High-resolution chemical sensor for unattended

High-resolution chemical sensor for unattended underwater networks Lori Adornato*a, Eric A. Kaltenbachera, Robert H. Byrneb, Xuewu Liub, Regina Easleyb a SRI International, 140 7th Avenue South COT 100, St. Petersburg, FL USA 33701; b University of South Florida College of Marine Science, 140 7th Avenue South, MSL 119, St. Petersburg, FL USA 33701

ABSTRACT Autonomous underwater sensors are the best solution for continuous detection of chemical species in aquatic systems. The Spectrophotometric Elemental Analysis System (SEAS), an in situ instrument that incorporates both fluorescence and colorimetric techniques, provides high-resolution time-series measurements of a wide variety of analytes. The use of Teflon AF2400 long-pathlength optical cells allows for sub-parts-per-billion detection limits. User-defined sampling frequencies up to 1 Hz facilitate measurements of chemical concentrations on highly resolved temporal and spatial scales. Due to its modular construction, SEAS can be adapted for operation in littoral or open ocean regions. We present a high-level overview of the instrument’s design along with data from moored deployments and deep water casts. Keywords: Spectrophotometric, long-pathlength, chemical sensing, in situ, colorimetric

1. INTRODUCTION High-resolution measurements of trace analytes in aquatic systems prove challenging for a number of reasons. Standard sampling techniques often involve manual water collection, storage, transportation from the sample site, and subsequent analysis in a shore-based facility. Samples are frequently poisoned or frozen immediately after collection to preclude microbial activity1, 2. Each time the sample is handled, the possibility of contamination is encountered from such disparate sources as ship stack gases, sunscreens or lotions, airborne particulates, and adsorption to or desorption from container walls1, 3–5. Should sample integrity problems come to light during analysis, which often occurs weeks or months after collection, the opportunity to resample would have long since passed. In addition, the number of samples collected is often limited by physical constraints such as the number of bottles on a rosette or the length of time available for a diver to rinse and fill sample bags. Furthermore, standard spectrophotometric cell lengths between 1 and 10 cm do not provide the sensitivity to detect many analytes at low concentrations. The Spectrophotometric Elemental Analysis System (SEAS) provides an alternative analytical tool for high-resolution, high-sensitivity analyses of chemical species in the water column. SEAS, an in situ instrument rated to 1,000 meters depth, performs sample collection and analysis while fully submerged, thereby precluding the sample contamination issues previously mentioned. With a sampling frequency of 1 Hz, SEAS offers very high spatial and/or temporal resolution, depending on the desired deployment mode 6. In addition, SEAS permits detection of parts per billion (nanomolar) levels of analytes through use of a long-pathlength cell.

*[email protected]; phone 1-727-553-1234; fax 1-727-553-3529

Unmanned/Unattended Sensors and Sensor Networks V, edited by Edward M. Carapezza Proc. of SPIE Vol. 7112, 71120R · © 2008 SPIE · CCC code: 0277-786X/08/$18 · doi: 10.1117/12.802616

Proc. of SPIE Vol. 7112 71120R-1 2008 SPIE Digital Library -- Subscriber Archive Copy

Spectrophotometric methods utilize the relationship established in the Beer Lambert Law, A = εbc, where A is the wavelength dependent absorbance, ε is the molar absorptivity (M-1cm-1), b is the pathlength (cm), and c is the molar scale analyte concentration (M). Absorbance is calculated by comparing the intensity of light, I, that passes through a sample before and after addition of a reagent that reacts with the analyte of interest to form a colored product. Precision and accuracy can be improved using multiple wavelengths. In particular, absorbance observations can be used to correct for baseline drift via the equation

⎡ I −I ⎤ ⎡ I −I ⎤ A = log ⎢ λ1 0 0 dark ⎥ − log ⎢ λ 2 0 0 dark ⎥ ⎣ λ 2 I − I dark ⎦ nonabsorbing ⎣ λ 1 I − I dark ⎦ sample where λ1 is the sample wavelength and λ2 is a wavelength not affected by the colored product, also called the nonabsorbing wavelength. The dark value (Idark) is a measurement from a detector element masked from all sources of light, and represents background electrical noise. I 0 is the intensity without reagent (reference intensity), and I is the intensity with reagent (sample intensity) for the various monitored wavelengths. The first term represents absorbance measured at the sample wavelength, and the second term represents absorbance measured at the non-absorbing wavelength. This second term is an effective absorbance related to changes in intensity from sources unrelated to analyte absorbance. Such sources can include air bubbles, lamp drift, inherent optical properties of the sample solution, absorbance from colored dissolved organic matter, and physical effects that can change the efficiency of light propagation through the waveguide. In contrast to standard spectrophotometric systems, which use 1–10 cm pathlength cells, SEAS provides one to two orders of magnitude lower detection limits through use of a novel optical cell (0.040" OD × 0.032" ID) made of a flexible fluoropolymer (Teflon AF 2400) with a refractive index (1.29) less than that of water (1.33) and seawater (1.34)7, 8. The lower index of refraction permits the Teflon tube to perform as a liquid core waveguide, with water acting as the core (Fig. 1). Because the material can be coiled in a small diameter cell holder, pathlengths of 5 meters or more can be achieved.

