Ar Measurements by

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Anal. Chem. 2009, 81, 1855–1864

Continuous High-Frequency Dissolved O2/Ar Measurements by Equilibrator Inlet Mass Spectrometry Nicolas Cassar,*,† Bruce A. Barnett,† Michael L. Bender,† Jan Kaiser,‡ Roberta C. Hamme,§ and Bronte Tilbrook| Department of Geosciences, Princeton University, Princeton, New Jersey 08544, School of Environmental Sciences, University of East Anglia, Norwich, NR4 7TJ, U.K., School of Earth and Ocean Sciences, University of Victoria, P.O. Box 3055 STN CSC, Victoria, British Columbia, V8W 3P6, Canada, and Commonwealth Scientific Industrial Research Organisation (CSIRO) Wealth from Oceans Research Program and Antarctic Climate and Ecosystem Cooperative Research Center, Hobart, 7001 Tasmania, Australia The oxygen (O2) concentration in the surface ocean is influenced by biological and physical processes. With concurrent measurements of argon (Ar), which has similar solubility properties as oxygen, we can remove the physical contribution to O2 supersaturation and determine the biological oxygen supersaturation. Biological O2 supersaturation in the surface ocean reflects the net metabolic balance between photosynthesis and respiration, i.e., the net community productivity (NCP). We present a new method for continuous shipboard measurements of O2/Ar by equilibrator inlet mass spectrometry (EIMS). From these measurements and an appropriate gas exchange parametrization, NCP can be estimated at high spatial and temporal resolution. In the EIMS configuration, seawater from the ship’s continuous intake flows through a cartridge enclosing a gas-permeable microporous membrane contactor. Gases in the headspace of the cartridge equilibrate with dissolved gases in the flowing seawater. A fused-silica capillary continuously samples headspace gases, and the O2/Ar ratio is measured by mass spectrometry. The ion current measurements on the mass spectrometer reflect the partial pressures of dissolved gases in the water flowing through the equilibrator. Calibration of the O2/Ar ion current ratio (32/40) is performed automatically every 2 h by sampling ambient air through a second capillary. A conceptual model demonstrates that the ratio of gases reaching the mass spectrometer is dependent on several parameters, such as the differences in molecular diffusivities and solubilities of the gases. Laboratory experiments and field observations performed by EIMS are discussed. We also present preliminary evidence that other gas measurements, such as N2/ Ar and pCO2 measurements, may potentially be performed with EIMS. Finally, we compare the characteristics of the EIMS with the previously described membrane inlet mass spectrometry (MIMS) approach.

physical and biological processes. O2 and the inert gas argon (Ar) have similar solubility properties. Measurements of Ar can therefore be used to remove the disequilibrium of dissolved O2 in waters of the oceanic mixed layer associated with heat fluxes, bubble injections, and variations in atmospheric pressure.1-3 The remaining O2 disequilibrium, “the biological O2 supersaturation”, reflects the competing influences of mixedlayer net community production (NCP), vertical mixing, and gas exchange. NCP corresponds to the difference between rates of oxygenic photosynthesis and respiration by the biological community. Biological oxygen is supersaturated (undersaturated) within the mixed layer if community photosynthesis/respiration is greater (less) than 1 (correcting for potential bias from entrainment, upwelling, or eddy diffusion of oxygen-undersaturated waters). With a gas exchange parametrization,4-8 net biological oxygen flux at the air-sea interface, and net community productivity (NCP) within the mixed layer, can be estimated. Because of the close stoichiometric link between O2 and organic carbon fluxes,9 and the approximately week-long residence time of oxygen within the mixed layer, NCP derived from O2/Ar measurements provides a unique measure of upper ocean carbon fluxes over the week or so prior to measurements. Several studies have reported discrete NCP estimates derived from in situ O2/Ar measurements.1,3,10-13 Continuous measurements offer the opportunity to capture the heterogeneity in oceanic (1) (2) (3) (4) (5) (6) (7) (8)

The departures from saturation in dissolved oxygen (O2) concentration in ocean surface waters are the consequence of * To whom correspondence should be addressed. E-mail: ncassar@ princeton.edu. Phone: 609-258-7435. Fax: 609-258-1274. † Princeton University. ‡ University of East Anglia. § University of Victoria. | Commonwealth Scientific and Industrial Research Organisation. 10.1021/ac802300u CCC: $40.75  2009 American Chemical Society Published on Web 02/04/2009

(9) (10) (11) (12) (13)

Craig, H.; Hayward, T. Science 1987, 235, 199–202. Emerson, S. J. Geophys. Res., [Oceans] 1987, 92, 6535–6544. Spitzer, W. S.; Jenkins, W. J. J. Mar. Res. 1989, 47, 169–196. Wanninkhof, R.; McGillis, W. R. Geophys. Res. Lett. 1999, 26, 1889–1892. Wanninkhof, R. J. Geophys. Res., [Oceans] 1992, 97, 7373–7382. Ho, D. T.; Law, C. S.; Smith, M. J.; Schlosser, P.; Harvey, M.; Hill, P. Geophys. Res. Lett. 2006, 33, L16611. Nightingale, P. D.; Liss, P. S.; Schlosser, P. Geophys. Res. Lett. 2000, 27, 2117–2120. Nightingale, P. D.; Malin, G.; Law, C. S.; Watson, A. J.; Liss, P. S.; Liddicoat, M. I.; Boutin, J.; Upstill-Goddard, R. C. Global Biogeochem. Cycles 2000, 14, 373–387. Shulenberger, E.; Reid, J. L. Deep-Sea Res., Part A 1981, 28, 901–919. Hendricks, M. B.; Bender, M. L.; Barnett, B. A. Deep-Sea Res., Part I 2004, 51, 1541–1561. Reuer, M. K.; Barnett, B. A.; Bender, M. L.; Falkowski, P. G.; Hendricks, M. B. Deep-Sea Res., Part I 2007, 54, 951–974. Cassar, N.; Bender, M. L.; Barnett, B. A.; Fan, S.; Moxim, W. J.; Levy, H.; Tilbrook, B. Science 2007, 317, 1067–1070. Hamme, R. C.; Emerson, S. R. J. Mar. Res. 2006, 64, 73–95.

