Yongming Huang

In-field determination of nanomolar nitrite in seawater using a sequential injection technique combined with solid phase enrichment and colorimetric detection.

A novel sequential injection method for the determination of nitrite at nanomolar level in seawater samples has been developed. The pink azo compound was formed based on the Griess reaction and quantitatively adsorbed onto a Sep-Pak C18 cartridge. The enriched azo compound was rinsed with water and ethanol (28%, v/v) in turn, and then eluted with an eluent containing 26.6% (v/v) ethanol and 0.108 mol L(-1) H(2)SO(4). Finally the azo compound was measured using a spectrophotometer at 543 nm. Under the optimized conditions, the linear calibration ranges were 0.71-42.9 nmol L(-1) for a 150-mL sample and 35.7-429 nmol L(-1) for a 15-mL sample. The relative standard deviation of 8 measurements was 1.44% for 14.3 nmol L(-1) nitrite. For the 150 mL sample, the detection limit was estimated to be 0.1 nmol L(-1). The throughput of the method was about 4 samples per hour. The proposed method has been successfully applied to the in-field determination of nanomolar concentrations of nitrite in seawater.

A sensitive flow-batch system for on board determination of ultra-trace ammonium in seawater: Method development and shipboard application

Combining fluorescence detection with flow analysis and solid phase extraction (SPE), a highly sensitive and automatic flow system for measurement of ultra-trace ammonium in open ocean water was established. Determination was based on fluorescence detection of a typical product of o-phthaldialdehyde and ammonium. In this study, the fluorescence reaction product could be efficiently extracted onto an SPE cartridge (HLB, hydrophilic-lipophilic balance). The extracted fluorescence compounds were rapidly eluted with ethanol and directed into a flow cell for fluorescence detection. Compared with the common used fluorescence method, the proposed one offered the benefits of improved sensitivity, reduced reagent consumption, negligible salinity effect and lower cost. Experimental parameters were optimized using a univariate experimental design. Calibration curves, ranging from 1.67 to 300nM, were obtained with different reaction times. The recoveries were between 89.5 and 96.5%, and the detection limits in land-based and shipboard laboratories were 0.7 and 1.2nM, respectively. The relative standard deviation was 3.5% (n=5) for an aged seawater sample spiked with 20nM ammonium. Compared with the analytical results obtained using the indophenol blue method coupled to a long-path liquid waveguide capillary cell, the proposed method showed good agreement. The method had been applied on board during a South China Sea cruise in August 2012. A vertical profile of ammonium in the South East Asia Time-Series (SEATS, 18° N, 116° E) station was produced. The distribution of ammonium in the surface seawater of the Qiongdong upwelling in South China Sea is also presented.

Simultaneous determination of nanomolar nitrite and nitrate in seawater using reverse flow injection analysis coupled with a long path length liquid waveguide capillary cell

A reverse flow injection analysis (rFIA) method coupled with 1m liquid waveguide capillary cell and spectrophotometric detection for simultaneous determination of nanomolar nitrite and nitrate in seawater was developed. The design of two analytical channels sharing the same detection system in the proposed method allowed the analysis of both nitrite and nitrate with single sample injection. Different strategies of reagent injection were investigated to obtain a higher sensitivity and a better peak shape. A dual-wavelength detection mode was chosen to eliminate the light source shifting and sample matrix interference. Experimental parameters were optimized based on a univariate experimental design and the matrix effect from seawater was preliminarily investigated. The proposed method had high sensitivity with detection limit of 0.6 nmol L(-1) for both nitrite and nitrate. The linearity was 2-500 nmol L(-1) for both analytes, and the upper limit could be extended by choosing a lower sensitivity detection wavelength. The analytical results of 26 surface seawater samples obtained with the proposed method showed good agreement with those using a reference method operated using an automated segmented flow analyzer. The proposed method could greatly minimize the trouble introduced by bubbles in the segmented flow analyzer. It also had the advantages of high precision and high sample throughput (nitrite and nitrate detected in triplicate; 5 h(-1)). Compared to normal flow injection analysis, the rFIA method is superior due to its lower reagent consumption, less dispersion of sample, as well as higher sensitivity.

