Environ. Sci. Technol. 2004, 38, 5729-5736
Pumping-Induced Ebullition: A Unified and Simplified Method for Measuring Multiple Dissolved Gases BRYANT A. BROWNE* College of Natural Resources, University of WisconsinsStevens Point, Stevens Point, Wisconsin 54481
The incorporation of multiple dissolved gas measurements in biogeochemical studies remains a difficult and expensive challenge. Incompatibilities in collection, handling, and storage procedures generally force the application of multiple sampling procedures for multiple gases. This paper introduces the concept and application of pumping-induced ebullition (PIE), a unified approach for routine measurement of multiple dissolved gases in natural waters and establishes a new platform for development of in situ real-time dissolved gas monitoring tools. Ebullition (spontaneous formation of bubbles) is induced by pumping a water sample through a narrow-diameter tube (a “restrictor”) to decrease hydrostatic pressure (PH) below total dissolved gas pressure (PT). Buoyancy is used to trap bubbles within a collection tower where gas accumulates rapidly (1 mL/min) to support multiple chemical analyses. Providing for field collection of an essentially unlimited and unified volume of gas sample, PIE afforded accurate and precise measurements of major (N2, O2, Ar), trace (CO2, N2O, CH4) and ultratrace (CFC11, CFC12, CFC113, SF6) dissolved gases in Wisconsin groundwater, revealing interrelationships between denitrification, apparent recharge age-dates, and historical land use. Compared to conventional approaches, PIE eliminates multiple gas-specific sampling methods, reduces data computations, simplifies laboratory instrumentation, and avoids aqueous production and consumption of biogenic gases during sample storage. A lake depth profile for CO2 demonstrates PIE’s flexibility as an in situ real-time platform for dissolved gas measurements. The apparent departures of some gases (SF6, H2, N2O, CO2) from solubility equilibrium behavior warrant further confirmation and theoretical investigation.
Introduction This paper introduces and demonstrates a device that facilitates routine measurement of multiple dissolved gases [noble gases, N2, N2O, O2, CO2, CH4, chlorofluorocarbons (CFCs), SF6, volatile organic compounds (VOCs), etc.] in natural waters and establishes a new platform for the development of in situ dissolved gas monitoring tools. Measurement of multiple dissolved solids (e.g., nutrients, redox indicators, weathering products, and reactants) along hydrologic gradients became fundamentally routine in geochemical (1-4) and biogeochemical (5, 6) studies more than four decades ago. Dissolved gases (natural and synthetic) offer an important complementary wealth of information * Corresponding author phone: (715)346-4190; fax: (715)346-3624; e-mail:
[email protected]. 10.1021/es035464m CCC: $27.50 Published on Web 09/29/2004
2004 American Chemical Society
about the hydrosphere. However, the incorporation of multiple dissolved gas measurements in biogeochemical studies remains a difficult and expensive challenge. Applications of dissolved gases include paleothermometry with noble gases (7, 8); age-dating of groundwater (9, 10); estimating groundwater recharge temperature (9-11); measuring advection and dispersion in rivers and stream (12); tracing ocean mixing (13); tracking volatile pollutants in groundwater (14); measuring groundwater denitrification on the basis of excess N2 (15-20); studies of greenhouse gases in groundwater (21) and surface water (22, 23); evaluation of terminal electron-accepting processes using dissolved H2 (24); and studies of biogenic gases (e.g., CO2, CH4, and N2O) in general. Despite obvious potential for powerful combinations of these and other gas measurements, the inclusion of direct measurements of multiple gases in studies such as refs 17-20 remains uncommon in routine water resource assessments. The challenge of performing multiple dissolved measurements is not one of developing or improving gas-specific detection technologies. Analytical capabilities for gaseous constituents are readily available due in part to enormous interest in biosphere-atmospheric gas exchange (25). The selection of methods to detect gases within atmospheric samples includes fairly standard analytical approaches (26) rooted in gas chromatography and optical spectroscopy and more sophisticated approaches (27), such as chemiluminescent detection, photofragmentation/two-photon laserinduced fluorescence detection, GC mass spectrometric detection, and membrane inlet mass spectrometry (MIMS; 28), which require more advanced, specialized instrumentation. Furthermore, recent advances in spectroscopic techniques (27), high-speed gas chromatography (29), and MIMS, among