between silicate (SRM 612) and borate glasses (AGV-1) obtained with both lasers indicate that the fractionation with the 266 nm system is similar for these two matrices, but this is not generally true for silicate minerals. The 193 nm system gives slightly better reproducibility between multiple analyses of one sample compared with the 266 nm system, yielding 2-5% RSDs for major and minor elements and 7-15% for concentrations below 10 ppm.