On-Line Solid Phase Extraction and Spectrophotometric Detection with Flow Technique for the Determination of Nanomolar Level Ammonium in Seawater Samples

The determination of low-level ammonium in seawaters suffers from low sensitivity and high contamination; therefore, it is desirable to develop highly sensitive methods for automatic measurements. A highly sensitive and automated flow technique system for nanomolar level ammonium measurement is described. Reagents for Berthelot reaction were automatically added into seawater samples. After a 10 min reaction at 40°C, the formed indophenol blue compound was on-line extracted onto an Hydrophilic-Lipophilic Balance (HLB) cartridge. The enriched compound was rinsed with water and ethanol solution (30%, v/v) and, in turn, eluted with an eluent containing 30% (v/v) ethanol and 5.0 mM of NaOH, and determined with a spectrophotometer at 640 nm. Parameter, including extraction conditions, reagent concentrations, pH, temperature, and reaction time, were optimized. Under the optimized conditions, the detection limit was 3.5 nM and the linear range was 0–428 nM. The relative standard deviations were 5.7% (n = 8) for 44.6 nM standard solution and less than 6.0% (n = 3–5) for samples within concentrations of about 52.4–288.7 nM; the recovery was in the range 93.6 to 108.5%. The sample throughput was 3 h−1. The proposed method provides a simple, cheap, and automatic way to determine ammonium in seawater samples without complicated sample treatment.

Reverse flow injection analysis method for catalytic spectrophotometric determination of iron in estuarine and coastal waters: A comparison with normal flow injection analysis

A method for determining iron in seawater had been developed by coupling reverse flow injection analysis (rFIA) and catalytic spectrophotometric detection with N,N-dimethyl-p-phenylenediamine dihydrochloride (DPD). With a seawater sample or a standard solution as the carrier, the mixture of DPD and buffer was injected into the carrier stream quantitatively and discretely. After mixing with H(2)O(2), the DPD was oxidized to form two pink semiquinone derivatives that were monitored at 514 nm wavelength with a reference at 700 nm. The method detection limit was 0.40 nmol L(-1), lower than half of that of normal flow injection analysis (nFIA) method. The sample throughput was 10h(-1) with triplicate determination, compared with 4h(-1) for nFIA-DPD method. The analysis results of the certified seawaters CASS-4 (12.33 ± 0.18 nmol L(-1)) and NASS-5 (3.47 ± 0.23 nmol L(-1)) well agreed with the certified values (12.77 ± 1.04 and 3.71 ± 0.63 nmol L(-1), respectively). The typical precision of the method for a 2.97 nmol L(-1) iron sample was 4.49% (n=8). Interferences from copper and salinity were investigated. An instrument was assembled based on the proposed method and applied successfully to analyze total dissolvable iron (TDFe) in surface seawater samples collected from the Pearl River Estuary, the results of which revealed non-conservative behavior of TDFe during the estuarine mixing. Results for these samples with both rFIA-DPD and nFIA-DPD methods showed good agreement with each other. The proposed method was superior to the currently used nFIA-DPD method, particularly when it is adapted for field and in situ deployment, due to its lower reagent consumption, higher sample throughput and keeping the manifold tubing clean.

Redox speciation analysis of dissolved iron in estuarine and coastal waters with on-line solid phase extraction and graphite furnace atomic absorption spectrometry detection.

An automatic on-line solid phase extraction (SPE) system employing the flow injection (FI) technique directly coupled to a graphite furnace atomic absorption spectrometer (GFAAS) was established for speciation and determination of dissolved iron in estuarine and coastal waters. Fe(II) was mixed with ferrozine solution in a sample stream to form the Fe(II)-ferrozine complex which was extracted onto a C18 SPE cartridge, eluted with eluent and detected with GFAAS. In a parallel flow channel, Fe(III) was reduced to Fe(II) with ascorbic acid and then detected in the same way as Fe(II). The home-made interface between FI-SPE and GFAAS efficiently realized the sample introduction to the furnace in a semi-automated way. Parameters of the FI-SPE system and graphite furnace program were optimized based on a univariate experimental design and an orthogonal array design. The salinity effect on the method sensitivity was investigated. The proposed method provided a detection limit of 1.38 nmol L(-1) for Fe(II) and 1.87 nmol L(-1) for Fe(II+III). With variation of the sample loading volume, a broadened determination range of 2.5-200 nmol L(-1) iron could be obtained. The proposed method was successfully applied to analyze iron species in samples collected from the Jiulongjiang Estuary, Fujian, China. With the 2-cartridge FI-SPE system developed, on-line simultaneous determination of Fe species with GFAAS was achieved for the first time.

