A new fluorescent reagent, 4,5-dimethoxyphthalaldehyde (M₂OPA) is synthesized for the determination of ammonium in environmental waters. M₂OPA has a similar molecular structure as o-phthalaldehyde (OPA), a commonly used fluorescent probe for ammonium, but M₂OPA has two methoxy groups joining the benzene ring of OPA. Ammonium is reacted with M₂OPA 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 M₂OPA 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 M₂OPA 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.

合成了一种新型的荧光试剂4,5-二甲氧基邻苯二甲醛（M ₂ OPA），用于测定环境水中的铵。M ₂ OPA具有与常用的铵荧光探针邻苯二甲醛（OPA）相似的分子结构，但是M ₂ OPA具有连接OPA苯环的两个甲氧基。铵与M ₂ OPA和亚硫酸盐反应形成荧光衍生物，其最大激发（_λex）和发射（_λem）分别位于380和485 nm，而OPA反应的_λex和_λem为360和430 nm。M ₂与基于OPA的测定相比，OPA测定具有快速的反应和增强的荧光强度。为了实现野外铵的检测，开发了便携式便携式激光二极管（LD）荧光计作为M ₂ OPA分析的检测器。并行使用两个405 nm的LD作为光源，从而使检测器可以在单激光和双激光光束模式下运行。这允许不同的线性范围（分别为0-5和0-2μM）。光电倍增管用于发射光检测。分别通过使用单激光和双激光光束模式获得6.5和3.5 nM的检测极限。