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Radiocarbon Analysis of Atmospheric Formaldehyde Using Cystamine Derivatization Haiwei Shen,† Ann P. McNichol,‡ Li Xu,‡ Alan Gagnon,‡ and Brian G. Heikes*,† Center for Atmospheric Chemistry Studies, Graduate School of Oceanography, University of Rhode Island, Narragansett, Rhode Island, and National Ocean Sciences Accelerator Mass Spectrometry Facility, Department of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts A compound specific radiocarbon analysis method was developed to evaluate the 14C composition of atmospheric formaldehyde. In this method, gaseous formaldehyde was collected with a high-volume air sampler using glass-fiber filters pretreated with sodium bisulfite. Collected formaldehyde was then released into water and derivatized with cysteamine to thiazolidine. The thiazolidine was extracted into dichloromethane and concentrated by evaporation. Concentrated thiazolidine was separated from other compounds using preparative capillary gas chromatography and uniquely collected as a phosphate salt with a fraction collector. The 14 C composition of the salt was analyzed by accelerator mass spectrometry after combustion to CO2 and subsequent reduction to graphite. In a pilot study, ambient formaldehyde samples collected on the roof of the CACS building at Narragansett Bay Campus, Narragansett, RI, showed a significantly larger fraction of fossil carbon than modern carbon. Formaldehyde (CH2O) is a key intermediate in tropospheric photochemistry and is the most abundant carbonyl compound in the ambient atmosphere. Its photolysis products impact HOx abundance, O3, and consequently the oxidation rates of other atmospheric gases like NOx and SO2. CH2O arises from the photooxidation of methane and other volatile organic compounds (VOCs). Methane is expected to be the main source of CH2O in the remote atmosphere,1 while over the continents, short- to moderately-lived biogenically and anthropogenically emitted VOCs are the dominant sources of CH2O.2 In turn CH2O column densities, measured from space with satelliteborne solar backscatter instruments, have been used as a topdown check on VOC emission estimates of, for example, isoprene.3-6 Note anthropogenic as used here and below implies fossil carbon sources. * Corresponding author. † University of Rhode Island. ‡ Woods Hole Oceanographic Institution. (1) Logan, J. A.; Prather, M. J.; Wofsy, S. C.; McElroy, M. B. J. Geophys. Res. 1981, 86, 7210–7254. (2) Shen, H. Dissertation, University of Rhode Island, 2008. (3) Abbot, D. S.; Palmer, P. I.; Martn, R. V.; Chance, K. V.; Jacob, D. J.; Guenther, A. J. Geophys. Res. Lett. 2003, 30, 1886, doi:10.1029/ 2003GL017336.
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The sources of CH2O are a complex mixture of biogenic and anthropogenic VOCs whose emissions and atmospheric processing remain to be fully understood. An identification tool to distinguish the relative source contributions of biogenic and anthropogenic carbon is radiocarbon (14C). 14C is naturally produced in the stratosphere and atmospheric CO2 contains the highest level of 14C. Biogenically derived VOCs are expected to reflect the atmospheric level of 14C due to the photosynthetic uptake of atmospheric CO2, and are referred to as radiocarbon “modern”. Conversely, VOCs from fossil fuels (including coal, petroleum, and natural gas) contain no detectable 14C due to radioactive decay (14C half-life is 5730 yr). Fossil VOCs are labeled as radiocarbon “dead” or “old”.7-9 With a known 14C composition of CH2O, the proportion of CH2O derived from biogenic sources or fossil sources can be estimated. This approach has been used in studies to determine the relative fraction of fossil to biogenic sources