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community production. We present a new method for measuring continuously the ratio of dissolved O2/Ar in seawater by equilibrator inlet mass spectrometry (EIMS). Seawater is pumped through an equilibrator cartridge, and the gases in the headspace of the cartridge assume partial pressures which reflect their dissolved concentrations. The headspace gases then flow through a capillary tube into the mass spectrometer, which measures the O2/Ar ion current ratio, among other properties. EIMS builds on the established methodology of membrane inlet mass spectrometry (MIMS).14-16 However, there are also important differences, which give each method its own niche. As opposed to MIMS, EIMS does not require liquid standards (e.g., air-equilibrated water) for calibration of ion current ratios. The measurements performed with a MIMS instrument can only be as accurate as the liquid standards.15,16 Calibration of the EIMS instrument is achieved by periodically sampling ambient air through a second capillary. EIMS also requires less peripheral instrumentation. The response time of EIMS is, however, significantly longer than MIMS. EIMS has now been successfully deployed on several cruises. Examples of the continuous in situ O2/Ar observations are reported below. MATERIALS AND METHODS Description of Equilibrator Inlet Mass Spectrometry. In the shipboard configuration of the EIMS, seawater from the ship’s underway system flows continuously through a gas equilibrator. Relatively rapid gas exchange and equilibration of the dissolved gases with the headspace of the equilibrator is ensured by the large surface area of the small, gas-permeable, hollow tubes packed into the equilibrator (see description of equilibrator below). The seawater flows perpendicular to, and outside of, the hollow fibers. The equilibrated gas phase in the headspace of the cartridge is continuously sampled through a fused-silica capillary (2 m in length and 0.05 mm diameter) for monitoring of gas ratios on a quadrupole mass spectrometer. The ion current measurements made by the mass spectrometer depend on the partial pressure of dissolved gases in the water flowing through the equilibrator. Because of the potentially large changes in room temperature on a ship, the ion source and mass filter of the quadrupole are temperature-controlled at 50 °C (±0.2 °C) with a custom-built, temperature-regulated, enclosure. The quadrupole mass spectrometer used in this study is a Pfeiffer Prisma model QMS 200 M1 with a Prisma gas-tight ion source with yttrium oxide coated iridium filament. The source emission is 0.50 mA. The detector is a continuous dynode secondary electron multiplier run at 1400 V amplification. The mass resolution is 50 (QuadStar software setting) with a dwell time of 50 ms. The pumping speed of the turbo pump is 60 L s-1. Pressure is measured with a compact full-range Pirani/cold cathode gauge (PKR 251). Below is a more detailed description of the various components of the equilibrator inlet. Upstream of the Equilibrator. A multiple-step filtration system upstream of the equilibrator ensures that the equilibrator (14) Kaiser, J.; Reuer, M. K.; Barnett, B.; Bender, M. L. Geophys. Res. Lett. 2005, 32, L19605. (15) Kana, T. M.; Darkangelo, C.; Hunt, M. D.; Oldham, J. B.; Bennett, G. E.; Cornwell, J. C. Anal. Chem. 1994, 66, 4166–4170. (16) Tortell, P. D. Limnol. Oceanogr., Methods 2005, 3, 24–37.

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Figure 1. The large seawater reservoir (A) sits in a sink. After going through an inline coarse filter (500 µm pore size), seawater flows into the inner reservoir (B) at a rate of 3-5 L min-1 (large arrow). Most of the water running into B overflows into A, which is used as a water bath thermostatted to the temperature of ambient seawater. A small fraction (100 mL min-1) of the high flow rate is pulled with a gear pump through a filter sleeve (C), with 100 and 5 µm pore size on the outside and inside, respectively. From the gear pump, the seawater flows through the equilibrator (D). The equilibrator sits in reservoir A to keep its temperature identical to that of the incoming seawater. A capillary, attached to the headspace of the equilibrator, leads to a multiport Valco valve. This valve alternates between admitting gas from the equilibrator and ambient air to the quadrupole mass spectrometer. An optode (not shown) in container B measures total oxygen saturation. Also not shown is a water flow meter located downstream of the equilibrator and thermocouples monitoring temperatures throughout the system.

