Highly sensitive spectral measurement of rubidium isotopes using open multi-pass cell in tunable diode laser absorption spectroscopy

Rubidium isotope analysis and isotope ratio determination are significant in the fields of environmental monitoring and geological dating. A measurement system for rubidium isotope analysis was constructed by combining tunable diode laser absorption spectroscopy with the redox reduction sample processing method. The system reduced the Doppler effect of the spectra via a micro-channel array structure and verified the collimation performance by the rubidium D1 line (794.98 nm, minimum isotope shift 2 pm). In addition, to increase the detection sensitivity of the system, the self-designed open optical multi-pass cell was used in rubidium absorption spectroscopy. The optimum distribution of the MPC spot was determined by analyzing the absorption intensity of atoms at various spatial positions. The results show that the multi-pass approach increased the spectral signal intensity by 6.3 times and signal-to-noise ratio by 3.8 times compared to single-path direct absorption. Based on optimized experimental conditions, the relative deviation and LOD of the system were 1.16% and 0.94 parts per thousand (3 sigma), respectively. This study applied the multi-pass cell to atomic absorption spectroscopy for the first time, providing an innovative and alternative high-sensitivity detection method for the ratio analysis of rubidium and other metal isotopes. An open multi-pass cell was constructed to extend the interaction distance between the light and Rb atoms in the TDLAS measurement system. Compared with single-path direct absorption, the SNR of the system was improved by 3.8 times.