other technologies, have improved the practicality for real-time detection and monitoring, expanding the flexibility and potential of in situ measurements of gaseous samples. Rather, a major impediment to multiple dissolved gases measurements in routine water resource assessments continues to be a relatively low technology problem: how to harvest multiple gases from water for analysis by the available detection technologies without invoking fairly overwhelming logistical and material requirements. Unfortunately, sample collection, handling, and storage procedures for individual gases are frequently unique and often incompatible with one another, a general situation that forces the application of multiple sampling procedures to measure multiple gases. Among many other techniques, existing field sampling methods include collecting water in sealed bottles (with or without chemical preservation) for headspace equilibration (22, 23, 30); flame-sealing water in glass ampules under highpurity gases to protect the sample from atmospheric gains or losses (10); collecting water in copper tubes with stainless steel pinch-offs (31); and use of bubble stripping, diffusion probes, and downhole samplers (32) and vacuum flasks (11). To complete, for example, measurements of Ar, N2, O2, CO2, N2O, CH4, chlorofluorocarbons (CFC11, CFC12, and CFC113), and SF6 in groundwater by conventional headspace and gasstripping approaches, using appropriate preservations techniques to avoid the potential for aqueous transformations during handling and storage of water samples, would require between five and eight separate sampling approaches (Supporting Information). An additional complication is that the laboratory instruments used to perform headspace and gas stripping measurements require in some cases [e.g., CFCs (9), SF6 (10), and noble gases (31)] modifications (e.g., fairly VOL. 38, NO. 21, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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sophisticated and expensive valving arrangements or other instrument add-ons) from their standard configurations. In this paper, the concept and application of pumpinginduced ebullition (PIE) are introduced for the routine measurement of multiple dissolved gases. PIE is shown to provide for field collection of an essentially unlimited volume of gas sample, affording analytical accuracy and precision across major (percent), trace (parts per million by volume, ppmv), and ultratrace (parts per trillion by volume, pptv) ranges. Advantages of PIE over gas-stripping and headspace equilibration procedures include the potential to (i) consolidate multiple analyte-specific gas sampling methods into a unified procedure, (ii) reduce data computations, (iii) simplify laboratory analytical instrumentation, and (iv) avoid aqueous production and consumption of gases (e.g., biogenic gases and volatile organic chemicals) associated with the handling and storage of water samples. Furthermore, PIE is shown to provide a flexible and simple platform for the development of new in situ dissolved gas monitoring tools. The reduced time, expense, and difficulties of dissolved gas measurements potentially afforded by PIE may increase the accessibility and practicality of multiple dissolved gas measurements to a broader range of scientists.
Experimental Section Principle of PIE. For a bubble to form in water, the sum of partial pressures of volatile species (ΣPi) must be in excess of the ambient hydrostatic pressure (PH) (33):
∑P ) P i
i
N2
+ PO2 + PAr + PH2O + ... > PH
(1)
Hence, theoretically, the spontaneous formation of bubbles (ebullition) can be induced in a water sample simply by mechanically decreasing PH. In a practical sense this can be accomplished by pumping water through a narrow-diameter tube (a “restrictor”) that produces a large frictional loss of head (a pressure drop). Once bubbles are formed, their buoyancy can be exploited to trap them within a collection tower (Figure 1). The gas aggregated during pumping can be harvested for multiple chemical analyses by available detection technologies. PIE Apparatus. A schematic of the PIE device is presented in Figure 1. A corresponding photoschematic of a working PIE device is provided in Figure S1 of Supporting Information. The focus of this paper is a configuration with the inlet and restrictor line upstream of the pump, and the collection tower downstream, but the tower could also be positioned upstream. A standard peristaltic pump head (Masterflex U-070 24-21) with PharMed or Viton tubing (6.4 mm i.d.) is operated at approximately 1000 rpm (in excess of the manufacturer’s recommended 600 rpm). The “restrictor tube” (3-m length, 2 mm i.d.) provides sufficient flow resistance to induce a partial vacuum (
