MS Detector System for

Jan 31, 2008 - Benjamin R. Miller,*,† Ray F. Weiss,† Peter K. Salameh,† Toste Tanhua,†,‡ Brian R. Greally,§. Jens Mu1hle,† and Peter G. S...
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Anal. Chem. 2008, 80, 1536-1545

Medusa: A Sample Preconcentration and GC/MS Detector System for in Situ Measurements of Atmospheric Trace Halocarbons, Hydrocarbons, and Sulfur Compounds Benjamin R. Miller,*,† Ray F. Weiss,† Peter K. Salameh,† Toste Tanhua,†,‡ Brian R. Greally,§ Jens Mu 1 hle,† and Peter G. Simmonds§

Scripps Institution of Oceanography, University of California, San Diego, La Jolla, California 92093, Leibniz-Institut fu¨r Meereswissenschaften, Marine Biogeochemie, Du¨sternbrooker Weg 20, D-241 05 Kiel, Germany, and School of Chemistry, University of Bristol, Bristol, BS8 1TS, U.K.

Significant changes have occurred in the anthropogenic emissions of many compounds related to the Kyoto and Montreal Protocols within the past 20 years and many of their atmospheric abundances have responded dramatically. Additionally, there are a number of related natural compounds with underdetermined source or sink budgets. A new instrument, Medusa, was developed to make the high frequency in situ measurements required for the determination of the atmospheric lifetimes and emissions of these compounds. This automated system measures a wide range of halocarbons, hydrocarbons, and sulfur compounds involved in ozone depletion and/or climate forcing, from the very volatile perfluorocarbons (PFCs, e.g., CF4 and CF3CF3) and hydrofluorocarbons (HFCs, e.g., CH3CF3) to the higher-boiling point solvents (such as CH3CCl3 and CCl2dCCl2) and CHBr3. A network of Medusa systems worldwide provides 12 in situ ambient air measurements per day of more than 38 compounds of part per trillion mole fractions and precisions up to 0.1% RSD at the five remote field stations operated by the Advanced Global Atmospheric Gases Experiment (AGAGE). This custom system couples gas chromatography/mass spectrometry (GC/MSD) with a novel scheme for cryogen-free low-temperature preconcentration (-165 °C) of analytes from 2 L samples in a two-trap process using HayeSep D adsorbent. The phase-out of the major chlorofluorocarbons (CFCs) and other chlorine and bromine containing compounds resulting from the Montreal Protocol and subsequent adjustments and amendments1 was followed by increasing atmospheric abundances of their replacements, the generally shorter-lived hydrochlorofluorocarbons (HCFCs) and hydrofluorocarbons (HFCs). As the longterm replacements for CFCs and HCFCs, HFCs show atmospheric * Corresponding author. E-mail: [email protected]. Phone: (303) 4976624. Fax: (858) 455-8306. † Scripps Institution of Oceanography. ‡ Now at Leibniz-Institut fu ¨ r Meereswissenschaften. § University of Bristol. (1) UNEP (United Nations Environment Programme). Handbook for the Montreal Protocol on Substances that Deplete the Ozone Layer, 7th ed.; Ozone Secretariat to the Vienna Convention and Montreal Protocol: Nairobi, Kenya, 2006.

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abundances that continue to rise rapidly and contribute to an increase in radiative forcing.2 Perfluorocarbons (PFCs) and SF6 have atmospheric lifetimes on the order of thousands of years and they, along with the HFCs, are three of the six main greenhouse gases targeted by the Kyoto Protocol.3 Accurate estimates of the atmospheric lifetimes of these compounds are essential in modeling the long-term chemical and radiative effects of their emissions on the global atmosphere. High-frequency in situ measurements of unpolluted, well mixed, or “baseline” air are essential to obtain reliable estimates of the regional long-term average lower tropospheric mole fractions of trace gas species. These measurements capture the fluctuations associated with synoptic-scale meteorological events as well as pollution episodes. They are used to determine rates of emission and/or chemical destruction, and hence the atmospheric lifetimes, of anthropogenic and natural compounds. Additionally, measurements of certain replacement species may be useful in future estimations of the global weighted-average lower atmospheric hydroxyl radical concentration, independent of those derived using CH3CCl34-9 and CHClF2,10 provided that their industrial emissions can be accurately characterized.11 (2) Velders, G.; Madronich, S.; Clerbaux, C.; Derwent, R.; Grutter, M.; Hauglustaine, D.; Incecik, S.; Ko, M.; Libre, J.-M.; Neilson, O.; Stordal, F.; Zhu, T.; Blake, D.; Cunnold, D.; Daniel, J.; Forster, P.; Fraser, P.; Krummel, P.; Manning, A.; Montzka, S.; Myhre, G.; O’Doherty, S.; Oram, D.; Parther, M.; Prinn, R.; Reimann, S.; Simmonds, P.; Wallington, T.; Weiss, R. Chemical and Radiative Effects of Halocarbons and Their Replacement Compounds. IPCC/TEAP Special Report on Safeguarding the Ozone Layer and the Global Climate System: Issues Related to Hydrofluorocarbons and Perfluorocarbons, IPCC Working Group I & III and TEAP; Metz, B., Kuijpers, L., Solomon, S, Anderson, S. O., Davidson, O., Pons, J., de Jager, D., Kestin, T., Manning, M., Meyer, L. A., Eds.; Cambridge University Press: Cambridge, U.K., 2005; pp 133-180. (3) Kyoto Protocol to the United Nations Framework Convention on Climate Change, United Nations, 1998. (4) Prinn, R.; Boldi, R.; Hartley, D.; Cunnold, D.; Alyea, F.; Simmonds, P.; Crawford, A.; Rasmussen, R.; Fraser, P.; Gutzler, D.; Rosen, R. J. Geophys. Res. 1992, 97 (D2), 2445-2461. (5) Prinn, R. G.; Weiss, R. F.; Miller, B. R.; Huang, J.; Alyea, F. N.; Cunnold, D. M.; Fraser, P. J.; Hartley, D. E.; Simmonds, P. G. Science 1995, 269, 187192. (6) Prinn, R. G.; Huang, J.; Weiss, R. F.; Cunnold, D. M.; Fraser, P. J.; Simmonds, P. G.; McCulloch, A.; Harth, C.; Salameh, P.; O’ Doherty, S.; Wang, R. H. J.; Porter, L.; Miller, B. R. Science 2001, 292, 1882-1888. 10.1021/ac702084k CCC: $40.75

