Theoretical description and experimental observation of aerosol transport processes in laser ablation inductively coupled plasma mass spectrometry

The transport efficiency of laser-induced aerosols mainly influences the detection capabilities for high spatial resolution laser ablation microanalysis. Therefore, the volume of the ablation cell, the geometry, the gas flow pattern and the transfer tube to carry the aerosol into the ICP are important influences on the transport process and the signal structure. Four different ablation cell volumes, ranging from 0.25 to 63 cm(3), in combination with various transfer tubes of different diameters and lengths were investigated. The signal structure was significantly modified by reducing the volume of the sample chamber by a factor of approximately 250. The peak height of single laser shot (aerosol density) was increased by a factor of 6, the signal width (aerosol dispersion) was reduced by a factor of 7 and the rinse time of the sample chamber was consequently shortened to approximately 1 s, thereby eliminating processes of sample recirculation within the cell. The gas flow pattern inside the cell was experimentally traced by introducing powder of different sizes (grain size <1 mum and < 65 mum) inside the cell. An angle of 40 degrees at the apex of the gas injection cone was observed at the tip of the inlet nozzle, for a 0.5 mm id nozzle and 1 L min(-1) gas flow. The gas inlet nozzle ensured a steady, high efficiency flow, influencing the precision of the measurements. The amount of transported material was constant within the precision of the ablation process for optimized transport and ionization conditions in all ablation cells. The ablation in a sample cell of volume of 0.25 cm(3) showed slightly reduced sample transport and a slightly longer rinse time than for a 1.5 cm(3) cell, which could be an indication that with extremely reduced cell volumes aerosol-wall interaction might limit the transport efficiency. The variation of the transport tube diameter (4-7 mm) and length (1.5-6 m) showed an influence on the signal structure, but the total amount of sample transferred was not influenced. Nevertheless, the variation of the dispersion as a function of the total volume of the transport system (cell and tube) showed that cell volume more significantly influenced the signal structure. A mathematical model was developed to describe the structure of the signal in relation to the parameters of the ablation set-up and its scope is discussed.