Entrainment and Transport of Laser Ablated Plumes for Subsequent Elemental Analysis

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Abstract
Several cell designs have been systematically evaluated for gas flow entrainment and transport of laser ablated material to a secondary excitation source for elemental analysis. The best cell is not limited to samples of particular size or shape and is insensitive to sample surface irregularity. An annular gas sheath around the cell results in a transient response sufficiently fast to permit depth and lateral sampling of single samples or rapid throughput of different samples but slow enough to give a steady signal with laser repetition rates ≥10 Hz. Entrainment and transport of ablated particulates have been investigated experimentally and by model calculation for a test material (Mo metal). The equations for predicting diffusive and gravitational loss of particles in a horizontal tube are presented and discussed. The major loss mechanism appears to be gravitational deposition of relatively large particles formed during ablation and possibly by coalescence within the transfer tube. Entrainment of ablated Mo by the cell and mass transport from the cell to the secondary source were determined to be ∼90% and ∼40% efficient, respectively. Shot-to-shot fluctuation in particle size may cause corresponding variation in transport efficiency when the upper end of the ablated particle size distribution exceeds the size limit for particle transport.