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The measurement of these trace gas species, which as a group exhibit a wide range of volatilities and atmospheric abundances, is a challenging task. The part per trillion (ppt, 10-12) ambient air mole fractions of these compounds, many of which exhibit poor sensitivity and/or specificity in traditional chromatographic detectors such as electron capture (ECD) or flame ionization (FID) detectors, require development of new instrumentation to obtain high-precision data suitable for the stated research goals. Signalto-noise ratios may be improved by preconcentration from large sample volumes (∼2 L). Cryogenic preconcentration using liquid nitrogen12-14 or liquid argon15 can be used, but the difficulty of obtaining these consumables at remote field locations necessitates the use of self-contained cryogenic technology. Simmonds et al.16 at the University of Bristol (UB) developed a Peltier-cooled preconcentration technique for the adsorptiondesorption system (ADS), coupled with a gas chromatograph/ mass spectrometric detector (GC/MSD). The Advanced Global Atmospheric Gases Experiment (AGAGE) began in situ trace gas measurements using the ADS at the remote field locations of Mace Head, Ireland, in 1994 and in Cape Grim, Tasmania, in 1998.16,17 Data from these instruments have been used to estimate global emissions for a number of the replacement compounds (e.g., HFC134a,18-20 HCFC-141b,18-20 HCFC-142b,18,19 HCFC-22,19 HFC152a,20,21 and HFC-12520). With a single-stage preconcentration trap at -50 °C, the ADS required a mixture of two strong Carboxen adsorbents (Supelco, Bellefonte, PA) to quantitatively trap the most volatile analytes.22 Nevertheless, the PFCs, SF6, and lowboiling point HFCs exhibited breakthrough on the trap with a nominal 2 L sample. (7) Prinn, R. G.; Huang, J.; Weiss, R. F.; Cunnold, D. M.; Fraser, P. J.; Simmonds, P. G.; McCulloch, A.; Harth, C.; Reimann, S.; Salameh, P.; O’Doherty, S.; Wang, R. H. J.; Porter, L.; Miller, B. R.; Krummel, P. B. Geophys. Res. Lett. 2005, 32, L07809, DOI:10.1029/2004GL022228. (8) Montzka, S. A.; Spivakovsky, C. M.; Butler, J. H.; Elkins, J. W.; Lock, L. T.; Mondeel, D. J. Science 2000, 288, 500-503. (9) Krol, M. C.; Lelieveld, J.; Oram, D. E.; Sturrock, G. A.; Penkett, S. A.; Brenninkmeijer, C. A. M.; Gros, V.; Williams, J.; Scheeren, H. A. Nature 2003, 421, 131-135. (10) Miller, B. R.; Huang, J.; Weiss, R. F.; Prinn, R. G.; Fraser, P. J. J. Geophys. Res. 1998, 103, 13,237-13,248. (11) Huang, J.; Prinn, R. G. J. Geophys. Res. 2002, 107 (D24), 4784. (12) Montzka, S. A.; Myers, R. C.; Butler, J. H.; Elkins, J. W.; Cummings, S. O. Geophys. Res. Lett. 1993, 20, 703-706. (13) Apel, E. C.; Hills, A. J.; Lueb, R.; Zindel, S.; Eisele, S.; Riemer, D. D. J. Geophys. Res. 2003, 108 (D20), 8794. (14) Goldan, P. D.; Kuster, W. C.; Williams, E.; Murphy, P. C.; Fehsenfeld, F. C.; Meagher, J. J. Geophys. Res. 2004 109, D21309. (15) Sturges, W. T.; Wallington, T. J.; Hurley, M. D.; Shine, K. P.; Sihra, K.; Engel, A.; Oram, D. E.; Penkett, S. A.; Mulvaney, R.; Brenninkmeijer, C. A. M. Science 2000, 289, 611-613. (16) Simmonds, P. G.; O’ Doherty, S.; Nickless, G.; Sturrock, G. A.; Swaby, R.; Knight, P.; Ricketts, J.; Woffendin, G.; Smith, R. Anal. Chem. 1995, 67 (4), 717-723. (17) Prinn, R. G.; Weiss, R. F.; Fraser, P. J.; Simmonds, P. G.; Cunnold, D. M.; Alyea, F. N.; O’Doherty, S.; Salameh, P.; Miller, B. R.; Huang, J.; Wang, R. H. J.; Hartley, D. E.; Harth, C.; Steele, L. P.; Sturrock, G.; Midgley, P. M.; McCulloch, A. J. Geophys. Res. 2000, 105, 17 751-17 792. (18) Simmonds, P. G.; O’Doherty, S.; Huang, J.; Prinn, R. G.; Derwent, R. G.; Ryall, D.; Nickless, G.; Cunnold, D. M. J. Geophys. Res. 1998, 103, 16 02916 038. (19) O’Doherty, S.; Cunnold, D. M.; Manning, A.; Miller, B. R.; Wang, R. H. J.; Krummel, P. B.; Fraser, P. J.; Simmonds, P. G.; McCulloch, A.; Weiss, R. F.; Salameh, P.; Porter, L. W.; Prinn, R. G.; Huang, J.; Sturrock, G.; Ryall, D.; Derwent, R. G.; Montzka, S. A. J. Geophys. Res. 2004, 109, D06310, DOI:10.1029/2003JD004277. (20) Reimann, S.; Schaub, D.; Stemmler, K.; Folini, D.; Hill, M.; Hofer, P.; Buchmann, B.; Simmonds, P. G.; Greally, B. R.; O’Doherty, S. J. Geophys. Res. 2004, 109, D05307, DOI:10.1029/2003JD003923.

