Determination of uranium isotopic ratios in biological samples using laser ablation inductively coupled plasma double focusing sector field mass spectrometry with cooled ablation chamber

An analytical procedure has been proposed for the determination of precise uranium isotope ratios in a thin uranium layer on a biological surface by laser ablation inductively coupled plasma sector field mass spectrometry (LA-ICP-MS). A cooled laser ablation chamber using a Peltier element was developed in order to analyze element distribution in thin cross-sections of frozen tissues with a lateral resolution in the μm range. In order to study the figures of merit of LA-ICP-MS with the cooled laser ablation chamber, one drop (20 μL, U concentration 200 ng mL−1), each of the certified isotope reference materials NIST U350 and NIST U930, the uranium isotopic standard CCLU-500 and also a drop of uranium with a natural isotopic pattern was deposited and analyzed on the biological surface (flower leaf). The precision and accuracy of isotope ratio measurements are significantly improved using cooled laser ablation chamber in comparison to non-cooled chamber. The precision of the measurements of isotope ratios in the range of 2.0-1.6% for 234U/238U, 1.3-0.4% for 235U/238U and 2.1-1.0% for 236U/238U in selected uranium isotopic standards reference material were determined by microlocal analysis (diameter of laser ablation crater: 15, 25 and 50 μm) using LA-ICP-MS with a cooled laser ablation chamber. The accuracy of 234U/238U, 235U/238U and 236U/238U isotope ratio measurements varied in the range of 4.2-1.1%, 2.4-0.5% and 4.8-1.1%, respectively, and were dependent on the diameter of the laser beam used.