Rapid and noncontaminating sampling system for trace elements in global ocean surveys

A system for the rapid and noncontaminating sampling of trace elements with volumes of up to 36 L per depth and including the dissolved and particulate phases has been developed for ocean sections that are a crucial part of programs such as International GEOTRACES. The system uses commercially available components, including an aluminum Seabird Carousel with all titanium pressure housings for electronics and sensors to eliminate zinc sacrificial anodes and holding twenty-four 12 L GO-FLO bottles, and a 7500 m, 14 mm Vectran conducting cable (passing over an A-frame with nonmetallic sheave) spooled onto a traction winch. The GO-FLO bottles are stored and processed in a clean lab built into a 20’ ISO container. To minimize contamination, the GO-FLO bottles are triggered when the carousel is moving upward into clean water at 3 m min–1. Analyses of salinity and nutrients in bottle samples from the stopped versus moving carousel show no detectable smearing, whereas the contamination-prone trace elements show the samples are uncontaminated when compared with other clean sampling methods. Based on the use of this system on three major cruises, the launch-sample-recover time for the carousel (2 bottles triggered per depth) is 1 h per 1000 m, and dissolved and particulate sampling time averages 6 h per hydrocast. Thus, the system described here meets all the requirements for ocean basin sampling for trace elements: rapid, good hydrographic fidelity, and noncontaminating. *Corresponding author: E-mail: gcutter@odu.edu Acknowledgments The authors thank Christopher Powell and Curtis Barnes for setting up and operating the US GEOTRACES carousel system, Geoffrey Smith, Peter Morton, and Jessica Fitzsimmons for clean lab operations, Jingfeng Wu (U. Miami) for unpublished Zn data from the SAFe Station, Mathew Brown for his unpublished dissolved aluminum data from BATS in 2008, and Rob Middag for his dissolved aluminum data from BATS in 2010. This work was supported by funding from the US National Science Foundation (Grant OCE-0648408 to G. Cutter and OCE-0648470 and OCE-0961579 to K. Bruland). This paper is part of the Intercalibration in Chemical Oceanography special issue of L&O Methods that was supported by funding from the U.S. National Science Foundation, Chemical Oceanography Program (Grant OCE-0926092 to G. Cutter). DOI 10.4319/lom.2012.10.425 Limnol. Oceanogr.: Methods 10, 2012, 425–436 © 2012, by the American Society of Limnology and Oceanography, Inc. LIMNOLOGY and OCEANOGRAPHY: METHODS This article describes a trace element sampling system based on that developed for the CLIVAR-CO2 Repeat Hydrography Program (Measures et al. 2008) for the upper 1000 m. The US GEOTRACES carousel can sample the full water column (to depths of 7500 m) with 24 12-L bottles per cast for dissolved and particulate (filtered) TEIs. It also uses commercially available components and hydrographic sensors (pressure, conductivity, temperature, oxygen, beam transmittance, and fluorescence) housed in titanium pressure cases to completely eliminate the need for zinc sacrificial anodes and therefore potential sample contamination (except perhaps for Ti). The carousel is lowered through the water with conducting Vectran cable with an extruded polyester outer jacket. The sampling system is flushed on the descent through the water column and samples are acquired while the carousel is being slowly raised into clean water. Thus, the focus of this paper is on the hydrographic fidelity of the samples (i.e., what depth?) as well as an assessment of their cleanliness (i.e., degree of any contamination). Because of the wide scope of assessing the performance of this sampling system, only results for dissolved (<0.2 or 0.4 μm) constituents are considered, whereas data for particulate TEIs are examined in Planquette and Sherrell (in press). Results for dissolved TEIs not included in this analysis are found in other articles in this volume. Materials and procedures Sampling system—carousel, winch and cable, and A-frame The sampling carousel itself is the prototype for what is now a “standard” item (Model 32G) from Seabird Electronics. The frame is one-piece, welded aluminum with a polyurethane electrostatic coating and a titanium lifting bail. The GO-FLO sampling bottles rest on titanium retaining pins fitted to pivoting Delrin blocks (with titanium bolts) that allow the GO-FLO bottles to be tilted in/out for installation and removal (Fig. 1). This minimizes damage to the GO-FLO mounting blocks (Measures et al. 2008). The pressure housings on the electronic release system that fires the bottles, the main 911 plus CTD unit, and SBE 43 oxygen, dual temperature, and dual conductivity sensors are titanium. The Seapoint fluorometer has a rigid polyurethane housing and the WET Labs C-Star 25 cm transmissometer has an anodized aluminum housing with its Zn anode removed. The titanium and aluminum conCutter and Bruland Rapid