Practical guide UK Ocean Acidification Research Programme (UKOARP) Carbonate Chemistry Facility

Transcription

1 Practical guide UK Ocean Acidification Research Programme (UKOARP) Carbonate Chemistry Facility

2 Practical guide for the UK Ocean Acidification Research Programme (UKOARP) Carbonate Chemistry Facility Scientific report on sampling procedures for DIC and alkalinity measurements by OA researchers for subsequent analysis at the National Oceanography Centre Southampton, University of Southampton Cynthia Dumousseaud, Eric Achterberg, Toby Tyrrell Contacts: Cynthia Dumousseaud: Eric Achterberg: Toby Tyrrell: National Oceanography Centre Southampton School of Ocean and Earth Science European Way Southampton SO14 3ZH UK January 10, 2011

3 Table of contents Introduction Sampling and storage recommendations Total Dissolved Inorganic Carbon Coulometric titration Infrared detection Determination of Total Alkalinity VINDTA 3C Apollo SciTech AS-ALK Precision and accuracy Calculation of the carbonate system parameters LIMS Software References... 11

4 Introduction The purpose of this guide is to provide the UK Ocean Acidification Research Programme community with information on the sampling procedures for carbonate system measurements, including sample preservation, storage and transport. The guide also provides a summary of the methods and the instruments used in the facility to determine Dissolved Inorganic Carbon and Total Alkalinity in oceanic and coastal samples, and in samples from experimental studies. In addition, we provide information about the online sample logging software developed for the carbonate facility.

5 1. Sampling and storage recommendations Borosilicate glass bottles (e.g. Schott Duran from Germany, or Pyrex USA) with glass stoppers are recommended and are available in different sizes (e.g. from Fisher Scientific). Oceanic and coastal samples should be taken in 250 ml bottles as the analysis of these samples will require a larger volume. Samples from experimental work can be collected in smaller volume (i.e. 100 ml). Sample bottles should always be filled to a near full level, whilst still leaving a small air space as recommended (see below). Filtering of the samples is recommended for experimental work with high cell density (e.g. cultures), but care should be taken not to expose the sample to ambient air during the filtration as this will affect the DIC content of the samples (Riebesell et al. 2010). The filtration should take place during sampling if possible or immediately after sampling. The sampling procedure used for the determination of Dissolved Inorganic Carbon (DIC) and Total Alkalinity (TA) should follow Dickson et al. (2007). At sea, CTD samples must be collected as soon as possible after the Niskin bottle has been opened on deck in order to prevent any gas exchange. A piece of Tygon tubing is to be used and care needs to be taken while filling the sample bottles in order to prevent any air bubbles being trapped in the sample bottle (N.B.: silicon tubing is recommended if Dissolved Organic Carbon samples are collected from the same bottle). The tubing is inserted at the bottom of the bottle and sea water is left to overflow by at least half a bottle volume. A glass stopper is inserted in the bottle in order to remove the stopper volume, and a volume of 1% (e.g. 2.5 ml for a 250 ml bottle) is then removed in order to allow a head space for water expansion. The sample should then be immediately poisoned with a saturated solution of mercuric chloride (7 g/100 ml deionized water) in a 0.02% volume ratio (50 µl for a 250 ml bottle) in order to prevent any further biological activity in the stored sample. Stoppers should be greased in order to form an airtight seal; however care should be taken in order not to introduce any grease into the sample (putting a few drops of grease at the top of the stopper before introducing it carefully in the bottle will help). The sample should be shaken thoroughly to mix the mercuric chloride homogeneously. Finally, seal the bottle and stopper by wrapping adhesive tape (e.g. electrical tape) around the bottle and stopper and clearly label the sample using a unique ID (the same ID will have to be entered on the LIMS (Laboratory Information Management System) website for shipment and analysis of the samples). Samples should be stored in a cool and dark place until analysis. A foam insert is recommended for shipping the samples in order to protect the samples and to keep the bottles upright (a nest attached lid plastic box is ideal for shipping). The salinity, temperature and nutrient (total nitrate, phosphate and silicate) concentrations of the samples at the time of collection are required in order to calculate the CO 2 system parameters (calcite/aragonite saturation state, ph, pco 2 ) following the DIC and TA analysis.