Several subsequent efforts have focused on achieving lower temperatures, also without cryogens, in an effort to extend the range of measured species. For example, Lamanna and Goldstein23 were able to trap C2-C10 volatile organic compounds, including many oxygenated compounds, on a multicomponent trap of fused silica wool, CarboSieve SIII, and Carbopak B cooled to -80 °C by contact with a two-stage, sealed immersion cooler (Neslab, CC100). Sive et al.24 reported a two-trap system for preconcentration of 98 analytes, with the most volatile being OCS (-50 °C bp), from air on glass beads chilled to -175 °C using the CC2202 Cryofocus system (MMR, Mountain View, CA). Yokouchi et al.25 combined low temperature (-150 °C) with strong adsorbents (Carboxen 1000 and Carbopak B) to extend the analyte volatility range to include three PFCs, four HFCs, and numerous HCFCs for in situ measurements at Hateruma Island, Japan, beginning in 2004. The Medusa GC/MSD system adds new compounds of interest in atmospheric research by extending the analyte range to those of lower volatility (e.g., CF4 -128 °C bp) while maintaining capability for species of higher boiling points (e.g., CHBr3, +149 °C bp). This is accomplished by a balanced combination of cold temperatures and relatively mild adsorbents with controlled removal of interfering bulk air compounds. The sampling frequency as well as the preconcentration, desorption and chromatographic characteristics of the AGAGE field GC/MSD instruments have been improved through this collaborative effort between research groups at the Scripps Institution of Oceanography (SIO) and the UB. The system described here reflects the optimizations required for AGAGE in situ field operations. However, the basic system accommodates a wide range of applications and types of samples, including air extracted from firn and samples from flux chambers used to study terrestrial trace gas emissions and sinks. An example of the use of Medusa in dissolved gas studies is exemplified by Deeds et al.’s26 measurements of gas extracts from groundwaters for estimation of natural fluxes of CF4 and SF6. Field operations of the Medusa instruments within the AGAGE program began in 2003, and systems are operational at all five AGAGE remote field stations,17 at an affiliated AGAGE station in Gosan, Jeju Island, South Korea, for monitoring Asian emissions, and at the AGAGE laboratories in La Jolla, California (SIO), and Aspendale, Australia (Commonwealth Scientific and Industrial Research Organization, CSIRO), where they are also used to measure urban air. In addition to these eight instruments, three more systems are under construction at SIO and UB. Another instrument has been constructed for deployment at the field station at Jungfraujoch, Switzerland, in cooperation with the System for Observation of Halogenated Greenhouse Gases in (21) Greally, B. R.; Manning, A. J.; Reimann, S.; McCulloch, A.; Huang, J.; Dunse, B. L.; Simmonds, P. G.; Prinn, R. G.; Fraser, P. J.; Cunnold, D. M.; O’Doherty, S.; Porter, L. W.; Stemmler, K.; Vollmer, M. K.; Lunder, C. R.; Schmidbauer, N.; Hermansen, O.; Arduini, J.; Salameh, P. K.; Krummel, P. B.; Wang, R. H. J.; Folini, D.; Weiss, R. F.; Maione, M.; Nickless, G.; Stordal, F.; Derwent, R. G. J. Geophys. Res. 2007, 112, D06308, DOI:10.1029/2006JD007527. (22) O’ Doherty, S. J.; Simmonds, P. G.; Nickless, G. J. Chromatogr. 1993, 657, 123-129. (23) Lamanna, M. S.; Goldstein, A. H. J. Geophys. Res. 1999, 104 (D17), 2124721262. (24) Sive, B. C.; Zhou, Y.; Troop, D.; Wang, Y.; Little, W. C.; Wingenter, O. W.; Russo, R. S.; Varner, R. K.; Talbot, R. Anal. Chem. 2005, 77, 6989-6998. (25) Yokouchi, Y.; Taguchi, S.; Saito, T.; Tohjima, Y.; Tanimoto, H.; Mukai, H. Geophys. Res. Lett. 2006, 33, L21814, DOI:10.1029/2006GL026403. (26) Deeds, D. A.; Vollmer, M. K.; Kulongoski, J. T.; Miller, B. R.; Mu ¨ hle, J.; Harth, C. M.; Izbicki, J. A.; Hilton, D. R.; Weiss, R. F. Geochim. Cosmochim. Acta, in press. DOI: 10.1016/j.gca.2007.11.027.

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Figure 1. Medusa flow scheme. The multiport valves (V1-V6) are shown in Phase I configuration in which ambient air (pathway shown in red) is introduced at 3 bar (controlled by the back pressure regulator, BPR) to port no. 3 of V1. The sample is dried using a Nafion membrane, the analytes are adsorbed on trap T1, and the volume is integrated by the mass flow controller (MFC). Subsequent sample pathways are shown in blue, which include fore flush of the traps to elute bulk air gases to vent, analyte transfer, and refocus on microtrap T2, followed by separations on the precolumn (CF4 only) and main column. Electronic pressure controllers (EPC3, EPC4, and EPC5) supply helium for analyte transfer throughout the system (helium only pathways are shown in green while air that is not sampled is shown in gray). Traps are chilled to -165 °C by a refrigeration cold-end, but set points up to +100 °C are controlled by resistive heating voltage applied from VAC through the trap tubing to the electrical ground on the opposite side of the adsorbent coils while a polymer union electrically isolates the remaining system.