does not clog. First, coarse filtration is performed with an inline reusable hose filter (pore size 500 µm) in the high flow rate seawater line (3-5 L min-1) (Figure 1). The high flow rate minimizes the rise in seawater temperature and decreases the residence time of the seawater in container B (Figure 1). A fraction of the seawater is pumped through the equilibrator at a nominal rate of 100 mL min-1 with a gear pump. Fine filtration is performed upstream of the equilibrator with a custom-made filter sleeve with 100 and 5 µm pore size polypropylene felt on the outside and inside, respectively (100 µm/5 µm filter bag, 1.5 in. width × 12 in. length, McMaster-Carr cat. no. 98315K99). Tygon silver antimicrobial tubing is used to inhibit biological growth within the lines upstream of the equilibrator. Equilibrator Description. The equilibrator is a small (26.9 mm × 73.1 mm), commercially available cartridge (http:// www.liquicel.com, MicroModule 0.75 × 1), similar to the one used by Hales et al.17 for pCO2 measurements by nondispersive infrared absorbance detection. Discs of hollow fiber arrays of gas-permeable and hydrophobic material (polypropylene/ epoxy) are tightly packed in a polycarbonate housing. This (17) Hales, B.; Chipman, D.; Takahashi, T. Limnol. Oceanogr., Methods 2004, 2, 356–364.

Table 1. Parameters Used in the Model, with Definitions and Units parameter Qw Qc F Vw Vhs ¯ C Cin Cout Chs A R ε P L r η Kw τ

identification volumetric seawater flow rate volumetric capillary flow rate out of the headspace to the mass spectrometer at headspace pressure sum of the volumetric efflux rates from the headspace volume on wet side of equilibrator volume on headspace of equilibrator average dissolved gas molarity within water side of equilibrator dissolved gas molarity entering equilibrator dissolved gas molarity exiting equilibrator gas concentration in the headspace of equilibrator surface area of the membrane contactor Ostwald solubility coefficient equilibration coefficient pressure within the headspace of equilibrator length of capillary radius of capillary gas dynamic viscosity liquid-phase mass transfer coefficient e-folding response time of equilibrator

units -1

Ls L s-1 L s-1 L L µmol L-1 µmol L-1 µmol L-1 µmol L-1 m2 dimensionless dimensionless Pa m m Pa · s (or kg m-1 s-1) m s-1 s

configuration provides a large surface area (392 cm2) for rapid gas exchange with a small headspace (2 mL), leading to relatively rapid gas equilibration between the headspace of the cartridge and the circulating seawater. The fibers have an internal diameter of approximately 200 µm and an outer diameter of approximately 300 µm, with a nominal wall thickness of 50 µm. Twenty-five percent of the surface is permeated with pores with an effective size of 0.04 µm. Gases flow to the mass spectrometer through a capillary inserted into the headspace of the equilibrator. Air Calibration Configuration. Relative to dissolved O2/Ar in surface seawater, the atmospheric O2/Ar is essentially constant. The ion current ratio (32/40) recorded when the mass spectrometer is sampling the equilibrator can be calibrated to compute the concentration ratio, by admitting ambient air through a second capillary. The capillary going to the mass spectrometer is connected to a Valco valve (HPLC Stream selector valve with 1000 psi rating, cat. no. C5-1306EMH2Y) that switches between two capillaries of identical dimensions, one coupled to the equilibrator, and one sampling ambient air. The air calibrations are implemented by switching the valve between the two capillaries. Switching occurs automatically every 2-4 h with a computer-controlled actuator, and ambient air is analyzed for 10 min. Downstream of Equilibrator and Ancillary Measurements. The water flow rate is monitored continuously with a Ryton 50-500 mL min-1 flow meter (Cole Parmer cat. no. C-3270352) downstream of the equilibrator. A continuous dissolved O2 monitor (Aanderaa optode model 3835), calibrated with Winkler titrations,18,19 sits in container B (Figure 1), next to the fine filter sleeve, and measures total oxygen concentration. Software Support. A single computer simultaneously logs data from several instruments. The ion currents measured by the (18) Williams, P. J. L.; Jenkinson, N. W. Limnol. Oceanogr. 1982, 27, 576–584. (19) Winkler, L. W. Ber. Deutsch. Chem. Ges. 1888, 21, 2843–2855.

mass spectrometer (Pfeiffer Prisma quadrupole) are recorded with the Quadstar 32-bit software with process control module and quantitative analysis module. The optode’s oxygen saturation and concentration observations are recorded with Oxyview (Aanderaa software supplied with the optode). Laboratory temperature, measured with several thermocouples, and the seawater flow rate are recorded with National Instruments LabVIEW. The latter program also controls the valve switching from water to ambient air for calibrations. A MATLAB script is used to consolidate all the parameters measured by these various programs, to average them into 2 min intervals, and to plot them along with ancillary data, in near real time, in a figure with multiple panels (Figure 2). The main advantage of this approach is that the EIMS operator can view diagnostics and results in real time, promptly respond if analytical issues arise, and also optimize discrete sampling. RESULTS AND DISCUSSION Theory. Dissolved gases in the seawater flowing through the equilibrator cartridge equilibrate with the headspace through the microporous membrane contactor. In the absence of a capillary or with a small capillary flow, the headspace gases approach equilibrium with the flowing seawater following Henry’s law. At steady state, the concentration gradient of dissolved gases between intake and outflow of water passing through the equilibrator is proportional to the net flux of gases across the membrane contactor. Headspace concentrations vary as a function of the net flux of gases across the membrane and the gas efflux associated with capillary flow to the mass spectrometer. At steady state, the net flux of gases across the membrane from seawater to the headspace must equal the capillary drawdown. As the capillary drawdown of gases from the headspace increases, the gases in the headspace become increasingly undersaturated relative to their predicted equilibrium partial pressures. To quantify the disequilibrium, we present a simple model that accounts for the kinetics of gas transfer across the membrane contactor and its dependence on various parameters such as seawater flow rate and headspace volume. 1. Kinetics of Equilibration. The gas mass balance on the water side of the equilibrator is