sampling system for trace elements 426 Fig. 1. US GEOTRACES carousel with GO-FLO bottles mounted and additional weights marked with red arrows, and close-up of sensor system and GO-FLO mounting blocks. struction of the system eliminates the need for sacrificial zinc anodes to prevent galvanic corrosion, a potential source of contamination. One modification to the Seabird carousel system is the addition of four 35 kg epoxy-encased lead weights (140 kg total) to the bottom of the vertical tubes on the carousel (Fig. 1) to make sure the conducting cable remains taut during lowering through the water column. These weights are made just like those on the CLIVAR rosette (Measures et al. 2008), except the lead shot/epoxy resin mixture is cast in a 7 L polyethylene bucket with a PVC pipe (3.2 cm OD) glued at the bottom in the center of the bucket. Before adding the epoxy and lead shot, the pipe and bucket are coated with silicone grease to ease removal after the mixture cures. After curing, the weight is cut in half length-wise, the cut surfaces recoated with epoxy resin, painted with 4 coats of white epoxy paint, and the matching halves placed around the vertical tubes of the carousel (arrows in Fig. 1). Each of the weights is then kept together with all 316 stainless steel, worm drive clamps (McMaster-Carr #5682K25). Besides these weights, a minor modification is the addition of 18 cm loops of 1 mm monofilament line below each trigger on the electronic release unit (Seabird Carousel manual, p. 22: www.seabird.com/ pdf_documents/manuals/32_018.pdf). Up to three triggering lanyards from the GO-FLO bottles can be placed over one of these loops, and the loop then put over the release trigger, allowing 2-3 bottles to be fired simultaneously. Twenty-four 12 L GO-FLO bottles (General Oceanics, Model 108012T) are mounted to the carousel. They are Teflon-coated by the manufacturer, all o-rings are Viton, the air bleed valve is replaced with a Swagelok Nylon fitting with o-ring (NY-6001-OR) to allow direct air-pressurization (a Teflon plug, Swagelok T-600-P, is used to cap this fitting), and the normal sample drain valve replaced with a 3/8” Teflon plug valve (Chemfluor/Cole Parmer # 06392-31). An ca. 25 cm piece of acid-cleaned 3/8” FEP Teflon tubing with one end cut at a 60° angle is attached to the inside of the plug valve so that the beveled end fits against the opposite side bottom of the bottle. This facilitates draining the entire sampler content and the collection of any particles that might sink below the level of the drain valve during sampling. New GO-FLO bottles are completely disassembled (including o-rings) and the o-ring grooves are wiped cleaned with 2-propanol and Kimwipes to remove remaining oil/grease and contaminants from manufacturing. The bottles are reassembled, filled with DI water and dilute Micro detergent, and allowed to soak for 24 h (Note: the Teflon drain valve is briefly opened to fill it with detergent solution). The bottles are then rinsed with DI water until all detergent is removed. Next, the bottles and valves are rinsed with 2-propanol to remove any organic contaminants (ca. 500 mL per bottle), then rinsed again with DI water. A final 24 h soak uses 0.3 M HCl; care should be taken to not expose the flexible nylon rods that hold the outer retaining rings to the HCl solution since they would become brittle and crack. After the HCl soak the GO-FLOs are thoroughly DI rinsed. They are then ready for use after proper conditioning with seawater (see below) and are not recleaned with this 3-step process unless major repairs occur or severe contamination is encountered. When not being sampled, the Teflon sampling valves on the GO-FLOs are plugged at the outlet to prevent contamination with a 3/8,” 2.5 cm Bev-a-Line IV tube (Cole Parmer, # S-06490-39) with one end melted shut. Lanyards for triggering the GO-FLO bottles are made of 1 mm monofilament line with stainless steel nicropress sleeves that are painted with white epoxy paint to minimize corrosion and contamination. The winch is a Dynacon Oceanographic unit (Bryan, Texas USA) with overhead electro-active level wind and cast nylon sheave (60 cm diameter), and line monitoring (length out/in, speed, tension). The winch drum is spooled with 7800 m of 14 mm conducting Vectran cable (braided) with four 18 gauge conductors and extruded polyester outer jacket (Cortland Cables). A Focal Technologies slip ring (Model 180) connects the paired cable conductors to a Seabird Deck Unit (11plus) and computer with Seabird Seasoft software. From the winch the Vectran cable runs through a Dynacon P19 style cast nylon sheave (71 cm diameter) mounted to an Allied Systems (Sherwood, Oregon USA), electrically driven, hydraulic Aframe. Both the winch and A-frame have remote controls so that a single operator can run the complete deployment system, although an operator for each is typically used. Clean van The clean laboratory “van” is an ISO-sized 20 ft aluminum container built to US UNOLS standards (htt