Europe (SOGE). Medusa measurements have already been used to address some of the scientific goals outlined above. Examples of time series and deduced growth rates from in situ observations of PFCs, SF6, and some of the more volatile HFCs in Ireland and Australia are given in Greally et al.27 and examples of hydrocarbon and halocarbon emission estimates from wildfires are given in Mu¨hle et al.28 AGAGE data are available online at the U.S. Department of Energy (DOE) Carbon Dioxide Analysis Center (CDIAC) website (http://cdiac.ornl.gov). Monthly mean mole fractions from 2004 through 2006 have been published for selected halocarbons from the in situ Medusa at Cape Grim, Tasmania.29,30 Details of the design and operational principles of the Medusa and its application within the AGAGE program are reported in the following sections. METHODS Overview. The Medusa sample analysis may be considered as three separate phases of operation. In Phase I, sample gas is (27) Greally, B. R.; Simmonds, P. G.; O’ Doherty, S.; McCulloch, A.; Miller, B. R.; Salameh, P. K.; Mu ¨ hle, J.; Tanhua, T.; Harth, C.; Weiss, R. F.; Fraser, P. J.; Krummel, P. B.; Dunse, B. L.; Porter, L. W.; Prinn, R. G. Environ. Sci. 2005, 2 (2-3), 253-261. (28) Mu ¨ hle, J.; Lueker, T. J.; Su, Y.; Miller, B. R.; Prather, K. A.; Weiss, R. F. J. Geophys. Res. 2007, 112 (D03307), DOI:10.1029/2006JD007350. (29) Krummel, P.; Fraser, P.; Porter, L.; Steele, P.; Rickard, C.; Dunse, B.; Derek, N. Baseline 2003-2004; Cainey, J., Derek, N., Krummel, P., Eds.; Australian Bureau of Meteorology/CSIRO Marine and Atmospheric Research: Melbourne, Australia, 2006; pp 73-78. (30) Krummel, P. B.; Fraser, P. J.; Steele, L. P.; Porter, L. W.; Derek, N.; Rickard, C.; Dunse, B. L.; Langenfelds, R. L.; Miller, B. R.; Baly, S.; McEwan, S. The AGAGE in Situ Program for Non-CO2 Greenhouse Gases at Cape, Grim 2005-2006: Methane, Nitrous Oxide, Carbon Monoxide, Hydrogen, CFCs, HCFCs, HFCs, PFCs, Halons, Chlorocarbons, Hydrocarbons and Sulfur Hexafluoride. In Baseline Atmospheric Program (Australia) 2005-2006; Cainey, J. M., Derek, N., Krummel, P. B., Eds.; Australian Bureau of Meteorology and CSIRO Marine and Atmospheric Research: Melbourne, Australia, 2007; pp 65-77.

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introduced to one of six inlet ports on the instrument and stripped of its water and then the analytes are preconcentrated by adsorption at low-temperature on a trap (T1) while the sample volume is determined on the trap effluent by an integrating mass flow controller (MFC). In Phase II, the most volatile analyte (CF4) and residual bulk air gases (mostly N2, O2, Ar, and Kr) are transferred to a second cold microtrap (T2) for refocusing, leaving all of the less volatile analytes and most of the CO2 behind on T1. Following desorption, the CF4 and remaining air components are separated on a precolumn before transfer to the main column and MSD detection. In Phase III, the compounds that still remain on T1 are desorbed and residual water is removed during analyte transfer for refocus on T2. Finally, the analytes are desorbed to the main column (precolumn bypassed) for separation before MSD detection. This division of analysis between the processing of CF4 and that of the remaining analytes is a consequence of the abundance of bulk air gases, including CO2, in the sample that must be separated carefully from the highly volatile CF4. The details of these three preparation phases are discussed in the next section. Instrument Description. The instrument flow scheme illustrated in Figure 1 shows the relationships of the various devices and the ways in which multiport valves (V1-V6) on multi- or twoposition microactuators (Valco Instrument Company Inc., Houston, TX) can be used to isolate or combine various sections of the sample pathways. The preconcentration traps T1 and T2 perform several roles in this scheme, and their optimization is critical to the performance of the system. The traps are cooled to -165 °C using a low-temperature refrigeration unit (Cryotiger, Polycold Division of Brooks Automation, Petaluma, CA). The stainless steel tubing of the traps are of dimension 1.60 mm i.d. by 76.8 cm long for T1 and 0.51 mm i.d. by 81.9 cm long for T2.

The tubing is tightly coiled onto 1.7 cm o.d. aluminum tube standoffs, one for each trap. The standoffs are attached to a copper baseplate, which is in turn cooled by the cold-end of the Cryotiger refrigeration unit. The thin wall thickness (0.25 mm) of the standoffs restricts the thermal conductivity between the trap and baseplate/cold-end assembly, permitting one trap to be heated without unduly warming the other (see Figure S-1a,b in the Supporting Information). An empty section of tubing at the front end of each trap serves to precool the sample and adsorb the higher-boiling point analytes and residual water. This section is followed by 200 mg of 100/120 mesh HayeSep D (HSD), a high purity divinylbenzene adsorbent (Hayes Separations, Inc., Bandera, TX), for retaining the more volatile analytes. The adsorbent is held in place on each end by “packing stops” of silanized glass wool and a 2.5 cm length of tightly rolled no. 200 mesh stainless steel wire cloth, both of which also serve as surfaces to weakly adsorb higher boiling point compounds. The refocusing microtrap, T2, contains 5.5 mg of HSD adsorbent, held in place on either side by packing stops made of concentric sections of stainless steel hypodermic tubing and a smaller o.d. rod. Trap temperatures are precisely and independently controlled at various set points between -165 °C and +100 °C by resistive heating through application of low voltage and high current across the length of the trap tubing itself. The temperatures of T1, T2, and the baseplate are sensed by chromel-constantan thermocouples (TCs) affixed to the standoffs and baseplate with Omega 200, an epoxy of high thermal conductivity and wide temperature range tolerance (Omega Engineering, Inc, Stamford, CT). These TCs are connected to Omega PID temperature/process controllers (CN77354-C2) that drive a custom-made power supply for the resistive heating of the traps. A thick hard-anodized layer on the standoff electrically isolates it from the heating voltage in the trap tubing while a PEEK (polyetheretherketone) union downstream in the trap transfer line plumbing prevents electrical continuity through to valves V3 and V5. This assembly of the cold-end, baseplate, standoffs, and traps is enclosed in a high vacuum chamber for thermal isolation. Phase I of sampling begins as air at 3 bar from a sample port on V1 is dried to a dew point of about -18 °C using Nafion no. 1. This 1.8 m length of Nafion membrane (PermaPure Inc., Toms River, NJ) is impermeable to Medusa analytes and bulk air gases but permeable to water, which is vented with a ∼250 cm3 min-1 (STP) countercurrent flow of dry, analyte-free air from a Balston TOC gas generator (Parker Hannifin Corporation, Cleveland, OH). The flow rate of a nominal 2 L of sample gas through T1 is controlled at 100 cm3 min-1 (STP) by an integrating mass flow controller (MFC) located downstream of T1. The subatmospheric pressure MEMS-Flow MFC (Redwood Microsystems Inc., CA, now defunct) used in Medusa has a manufacturer-specified repeatability of (0.5%, but the overall analysis precisions of Medusa indicate a significantly higher precision in volume measurement. Another mass flow controller of comparable precision, the Red-Y Smart Controller GSC (Vo¨gtlin Instruments AG, Switzerland), has been demonstrated to be a suitable replacement for the MEMS-Flow in Medusa. An All Sensor pressure transducer (100PSI-A-DO, All Sensor Corporation, Morgan Hill, CA), denoted P1 in Figure 1, located immediately upstream of the MFC allows detection of flow anomalies that might arise from malfunctions