Vw

¯ dC ¯ - RChs) ) Qw(Cin - Cout) - KwA(C dt

(1)

where Kw, Vw, Qw, A, and R are the liquid-phase mass transfer coefficient, volume of the water side of the equilibrator, the seawater flow rate, the surface area of the membrane contactor, and the Ostwald solubility coefficient (dimensionless) for the given gas, respectively (see Table 1 for summary of parameters used in the model). R is in effect the reciprocal of HL, the ¯ , Cin, Cout, and Chs are dimensionless Henry’s law constant.20 C the molar concentrations within the equilibrator on the water side, flowing in and out of the equilibrator, and within the headspace of the equilibrator, respectively. The term on the left-hand side of eq 1 represents the rate of change of the standing stock of a given dissolved gas in the water flowing through the (20) Schumpe, A.; Quicker, G.; Deckwer, W. D. Adv. Biochem. Eng. 1982, 24, 1–38.

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Figure 2. Time-series of EIMS 2 min averaged shipboard results during the Southern Ocean GasEx cruise on the NOAA ship Ronald H. Brown on April 4-5, 2008. Measurements presented in the figure were performed in the Western South Atlantic, from 51.46° S, 37.33° W to 49.63° S, 40.43° W. (a) Pressure (mbar) in the quadrupole mass spectrometer (QMS), (b) percent total O2 saturation (with preliminary optode calibration) and EIMS biological supersaturation (primary ordinate, in blue and black, respectively) and O2/Ar ion current ratio from MS measurements (secondary ordinate, in gray), (c) N2/Ar ion current ratio, (d) optode measurement of seawater temperature (°C) at EIMS inlet, and (e) seawater flow rate through the equilibrator (mL min-1). The red portions of the signals in panels a-c represent air calibrations. The magenta and cyan circles in panel b represent discrete isotope ratio mass spectrometry (IRMS) O2/Ar and Winkler O2 measurements, respectively. In this example, the O2/Ar ion current ratio of air calibrations was 22.54 ( 0.01 (error bounds represent the standard deviation, n ) 12 over a period of 24 h). The O2/Ar saturation is calculated from the O2/Ar ion current ratio divided by the linearly interpolated O2/Ar ion current ratios of bounding air calibrations. Flow was stopped and the optode was removed from water at time 00:00, which explains the temperature (panel d) and flow rate (panel e) signal excursions at that time. Achieved accuracy for field dissolved O2/Ar measurements by EIMS is (0.3% (standard deviation). On this cruise, no offset in O2/Ar was observed between discrete samples collected from Niskin bottles and from the ship’s underway system.

equilibrator. The first term on the right-hand side of eq 1 represents the net flux of a given dissolved gas as water flows through the equilibrator, whereas the second term equals the net flux across the membrane. The headspace concentration is assumed to be uniform, whereas the water side of the equilibrator has a concentration gradient from the inlet to the outlet. We also first assume that the net gas exchange across the membrane contactor of the equilibrator is proportional to the concentration gradient between the seawater and the headspace (i.e., the stagnant boundary layer model21). 1858

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Likewise, the mass balance on the headspace side of the equilibrator is Vhs

dChs ¯ - RChs) - QcChs ) KwA(C dt

(2)

where Qc is the capillary flow rate (i.e., volume of gas flowing out of the headspace at headspace pressure). As in eq 1, the term on the left-hand side of the equation represents the rate of change of the standing stock of a given gas. The second term on the right-hand side of the equation is the molar flow rate of a

given gas down the capillary to the mass spectrometer. We assume the capillary flow rate to be

( )

Qc ≈ 103

πr4P 16Lη

(3)

where r and L are the radius and length of the capillary (m). P is the absolute pressure (Pa) of the headspace gas, and η is the gas dynamic viscosity (in (Pa · s) or (kg m-1 s-1)). The 103 multiplier is a unit conversion factor from cubic meters to liters. Equation 3 is a simplification of the fluid dynamic law of Hagen-Poiseuille22 for viscous compressible fluids with negligible pressure at the outlet. The viscosities of Ar, O2, and N2 at 300 K are about 2.27 × 10-5, 2.06 × 10-5, and 1.79 × 10-5 kg m-1 s-1, respectively.23 The viscosity of standard air at the same temperature is approximately 1.87 × 10-5 kg m-1 s-1. Under our experimental conditions, the pressure in the headspace of the equilibrator is close to 105 Pa (1 atm). On the basis of the capillary’s dimensions, the volumetric flow rate to the mass spectrometer should be around 0.2 µL s-1 or 18 mL day-1 (standard temperature and pressure). We measured the flow rate to be 19 mL day-1 by determining the decrease in pressure of an isolated gas volume as its air was allowed to flow through a capillary to vacuum. Hence, our simplified equation seems to provide a reasonable estimate of capillary flow for the conditions used during these experiments. We neglect the slight decrease in headspace pressure that comes from the fact that the gas flux down the capillary is significant relative to the gross flux across the membrane contactor into the headspace. As the capillary flow increases relative to the gas fluxes across the membrane (e.g., shorter capillary), the pressure decreases within the headspace. This decrease in pressure reduces the capillary flow rate (see eq 3) and increases the net flux across the membrane, partly compensating for the effect of the capillary flow. Our calculations of the degree of disequilibrium should therefore be regarded as upper limits. Depending on the configuration, the pressure in the headspace of the equilibrator is within ±10% of 1 atm. Assuming that the change with time of the standing stock of a given gas on the water side of the equilibrator is negligible, and assuming that the dissolved gas exiting the equilibrator has fully equilibrated with the headspace, and rearranging eqs 1 and 2, the change in the headspace standing stock of a given gas can be approximated as

