A sensitive and automated method for underway and onboard analysis of trace ammonium in seawater was established using on-line solid phase extraction and fluorescence detection. Ammonium reacted with o-phthalaldehyde and sulfite to produce isoindole derivative that was concentrated on a hydrophilic lipophilic balanced (HLB) cartridge and then rapidly eluted with ethanol and measured using a fluorescence detector. The work was modified from our previous developed method, where 30 mL of a seawater sample was introduced into a reaction bottle with a 10 mL pipette. In the modified method a 10 mL sample loop was adopted for sample introduction to reduce possible contamination caused by manual operation. More importantly, the present method makes underway monitoring much more automatic. The detection limit was 3.5 nmol L⁻¹, and the relative standard deviation was 3.52% (n = 12) for a sample spiked with 40 nmol L⁻¹ ammonium standard. The method was free of the effects of salinity, carry-over and dissolved organic matter, with recoveries between 94.0% and 106.1%. An equation was introduced to correct matrix effects, and pure water and seawater blank were found to be equivalent to ammonium concentration of 8.1 ± 2.2 nmol L⁻¹ and 6.0 ± 0.8 nmol L⁻¹, respectively. The correction method can also be introduced to determination of other trace analytes where matrix effect cannot be neglected. A technique of stopped-flow was adopted and stopped-flow time could be adjusted depending on the concentration of ammonium in the seawater samples. The sample throughput was 7–9 h⁻¹, and the method permitted the analysis of samples over a wide range of concentrations. The modified method has been successfully applied for shipboard underway determination of ammonium in a coastal area and shipboard determination the ammonium concentration of discrete samples in three vertical profiles during a cruise in the South China Sea.

使用在线固相萃取和荧光检测，建立了一种进行海水中痕量铵的自动，灵敏的分析方法。铵与邻反应-邻苯二甲醛和亚硫酸盐制得异吲哚衍生物，将其浓缩在亲水亲脂平衡（HLB）柱上，然后用乙醇快速洗脱，并使用荧光检测器进行测量。这项工作是根据我们先前开发的方法进行修改的，该方法是用10 mL移液管将30 mL海水样品引入反应瓶中。在改进的方法中，采用了10 mL样品定量环进行样品引入，以减少手动操作可能造成的污染。更重要的是，本方法使进行中的监视更加自动化。检测限为3.5 nmol L ^-1，加标有40 nmol L ^-1的样品的相对标准偏差为3.52％（n = 12）铵标准。该方法不受盐度，残留和有机物溶解的影响，回收率在94.0％至106.1％之间。引入了一个方程式来校正基质效应，发现纯水和海水空白分别相当于铵浓度为8.1±2.2 nmol L ^-1和6.0±0.8 nmol L ^-1。校正方法还可以用于确定其他痕量分析物，其中基质效应不可忽略。采用了停流技术，可以根据海水样品中铵的浓度调节停流时间。样品通量为7–9 h ^-1，该方法可以分析多种浓度的样品。改进后的方法已成功应用于沿海地区船上正在进行的铵的测定，以及在南海航行中船上测定三个垂直剖面中离散样品的铵浓度。