such as trap obstruction and Valco valve failures. At the end of Phase I, all of the analytes and some residual air gases from the sample are concentrated on T1. Three electronic pressure control (EPC) devices of the Agilent 6890 GC control the research grade 6.0 helium carrier gas in the remaining phases of sample preparation. Phase II for CF4 processing and analysis involves transfer to T2. Note that V5 is a nonstandard configuration of a multiposition actuator mated to a two-position valve. Furthermore, custom rotor engravings for V5 allow either isolation or in-line positioning of T2 while maintaining carrier flow through the main column in any one of three valve positions, as indicated by the (30° rotor rotation illustrated in Figure 1. The fore flush transfer of CF4 from T1 to T2 begins when T2 at -165 °C is put in-line and T1 is warmed to -65 °C. A 5 min helium fore flush of T1 from EPC4 in this configuration results in near 100% CF4 transfer (see Results) but also transfers significant amounts of CO2 and other bulk air gases along with the CF4. The flow rate, temperature, and time of this T1 to T2 transfer are optimized so that transfer of the broad CF4 peak is maximized while transfer of the relatively sharp CO2 peak that follows is minimized. A second All Sensor pressure transducer (5INCH-D-DO, P2 in Figure 1) located between V5 and a flowmeter provides a sensing of the transfer flow during this process. Following a 3 min EPC4 helium fore flush of T2 at -115 °C to reduce bulk air gases other than CO2, T2 is taken out of line and isolated while heated to +100 °C within 30 s. The contents of T2 are then back flushed using EPC3 to a micropacked precolumn for separation of CF4 from CO2 and the remaining air gases. Although major amounts of the bulk air gases have been removed in previous helium fore flush steps, a chromatographic separation of the low-abundance and most volatile analyte CF4 from the Ar, Kr, O2, N2, and CO2 remaining on T2 is necessary to avoid interferences in the MSD. These separations are achieved on a custom micropacked precolumn of 40 mg of 100/120 mesh molecular sieve 4 Å (MS 4 Å) followed by 160 mg of 100/120 mesh HiSiv-3000 (HiSiv, UOP, Des Plaines, IL) in a 80 cm long by 0.75 mm i.d. stainless steel column (Restek Corp., Bellefonte, PA) under isothermal conditions at 40 °C. The MS 4 Å section of this column retains the CO2, precluding its elution onto the HiSiv section with CF4. The HiSiv has a higher affinity for the CF4 than for the remaining air gases. This separation is largely preserved during the chromatography that follows at 40 °C on a CPPoraBOND Q fused silica PLOT main column (25 m long, 0.32 mm i.d., 5 µm film thickness, Varian Inc., Palo Alto, CA). Thus the MS 4 Å/HiSiv column is the dominant factor in these separations, with O2 and Ar eluting early at 121 s after injection, followed by N2 at 128 s, Kr at 202 s, and then CF4 at 410 s. Between analyses, CO2 is removed from the MS 4 Å by helium back flush at 200 °C. Throughout Phase II, all analytes other than CF4 have remained trapped on T1. Midway through Phase II, Phase III begins with an additional fore flush of T1 at -58 °C with helium from EPC4 to further reduce residual air gases. In this step, it is critical to fore flush the trapped xenon to waste to reduce its chromatographic interference with the SF6 (m/z ) 127). To achieve this separation, the least-retained analyte, CF3CF3, is driven almost to breakthrough on T1 by fore flush before the flow Analytical Chemistry, Vol. 80, No. 5, March 1, 2008

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Figure 2. A strip chart for a typical air analysis illustrates the amount and detail of diagnostic information stored for each analysis. These plots may be used to detect performance issues before they become apparent in the chromatography. The pressures of helium used for carrier gas, fore flush and back flush of traps T1 and T2 are controlled by electronic pressure controllers (EPC3, EPC4, and EPC5). Two pressure transducers (P1 and P2) are used to monitor sample transfer flows. The temperature of the thermostated sample module and mass flow controller (MFC) upstream and downstream pressures are also stored, but here they are omitted for clarity. The three phases of operation (sample preconcentration, CF4 analysis, and analysis of all other analytes) are labeled as Phase I, II, and III.

is reversed and the analytes are desorbed at +100 °C for transfer to and refocus on T2 at -165 °C. During this transfer, the desorbed sample passes back through Nafion no. 2, efficiently removing more of the residual water that had been preconcentrated on T1. After transfer, T2 is isolated and heated to +100 °C before a back flush with helium from EPC3 transfers the analytes onto the CP-PoraBOND Q column for separation and subsequent MSD detection. The column temperature is ramped (40 °C isothermal for 1 min, then ramp at 22.9 °C/min and hold at 200 °C) to maintain narrow peak widths and chromatographic resolution. The EPC3 pressure is correspondingly ramped (1.44 bar for 1 min, then ramp at 0.084 bar/min to hold at 2.03 bar) to compensate for changing carrier gas viscosity and maintain constant flow rate. The chromatographic separations overlap by 10 min with the sample preparation process of the next analysis, resulting in an analysis time of 60 min and 24 analyses per day. Instrument Control and Data Acquisition. Medusa software is based on a previous version developed for the AGAGE multidetector instrument,17 with additional support for the Agilent 5973/5975 quadrupole MSD and improvements in the structure required for the more complex Medusa operation. This Linuxbased software allows complete automated control of 17 devices (including multiport valves, MFC, trap temperature set points, GC oven temperature and EPC pressure ramps, and MSD), display of chromatograms and instrument parameters, peak integration, and graphical and tabulated display of all results. Once per week a special sequence of analyses measures a system blank and the laboratory air to check for contamination of the instrumental environment. Several improvements were made in the 5973/5975 MSD control and data acquisition software with the aid of Agilent’s proprietary protocol. Biweekly MSD tunes are performed to optimize the detector parameters for the specific range of ion 1540