Vhs

dChs ) QwCin - (QwR + Qc)Chs dt

(4)

Defining F ) (QwR + Qc) (i.e., the sum of the volumetric efflux rates from the headspace) and integrating relative to time (i.e., time after change in gas concentration of seawater entering equilibrator) 1 Chs(t) ) [(FChs0 - QwCin)e-(F/Vhs)t + QwCin] F

(5)

(21) Lewis, W. K.; Whitman, W. G. Ind. Eng. Chem. 1924, 16, 1215–1220. (22) Golubev, I. F. Viscosity of Gases and Gas Mixtures; Fizmat Press: Moscow, 1959. (23) Lemmon, E. W.; Jacobsen, R. T. Int. J. Thermophys. 2004, 25, 21–69.

where Vhs/F is the e-folding response time (τ) of the equilibrator. The transit time for gases through the capillary is on the order of 10 s and hence is short compared to the response time associated with the headspace equilibration. If there is no flux of gas down the capillary, this equation simplifies to the model presented by Johnson24 for a showerhead CO2 equilibrator. The e-folding residence time calculated based on the above equation (approximately 0.7 min) is shorter than our empirical observations (see results below), which suggests that the assumption in eq 4 that the seawater flowing out of the equilibrator has fully equilibrated with the headspace is invalid. In order to account for the relatively slow exchange of gases across the membrane, a dimensionless equilibration coefficient (ε) between 0 and 1 (1 being a full equilibration) may be applied24 to the gas exchange across the membrane. Taking into account the kinetics of gas transfer across the membrane, eq 5 becomes 1 Chs(t) ) [(FChs0 - εQwCin)e-(F/Vhs)t + εQwCin] F

(6)

where F now equals (εQwR + Qc). We assume that most of the resistance to mass transfer is from the aqueous phase (Kw) and is mediated by molecular diffusion on the water side of the membrane. Kw is then a function of the molecular diffusivity coefficients of the gases. Differences in diffusivity between gases can be accounted for by normalizing the equilibration coefficients of the various gases to their relative molecular diffusivity coefficients. The molecular diffusivity coefficients of N2, O2, and Ar, as compiled by Broecker and Peng,25 are 2.1 × 10-5, 2.3 × 10-5, and 1.5 × 10-5 cm2 s-1 at 24 °C, respectively. However, the coefficients reported in the literature vary substantially.26-28 A low equilibration coefficient would be consistent with significant resistance to the gas mass transfer across the membrane from the aqueous boundary layer, the air-filled pores (hydrophobic material) of the membrane, and the gas boundary layer in the headspace. The reciprocal of the overall mass transfer coefficient, known as the resistance, is a function of the resistance from these various mediums. Yang and Cussler29 expressed the overall resistance with a series model analogous to Ohm’s law for electrical resistors in series where the overall mass transfer coefficient (K) is a function of the aqueous medium, the microporous membrane, and the headspace transfer coefficients. In most applications, a sweep gas is applied to the other side of the membrane contactor. Under such conditions, the resistance to mass transfer across the hollow fiber contactor is generally dominated by the aqueous phase.29 This is because molecular diffusion of gases in water is orders of magnitude slower than in air. However, under certain conditions, such as low flow rate of the sweep gas relative to the water flow rate, the air-filled pores and the headspace may represent significant resistances to the (24) Johnson, J. E. Anal. Chim. Acta 1999, 395, 119–132. (25) Broecker, W. S.; Peng, T. H. Tracers in the Sea; Eldigio Press: New York, 1982. (26) Baird, M. H. I.; Davidson, J. F. Chem. Eng. Sci. 1962, 17, 87–93. (27) Wise, D. L.; Houghton, G. Chem. Eng. Sci. 1966, 21, 999–1010. (28) Ferrell, R. T.; Himmelblau, D. M. J. Chem. Eng. Data 1967, 12, 111–115. (29) Yang, M. C.; Cussler, E. L. AIChE J. 1986, 32, 1910–1916.

