Fig. 2. Fluorescence intensities and intensity ratios of aqueous phosphate-buffered standards upon excitation of SNARF-1 (A) at 488 nm (dual-emission detection at 590-610 nm and >630 nm) and of SNAFL-2 (B) at 488 nm and 543 nm (emission detection at >570 nm). The extensive long-term laser instability is compensated for by ratiometric analysis (±2%) with the dual emission dye SNARF-1 (A), but is additively transmitted into the intensity ratio of the dual excitation dye SNAFL-2 (B). Referencing SNAFL-2 signals to quasi-simultaneous measurements of a fluorescent uranium glass standard is suitable to reduce fluctuations to ±6%. Abbreviations: exc., excitation wavelength; em, emission wavelength.