The quality of the 2D element maps depends on the measurement conditions (laser fluence, beam diameter, repetition rate, scanning speed, laser ablation chamber volume, flow rate and acquisition time). Optimizing these parameters is often a matter of trial and error since the quality criteria for elemental imaging (sensitivity, spatial resolution, noise and analysis time) are intricately linked. A simple 1D animation shows how the ICP-MS signal might degrade as a result of the physical size of the laser beam and the slow washout of aerosol particles from the ablation chamber (Figure 1). A simple mathematical model, and ensuing software, was developed [1] to simulate the LA-ICP-MS output upon virtual rastering of a digital signal image of a cross-section of a sample (a copy of the software can be downloaded here). Even though the LA-ICP-MS maps are not directly correlated with elemental imaging, element distributions are often related to visual features in the sample, allowing optimization of the LA-ICP-MS settings for the desired quality criteria (samples without any visual features can not be optimized in this way). A demo of this software can be found in Figure 2 to find the optimal LA-ICP-MS mapping conditions of a star; it can be seen that reducing the laser beam diameter is not always beneficial to increase the lateral resolution, at least when the analysis time is constant.

Figure 1: 1D animation showing the degradation of the original signal upon traversing of the laser beam across two objects.

Figure 2: Demo of the 2D LA-ICP-MS mapping software.

References

  1. TRIGLAV, J., ELTEREN, J.T. van, Ĺ ELIH, V.S.. Basic modeling approach to optimize elemental imaging by laser ablation ICPMS. Analytical chemistry, ISSN 0003-2700. [Print ed.], 1. okt. 2010, vol. 82, no. 19, str. 8153-8160, ilustr., doi: 10.1021/ac1014832.