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Figure 3. Modeled relaxation of O2/Ar (A and B) and N2/Ar (C and D) in the headspace as a function of seawater flow rate (A and C) and capillary length (B and D). Equilibration coefficient (ε) of O2 is assumed to be 1 (seawater flowing out is fully equilibrated with the headspace). Model is based on eq 6 with an equilibration coefficient (ε) for N2 and Ar normalized to the molecular diffusivity of each gas relative to O2 at 24 °C. The molecular diffusivity coefficients of N2, O2, and Ar used in this example are 2.1 × 10-5, 2.3 × 10-5, and 1.5 × 10-5 cm2 s-1, respectively (ref 25). The difference in diffusivity of oxygen and argon may, however, be smaller (ref 27), in which case, the steady-state O2/Ar supersaturation presented in this figure would be overestimated. The headspace gas ratios are 10% supersaturated in O2/Ar and N2/Ar at time 0, at which point water flow is switched to air-equilibrated seawater. The varying seawater flow rate (A and C) is modeled with a 2 m long capillary. The varying capillary length (B and D) is modeled with 100 mL min-1 seawater flow rate through the equilibrator.

mass transfer of gases.30,31 The headspace of our equilibrator is effectively stagnant. Hence, the membrane and the headspace may also represent a substantial resistance to the mass transfer of gases in EIMS. 2. Steady-State Conditions. At steady state (i.e., t f ∞), Chs tends toward a concentration that is proportional to the ratio of the volumetric influx and efflux from the headspace: Chs(t f ∞) )

εQw C (εQwR + Qc) in

(7)

If the capillary flow is small relative to the gas exchange across the membrane, the gas concentration in the headspace tends toward gas equilibration, i.e., Cin/R. The steady-state ratio of two gases (gas 1 and gas 2) in the headspace is therefore (Figure 3, parts A and C) Chsgas1 Chsgas2

)

( )[ Cingas1

(Rgas2 + Qc(εgas2Qw)-1)

Cingas2

(Rgas1 + Qc(εgas1Qw)-1)

]

(8)

(30) McDermott, C. I.; Tarafder, S. A.; Kolditz, O.; Schuth, C. J. Membr. Sci. 2007, 292, 17–28. (31) Tarafder, S. A.; McDermott, C.; Schuth, C. J. Membr. Sci. 2007, 292, 9– 16.

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For two gases with similar solubilities and molecular diffusivities, the capillary drawdown of gases does not significantly change the steady-state headspace gas ratio from equilibrium gas ratio predictions (Figure 3B). This is because the numerator and denominator within brackets in eq 8 in effect cancel out. On the other hand, for two gases such as N2 and Ar, with very different solubilities, an increase in capillary flow relative to the gas equilibration rate increases the calculated disequilibrium in the headspace gas ratio with the flowing seawater (Figure 3, parts C and D). To approach conditions of a fully equilibrated headspace, more than twice as much N2 as Ar must cross the membrane relative to their concentrations in seawater. The flux of gas to the mass spectrometer depletes headspace concentrations. The flux of gases across the membrane cannot quite maintain gases within the headspace at the partial pressures in seawater. The lower the solubility of a gas, the greater is its partial pressure deficiency relative to equilibrium in the headspace. N2, which has a lower solubility than Ar, is thus more deficient in the headspace, accounting for the observed reduction in N2/Ar ratio of headspace gases compared with full equilibration (Figure 3, parts C and D). The effect is smaller for the O2/Ar ratio (Figure 3B) because these two gases have similar solubilities. However, it is possible to observe an O2/Ar

supersaturation in the headspace if the oxygen has a greater molecular diffusivity than argon25 and if the drawdown of gases from the headspace is significant relative to the gas flux across the membrane (Figure 3B). The effect is, however, smaller than for N2/Ar (Figure 3). On the basis of eqs 7 and 8, it can be shown that the level of undersaturation relative to equilibrium for a given gas in the headspace is proportional to the ratio of capillary drawdown to gross flux across the membrane into the headspace. Similarly, the under- or supersaturation relative to equilibrium in elemental ratio of two gases in the headspace is proportional to the difference of the ratios of capillary flows to gross fluxes into the headspace for the two gases measured. In our current configuration, we estimate the capillary flux to be roughly 1% of the gross flux across the membrane into the headspace for oxygen. For accurate EIMS measurements of gases with different solubilities, such as N2/Ar, the capillary flow must be negligible relative to the gas equilibration rate. Alternatively, a correction to the N2/Ar measurements could potentially be applied, assuming the seawater flow rate, gas flow across the membrane (boundary layer conditions), and capillary gas drawdown from the headspace are well constrained by measurements or theory and validated by calibrations. Because boundary layer conditions are affected by many factors such as changes in temperature and flow rate, such a correction may be difficult to apply when capillary flow is significant relative to the gas flux across the membrane. Laboratory Tests of EIMS. 1. O2/Ar: Kinetics of Equilibration. The response time of EIMS to a change in gas concentrations, as well as its linearity, were tested in the laboratory by switching the source of water flowing through the equilibrator from air-equilibrated water (i.e., dissolved gas pressure in equilibrium with atmospheric gas pressure) to waters with elevated O2 concentrations (Figure 4A). O2 supersaturated waters, with O2/Ar ratios elevated above saturation ratios by 2.2%, 10.1%, and 12.4% were prepared by adding different amounts of oxygen to the headspace of Tedlar bags containing air-equilibrated water. The e-folding response time (τ) (i.e., t1/2/ln(2)) was determined by fitting the O2/Ar ion current ratio I(m/z ) 32)/ I(m/z ) 40) (denoted 32/40) to a reaction progress equation based on eq 6

( 4032 ) ) [( 4032 ) - ( 4032 ) ]e

(-t/τ)

(t)

i

f

+

( 4032 )

f

(9)

where (32/40)i and (32/40)f are the initial and final 32/40, respectively. τ is estimated as the reciprocal of the slope of a linearized form of eq 9 with time as the independent variable