of trace gases such as CH4,10-12 VOCs,,13,14 and carbonyls.15-17 (4) Palmer, P. I.; Abbot, D. S.; Fu, T. M.; Jacob, D. J.; Chance, K.; Kurosu, T. P.; Guenther, A.; Wiedinmyer, C.; Stanton, J. C.; Pilling, M. J.; Pressley, S. N.; Lamb, B.; Sumner, A. L. J.Geophys. Res. 2006, 111, D12315, doi: 10.1029/2005JD006689. (5) Millet, D. B.; Jacob, D. J.; Turquety, S.; Hudman, R. C.; Wu, S. L.; Fried, A.; Walega, J.; Heikes, B. G.; Blake, D. R.; Singh, H. B.; Anderson, B. E.; Clarke, A. D. J. Geophys. Res. 2006, 111, D24S02, doi:10.1029/ 2005JD006853. (6) Fu, T. M.; Jacob, D. J.; Palmer, P. I.; Chance, K.; Wang, Y. X. X.; Barletta, B.; Blake, D. R.; Stanton, J. C.; Pilling, M. J. J. Geophys. Res. 2007, 112, D06312, doi:10.1029/2006JD007853. (7) Conny, J. M.; Currie, L. A. Atmos. Environ. 1996, 30, 621–638. (8) Goldstein, A. H.; Shaw, S. L. Chem. Rev. 2003, 103, 5025–5048. (9) McNichol, A. P.; Aluwihare, L. I. Chem. Rev. 2007, 107, 443–466. (10) Lowe, D. C.; Brenninkmeijer, C. A. M.; Manning, M. R.; Sparks, R.; Wallace, G. Nature 1988, 332, 522–525. (11) Quay, P. D.; King, S. L.; Stutsman, J.; Wilbur, D. O.; Steele, L. P.; Fung, I.; Gammon, R. H.; Brown, T. A.; Farwell, G. W.; Grootes, P. M.; Schmidt, F. H. Global Biogeochem. Cycles 1991, 5, 25–47. (12) Lassey, K. R.; Lowe, D. C.; Smith, A. M. Atmos. Chem. Phys. 2007, 7, 2141– 2149. (13) Klouda, G. A.; Lewis, C. W.; Rasmussen, R. A.; Rhoderick, G. C.; Sams, R. L.; Stevens, R. K.; Currie, L. A.; Donahue, D. J.; Timothy Jull, A. J.; Seila, R. L. Environ. Sci. Technol. 1996, 30, 1098–1105. (14) Klouda, G. A.; Lewis, C. W.; Stiles, D. C.; Marolf, J. L.; Ellenson, W. D.; Lonneman, W. A. J. Geophys. Res. 2002, 107, 4072, doi:10.1029/ 2001JC000758. (15) Tanner, R. L.; Zielinska, B.; Uberna, E.; Harshfield, G.; McNichol, A. P. J. Geophys. Res. 1996, 101, 28,961–28,970. (16) Larsen, B. R.; Tudos, A.; Slanina, J.; Van der Borg, K.; Kotzias, D. Atmos. Environ. 2001, 35, 5695–5707. (17) Kato, Y.; Shinohara, N.; Yoshinaga, J.; Uchida, M.; Matsuda, A.; Yoneda, M.; Shibata, Y. Atmos. Environ. 2008, 42, 1049–1056. 10.1021/ac9004666 CCC: $40.75 2009 American Chemical Society Published on Web 07/02/2009
The radiocarbon content of atmospheric CH2O has been reported from a single study at Chebogue Point, Nova Scotia in the summer during NARE-1993.15 They found ambient CH2O to be ∼87% modern carbon and concluded the majority of the CH2O measured there was derived from biogenic precursors. Sodium bisulfite-coated cellulose filters were used to collect CH2O. Collected sodium hydroxymethanesulfonate (HMSNa) was dissolved into an acidic solution and heated at 60 °C for 10-15 min. Aliquots of this solution were prepared for radiocarbon analysis. These filters collect CH2O and other aldehydes, such as acetaldehyde,18,19 the second most abundant carbonyl in air, and possibly acetone,19 all of which would then contribute to the measured radiocarbon signal. An alternative compound specific method for 14CH2O was sought. Routine procedures for compound specific radiocarbon analysis of atmospheric trace species do not exist. Technical challenges to such a measurement of CH2O include (1) the natural abundance of 14C is low and varies from 0 to 1 ppt (part per trillion) in even biogenically derived samples, (2) the most sensitive system for 14C measurement, accelerator mass spectrometry or AMS, requires a sample to have approximately 20 µg of carbon20-22 which, assuming an average CH2O concentration of 1 ppb in the air, requires a sample volume on the order of 50 m3 to provide enough CH2O for radiocarbon AMS measurement, (3) sampling methods for such large air volumes collect other carbon containing compounds in addition to CH2O and isolation of CH2O from these other ambient carboncontaining compounds