| Phosphorus is an essential nutrient for living organisms in marine environment,and plays a crucial role in driving biogeochemical cycles.The concentration of dissolved reactive phosphorus(DRP)has significantly spatial and temporal variability.Due to the uptake of DRP by phytoplankton,its concentration can be as low as the nanomolar level in some oligotrophic areas,which is lower than the detection limit of conventional methods,causing the obvious sparsity of in-field data.Therefore,automated methods for measuring DRP are in high demand for the purpose of better understanding the biogeochemistry of phosphorus in the open ocean.Although methods for determination of nanomolar level DRP have been increasingly developed and promoted in recent decades,there are still some issues related to preconcentration volumes change easily,large consumption of reagents as well as insufficient automation.To overcome those shortages of the current continuous flow analyzers and other methods,we developed two sensitive,accurate,and low reagent consumption methods for determining nanomolar level DRP in seawater based on flow batch analysis using an integrated syringe pump-based environmental water analyzer(iSEA).The main contents and results are as follow:(1)On the basis of the researches of others,when iSEA system combined with a solid phase extraction(SPE)cartridge,an automated portable and robust analyzer was described for measuring nanomolar level DRP in seawater.PMB can be loaded into a hydrophilic-lipophilic balance SPE cartridge under acidic conditions,then eluted using 0.2 mol/L NaOH solution,and measured in the spectrophotometric system at 700 nm.The limit of detection(LOD)was 3.2 nmol/L with preconcentration of 5 mL sample,and the analysis time was 3 min per sample.The relative standard deviations(RSDs)at different concentrations(200,500,800 nmol/L phosphate)were 3.8%,2.4%and 3.8%(n=11),respectively.When preconcentration of 25 mL sample,the LOD was 1.7 nmol/L with analysis time of 8 min per sample.The RSDs for 20,50 and 80 nmol/L phosphate were 4.9%,3.0%and 5.7%(n=11),respectively.The recoveries for detection varied from 92.8%to 105%.The proposed method shows multiple advantages,including stability of preconcentration volume,high accuracy and reproducibility;no interference from salinity,silicate,arsenate,and P-containing compounds.Three types of phosphate reference standards were measured to evaluate the accuracy of this method.The determinations show no significant difference between the results obtained with this method and reference methods.The proposed method was successfully applied to measure DRP concentration in samples from Taiwan Strait,the South China Sea and the Pacific Ocean.(2)An automated method using liquid waveguide capillary cell(LWCC)combined with iSEA system for the determination of nanomolar level DRP in seawater was developed.The sensitivity could be easily improved by the 2.5 m LWCC.Under the optimized conditions,the LOD was 0.6 nmol/L with the analysis time of 5 min per sample.The consumption of sample and reagents were largely decreased,only 7 mL sample(including 2 mL for detection and 5 mL for cleaning)and 0.04 mL regents for each measurement.The effect of arsenate was evaluated in detail.The method shows high reproducibility with RSDs of 0.5%-1.7%(n=11).The recoveries varied from 94.3%to 106%for spiked sea water samples.Comparison of the analytical results with certified concentrations of standard solutions and reference methods shows no significant difference.This method demonstrates the high sensitivity and reproducibility,low reagent consumption(0.04 mL per sample)and was successfully applied for determining the DRP concentration in samples(n=235)collected from the South China Sea. |
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