Development And Application Of The Analytical Technique And Shipboard Instrumentation For The Determination Of Ultra-trace Soluble Reactive Phosphorus In Seawater

Phosphorus is an essential nutrient element for phytoplankton in marine environment.As the major form of inorganic phosphorus,soluble reactive phosphorus (SRP) plays an important role in the marine biogeochemical cycle of phosphorus.In oligotrophic open-ocean waters,SRP concentrations are down to nanomolar levels.At present,most of the reported determination techniques for SRP have not been successfully applied in the field of marine environmental science because of issues related to sensitivity,reproducibility,chemical interference and ease of use.It is thus increasingly demanded to establish sensitive and.reliable SRP detection technique and related shipboard instrument for the use in the oligotrophic open-ocean waters.To meet these requirements,the following studies have been carried out,and progresses have been made as follows:(1) A shipboard C18 solid phase extraction(SPE) method coupled with flow injection analysis(FIA) and colorimetric detection had been developed for the determination of ultra-trace amount of SRP in seawater.In the previous work,it was found that phosphomolybdenum blue(PMB) anion could react with a cationic surfactant, cetyltrimethylammonium bromide(CTAB) to yield a PMB-CTAB ion-pair compound, which could be efficiently extracted on a Sep-Pak C18 cartridge and eluted by 0.56 mol/L sulfuric acid ethanol solution,and thus determined with a spectrophotometer at 700 nm.In the present work,this previous method was further improved for field analysis on shipboard by optimizing the mode of adding reagents,the method of minimizing Schlieren effect,the parameters for reducing time for PMB-CTAB formation and sample loading.Under the optimized conditions,using seawater with salinity of 35 as a matrix,the linearity and the detection limit of the proposed method were found to be 3.4 to 515 nmol/L and 1.57 nmol/L,respectively.For samples with SRP concentration higher or lower than 20 nmol/L,the analysis time was 10 min/sample or 30 min/sample,respectively.The proposed method was applied on field during a cruise for one month in South China Sea.More than 200 samples collected from 32 stations were analyzed on shipboard laboratory.The relative standard deviation(RSD) ranged from 4.45 to 6.75%for the field analysis.In land-base laboratory,the seawater samples were analyzed with both the proposed method and the magnesium hydroxide-induced coprecipitation(MAGIC) method,and the results showed no significant difference with t test at the confidence interval 95%.(2) By the use of an 8-position valve and a lab-made spectrophotometer,a shipboard C18 SPE method coupled with sequential injection analysis(SIA) and colorimetric detection had been developed for the determination of ultra-trace amount of SRP in seawater.Using seawater with salinity of 35 as a matrix,the linearity and the detection limit of the proposed method were found to be 3.4 to 515 nmol/L and 1.26 nmol/L,respectively.For samples with SRP concentration higher or lower than 10 nmol/L,the analysis time was 10 min/sample or 30 min/sample,respectively.The proposed method was automatic,less reagents consuming and had been applied successfully during a one-month cruise in South China Sea for shipboard analysis of more than 500 samples.The RSD ranged from 4.07 to 4.68%.(3) A novel on-line HLB(Hydrophilic-lipophilic balance) SPE method coupled with SIA and colorimetric detection had been developed for the determination of ultra-trace amount of SRP in seawater.Under acidic condition,PMB could be first extracted on HLB solid phase,and then the adsorbed PMB could be rapidly eluted by a 0.15 mol/L NaOH solution,and thus determined with a lab-made detector at 740 nm. Experimental parameters,including the reagent concentration,the reaction time and temperature,the eluent concentration,the sample loading flow rate,and the eluting flow rate,were optimized with the experiments based on univariate experimental design.Under the optimized conditions,the calibration curve showed a linear range between 3.4 and 1134 nmol/L,and the detection limit and the recovery of the proposed method were found to be 1.42 nmol/L and 94.35%,respectively.The analysis time was 6～10 min/sample.Silicate concentration at 5000 times higher than that of phosphate would not interfere with the determination of SRP.The interference of 100 nmol/L arsenate could be masked by a reduction reagent.The RSD(n=7), which was determined at different time,was 2.50%for a sample with concentration of 31 nmol/L phosphate.Seawater samples were analyzed using both the proposed method and the MAGIC method,and the results of the two methods showed no significant difference using the t test at the confidence interval 95%.The proposed method was successfully applied in the land-base laboratory to determine about 200 seawater samples obtained from South China Sea.The proposed method has the potential to be developed as an in situ method.(4) A novel reverse flow injection analysis(rFIA) method coupled with liquid waveguide capillary cell(LWCC) and colorimetric detection for the determination of ultra-trace amount of SRP in seawater was established.Experimental parameters, including the reagent concentration,the sample:injection volume,the length of mixing coils and the sample flow rate,were optimized with the experiments based on univariate experimental design.Under the optimized conditions,the calibration curve showed a linear range between 8.25 and 165 nmol/L,and the detection limit and the recovery of the proposed method were found to be 0.5 nmol/L and 87.8～101.8%, respectively.The analysis time was 4 min/sample.240μmol/L silicate and 53.3 nmol/L arsenate showed no statistically significant effect on the signals of a blank and an 82.5 nmol/L sample.The RSDs for the determination of the samples at 24.7 and 82.5 nmol/L were 1.54%and 1.86%(n=9),respectively.Using the t-test at the 95% confidence level,the results of the proposed method and a segmented flow analyzer reference method for the determination of 2 samples showed no significant difference. The proposed method has the potential to be developed as an in situ method.(5) A shipboard system for ultra-trace SRP determination was designed and assembled.By the use of optical fibers,nuts and tubing,valves,pump,the light source and the detector could be integrated as a system.The controlling software programmed by Visual Basic 6.0 had a friendly interface and was easy to use.The lab-made detector was steady,and the RSD for the continuous determination of light intensity of a blank sample within 120 min was 0.026%(n=240),the biggest relative deviation was less than 0.06%.The experimental results of the accuracy for the lab-made detector and a commercial spectrophotometer for the determination of SRP showed no significant difference.