Kunning Lin

Automated determination of nitrate plus nitrite in aqueous samples with flow injection analysis using vanadium (III) chloride as reductant

Determination of nitrate in aqueous samples is an important analytical objective for environmental monitoring and assessment. Here we report the first automatic flow injection analysis (FIA) of nitrate (plus nitrite) using VCl3 as reductant instead of the well-known but toxic cadmium column for reducing nitrate to nitrite. The reduced nitrate plus the nitrite originally present in the sample react with the Griess reagent (sulfanilamide and N-1-naphthylethylenediamine dihydrochloride) under acidic condition. The resulting pink azo dye can be detected at 540 nm. The Griess reagent and VCl3 are used as a single mixed reagent solution to simplify the system. The various parameters of the FIA procedure including reagent composition, temperature, volume of the injection loop, and flow rate were carefully investigated and optimized via univariate experimental design. Under the optimized conditions, the linear range and detection limit of this method are 0-100 µM (R(2)=0.9995) and 0.1 µM, respectively. The targeted analytical range can be easily extended to higher concentrations by selecting alternative detection wavelengths or increasing flow rate. The FIA system provides a sample throughput of 20 h(-1), which is much higher than that of previously reported manual methods based on the same chemistry. National reference solutions and different kinds of aqueous samples were analyzed with our method as well as the cadmium column reduction method. The results from our method agree well with both the certified value and the results from the cadmium column reduction method (no significant difference with P=0.95). The spiked recovery varies from 89% to 108% for samples with different matrices, showing insignificant matrix interference in this method.

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).

Determination of Nitrite, Phosphate, and Silicate by Valveless Continuous Analysis with a Bubble-Free Flow Cell and Spectrophotometric Detection

ABSTRACT A continuous flow analysis system, composed of a 1.2-cm laboratory-made antibubble flow cell and a spectrophotometer, was established. The system was evaluated for the determination of nitrite, phosphate, and silicate. Different from flow injection analysis and other flow analysis modes, an injection or multiposition valve was not needed. Even better, the system was free from interferences from air bubbles without the use of a debubbler device or electronic bubble gate. Without the formation of air bubbles, the chemical reaction was accelerated using a water bath. The experimental parameters for nutrient analysis, including reagent concentration, flow strategy, flow rate, and reaction temperature, were optimized based on a univariate experimental design. The carry-over effect was comprehensively evaluated and may be ignored using this protocol. The established system and analytical methods were especially suitable for laboratories with only basic instruments and limited budgets. The system had the advantages of high sample throughput (>60 h−1); great convenience without valve utilization; long linear dynamic ranges (0.2–80 µM for nitrite, 0.3–14 µM for phosphate, and 0.5–120 µM for silicate); low detection limits (0.06 µM for nitrite, 0.08 µM for phosphate, and 0.11 µM for silicate); and high recovery values (91.5 ± 1.01 to 108.7 ± 3.18%). In addition to water samples, national reference materials were analyzed, and the results were in good agreement with the certified values.

Optimization of a salinity-interference-free indophenol method for the determination of ammonium in natural waters using o-phenylphenol

The indophenol blue (IPB) method based on Berthelots reaction is one of the most widely used methods for the determination of ammonium in natural waters. This study comprehensively optimized the kinetics of the IPB reaction under different reagent concentrations, temperature and salinity. The normally used toxic and odorous phenol was replaced by the less toxic, stable flaky crystalline compound, o-phenylphenol. With the application of nitroprusside as the catalyst, the reaction can be finished within 20min at room temperature and the formed color compound is stable for 24h. Under the optimized conditions, the method shows high reproducibility (relative standard deviations of 0.64-1.71%, n = 11), highly linear calibration up to 100μM (R2 = 0.9995, n = 165, 17 days) and a low detection limit of 0.2μM. This method was successfully applied to measure ammonium in estuarine and coastal surface water (n = 63). The results showed insignificant differences with the results obtained using both the standard AutoAnalyzer method and a fluorometric o-phthaldialdehyde method at the 95% confidence level. Compared with previous studies, this method shows the advantages of relatively fast reaction, low toxicity and easy reagent preparation. It is salinity-interference-free and robust (no temperature control is required, reagents can be stored up to 10 days), and suitable for routine analysis under harsh field conditions.

