Design and evaluation of a new Peltier-cooled laser ablation cell with on-sample temperature control.

A new custom-built Peltier-cooled laser ablation cell is described. The proposed cryogenic cell combines a small internal volume (20 cm(3)) with a unique and reliable on-sample temperature control. The use of a flexible temperature sensor, directly located on the sample surface, ensures a rigorous sample temperature control throughout the entire analysis time and allows instant response to any possible fluctuation. In this way sample integrity and, therefore, reproducibility can be guaranteed during the ablation. The refrigeration of the proposed cryogenic cell combines an internal refrigeration system, controlled by a sensitive thermocouple, with an external refrigeration system. Cooling of the sample is directly carried out by 8 small (1 cm×1 cm) Peltier elements placed in a circular arrangement in the base of the cell. These Peltier elements are located below a copper plate where the sample is placed. Due to the small size of the cooling electronics and their circular allocation it was possible to maintain a peephole under the sample for illumination allowing a much better visualization of the sample, a factor especially important when working with structurally complex tissue sections. The analytical performance of the cryogenic cell was studied using a glass reference material (SRM NIST 612) at room temperature and at -20°C. The proposed cell design shows a reasonable signal washout (signal decay within less than 10 s to background level), high sensitivity and good signal stability (in the range 6.6-11.7%). Furthermore, high precision (0.4-2.6%) and accuracy (0.3-3.9%) in the isotope ratio measurements were also observed operating the cell both at room temperature and at -20°C. Finally, experimental results obtained for the cell application to qualitative elemental imaging of structurally complex tissue samples (e.g. eye sections from a native frozen porcine eye and fresh flower leaves) demonstrate that working in cryogenic conditions is critical in such type of direct sample analysis.