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masses of interest in AGAGE. For investigative purposes, the MSD may be operated in SCAN mode that cycles incrementally through a mass range. For maximum sensitivity in routine field monitoring, the MSD is operated in selective ion mode (SIM). In this mode, the MSD cycles through a set number (2-14) of target and qualifier ion masses during a fixed period (SIM window) of chromatographic elution, with 13 optimally configured SIM windows per chromatogram and yielding analysis of more than 38 analytes. The range of ion masses of interest in AGAGE (26181 amu) is relatively small compared to the much larger capability range of Agilent’s 5973/5975 MSD and results in relatively fast signal stabilization when changing from one ion mass to the next. When stepping to the next ion mass, the custom software shortens the SIM delay (period of discarded signal during stabilization) from Agilent’s default value of 15 ms to approximately 3 ms, thus recovering 12 ms of dwell time per SIM ion (in newer Agilent models, the default SIM delay is now below 3 ms). This results in approximately 37% more available dwell time for a typical SIM window with 12 SIM ions and dwell time of the order of 10 ms (short dwell times are required to define the sharp peaks with width at half-height approximately 2 s). This allows monitoring of additional ion masses per SIM window without sacrificing sensitivity or loss of peak definition. By this means, one or more qualifier ions are monitored in addition to target ions for most of the analytes, yielding an improved diagnostic for determining chromatographic coelution. In complement to each sample chromatogram, a “strip chart” record is stored with data from 13 temperature, pressure, and flow transducers that monitor almost every major device and process described in the analyses above (see Figure 2). This information may be used to detect instrument performance anomalies before they become apparent in the chromatograms.

Sample Acquisition and Calibration. In situ air sampling is performed in such a way as to minimize manipulation of the gas and thus reduce the potential for sample degradation or contamination. Each station has a 10-75 m high tower equipped with a sample inlet and a connected length of tubing that delivers ambient air directly into the nearby analysis laboratory. Air is drawn through this tubing, which is chosen for its inertness (instrumentgrade stainless steel, Restek and/or type 1300 Synflex tubing from Eaton Corporation, Cleveland, OH), at a high total flow rate of ∼10 L min-1 to reduce residence time using a custom air pump module. The main flow of air goes to a Gast oil-free linear pump (Gast Manufacturing, Inc., Benton Harbor, MI), with a tee in the line upstream of the pump to divert the sample portion of the gas to a separate section pressurized by a KNF UN05-SVI diaphragm pump (KNF Neuberger, Trenton, NJ). The output of the KNF pump is split between a transfer line that directly delivers the actual air to be sampled to an inlet on the Medusa (port no. 3 of V1 in Figure 1) and a backpressure regulator that controls the sample pressure at 3 bar and vents at ∼2 L min-1. The original Viton (fluoropolymer) diaphragm and valve plate of the KNF pump were found to contaminate the sample with CHF3 by ∼25%-130% above ambient values. In mid-2007, this material was replaced with Neoprene (chloropolymer) that showed no contamination for Medusa analytes. In order to achieve the previously stated scientific goals, the ambient air measurements from the AGAGE global network must be calibrated relative to a single reference scale. Accurate absolute calibration for ppt levels of trace gases requires intensive preparations of “primary standards” from pure analytes in “airlike” mixtures to minimize analytical artifacts arising from differences between real air and these synthetic blends. To date, primary standards have been gravimetrically prepared at SIO17,31 for 25 of the halocarbon and sulfur compounds measured by Medusa. While absolute calibration for all analytes is the ultimate goal, and such preparation work continues to add new compounds and refine previous absolute scales, it is necessary to maintain a “relative scale” of compressed real air standards (thus containing all analytes) that are collected and archived periodically to minimize differences between these standards and the changing atmosphere. The AGAGE relative scale is denoted “SIO-R1” and is comprised of a hierarchy of compressed (40-60 bar) whole air samples (denoted secondary, tertiary, and quaternary gases) stored in 35 L electropolished stainless steel tanks humidified to ∼0.03 atm for passivation against analyte degradation. Medusa ambient air measurements are made alternately with analysis of the quaternary gas, a comparison that permits detector drift correction. The quaternary gases are filled on-site at three of the AGAGE remote field stations (Trinidad Head in California, Mace Head in Ireland, and Cape Grim in Tasmania), thereby assuring similarity between this reference gas and ambient air samples as well as simplifying the supply of this rapidly consumed gas. Calibration of the quaternary gas is achieved on-site by weekly quadruple comparisons with a tertiary gas. While a given tank of quaternary gas is consumed within about 2 months, the tertiary gases are sampled infrequently enough that they provide the continuity of a single calibration gas for about 9 months. Tertiary (31) Miller, B. R. Abundances and Trends of Atmospheric Chlorodifluoromethane and Bromomethane. Ph.D. Thesis, University of California, San Diego, CA, 1998.

