CICEET Progress Report for the period 9/01/01 through 1/31/02

Project Title: In-situ monitoring of a reactive trace metal in riverine and estuarine mixing zones
Principal Investigator(s): Dr. Robert H. Byrne, Dr. Michael R. Callahan, Dr. Eric T. Steimle

Accomplishments
Scheduled Tasks:
The primary objectives for the first half of proposal Year 1 were (a) to configure our SEAS (spectrophotometric elemental analysis system) instrument for copper measurements, and (b) to begin autonomous in-situ deployments of the developing SEAS-Cu sensor.

Progress on Tasks
We are on our time-line schedule and have 2.5 months of work completed since procuring project funds. To date, significant progress has been made in the specific task areas required to meet our primary objectives, including: (1) selection and acquisition of materials, (2) assembly of a SEAS-Cu sensor, and (3) in-lab testing of the instrument. The current work completed has produced an operational SEAS-Cu sensor ready for in-field testing and deployment.

Difficulties Encountered
No unforeseen difficulties have been encountered in completing the tasks outlined above. The most significant obstacle to date has involved in-line contamination associated with certain copper containing components of the SEAS instrument. In-line contamination issues have been resolved by component replacement. Specifically, all Nitrile® based o-rings in contact with reagents have been replaced with Viton® o-rings (McMaster Carr). Similarly, the metallic SMA fiber-optic connectors (Amphenol) in contact with sample or reagents have been replaced with plastic fiber-optic couplers (custom in-house assembly).

Anticipated Success in Meeting Project Objectives in Scheduled Project Period
We anticipate that all project objectives will be met according to the proposed project timeline of 1 year.

Preliminary Results
The first generation SEAS(I) was designed for autonomous mixing and delivery of a single combined reagent for each sample. A necessary component of configuring the SEAS instrument for copper analysis involved the redesign of the reagent delivery system. Specifically, our LPAS (Long Pathlength Absorbance Spectroscopy) method for copper analysis (Callahan, submitted) employed manual stepwise addition of reagents. As a validation, a known concentration of copper was analyzed in triplicate using both stepwise addition of reagents and a single addition of a combined reagent ( see Figure 1: BDS-CuI complexation - stepwise versus mixed reagent addition for 40 nM copper solutions (n = 3) measured using a 4.4 meter LPAS system). No statistical difference was observed in the efficiency of the reaction between copper and bathocuroine disulfonic acid disodium salt (BDS) for the stepwise (e = 13,546 + 196 L mol^-1 cm^-1) versus the single combined (13,413 + 131 L mol^-1 cm^-1) addition of reagents.

Following assembly of a functional SEAS-Cu sensor, in-lab testing was done to identify and replace components of SEAS instrument contributing to in-line contamination of the system. As an added precaution many of the metal containing components of the SEAS assembly were eliminated. Figure 2 shows a picture of the current SEAS-Cu configuration featuring plastic fiber-optic couplers at the sample input and exit ends of the LCW (liquid core waveguide), Teflon end caps, a Teflon reagent cap, and a PVC pressure housing. Not shown in Fig. 2 is a Viton® o-ring used to seal the reagent dye bag. This o-ring is located just under the Teflon reagent cap.

Quality control testing of the electrical and mechanical components of SEAS revealed the system to be stable. The difference in the electronic signal between the sampling (484 nm) and monitoring (675 nm) wavelengths used for copper analysis was found to be + approximately 5 counts over the course of 500 samples. Baseline absorbance measurements (A₄₈₄, pathlength (_) = 90 cm) in an autonomous mode and using Milli-Q water for the sample and reagent solutions was found to be 0.009 + 0.004 AU.

The second phase of in-lab testing established a set of SEAS-Cu mission parameters that would produce a colorimetric reaction with an efficiency equivalent to the current LPAS-Cu method. The parameters included (a) tubing diameters and pump speeds to ensure the proper combining ratio of reagent to sample, and (b) settling time needed for full color development.

Tubing and pump speeds were selected to give a reagent:sample combining ratio of 0.16, a value in good agreement with the ratio (0.17) used in the established Cu-LPAS method. Optimal color development (see Figure 3) Cu-BDS kinetics observed in SEAS for 100 nM copper solution in Milli-Q water. A concentration response of 84 nM copper was expected based on the reagent dilution effect determined for the current SEAS-Cu configuration) was achieved using a 80 s mixing period (sample and reagent pumps on, followed by a 80 s settling period (sample and reagent pumps off).

Using the optimized parameters the absorbance (A₄₈₄) versus concentration response of the Cu-BDS complex in the SEAS-Cu sensor was evaluated. The standard regression curve (see Figure 4: Cu-BDS standard curve in Milli-Q water measured using a 90.17 cm pathlength SEAS system) was linear (R² = 0.9931) for added copper concentrations between 50 and 500 nM. The molar absorptivity (13,786 L mol^-1 cm^-1) of the Cu-BDS complex determined from the slope of the regression (_ = 90 cm) was excellent agreement with the value (13,863 + 45 L mol^-1 cm^-1) observed for the standard LPAS-Cu method.

Having successfully completed a set of validation and calibration tasks, we are currently undertaking short-term test deployments of our SEAS-Cu sensor in Tampa Bay, FL.

Tasks and activities for next reporting period

Tasks for the next reporting period
In-field testing, optimization and continued deployment of our SEAS-Cu sensor are the principal tasks for the next reporting period. Data collected during SEAS-Cu deployments in Tampa Bay and Rookery Bay, FL will eventually be disseminated through publication in a peer-reviewed journal.

Work plan to accomplish tasks
A minimum of 10 short-term deployments will be used to collect data on copper gradients in Tampa Bay and Rookery Bay, FL. The collected data will be analyzed in order to demonstrate that the current SEAS-Cu sensor is capable of detecting and measuring Cu in-situ. Additionally, the data will be used to understand the capabilities of the current SEAS-Cu sensor and to optimize the sensor for data collection in riverine and estuarine environments. Long-term field deployments will follow. Finally, collected data will be analyzed and prepared for publication.

Concerns or difficulties
Given the four month delay in funding, we are working very hard to find ways to compress the original proposed timeline.

Expenditures
Expenditures were in the range anticipated for the work completed to date.