Normal

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Fig. 1. Total internal reflection occurs when light is introduced at an angle (θ1) greater than the critical angle (θ C). Light introduced at the critical angle propagates down the waveguide/water interface, and light introduced at less than the critical angle is lost from the system.

Proc. of SPIE Vol. 7112 71120R-2

2. INSTRUMENT DESIGN In this section we present an overview of the SEAS instrument design. The SEAS (Fig. 2) is equipped to perform complete and precise spectrophotometric analyses in situ. The primary elements comprising SEAS are optics, fluidics, mechanics, electronics, and software. (a)

(b) Battery

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Fig. 2. (a) Diagram of the assembled SEAS instrument. (b) Exploded view of the instrument showing the modularity of the system. Underwater cables are not displayed.

2.1 Optics In conjunction with the liquid core waveguide described above, SEAS utilizes a broadband lamp matched with a miniature spectrometer to provide a flexible and powerful optical system. One of the benefits of this broadband approach is that a wide range of analytes can be studied without modifying the instrument. Since the lamp and detector are sealed within a pressure vessel while the waveguide is mounted externally, coupling optics were designed to transfer light from the lamp into the waveguide and subsequently from the waveguide into the spectrometer. Nominally, the instrument operates in absorbance mode whereby optical absorption is measured through the length of waveguide. A unique feature of SEAS is that fluorescence measurements can be performed almost simultaneously with absorption measurements. Radiation from solid-state emitters is used to excite fluorescence in samples contained within the waveguide. Fluoresced radiation that propagates along the waveguide is collected by the coupling optics and detected by the spectrometer.


2.2 Electronics The SEAS electronics are custom designed to provide instrument control and data processing, and to interface with peripherals. Motorola microcontrollers, used as the main control elements in the circuitry, provide highly adaptable control of the instrument functions as well as real-time processing of measured data. On-board memory is used to store absorbance data and ancillary data for future download. The circuitry provides a user interface to the instrument using either RS-232 or Ethernet communications. Additionally, four RS-232 channels, coupled with a 12 VDC supply, are available to control external peripherals attached to SEAS. Typical peripherals used with SEAS include a conductivity temperature depth (CTD) meter, a photosynthetically available radiation (PAR) sensor, a chlorophyll fluorescence meter and a transmissometer. The data from these peripherals are merged and stored with SEAS-generated data. Additional circuitry is utilized to condition power for the lamp and spectrometer, and provide closed-loop control of the fluid heater. 2.3 Mechanics The SEAS design is highly modular, allowing it to be readily adapted to different deployment modes. Two pressure vessels, one at each end of the instrument, contain the instrument electronics. The detector and main processor are located within one vessel, and the pump controller and batteries are located within the other vessel. All fluidic components mount to a plate between these two pressure vessels. This layout provides convenient access to the components most frequently adjusted by users. Because the central plate easily disconnects from the two pressure housings, the pump layout can be condensed. Also, the battery housing can be replaced with an endcap equipped with a connector for external power. By reorganizing the relative positions of the pressure vessels and eliminating the battery, the instrument is readily configurable to fit within an AUV or ROV payload. Depending on the network configuration, SEAS can readily be deployed as part of an unattended network, either in its default form or with the modular SEAS components reconfigured. In addition to the modular design, SEAS is relatively rugged. Most of its components are machined from Titanium or aluminum for strength and corrosion resistance. With batteries installed, SEAS measures roughly 50 inches long, with a diameter varying from 4.5 inches to 7.2 inches. SEAS is rated for operation to depths of more than 3000 feet. 2.4 Fluidics The SEAS fluidic system is comprised of pumps, a heater, reagent reservoirs, the waveguide, and connecting tubing. The entire fluidic path is maintained at ambient pressure to eliminate high-pressure differentials and variable flow rates with changes in depth. Three peristaltic pumps, each housed in its own pressure vessel, are clamped to the central plate. Each pump holds two separate tubing lines, providing a total of six possible fluid lines of varying size. Inclusion of a heater cartridge allows colorimetric reactions to proceed at constant or elevated temperature. This is especially important when the instrument experiences large temperature variations during deployment. The heater is comprised of a copper core, a resistive heating cartridge, and a resistive temperature sensor to inform the drive circuitry that maintains the user-defined temperature. Temperature control has been demonstrated within ± 1 degree Celsius. Reagents are stored in collapsible containers and are fastened to the instrument during deployment. Depending on the number and size of these containers, they either fit within the central plate or are separately held adjacent to the instrument. The separate elements (pumps, heater, waveguide, reagents) are plumbed together using connecting tubing. Both rigid and flexible tubes are used for this purpose and are selected for each measurement. The configuration of the connecting tubing depends on deployment objectives and is considered part of the instrument’s flexibility. 2.5 Software SEAS software consists of two elements: the firmware controlling instrument operations, and a graphical user interface (GUI) operating on a personal computer. The firmware running on the microcontrollers governs the instrument operations. In manual mode, the firmware receives commands from the user and executes them. It can also operate autonomously by executing a command-set programmed by the user. Basic instrument functions (e.g., turn on pump) are organized as user-accessible commands. Using an editor, the user assembles a list of commands into a method file that is loaded into the instrument for autonomous execution. The GUI provides an intuitive connection to all the commands and displays spectral and chemical data.