6 2. Total Dissolved Inorganic Carbon 2.1. Coulometric titration The Versatile INstrument for the Determination of Total inorganic carbon and titration Alkalinity (VINDTA 3C, Marianda, Germany) is used for the determination of DIC in oceanic and coastal samples (Figure 1). The VINDTA is connected to a coulometer (5011, UIC, USA). The measurement consists in the extraction of CO 2 from a sea water sample and quantitative transfer of released CO 2 to the coulometer using a carrier gas. Gas calibration of the system with pure CO 2 is not yet available on the VINDTA 3C in use at the National Oceanography Centre, but will be installed early The sampling, titration and DIC calculation of the sea water samples is controlled by LabVIEW TM software. All samples are measured at 25 ºC with temperature control using a water-bath (F12, Julabo, Germany). The sea water sample (20 ml) is acidified with phosphoric acid (10% in 0.7 M NaCl), which converts all carbonate species into CO 2 gas. The generated CO 2 is carried into the coulometric cell using an inert gas (N 2 ), and titrated coulometrically. In order to remove any water vapor from the carrier gas, a condenser is placed before the coulometric cell and connected to a Peltier cooler (custom-made) set to a temperature just above freezing (ideally between 1 and 2 C). The coulometric titration can last for up to 20 min, or until 4 endpoints are found. The endpoint is defined as an increment lower than the blank determined at the beginning of each day (blank ideally comprised between counts per min, with an upper limit set to 100 counts per min). Figure 1. Front view of the VINDTA 3C and the UIC coulometer in use at the Facility

7 2.2. Infrared detection The Apollo SciTech DIC analyzer (AS-C3) is used to measure the DIC content of experimental samples (Figure 2). The system uses a LI-COR (7000) CO 2 infrared analyser as a detector, a mass-flow-controller to precisely control the carrier gas (N 2 ) flow, and a digital pump for transferring accurate amounts of reagent and sample. The analyzer is controlled by a PC. The sample and standard volume can be changed, although reducing the sample volume will result in a slight decrease of precision. The system generally achieves a precision of 0.1% or better. As for the coulometric method, phosphoric acid (10% in 0.7 M NaCl) is used to convert all the CO 2 species. The titration time is also reduced with this method (down to 1 to 2 min per sample). Figure 2. AS-C3 DIC Analyser (Apollo SciTech; USA) in use at the Facility 3. Determination of Total Alkalinity 3.1. VINDTA 3C Total Alkalinity of oceanic and coastal samples is determined on the VINDTA 3C (Figure 1) using a closed-cell titration (Dickson et al. 2007). The sample of seawater (100 ml) is titrated with 0.1 M hydrochloric acid (HCl) prepared in 0.7 M sodium chloride (NaCl). The acid is added in small increments (150 µl) until the carbonic acid equivalence point is reached (protonation of carbonate and bicarbonate ions). A full TA titration takes approximately 20 minutes. A glass electrode/reference electrode system monitors the titration (measurement of the electromotive force). The end-point is determined using the change in ph, measured with the electromotive force (emf), against the volume of acid added to the sample. The emf and

8 the total volume of acid added allow the calculation of total alkalinity based on a non-linear curve fitting (least-squares) or a Gran plot approach (Dickson et al. 2007). The cell (100 ml) for the TA determination is equipped with a ph half cell electrode (glass bodied Orion 8101SC, Ross, USA) and an Ag/AgCl reference electrode (model , Metrohm, Switzerland) connected to a titration unit (Titrino, Metrohm, Switzerland) with stirrer and dispensing burette unit. All samples are analyzed at 25 ºC (±0.1 ºC) with temperature regulation using a water-bath (Julabo F12 MB, Germany). The sampling, titration and calculation of the sea water samples are controlled by LabVIEW TM software Apollo SciTech AS-ALK2 Total Alkalinity of samples from experimental work is determined using an open-cell titration (Dickson et al. 2007). The Apollo SciTech s AS-ALK2 Alkalinity Titrator (Figure 3) conducts an automated Gran titration and calculates the TA result. It achieves a precision of 0.1% or better in laboratory conditions. AS-ALK2 is designed for both laboratory and shipboard use with small sample volume (5-25 ml) and high precision (± 0.1%). The titrator is fully-automated with standardization and sample analysis. The system is equipped with a combination ph electrode (8102BNUWP, Thermo Scientific, USA) and temperature probe for temperature control (Star ATC probe, Thermo Scientific, USA) connected to a ph meter (Orion 3 Star benchtop ph meter, Thermo Scientific, USA). The analyzer is controlled by a PC. All samples are analyzed at 25 ºC (±0.1 ºC) with temperature regulation using a water-bath (F12 MB, Julabo, Germany). Figure 3. Total Alkalinity Titrator system (AS-ALK2, Apollo SciTech, USA) in use at the Facility

