Overview

Common ions can interfere with the nitrate detection. The interference for 10 of the common ions present in groundwater (NO₂^-, Cl-, PO₄^3-, SO₄^2-, F^-, CO₃^-, BO₂^-, K⁺, Ca²⁺, and Sr²⁺) was quantified in the presence and absence of nitrate. A modified sensor design is proposed for more reliable operation in the field: flow-injection-analysis system with commercial working, reference and counter electrodes and computer-contolled fluidic handlings.

Accomplishments

Two main accomplishments made during this research period are:

• Characterizaion of ionic interference to the nitrate sensor
• Modification of nitrate sensing system design for more reliable operation

Interference from ions in groundwater

Interference from ions in commonly present in groundwater was determined with the same electrochemical techniques. 1 M standards of each of seven anions (NO₂^-, Cl^-, PO₄^3-, SO₄^2-, F^-, B (as BO2^-), and CO₃^2-), and three cations (K⁺, Ca²⁺, and Sr²⁺) were prepared. Two kinds of interference were studied: (1) how much the electrochemical system mistakenly could sense nitrate from the interfering ions when nitrate is absent and (2) how much the nitrate measurement could be affected by the interfering ions when nitrate is present.

For the first study, a calibration curve was determined using nitrate standards before each measurement of interfering ions. Then, 1000 μM solutions of each of the interfering ions in fresh electrolyte were measured. An analytic signal was determined and related to an equivalent nitrate concentration using the nitrate calibration curves. The interference is, in general, minimal (i.e., less than 1% of response relative to 1000 μM nitrate). NO₂^- is a common problem in nitrate analysis but the interference is equivalent to 3.9 μM nitrate in this study. The largest interference arises from PO₄^3- (equivalent to 6.9 μM nitrate) but still a small value. The signal from SO₄^2-, F^-, CO₃^2-, BO₂^-, K⁺, and Sr²⁺ are smaller than the average nitrate detection limit. 

For the second study, a calibration curve was obtained in the same way as the first study. 1000 μM nitrate solution in the electrolyte was prepared and the analytic signal was measured. Then, a sufficient amount of 1 M standard of one of the interfering ions was pipetted to the nitrate solution to yield 1000 μM of the interfering ion. The analytic signal was again measured and the equivalent nitrate concentration was calculated using the nitrate calibration curve. The previously measured concentration of nitrate-only solution was compared to the equivalent concentration determined for the solution with interfering ions. The result is depicted in Figure 1. In general, the equivalent nitrate concentration increases or decreases by less than 10%. PO₄^3- anions and Ca²⁺ and Sr²⁺ cations can be potential problems since these either increase or decrease the equivalent nitrate concentration by more than 20%. Acetate salts were used for cation standards because the acetate ion (as 1 mM NaC₂H₃O₂) showed negligible interference by itself. It is worthwhile to note that a 2.4% increase is observed with acetate ion for the second study.

Modified nitrate sensing system design for more reliable operation

Even though microfabricated thin-film electrodes on the silicon chip shows large sensitivity, low detection limit, and wide dynamic range, short life-time of working electrodes and potential drift of silver oxide reference electrode were a major road block to the field operation. Thus, new design is proposed to alleviate reliability problems.  

The entire setup is depicted in Figure 2. The sensing system consists of sensor housing, miniature peristaltic pumps, calibration standards, valve manifolds with control circuitry, data acquisition board, and LABVIEW software. A sensor incorporates commercial working, reference and counter electrode in plexi-glass housing. Life time of the working and reference electrode is virtually infinite if the electrode surface is reactivated. All electrodes are electrically connected to a potentiostat. Electrochemical cell volume inside the housing is about 0.6 ml. P625 miniature peristaltic pumps (Instech Solomon) provide the nitrate sample into the housing at maximum flow rate of 1.7 ml/min. The sensor can be calibrated in-situ with seven nitrate standards (0 μM – 1000 μM) of 100ml. Control circuitry opens an appropriate bi-stable solenoid valve in the manifold to select nitrate standard of increasing concentration when the sensor calibration is required. A LABVIEW program controls each component of sensing system through D/A and A/D channels in the data acquisition board and processes analytic signal transferred from the potentiostat. Nitrate in groundwater sample is measured continuously each predetermined time interval (15 minutes in this instance) and sensor is automatically calibrated every 20 measurement. Operator’s intervention can be minimized with LABVIEW program.

Calibration curve was obtained automatically with the setup as in Figure 3. The detection limit was 1.2 μM and it was linear up to 1000 μM (r²=0.999).

Figure 1. Measured nitrate concentration when 1000 μM of interfering ionic species are added to 1000 μM nitrate

Figure 2. Experimental Setup for Automatic Nitrate Sensing System

Figure 3. Calibration with Automatic Nitrate Sensing System

Future Directions and Objectives

Design and testing a Donnan dialysis unit for increased selectivity

As Figure 1 indicated, the sensor suffers a major interference from PO₄^3- ,Ca²⁺ and Sr²⁺, and minor from NO₂^-  and Cl^-. Selectivity could be improved using a separate dialysis unit that uses a monovalent-anion-permeable membrane (ACS membrane, Tokuyama Soda) because the monovalent anion such as nitrate transport through membrane much faster than any other ionic species.

 Sensor Operation in Wireless Network

The micro-potentiostat board (CENS sample preparation project) will be used to interface present sensor setup to wireless motes. The board has potentiostat functionality for electrochemical measurement, valve-manifold control circuitry, and microcontroller for data processing. It is also designed to communicate with wireless motes. Board is completed and operating software is currently being written.

 Nitrate Sensor Field Test

After completion of micro-potentiostat programming and dialysis unit fabrication, nitrate sensor units will be deployed in the field to measure concentration of nitrate in groundwater.

Integration of Ag/AgCl Reference Electrode

PU-coated Ag/AgCl reference electrodes show reliable reference potential. We will work to integrate this electrode into microsensor chip