Measurement of 7Li/6Li isotope ratios by resonant Doppler-free two-photon diode laser atomic absorption spectroscopy in a low-pressure graphite furnace

Resonant two-photon absorption spectroscopy with laser diodes is applied for isotope selective analysis of 6Li and 7Li in a low-pressure graphite furnace atomizer. The detection limits for both isotopes and the selectivity in form of the relative absorption sensitivity for 6Li are determined under large optical saturation conditions with co-propagating laser beams as well as for small optical saturation with counter-propagating beams. The influence of collisional energy transfer and radiative processes are studied and taken into account. Absolute detection limits lower than 1 pg are found for both isotopes. The selectivity for 6Li is approximately 4.8×103 in the high saturation and approximately 4.6×104 in the low saturation case. The isotope ratio of a highly 7Li enriched stock solution was measured and found to agree with previous results obtained by wavelength modulation diode laser atomic absorption spectrometry of the Li resonance lines.