Determination of Nitrate in Seawater with Valve-Free Continuous Flow Analysis

Abstract A valve-free continuous flow method and its corresponding instrument were established, with only two multi-channel pump for delivering the sample and reagent, without any injection or solenoid valves and sample loop for selecting and adding the sample or reagent. Nitrate was reduced to nitrite with Cu-Cd reductant column, and then detected with spectrophotometric detector. The proposed method was suitable for determination of nitrate at normal level in most of estuary and coastal seawaters. With the optimum parameters, the linear range and detection limit were 5–180 μM and 0.27 μM, respectively. The nitrate samples (10 and 80 μM) were continually measured for 11 times, and the relative standard deviations were 1.4% and 1.3%, respectively. The recovery of real samples at different salinity was ranged from 99.4% to 106.1%. There were no significant differences in the analytical results between the proposed method and a reference method, i.e., flow injection analysis. In comparison with flow injection analysis, the method and instrument were less cost and easy to operate, and was suitable to be applied in general laboratories and field for continuous monitoring. The method was successfully applied to measure nitrate of seawater samples in Xiamen western harbor and underway monitor nitrate in the Jiulongjiang estuary.

Simultaneous determination of total dissolved nitrogen and total dissolved phosphorus in natural waters with an on-line UV and thermal digestion

A flow injection method combined with an on-line UV and thermal digestion for simultaneous determination of total dissolved nitrogen (TDN) and total dissolved phosphorus (TDP) in natural waters was established in this study. A novel flow manifold made the proposed system compact and automatic. The conversion rates of various nitrogen and phosphorus compounds to their nitrate and phosphate forms with different digestion models and different concentrations were well investigated using the flow injection technique. The reagent concentrations for colorimetric analysis were optimized based on a univariate experimental design. The detection limits were 0.8 μmol L-1 and 0.2 μmol L-1, and linear analytical ranges were up to 300 μmol L-1 and 25 μmol L-1 for TDN and TDP, respectively. The sample throughput was ~ 5 h-1. The recovery of spiked natural water samples varied from 86.8% to 102.6% for TDN and 88.0% to 102.0% for TDP. The present approach was successfully applied for the determination of TDN and TDP in natural water samples and was found to have good agreement with reference methods. The outcomes of present study indicated that the proposed method is suitable for routine analysis as well as for potential on-line monitoring.

Development of an Integrated Syringe-Pump-Based Environmental-Water Analyzer (iSEA) and Application of It for Fully Automated Real-Time Determination of Ammonium in Fresh Water

The development of a multipurpose integrated syringe-pump-based environmental-water analyzer ( iSEA) and its application for spectrophotometric determination of ammonium is presented. The iSEA consists of a mini-syringe pump equipped with a selection valve and laboratory-programmed software written by LabVIEW. The chemistry is based on a modified indophenol method using o-phenylphenol. The effect of reagent concentrations and sample temperatures was evaluated. This fully automated analyzer had a detection limit of 0.12 μM with sample throughput of 12 h-1. Relative standard deviations at different concentrations (0-20 μM) were 0.23-3.36% ( n = 3-11) and 1.0% ( n = 144, in 24 h of continuous measurement, ∼5 μM). Calibration curves were linear ( R2 = 0.9998) over the range of 0-20 and 0-70 μM for the detection at 700 and 600 nm, respectively. The iSEA was applied in continuous real-time monitoring of ammonium variations in a river for 24 h and 14 days. A total of 1802 samples were measured, and only 0.4% was outlier data (≥3 sigma residuals). Measurements of reference materials and different aqueous samples ( n = 26) showed no significant difference between results obtained by reference and present methods. The system is compact (18 cm × 22 cm × 24 cm), portable (4.8 kg), and robust (high-resolution real-time monitoring in harsh environments) and consumes a small amount of chemicals (20-30 μL/run) and sample/standards (2.9 mL/run).