is required, and (4) the ubiquitous presence of carbon-containing compounds in air requires meticulous measures of cleanliness to prevent contamination during sample preparation, handling, and analysis. Multiple collection schemes exist for isotopic CH2O measurements. DNPH (2, 4 dinitrophenylhydrazine) cartridges, cryotraps, and sulfite-coated filters have been used to collect gaseous CH2O for 13C measurements.15,23-26 While sensitive and specific for CH2O by using DNPH collection and gas chromatography separation, DNPH cartridges can suffer from an ozone interference even with an ozone scrubber.27 Furthermore, this method introduces six additional carbon atoms for each carbon atom from CH2O into the carbon isotopic analysis,24 which dilutes the 14C from CH2O and decreases sample 14C sensitivity of the DNPH method. Cryogenic collection using Tenax has been used for 13C analysis of atmospheric CH2O.26 Tenax traps CH2O for short intervals (i.e., 99.9% fine wires, 4 × 0.5 mm >98% GC grade 85%, puriss, p.a. for HPLC silver powder, 10-20 mesh ACS reagent 95% 1200 CO, binderless 2500 QAT
Sigma-Aldrich Fisher Scientific Sigma-Aldrich Anachemia Elemental Microanalysis Limited Alfa Aesar Burdick and Jackson Fluka Spectrum Quality Products Alfa Aesar Sigma-Aldrich Sigma-Aldrich Pall Corporation Pall Corporation
higher levels of pollution; pollution containing fossil carbon VOCs and which may lead CH2O to have a low modern carbon content. Alternatively, under northerly flow, vegetative VOC emissions of species like isoprene are expected to cause CH2O to have a predominantly biogenic source with high modern carbon content. We conclude with an initial set of 14CH2O measurements made in Narragansett, RI. Their interpretation is presented elsewhere.2,35 EXPERIMENTAL SECTION Reagents and Materials. Chemical reagents and materials used in the experiment are listed in Table 1. Anhydrous sodium sulfate was precombusted at 450 °C for 5 h before use. Copper oxide and silver were precombusted at 850 °C for 5 h. Methodology for CH2O Collection and 14C Analysis. Preparation of NaHSO3-Coated Filter. NaHSO3-coated filters were prepared in a manner analogous to previous studies.15,23,28 First, 8 × 10 in. glass-fiber filters were precombusted at 450 °C in a muffle furnace for 3 h. Each precombusted filter was wetted with 20 mL of 10% w/w sodium bisulfite (NaHSO3) solution, and subsequently dried under a continuous flow of CH2O-free air at ∼2 L/min. CH2O-free drier air was prepared in-house as follows: air was supplied by either an Aadco 737 (Aadco Instruments, Inc.) pure air generator or by passing room air through an Alltech Hi-EFF organic trap. The air stream was then passed through a trap containing pH-2 water to remove any residual CH2O.36 Water in the air stream was removed using a -30 °C cold trap or a -78 °C cold trap connected to the outflow of the CH2O-trap. Filter blanks from the two drying setups were tested and showed no evidence of CH2O contamination. The coated filters were stored in a capped glass bottle at 4 °C until use. Air Sampling. The air sampling setup was adapted from Johnson and Dawson23 and Tanner et al.15 A mass-flow controlled high volume air sampler (TE-PNY1123, Tisch Environmental) was used. Two filters, an untreated quartz filter (to remove particles (33) Angevine, W. M.; Senff, C. J.; White, A. B.; Williams, E. J.; Koermer, J.; Miller, S. T. K.; Talbot, R.; Johnston, P. E.; McKeen, S. A.; Downs, T. J. Appl. Meteor. 2004, 43, 1425–1437. (34) Mao, H. T.; Talbot, R. J. Geophys. Res. 2004, 109, D20305, doi:10.1029/ 2004JD004850. (35) Shen, H.; Heikes, B.; Merrill, J.; McNichol, A. P.; Xu, L. Geophys. Res. Lett. 2009, submitted. (36) Lazrus, A. L.; Fong, K. L.; Lind, J. A. Anal. Chem. 1988, 60, 1074–1078.
