Fundamental studies of laser ablation ICPMS using a nitrogen plasma source and helium, argon and nitrogen as carrier gas†

Laser-generated aerosols in helium, argon and nitrogen were directly compared using a nitrogen microwave inductively coupled plasma in combination with a mass spectrometer. An increased surface darkening of the sample was observed during ablation in nitrogen, suggesting a more pronounced particle deposition in comparison to helium, but only 30% lower sensitivities were obtained for nitrogen. Crater depth measurements proved that these variations were not the consequence of a difference in ablation rate (271 ± 17 nm per pulse for He vs. 267 ± 12 nm per pulse for N₂). Particle size measurements and electron microscopy images indicated that the aerosol generated in nitrogen or argon tends to form smaller agglomerates in comparison to helium. This dependence in agglomeration however appeared to be dependent on the ablation cell used. Furthermore, the fluence of the 193 nm laser system used for the experiments significantly influenced the temporal stability of the signals (e.g.²³⁸U⁺/²³²Th⁺). Finally, the quantification capabilities of nitrogen as an ablation environment and carrier gas were studied and compared to helium generated aerosols transported in nitrogen. A selection of samples (NIST SRM 612, USGS BCR-2G, USGS GSD-1G and GSE-1G) were quantified using NIST SRM 610 as an external standard. The potential of nitrogen as an alternative carrier gas was highlighted with deviations from the reference values calculated between −13% and +16% in comparison to the deviations between −15% and +17% obtained with helium in a cylindrical ablation cell for most elements in the different reference materials analyzed.