Real-Time Redox Speciation of Iron in Estuarine and Coastal Surface Waters

An automated, shipboard-use system was developed for real-time speciation of iron in coastal surface waters. It comprised a towed Fish underway sampler and a modified reverse flow injection analysis system with a liquid waveguide capillary flow cell-spectrophotometric detection device. The detection was based on the reaction between ferrozine and Fe(II). The detection limits of 0.3 and 0.7 nM were achieved for Fe(II) and Fe(II+III), together with their respective dynamic linear ranges of 0.5-250 and 0.9-250 nM. The system was successfully deployed and run consecutively for about 1 week during a cruise in August 2009 to the East China Sea off the Changjiang Estuary. The distribution of operationally defined field dissolvable Fe(II) and Fe(II+III) (expressed as Fea(II) and Fea(II+III)) in these areas was obtained, which showed that both Fea(II) and Fea(II+III) concentrations decreased with salinity when there were relatively high Fea(II) concentrations (up to about 120 nM) near shore. A distinct distribution of Fea(II) to Fea(II+III) ratios was also revealed, with a ratio of 0.58 in the water off Changjiang Estuary and 0.19 in the open ocean.

Rapid Analysis of Heavy Metals in Coastal Seawater Using Preconcentration with Precipitation/Co-precipitation on Membrane and Detection with X-Ray Fluorescence

State Key Laboratory of Marine Environmental Science at Xiamen University through MEL [MELRI1001]

Solid phase extraction coupled with a liquid waveguide capillary cell for simultaneous redox speciation analysis of dissolved iron in estuarine and coastal waters

A portable automatic flow injection (FI) based system incorporating on-line C18 solid phase extraction (SPE) cartridges and a 2 m long liquid waveguide capillary cell (LWCC) is established for simultaneous redox speciation analysis of dissolved iron in estuarine and coastal waters. Utilization of the SPE preconcentration and the LWCC enhanced the sensitivity of the ferrozine method for Fe(II) analysis. The Fe(II)–ferrozine complex was formed and extracted onto a C18 cartridge, and eluted with an HCl–ethanol solution for spectrophotometric detection with an LWCC. The determination of total Fe(II + III) was realized in a parallel flow channel after the reduction of Fe(III) to Fe(II) with ascorbic acid. The optimal combination of the pre-eluting solution and eluent was investigated to eliminate the Schlieren effect. The parameters of the FI-SPE-LWCC system were optimized based on a univariate experimental design. The effect of salinity on the method sensitivity was low enough to apply the system in both estuarine and coastal waters without adjustment. The limit of detection was 0.056 nmol L−1 for Fe(II) and 0.096 nmol L−1 for Fe(II + III). A linear determination range of 0.5–50 nmol L−1 iron was obtained with a sample loading volume of 5 mL and a sample throughput of 6 h−1. The system was successfully applied in situ in Wuyuan Bay, Xiamen, for the continuous monitoring of dissolved iron species for 20 h.

Sequential determination of multi-nutrient elements in natural water samples with a reverse flow injection system

An integrated system was developed for automatic and sequential determination of NO2-, NO3-, PO43-, Fe2+, Fe3+ and Mn2+ in natural waters based on reverse flow injection analysis combined with spectrophotometric detection. The system operation was controlled by a single chip microcomputer and laboratory-programmed software written in LabVIEW. The experimental parameters for each nutrient element analysis were optimized based on a univariate experimental design, and interferences from common ions were evaluated. The upper limits of the linear range (along with detection limit, µmolL-1) of the proposed method was 20 (0.03), 200 (0.7), 12 (0.3), 5 (0.03), 5 (0.03), 9 (0.2) µmolL-1, for NO2-, NO3-, PO43-, Fe2+, Fe3+ and Mn2+, respectively. The relative standard deviations were below 5% (n=9-13) and the recoveries varied from 88.0±1.0% to 104.5±1.0% for spiked water samples. The sample throughput was about 20h-1. This system has been successfully applied for the determination of multi-nutrient elements in different kinds of water samples and showed good agreement with reference methods (slope 1.0260±0.0043, R2=0.9991, n=50).