3. APPLICATIONS SEAS is a versatile instrument that can readily adapt to a variety of deployment methods. In this section, we discuss several of the ways in which we have deployed SEAS. While SEAS has not been directly interfaced with an unattended network, the diverse applications of SEAS presented here suggests it would work well as part of a network. 3.1 Tethered Using external power in lieu of a battery, SEAS can conduct long-term time-series measurements (Fig. 3). In this example, 24 VDC is supplied through a cable connected to a shore-based power supply. The instrument can be placed on the seabed or suspended from a cable. In one configuration, SEAS can be connected to an Ethernet transceiver so realtime data and spectral quality can be monitored remotely. The GUI is programmed to automatically reconnect to the instrument when the user enters the Ethernet transmission range. This connectivity permits the user to spot-check instrument performance without having to restart the program.

Fig. 3. Two SEAS instruments lowered into Bayboro Harbor, St. Petersburg, Florida, via a seawall davit.

3.2 Profile Vertical profiling of the water column can be achieved in a number of ways. One profiling technique involves SEAS mounted on a custom-made frame and lowered through the water column on a ship hydrowire (Fig. 4). The versatile frame holds between one and three SEAS instruments as well as optional peripheral instruments by means of exchangeable support plates attached to a central, load-bearing tube. As the instrument returns to the surface, autonomous operation can be terminated in three ways: (1) power can simply be turned off, (2) the instrument can be kept in the water until autonomous operation is allowed to fully execute, or (3) using depth data read from the attached CTD, the method can be terminated using a programmed command that terminates the program at a predetermined depth. In the event that the method is not complete at the end of a cast, the third termination option precludes air introduction into the fluid line by stopping the pumps before the instrument exits the water.


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Fig. 4. Two SEAS plus several peripheral instruments including two CTDs, a fluorometer, and a PAR sensor are affixed to a custom-made frame and lowered through the water via a ship hydrowire.

Another profiling technique involves attaching the SEAS instrument to the ship rosette frame (Fig. 5a). This provides several advantages. First, SEAS data can be transmitted real-time through the data cable used to monitor the rosette’s CTD system (Fig. 5b). Second, analysis of discrete water samples collected in the attached Niskin bottles during the cast permits verification of SEAS data within the contamination limits described earlier. Third, valuable ship time is saved by obtaining concurrent measurements. Deployment of SEAS on the rosette is, however, only advantageous when the kinetics of the chemical method are fast enough to be compatible with the high vertical profiling rates (~30 m/min) typically used on CTD casts. (a)

(b)

Fig. 5. (a) SEAS pH instrument (black) attached to the inner ring of a CTD rosette using a custom-made bracket. (b) Data from the vertical cast were transmitted to the ship’s lab along with data from the rosette CTD.


3.3 Seafloor Monitoring of benthic (seafloor) chemical time-series involves mounting SEAS on a cradle and powering the instrument either by battery (8–16 hours deployment time) or via the external power cable (Fig. 6). Data can be monitored via a cable to a monitoring station, or by a diver using a hand-held device connected to the instrument. For any time-series deployment, standards should be used regularly to ensure data quality. This can be done by switching fluid flow from the sample line to a standards line or by using valves to inject a standard into the sample line. Further research is required to determine the effects of bio-fouling and methods for bio-fouling mitigation. Preliminary investigations indicate that periodic acid/base rinses or use of copper intake tubing is helpful.

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,/i Fig. 6. Two SEAS instruments sample ambient seawater and excurrent water from a barrel sponge (located behind the SEAS instruments and under the acoustic Doppler velocimeter). The black SEAS instrument measured pH and the silver SEAS instrument measured nitrate. (Photo courtesy of Chris Martens.)

