Secondary shock wave: Implication for laser ablation inductively coupled plasma mass spectrometry

Shadowgraphs of the dynamic evolution of nanosecond laser-induced ablation plumes outside of BCR-2G and BIR-1G geological standard samples at atmospheric pressure are captured. The model of the secondary shock wave is proposed and experimentally verified. Experimental results indicate that the secondary waves appear at 74 ns for the BCR-2G sample and at 98 ns for the BIR-1G sample, which result from the reflection of the backward moving gas on the sample surface. Under the same condition, the ablation threshold of the BCR-2G sample is lower than that of the BIR-1G sample. Furthermore, the ablation rate (crater depth per laser pulse) of the BCR-2G sample is higher than that of the BIR-1G sample. The secondary shock wave induces larger-diameter particles or clusters which have been regarded as a fractionation source. Debris redeposition plays a role in the laser ablation and inductively coupled plasma mass spectrometry. The suppression of the secondary shock wave effect is possible using low viscosity ablation gas which can expand the shock wave front, leading to less collision and aggregation of the ablated particles. As a result, the debris redeposition will be attenuated and the larger particle or cluster induced elemental fractionation can be expected to be suppressed in practice.