In-Depth Assessment of the Effect of Sodium Azide on the Optical Properties of Dissolved Organic Matter

Treatment and preservation of samples are critical issues in measuring the optical properties of dissolved organic matter (DOM) due to their high sensitivity to physical and chemical changes upon sample handling. In this study, we rigorously assessed the potential interferences of sodium azide (NaN3) on DOM absorption and fluorescence. A wide range of different samples were poisoned with varying NaN3 concentrations. Several commonly used optical parameters derived from absorbance and fluorescence spectroscopy were compared at different samples and conditions to assess the interfering effect of NaN3. Our results showed that NaN3 altered the original features of absorbance and fluorescence even at the lowest level of the addition. The absorption coefficients of NaN3-treated samples increased up to 2608% at 254 nm and 66% at 280 nm relative to the untreated control. Fluorescence data revealed both a quenching effect and an enhancement in fluorescence. The effect of NaN3 on fluorescence was highly variable and affected by the NaN3 concentrations added, and the sources and the concentrations of DOM samples. None of these factors exhibited a clear linear behavior with NaN3 levels, making it difficult to develop a correction method. It can be recommended from the findings not to use NaN3 in preserving DOM samples for optical measurements.