gases are given before- and after-field-service calibrations on a single instrument (Medusa1 at SIO) relative to the secondary gases, which are permanently archived at SIO. Tanks of tertiary and secondary gases are filled yearly at Trinidad Head to incorporate new members into the relative scale that reflect the changing atmosphere. One of the secondary gases from April 2003 was designated “R1” and it is the ratios of ambient air analyses to R1 that are ultimately calculated as the SIO-R1 relative scale values for field data. Absolute calibration of the SIO-R1 scale is achieved by Medusa comparison of R1 and/or secondary gases with primary standards. The resulting “R1 factors” allow the measurements from all stations to be placed on a common traceable absolute calibration scale. RESULTS The CP-PoraBOND Q capillary column, when combined with the specificity of the MSD, sufficiently resolves most compounds of interest in AGAGE. Separation performance of this column does change with time, despite the relatively clean samples that are analyzed, and column replacement is required every few years. For example, on a new column, CH2Cl2 and CCl3F are resolved. But they begin to coelute with column aging, with the two minor ions (49 and 84) of CCl3F interfering with these major ions of CH2Cl2. In most cases involving coelution on a new column, conflicts can be resolved by using a less abundant ion. In the few instances where baseline resolution is not achieved using alternate ions, peaks are quantitated by height instead of the generally more precise area measurement. Table 1 lists the Medusa target and qualifier ions and denotes those requiring quantitation by peak height on a new CP-PoraBond Q column. Figure S-2a-g in the Supporting Information shows typical chromatograms for all compounds listed in Table 1. As demonstrated by the precisions listed in Table 1 for 1 year of data (∼3600 measurements) from the remote station at Trinidad Head, California, Medusa yields long-term stability and high precision. For example, the abundant analytes CF4 and CCl2F2 yield percent relative standard deviations (%RSD) of standard gas analyses of ∼0.1% or better. Less abundant compounds, or those with chromatographic interferences that require use of a minor ion, exhibit poorer precisions due to decreased signal-to-noise ratio. The proportionality of instrument response to variations in sample composition, often simply termed “linearity”, must be assessed in applications for which differences in analyte or other constituent mole fractions are anticipated between standard and sample. Such differences may arise from, for example, hemispheric gradients, seasonal cycles, pollution events, and/or large trends in atmospheric abundances. Instrument nonlinearity may be considered in terms of response of the detector as distinct from effects induced by sample preparation prior to detection (e.g., breakthrough during trapping, contamination, degradation/losses, and coelution interferences) and peak integration method. In AGAGE, the close similarity in composition between the real air standards and ambient air samples reduces the affect of nonlinearity on the in situ field measurements, but it is essential to quantify and correct for nonlinearity when it is significant. To assess linearity, instrument responses (peak areas or heights) are determined for a range of moles of analyte and the corresponding sensitivities (responses per mole) are evaluated Analytical Chemistry, Vol. 80, No. 5, March 1, 2008

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Table 1. Selected Medusa Analytes, Main Column Retention Times, Target and Qualifier Ion Masses, Mid-2006 Abundances at Trinidad Head, Northern California, and Standard Precisions chemical formula

industrial name

CP-PoraBOND Q retention time (s)

target (qualifiers) ions (amu)

Trinidad Head abundance (mid-2006) (ppt)b

2006 annual mean standard precision (% RSD)c

CF4 CF3CF3 CHF3 CHtCH SF6 CH2F2 CClF3 C2H6 SO2F2 CF3CF2CF3 CH3CF3 COS CBrF3 CHF2CF3 CClF2CF3 CH3CHF2 CH2FCF3 CHClF2 CH3Cl CCl2F2 CH3Br CH3CClF2 CHClFCF3 CBrClF2 CClF2CClF2 CH3I CH2Cl2 CCl3F CH3CF2CH2CF3 CH3CCl2F CCl2FCClF2 CBrF2CBrF2 CHCl3 C6H6 CCl4 CH3CCl3 CCl2dCCl2 CHBr3

carbon tetrafluoride PFC-116 fluoroform acetylene sulfur hexafluoride difluoromethane CFC-13 ethane sulfuryl fluoride PFC-218 HFC-143a carbonyl sulfide halon 1301 HFC-125 CFC-115 HFC-152a HFC-134a HCFC-22 methyl chloride CFC-12 methyl bromide HCFC-142b HCFC-124 halon 1211 CFC-114 methyl iodide dichloromethane CFC-11 HFC-365mfc HCFC-141b CFC-113 halon 2402 chloroform benzene carbon tetrachloride methyl chloroform perchloroethylene bromoform

289a 254 255 258 258 264 265 266 271 304 304 309 313 318 334 337 340 342 351 375 417 418 431 448 457 503 505 507 519 526 570 580 583 622 623 623 695 776

69 (50) 119 (69) 51 (69) 26 (27) 127 (89) 33 (51) 85 (69) 26 (27, 28) 83 (85) 169 65 (64) 60 (64) 129 (148, 131) 101 (51) 119 (135) 65 (46) 83 (33) 67 (50, 52) 52 (50) 85 (87, 101) 94 (96) 65 (85) 67 (101) 85 (129) 135 (137) 142 (127) 84 (86, 85) 103 (105, 101) 133 (65) 81 (83, 61) 153 (155, 85) 179 (181) 83 (85) 78 82 (84) 99 (97) 166 (164) 173 (171, 175)

76 3.8 22 NA 6.1 2.3 2.9 1200 1.5 0.48 7.4 500 3.2 5.1 8.4 7.0 45 190 540 540 8.5 18 1.7 4.5 17 1.6 34 250 0.41 20 78 0.49 11 80 89 16 3.6 NA

0.11 0.84 0.46 0.44 (ht) 0.51 3.3d 1.4 (ht) 0.19 1.0 2.7 0.88 0.17e 1.7 0.67 0.86 0.58 0.43 0.26 0.19 0.12 0.40 0.32 1.7 (ht) 0.35 0.25 1.7 0.35 0.18 4.9 0.41 0.22 1.8 0.33 0.25 1.5 (ht) 1.3 (ht) 0.44 1.2

a CP-PoraBOND Q column retention times for all compounds are relative to the air peak from the second desorption of trap T2, except for CF , 4 which is relative to the air peak of the first desorption. b Approximate 2006 annual mean clean air mole fractions (ppt, 10-12) observed at Trinidad Head, CA, listed for comparison with standard precisions. The 25 compounds listed in bold are on SIO gravimetric absolute calibration scales, nonbold values are calibrated relative to other scales, and those listed as “NA” lack absolute calibration. c Annual mean percent relative standard deviations (%RSD) of ∼3600 standard gas analyses from 1 year (2006) of data from Trinidad Head, CA. Peak areas were used unless noted as (ht) for peak heights. d CH2F2 measurements using the target ion mass 33 began in late 2007 (∼790 standard gas analyses). e COS measurements using the target ion mass 60 began in early 2007 (∼1700 standard gas analyses).

