Sequential injection analysis of nanomolar soluble reactive phosphorus in seawater with HLB solid phase extraction

A cartridge of solid phase extraction (SPE), hydrophilic–lipophilic balance (HLB), has been used to enrich phosphomolybdenum blue (PMB) from water samples without any other additives. Based on this, the previous on-line SPE method established for the determination of nanomolar soluble reactive phosphorus (SRP) in seawater has been greatly improved. Cetyltrimethylammonium bromide (CTAB), the cationic surfactant needed for the formation of the PMB-CTAB paired compound that could be extracted on a Sep-Pak C18 cartridge using the previous method [Liang, Y., Yuan, D.X., Li, Q.L., Lin, Q.M., 2007. Flow injection analysis of nanomolar level orthophosphate in seawater with solid phase enrichment and colorimetric detection. Marine Chemistry 103, 122–130.], was not necessary. Thus the longer time and higher temperature required for the complete formation of the PMB-CTAB compound were no longer needed. In addition, with application of the sequential injection analysis technique the proposed method showed the advantages of being much faster, simpler, sample and reagent saving, as well as more convenient in operation. The PMB compound formed under room temperature was efficiently extracted on an in-line HLB cartridge, rapidly eluted by 0.15 mol/L NaOH solution, and finally determined with a laboratory-made spectrophotometer at 740 nm. Experimental parameters, including the volume of reagents added, sample loading flow rate, and eluting flow rate, were optimized. Time and temperature for the PMB reaction, and salinity effect were also studied, and these were found to have no severe effect on the detection. With variation of sample loading time at a fixed flow rate, a broadened determination range of 3.4 to 1134 nmol/L phosphate could be obtained. The recovery and the method detection limit of the proposed method were found to be 94.4% and 1.4 nmol/L, respectively. The relative standard deviation (n = 7) was 2.50% for the sample at a concentration of 31 nmol/L phosphate. Two typical seawater samples were analyzed with both the proposed method and the magnesium hydroxide-induced coprecipitation method and, using the t-test, the results of the two methods showed no significant difference.