9 4. Precision and accuracy Repeated measurements on the same batch of seawater (n 3) are conducted every day of analysis in order to assess the precision of the method for both DIC and TA. The repeated measurements are conducted prior to analysis of experimental or oceanographic samples. Certified Reference Materials (CRM) from A.G. Dickson (Scripps Institution of Oceanography) are used as standards to calibrate the system at the beginning of each day of analysis. A CRM is also run at the end of each day of analysis to check for any instrument drift. As no gas calibration is yet available for the coulometric VINDTA 3C measurements, a correction factor is currently applied to all measured values in order to normalize the measured values (for both TA and DIC): Corrected Value = Measured Value x (CRM cert /CRM meas ) Where CRM cert corresponds to the certified value of the CRM used. The Apollo SciTech instrument is used for volume-limited experiments (e.g. cultures). The precision of the system is possibly slightly reduced with smaller volumes. However, we will investigate this over the coming months. The recommended volumes are 25 ml for TA and 0.75 ml for DIC, for a precision of 0.1%. The VINDTA instrument will be used for open ocean water samples which require a higher analytical precision. The precision of the DIC and TA measurements using the VINDTA 3C typically reaches ± 1 to 2 µmol kg -1 (± 0.05% or better). 5. Calculation of the carbonate system parameters The measurement of any two variables of the carbonate system allows the calculation of the other variables (including the saturation state of calcite and aragonite, the carbonate and bicarbonate ion concentrations, etc ) through the use of the appropriate thermodynamic constants. The CO 2 SYS program (Lewis and Wallace 1998) is used for the recalculation of the carbonate system variables using the thermodynamic constants of Mehrbach et al. (1973) refitted by Dickson and Millero (1987). The salinity, temperature and nutrient (total nitrate, phosphate and silicate) concentrations of the sample at the time of collection are required in order to calculate the CO 2 system variables. These variables will not be measured at the Carbonate Facility and will need to be entered in the LIMS software (see section 6).

10 6. LIMS Software A LIMS website has been set up for researchers to enter samples and for the Facility to report back analyses results ( Figure 4). The website is password protected and a username and password will be required in order to enter the list of samples and to access the analyses results. All the additional carbonate system parameters data will be available after analysis (i.e. calcite and aragonite saturation state, ph, pco 2 ) in addition to DIC and TA. Following sample analysis, the Carbonate Facility will use the LIMS website to report DIC and TA results, and the other carbonate system variables calculated with the CO 2 SYS program (see section 5). Figure 4. Screenshot of the Carbonate Facility LIMS website (

11 References Dickson, A.G., Millero, F.J., A comparison of the equilibrium constants for the dissociation of carbonic acid in seawater media. Deep-Sea Res. 34, Dickson, A.G., Sabine, C.L., Christian, J.R. (Eds.) Guide to best practices for ocean CO 2 measurements. PICES Special Publication 3, IOCCP report No. 8, 191 pp. Lewis, E., Wallace, D.W.R., Program Developed for CO2 System Calculations. ORNL/CDIAC-105. Carbon Dioxide Information Analysis Center, Oak Ridge National Laboratory, U.S. Department of Energy, Oak Ridge, Tennessee. Mehrbach, C., Culberson, C.H., Hawley, J.H., Pytkowicz, R.M., Measurement of the apparent dissociation constants of carbonic acid in seawater at atmospheric pressure. Limnol. Oceanogr. 18, Riebesell, U., Fabry, V.J., Hansson, L., Gattuso, J.-P. (Eds.) Guide to best practices for ocean acidification research and data reporting. EUR24328 EN, Publication Office of the European Union, 260 pp.