Qualitative imaging of elemental spatial distribution in stalagmites through laser ablation inductively coupled plasma mass spectrometry analysis

The laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) technique is considered versatile for multi-elemental analysis and imaging because it is easy to handle, compatible with different types of solid samples, requires minimal sample preparation, and provides high spatial resolution and sensitivity. One of the challenges of imaging analysis is to obtain accurate and precise spatial information of elements distribution in the sample, so the optimization of the laser ablation (LA) parameters is essential. In this context, this study aimed to optimize the LA parameters for direct analysis of speleothem samples. Laser intensity, frequency, and spot diameter were evaluated through multivariate experimental design and multi-response data, the influence of ablation scan speed and the use of 44Ca as an internal standard (IS) for the qualitative image of 66Zn, 137Ba, 55Mn, 57Fe, 88Sr, 60Ni, and 26Mg distribution in the sample were also evaluated. The multivariate optimization revealed positive interactions between the parameters evaluated, i.e., the greater the laser intensity, LA frequency, and spot diameter, the greater the analyte signal and, thus the sensitivity. Therefore, 90% laser ablation intensity, 20 Hz repetition rate, and 100 µm spot diameter were selected. In the scan speed evaluation, the images obtained with 40 and 20 µm s-1 were very similar for all isotopes. The use of 44Ca as IS did not impact the resolution of the images. The use of 44Ca can provide important information about the speleothem formation and paleoclimate changes.