Toward sensitive determination of ammonium in field: A novel fluorescent probe, 4,5-dimethoxyphthalaldehyde along with a hand-held portable laser diode fluorometer

A new fluorescent reagent, 4,5-dimethoxyphthalaldehyde (M 2 OPA) is synthesized for the determination of ammonium in environmental waters. M 2 OPA has a similar molecular structure as o-phthalaldehyde (OPA), a commonly used fluorescent probe for ammonium, but M 2 OPA has two methoxy groups joining the benzene ring of OPA. Ammonium is reacted with M 2 OPA and sulfite to form a fluorescent derivative, which has a maximum of excitation (λ ex ) and emission (λ em ) located at 380 and 485 nm, while the λ ex and λ em of OPA reaction are 360 and 430 nm. The M 2 OPA assay has a rapid reaction and enhanced fluorescent intensity, compared with the OPA-based assay. To realize the ammonium detection in field, a hand-held portable laser diode (LD) fluorometer is developed as the detector for the M 2 OPA assay. Two LDs of 405 nm in parallel are used as the light source, allowing the detector to operate in single- and dual-laser beam modes. This allows for different linear ranges (0–5 and 0–2 μM, respectively). A photomultiplier tube is used for emission light detection. Limits of detection of 6.5 and 3.5 nM are obtained by using single- and dual-laser beam modes, respectively.