[

32 40 y ) ln 32 40

32 (t) 40 32 (i) 40

( ) ( ) ( ) ( ) -

f

f

]

)-

( 1τ )t

(10)

Kinetics of equilibration were estimated from the response of the 32/40 as we switched the flow through the equilibrator cartridge between air-equilibrated water and O2/Ar-elevated waters. From these experiments, the e-folding response time is found to be on average 7.75 ± 0.25 min (± standard deviation)

Figure 4. EIMS O2/Ar response time and linearity when switching from equilibrated water to waters with various O2/Ar supersaturations. Experiments were performed with a seawater flow rate of 100 mL min-1. O2/Ar supersaturated waters were prepared by injecting 10 (green), 50 (red), or 100 mL (blue) of pure O2 to the small headspace of 3 L of equilibrated water contained in Tedlar bags. (A) EIMS O2/Ar ion current ratio (32/40) as a function of relative time for the various O2 injections. The three experiments were performed on different days. (B) y ) ln[((32/40)(t) - (32/40)f)/((32/40)(i) - (32/40)f)] ) -(1/τ)t (from eq 10) for the red and blue experiments. The upper blue and red curves (1 and 3) are for the kinetics of perturbation of O2/Ar equilibrium, whereas the lower blue and red curves (2 and 4) are for the kinetics of relaxation to O2/Ar equilibrium. The time of onset of perturbation and relaxation has been normalized to zero. For clarity, the intercept of each curve has been modified to avoid overlapping of curves. The dashed lines represent linear least-squares values of the slopes. From top to bottom, the e-folding times, the reciprocals of the slopes, are 7.80 (r2 ) 0.98), 7.41 (r2 ) 0.97), 8.01 (r2 ) 0.98), and 7.78 (r2 ) 0.97) min. (C) Percent deviation from equilibrium in the O2/Ar ratio of waters with various O2/Ar mixtures as measured on discrete samples by isotope ratio mass spectrometry (abscissa) and as measured on the EIMS (ordinate). Each data point on the graph represents one switchover from air-equilibrated water to water with a different O2/Ar mixture. The black filled circle at the origin represents comparisons of equilibrated waters vs ambient air (Figure 5, where the mean difference in %O2/Ar between equilibrated water and ambient air is less than 0.1%). The slope of the comparison for the colored points is not significantly different from the identity slope, represented by the black line (t value ) 0.94, p ) 0.48, DF ) 1).

(Figure 4B). On the basis of these results, a change in the O2/ Ar ratio of 3% will be measured with a bias (i.e., memory effect) of 0.06% after 30 min of equilibration. The response is slower than predicted based on the model presented in eq 5. Observations can be reconciled with the model by invoking an equilibration factor of 0.1 (ε in eq 6), implying that the kinetics of gas transfer are relatively slow. O2/Ar: Steady-State Conditions. The linearity of the response was determined by comparing the relative change in signal on the EIMS to values measured independently in Analytical Chemistry, Vol. 81, No. 5, March 1, 2009

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discrete samples analyzed by isotope ratio mass spectrometry (IRMS), as described by Emerson et al.32 The two methods measure similar fractional changes when switching among solutions of different O2/Ar ratios (Figure 4C). The linearity of the response is further supported by field observations. O2/Ar saturation measurements, derived from 5 min time-averaged O2/Ar ion current ratios and air calibrations, are within 0.3% (standard deviation) of corresponding IRMS measurements on discrete samples (mean difference ) 0.07%, n ) 75, example shown in Figure 2). This is similar to the precision achievable by MIMS.14 In addition, under circumstances where variability in total oxygen concentration in the surface ocean is driven by biological activity, the total and the biological oxygen supersaturations are expected to covary. This statement is true if all the oxygen supersaturation within the surface of the ocean is derived from NCP or if physical processes such as bubble injection impart a constant oxygen supersaturation to the biological signal. In line with this expectation, we find that changes in O2/Ar ion current ratio closely track changes in percent O2 saturation as observed by the optode (Figure 2B). This observation suggests that, over the time scale of a day or so, most variability in the 32/40 ion current ratio derives from changes in biological O2 supersaturation and that the 32/40 ratio measured with EIMS responds linearly to the seawater ratio. The unlikely alternative