3.4 Other deployment modes In addition to the methods described above, SEAS can be configured to operate as part of an AUV or ROV payload. Because instrument length is a critical parameter, certain mechanical modifications, including reconfiguration of the pump layout and replacement of some underwater connectors, would be required to reduce the overall instrument length for these applications. The SEAS power requirement of approximately 15 W can be provided by the vehicle itself, thereby precluding the need for a separate battery. SEAS can also operate in a buoy-mounted configuration. The benefits of buoy-based operation include observation in infrequently visited locations, near-real-time communication via data-packet telemetry, and long-term, high temporal resolution data collection. Under these circumstances, efforts to reduce the power requirements would be beneficial.

4. DATA In this section, we present samples of data collected using SEAS. The intent here is not a scientific discussion of the meaning of these data, but rather a demonstration of instrument efficacy and versatility. In the examples below, commercial instruments were used to measure conductivity, temperature, depth, and dissolved oxygen.


4.1 Bayboro Harbor SEAS was used to measure phosphate in Bayboro Harbor, St. Petersburg, Florida (March 14– 16, 2006). Bayboro Harbor is a small, semi-enclosed embayment within the Tampa Bay estuary. Phosphate concentrations, in general, varied inversely with dissolved oxygen, although the correlation was stronger after sunset (Fig. 7). Much of the temporal variability appeared to be due to tidal movement of water rather than photosynthesis/respiration relationships, which would have shown a stronger day/night cycle. Concentrations ranged from 3.2 to 4.3 µm, corresponding well with previously reported spring dry season concentrations for the Hillsborough River10, a major source of nutrients to Tampa Bay. Optical quality of Tampa Bay water improves in the dry season (winter-spring) and deteriorates in the wet season (summer-fall), due to seasonal phytoplankton blooms. In-line filters can be used to remove most of the particulates that interfere with spectrophotometric measurements, although care must be taken to ensure that sample flow rates do not deteriorate over time. 350

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Fig. 7. Phosphate concentrations (using a SEAS instrument) and dissolved oxygen (using a Sea-Bird SBE 52) were measured overnight in Bayboro Harbor, Florida. Depth varied from 0.5 to 1.0 meters, reflecting the tidal nature of the deployment site.

4.2 Conch Reef SEAS was used to compare nitrate 9 in ambient and excurrent water from a barrel sponge on Conch Reef (Atlantic side of the Florida Keys) as part of a larger study that included measurements of pH and water velocity (Fig. 8). Prior studies indicated that sponges provide a source of nitrate to the reef via nitrification by microbes in the sponge tissue11. Sampling ambient and excurrent water masses was accomplished by alternating pumps on a SEAS pH instrument located adjacent to the nitrate instrument (Fig. 6). The nitrate instrument intake line was connected with a T-connector to the pH sample line so that both instruments sampled the same water mass. Ambient nitrate concentrations of 200–300 nM and excurrent concentrations of approximately 900 nM to 1.2 µM agreed well with previously published results (Fig. 8)11. Future work at the Aquarius station will involve a sensor network that includes two SEAS instruments (pH and nitrate), two underwater mass spectrometers (measuring dissolved gases such as argon, carbon dioxide, nitrogen, and oxygen), a CTD (salinity, temperature, depth, and dissolved oxygen), and an acoustic Doppler velocimeter (water velocity exiting the sponge).


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4.3 Gulf of Mexico SEAS can be also used to measure spatial, rather than temporal, chemical distributions. In November 2006, SEAS measured vertical nitrate profiles at the West Florida shelf break in the Gulf of Mexico, located approximately 100 miles west of Tampa Bay. The instrument was attached to a custom-made frame and lowered through the water column at approximately 5 m/min via the ship’s hydrowire. Because Beer-Lambert linearity typically extends to absorbances between 1 and 2, a multiple wavelength technique was used to capture the range of concentrations found in this region 9. Each of the selected wavelengths produced linear calibrations within the concentration range depicted in Figure 9. Concurrent CTD data show the commonly observed relationship between temperature gradients and nutrient gradients.

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Fig. 9. Vertical nitrate and temperature distributions measured in the Gulf of Mexico (November 2006).


5. CONCLUSION The SEAS instrument provides a robust, versatile platform for both monitored and unattended chemical sensing in aquatic environments. A variety of chemical species can be detected by selecting appropriate reagents and optimizing reaction rates. Deployment duration is governed by a variety of factors such as reagent usage, waste production, power availability, and sampling frequency. Future instrument design will focus on package miniaturization, power reduction, and reduced reagent consumption.

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