| Ammonium,as one of the most important nutrients,occupies a central role in nitrogen biogeochemical cycles.It is also an important water quality indicator because of its toxicity in aquatic ecosystems.Accurate determination of ammonium is of great interest for both environmental scientists and oceanographers.Among multiple methods for ammonium measurement,the indophenol blue(IPB)method based on Berthelot's reaction is one of the most widely used methods.Routine analysis based on IPB method and flow techniqeus is the most popular technique for the automated ammonium measurement.However,the current IPB method has the shortages of high toxicity,interference from salinity,high reagent consumption and difficulties in field application.Herein,a new analytical method and a relative compact,robust and portable instrument for ammonium determination have been developed.The main contents and results are as follows:(1)A salinity-interference-free indophenol method for the determination of ammonium in natural waters using o-phenylphenol(OPP)was developed.The normally used toxic and odorous phenol was replaced by the less toxic,stable flaky crystalline compound,OPP.With nitroprusside as the catalyst,the reaction could be finished within 20 min at room temperature and the formed color compound was stable for 24 h.Under the optimized conditions,the method showed no interference from salinity,high reproducibility with relative standard deviations(RSD)of 0.64-1.7%(n=11),highly linear calibration up to 100?M and a low detection limit(LOD)of 0.21 ?M.This method was successfully applied to measure ammonium in samples from Minjiang river,Jiulongjiang Estuary and Xiamen coastal area.(2)An automatic ammonium analyzer in fresh water was presented.The analyzer consisted a mini-syringe pump equipped with a selection valve.The principle of chemistry was based on the IPB method with OPP shown in(1).The instrument has dimension of 18 cm×22 cm×24 cm(length×width×height)and weighs 4.8 kg.This fully automated analyzer had a LOD of 0.12?M with sample throughput of 12 h-1 and consumes 2.9 mL sample/run and 20-30 ?L chemicals/run.RSDs at different concentrations(0-20 ?M)were 0.23-0.52%(n=11)and 1.0%(n=144,in 24-h continuous measurement,~5 ?M).Calibration curves were linear(R2=0.9998)over the range of 0-20 ?M and 0-70 pM for detection at 700 nm and 600 nm,respectively.The analyzer was applied in continuous real-time monitoring of ammonium variations in a river for 24 h and 14 days with excellent long-term stability and accuracy.(3)An automatic ammonium analyzer was developed for underway analysis in estuary and coastal waters.The chemistry and analytical flow technique were based on(1)and(2),respectively.The LOD was 0.15 ?M with sample throughput of 12 h-1.When measuring at insensitive wavelength(580 nm)and automated dilution mode,the upper limit of the linear analytical range can be extended to 200 ?M.The RSDs at different concentrations(2,10,and 20?M)were 2.2%,0.33%,0.32%(n=11).For 24-h continuous repeated analysis(n=288),the RSD was 0.85%.The effects of sample temperature,sample salinity and organic nitrogen compounds were evaluated and showed no interference using the proposed protocol for ammonium analysis.Between results obtained by reference and the present methods,there were no signifcant differences in the measurements of reference materials and different aqueous samples(P>0.05,n=51).After integration with Ferrybox and an on-line filtration system,the analyzer was successfully applied in underway ammonium analysis in the Jiulongjiang Estuary,Xiamen Bay and Dongshan Bay,demonstrating the capability of the analyzer in automated real-time mapping of ammonium distribution in a shipboard laboratory. |
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