is that the physical supersaturation of Ar exactly compensates for nonlinearities in the mass spectrometer. Although the slow kinetics of gas transfer across the membrane contactor affect the equilibration rate of the O2/Ar ratio, they do not significantly fractionate the O2/Ar once steady state is achieved. This conclusion is validated by an experiment in which we switched between a capillary sampling headspace gas from an equilibrator through which air-equilibrated water flowed and capillaries of the same length sampling ambient air (Figure 5). It should, however, be noted that, if the molecular diffusivities of O2 and Ar in fact differ,25 a small but significant departure from equilibrium O2/Ar at steady state may occur if the ratio of capillary to gas flux across the membrane is greater than in our current configuration. To test the dependency of steady-state O2/Ar measurements on seawater flow rate, we measured the O2/Ar ion current ratios as we varied the flow of water through the equilibrator. No discernible differences in the mean O2/Ar ion current ratios (n ) 150) were observed at flow rates of 90, 110, and 135 mL min-1 (mean (32/40) of 25.64 for each flow rate). O2/Ar: Air Calibrations of O2/Ar Ion Current Ratio. In order to test ambient-air calibrations, we examined the change in the O2/Ar ratio when switching from air-equilibrated water (sampled from the headspace of the equilibrator cartridge) to ambient air (see above). This change was very small. Because of potential drift in the instrument, the mean O2/Ar ion current ratio of equilibrated water and dry air were calculated only from the 200 measurement cycles immediately preceding and following the switch from equilibrated water to dry air. Six comparisons were performed (see Figure 5). A 95% confidence interval for the true difference in mean ion current ratio between air-equilibrated water and dry air shows that, although the difference in O2/Ar ion current ratio is significant, the discrepancy between the two measurements is small: 0.04-0.12%. Our measurements therefore show that the O2/Ar partial pressure ratio measured with EIMS is negligibly fractionated between water and headspace in our current configuration. For comparison, the achievable precision with the standard IRMS method on discrete samples is ±0.1-0.35%.11,32 Water vapor decreases the mass spectrometer’s ionization efficiency and reacts in the mass spectrometer to form O2.33 In the EIMS configuration, the headspace of the equilibrator is water-saturated, whereas the water vapor pressure of the calibration gas, ambient air, is variable. To test the effect of varying water partial pressure, the 32/40 ion current ratio of water-saturated air (i.e., headspace air of a sealed container with water) and dry air (i.e., air in a container with some calcium sulfate desiccant) were sequentially measured. Six comparisons of means of dry and water-saturated air ion current ratios were performed by sequential EIMS measurements of dry and water-saturated air (Figure 5). The differences in mean O2/Ar ion current ratios of water-saturated and dry air were not statistically significant (p > 0.05, DF ) 398) (Figure 5A). A variety of processes have the potential to alter the dissolved O2 concentration of seawater as it is pumped through the ship’s plumbing into the laboratory.14 These include heterotrophic

(32) Emerson, S.; Stump, C.; Wilbur, D.; Quay, P. Mar. Chem. 1999, 64, 337– 347.

(33) Orsnes, H.; Bohatka, S.; Degn, H. Rapid Commun. Mass Spectrom. 1997, 11, 1736–1738.

Figure 5. Laboratory assessment of air calibrations of ion current ratios. (A) O2/Ar ion current ratio (32/40). (B) N2/Ar ion current ratio (28/40). (C) Pressure recorded in the mass spectrometer. Interfaced with a Valco valve, the quadrupole mass spectrometer sequentially measures (1) air from the equilibrator through a 480 mm capillary (blue), (2) ambient dry through a 480 mm capillary (red), (3) ambient wet through a 480 mm capillary (green), (4) ambient dry air through a 580 mm capillary (black), and (5) ambient dry air through a 380 mm capillary (cyan). Air-equilibrated water is flowing through the equilibrator and the headspace gas is measured in (1) (blue).

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activity within the lines, rust, warming and degassing, and cavitation. Hence, discrete sampling independent of the ship’s seawater underway sampling system (e.g., Niskin bottles) is needed to correct for any O2 depletion within the ship’s lines and within the EIMS lines upstream of the equilibrator. 2. Preliminary Test of N2/Ar Measurements by EIMS. The N2/Ar concentration ratio in normal air is approximately 83.6 compared with 37.9 in air-equilibrated seawater (S ) 35, T ) 20 °C). The dissolved N2/Ar naturally varies in the surface oceans predominantly because of bubble injection and temperature changes.13 Cooling and bubble injection cause N2/ Ar in the mixed layer to exceed the saturation value. The slow kinetics of gas exchange across the membrane contactor, together with the significant drawdown of headspace gases from the capillary, may explain the observed undersaturated N2/Ar gas ratio observations (Figure 5). When the capillary drawdown of headspace gases is stopped (e.g., switch from equilibrator measurements to air calibrations depicted in Figure 5), the N2/Ar in the headspace increases due to the replenishment of N2 and Ar from gas exchange across the membrane. As the capillary flow is reestablished (e.g., switch from air calibrations to equilibrator measurements depicted in Figure 5), the N2/Ar in the headspace starts out very close to the equilibrium but then decreases by about 1%. An alternative explanation for the anomalous 28/40 ion current ratio might be fractionation in the source due to the difference in source pressures between sample and air analyses. However, the 28/40 ion current ratio actually rises rather than falls as flow rate to the mass spectrometer decreases. This result is shown in Figure 5: with the capillary sampling ambient air, the shorter the capillary, the greater the pressure in the mass spectrometer, and the lower the 28/40 signal (Figure 5C). In our field observations (Figure 2), the departure from equilibrium due to the difference in solubility between N2 and Ar is exacerbated by the slight pressurization of the headspace. 3. Preliminary Test of CO2 Measurements by EIMS. The CO2 in the equilibrated headspace of the EIMS should be reflective of the dissolved CO2 partial pressure. Initial field comparison of EIMS ion current ratio 44/40 (i.e., CO2/Ar) to CO2 measured by the standard showerhead equilibrator inlet nondispersive infrared (NDIR) analyzer were performed on the Australian icebreaker Aurora Australis in January-February 2007 in the Australian subantarctic zone. Because the (44/40) ion current ratio may drift over time, we compared EIMS measurements with NDIR CO2 estimates over 6 h intervals. The median 6 h Pearson correlation coefficient between the two measurements for the last 10 days of the cruise was 0.97. (44/40) measurements divided by independent NDIR pCO2 estimates scatter about a linear trend by 0.2% (