Development of flame frequency stabilized-continuous wave laser excited atomic fluorescence spectrometric (flame-cw-LEAFS) technique and its applications for ultra-trace level determination of rubidium in spodumene

A frequency-stabilized continuous wave laser-excited atomic fluorescence spectrometry (flame-cw-LEAFS) method has been developed in an air-acetylene flame for ultra-trace determination of rubidium (Rb) in spodumene and related matrices. In contrast to conventional wavelength-scan LEAFS, which suffers from frequency drifts, laser power fluctuations, high sample consumption, and long acquisition times, the present approach stabilizes a cw-ring Ti:sapphire (Ti:Sa) laser to the Doppler-broadened hyperfine component of the ⁸⁵Rb D2 transition. Stabilization was achieved through frequency modulation of the intracavity Brewster plate and phase-sensitive detection of Rb fluorescence in a reference cell. Approximately 100 mW power of the stabilized laser output was directed into a modified commercial air-acetylene flame atomizer, and resonance fluorescence was collected orthogonally with a photomultiplier tube and narrow-band interference filter. Optimization of laser power, beam geometry, and collection optics suppressed scattering and maximized the signal-to-noise ratio. Under optimized conditions, the method achieved a limit of detection of ∼0.8 ng mL^-1, a linear dynamic range of 5-250 ng mL^-1, and precision of 1 % RSD at 50 ng mL^-1. Long-term stability studies showed wavelength fluctuations of only ∼3 fm over 1200 s of closed-loop operation. Accuracy of the developed analytical technique was confirmed by analysis of NIST SRMs and cross-validation with graphite furnace AAS (GFAAS), while spike recoveries from aqueous and spodumene digests exceeded 90 %. These results demonstrate that flame frequency-stabilized-cw-LEAFS provides a sensitive, precise, and practical alternative to conventional AAS and GFAAS, while avoiding the complexity and cost of ICP-MS, making it a promising tool for trace alkali metal analysis in complex matrices.