Femtosecond laser-induced-breakdown spectrometry for Ca2+ analysis of biological samples with high spatial resolution

We describe an analytical method for element-specific in situ investigations of biological samples with high spatial resolution. using laser-induced-breakdown spectroscopy (LIBS). The prerequisites for spatially highly resolved LIBS are an appropriate analytical performance in combination with precise micro-ablation conditions, both laterally and axially. In order to identify the best suitable laser source, the analytical performance and the ablation process were studied for ns-LIBS and fs-LIBS. The analytical performance was studied on an aqueous solution of CaCl2, where water served as a first substitute for biological material. The ablation process was investigated in the outer epidermal wall of a sunflower seedling stem. Besides direct measurements, the physiology of the sunflower seedling can be used in order to estimate the ablation depth. Our results show that analytical measurements with high spatial resolution can be performed on biological samples using fs-LIBS. This was demonstrated by in situ measurements of wall-associated calcium ion (Ca2+) distributions within the peripheral cell wall of the sunflower seedling (Helianthus annuus L.) stem. In this biological sample an axial resolution of about 100 nm was achieved.