for linearity. In the example shown in Figure 3, these sensitivities and responses have been normalized with respect to alternated analyses of a reference standard to which the analyte mole ratios are accurately known. In such a representation, a “linear” instrument yields values of normalized sensitivity equal to unity for the range of normalized responses. Experimentally, the moles of analyte can be varied by analyses of samples of various analyte mole fractions (“variable mole fraction” method). To produce such samples, seven subsamples from a single ambient air sample were diluted into fused-silica lined SilcoCan flasks (Restek Inc.) with dry analyte-free or “zero” air (Scott-Marrin, Inc., Riverside, CA) and the dilution factors (ranging 5-75% ambient mole fractions) were determined by CH4 analyses on a GC-FID instrument of known linearity.32 Prior to the zero air addition step, the zero air was repurified using a 8.6 m long column of glass beads, activated charcoal, molecular sieve 13X, and Carboxen 1000 at -97 °C and then verified analyte-free on Medusa. While many analytes showed 1542 Analytical Chemistry, Vol. 80, No. 5, March 1, 2008

linearity within measurement precision, the CCl2F2 example in Figure 3 shows a slight nonlinearity of ∼1% increased sensitivity for samples that are less than half the CCl2F2 mole fraction of the standard. The routine application of the above method for periodic linearity determination of Medusa systems at all seven AGAGE installations is not feasible due to the cost and intensive labor involved in maintaining sets of mixtures. Furthermore, preparations of mixtures of mole fractions greater than the standard are required to extend the range to higher responses. Therefore, for routine work, a technique was adapted from previous applications (32) Francey, R. J.; Steele, L. P.; Langenfelds, R. L.; Lucarelli, M. P.; Allison, C. E.; Beardsmore, D. J.; Coram, S. A.; Derek, N.; de Silva, F. R.; Etheridge, D. M.; Fraser, P. J.; Henry, R. J.; Turner, B.; Welch, E. D.; Spencer, D. A.; Cooper, L. N. Baseline Atmospheric Program Australia 1993; Francey, R. J., Dick, A. L., Derek, N., Eds.; Bureau of Meteorology and Commonwealth Scientific and Industrial Research Organization, Division of Atmospheric Research: Melbourne, Australia 1996; pp 8-29.

Figure 3. Instrument “linearity” plotted as the unitless quantities of normalized sensitivity versus normalized peak response. Sensitivity and response are normalized to samplings of standard for which these quantities are defined as unity (O). A “linear” instrument thus yields normalized sensitivity values of unity over a range of responses. In this example for CCl2F2, the instrument exhibits a ∼1% greater sensitivity for analyte amounts of less than one-half that of the standard. Two independent methods of varying the moles of analyte for linearity assessment are shown here to be in agreement: a “variable volume method” (blue dot means, 1 σ error bars) in which different volumes (0.1-4 L) of a single air standard are analyzed and a “variable mole fraction method” (red dot means, 1 σ error bars) involving 2 L analyses of seven flask subsamples of a single air standard that has been diluted with different amounts of analyte-free air. The blue and red lines are power series polynomial fits of the variable volume and variable mole fraction methods, respectively.

involving cryo-focus of sample for GC analysis31,33 that varies the analyte moles by variation of the volume sampled from a single standard of compressed air. Thus the quaternary gas at each station is used to periodically assess instrument linearity over a wide range of anticipated analyte moles using this “variable volume method.” Figure 3 shows an example of the statistical agreement observed for nonlinearity determination between these two independent methods of varying analyte moles. It should be noted that the sample volume indicated by the MFC differs from the total sample volume that actually passes through T1 by an amount Vcorr. This empirically determined additive correction is a function of adsorbent mass, temperature and pressure changes in T1 during sampling, and the relative sensitivity of the MFC to different gas compositions (helium carrier gas versus air), as illustrated for the MEMS-Flow MFC in Figure 4. Because of these MFC sensitivity differences, the Vcorr from use of the Red-Y MFC was found to be ∼18% larger than that from use of the MEMS-Flow. This implies a helium to air response ratio in the Red-Y that is about 6 times greater than that of the MEMS-Flow, consistent with the respective thermal conductivity versus pressure difference principles of operation of the two MFCs. The use of Nafion dryers for water removal in GC analyses of hydrocarbons, particularly VOCs, has the potential for artifact formation. Nafion, a fluoropolymer that contains sulfonic acid groups, is permeable to many alcohols. Furthermore, Nafion can function as a strong acid catalyst and transform carbonylcontaining compounds to alcohols. Also, organic compounds that contain double or triple bonds between carbon atoms, or between (33) Bullister, J. L.; Weiss, R. F. Deep-Sea Res. 1988, 35, 839-853. (34) Draxler, R. R.; Hess, G. D. Aust. Meteorol. Mag. 1998, 47, 295-308.

Figure 4. The sample volume indicated by the mass flow controller (MFC) differs from the total sample volume that actually passes through trap T1 by an amount Vcorr. On the basis of instrument linearity assessments involving different volume samplings of the same air standard, Vcorr is determined empirically as the constant that must be added to the MFC-measured volumes in order to minimize the deviations of the resulting sensitivities from linearity. With the use of the MEMS-Flow MFC, Vcorr values (b) of 33 and 42 cm3 were determined for 133 and 200 mg of HayeSep D (HSD) adsorbent, respectively. Vcorr has three components, each of which varies linearly with the amount of HSD in T1. VdPT is the difference between the volume of air in T1 at the conclusion of sampling minus the volume of helium carrier gas in T1 prior to the beginning of sample flow. VMFC is a correction for the difference in MEMS-Flow response to helium versus air (factor of ∼2.9) for that volume of helium initially in T1. Vads, the volume of air retained by adsorption on the HSD, is determined by difference. Note that Vads becomes negligible for zero adsorbent mass.

carbon and other elements, can undergo acid catalysis causing an apparent loss of analyte. Simple tests (see The Effects of Nafion Dryer on Medusa Analytes in the Supporting Information) were performed for the 38 Medusa analytes listed in Table 1 to determine the potential for artifact formation from use of Nafion dryers. When a whole air sample was repeatedly analyzed, first on one sample inlet (reference) and then on a second inlet that was fitted with one or more Nafion dryers (treated sample), comparison of these results showed no discernible affects within standard errors for 15 Medusa analytes. The remaining compounds showed small enhancements and losses (generally