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from the air stream) and a NaHSO3-coated glass fiber filter, were placed in series to collect ambient CH2O. Initial and final orifice manometer readings and air temperature were recorded to calculate total sample air volume. The average flow rate for all samples was ∼70 slpm (standard liters per minute). Sample duration was 9-13 h. After sampling, the NaHSO3-coated filter was folded with a piece of aluminum foil (precombusted at 450 °C for 3 h) into a small roll and stored in a capped glass bottle at 4 °C until analyzed. Collections were made on the roof of the CACS building (∼15 m above ground) at the University of Rhode Island Narragansett Bay Campus, Narragansett, RI. Cysteamine Derivatization of Collected CH2O. The CH2O (as HMSNa) filter samples were extracted with water and then derivatized to thiazolidine.25,37-39 Water extraction of HMSNa was carried out using an ASE 200 solvent extraction system (Dionex). Each filter was loaded into a 33 mL extraction cell and extracted with pure water at room temperature and 1000 PSI. The pure water was from a Gradient A10 Milli-Q whose feedwater was from a Barnstead FiSTREEM II Glass Still. The aqueous extract was collected in a 60 mL collection tube. After extraction, the HMSNa solution was adjusted to pH 1.5-2 with 1N HCl and placed in a 60 °C water bath for 20 min. Then 0.2 mL of 0.36 M cysteamine hydrochloride solution (14C value determined before use) was added to each HMSNa sample, the pH was adjusted to 9-10 with 1 N NaOH and 0.322 M Na2HPO4.39 This solution was left to react for at least 4 h to yield thiazolidine. The product thiazolidine was extracted from aqueous solution into 6 mL of dichloromethane three times. The dichloromethane extracts from each sample were combined and dried over precombusted anhydrous sodium sulfate, decanted, and evaporated down to ∼0.5 mL of liquid. The thiazolidine concentration was determined by GC-FID before PCGC separation. Thiazolidine Isolation Using PCGC System. An automated PCGC system (Gerstel, Inc.) was used to separate the CH2O-derived thiazolidine from other carbonyl derived thiazolidines or other organic compounds carried into the extract. Thiazolidines denote a class of heterocyclic organic compounds with a 5-membered saturated ring with an amine group and a thioet(37) Yasuhara, A.; Shibamoto, T. J. Assoc. Off. Anal. Chem. 1989, 72, 899–902. (38) Hayashi, T.; Reece, C. A.; Shibamoto, T. J. Assoc. Off. Anal. Chem. 1986, 69, 101–105. (39) Huang, T. C.; Huang, L. Z.; Ho, C. T. J. Agric. Food Chem. 1998, 46, 224– 227.
Figure 1. Schematic diagram of the preparative capillary gas chromatography instrument (adapted from Eglinton et al.29), comprised of a conventional HP gas chromatograph, programmable cooled injection system (CIS), and a preparative fraction collector (PFC).
her group in the 1 and 3 positions. Henceforth, thiazolidine specifically refers to formaldehyde-thiazolidine. The PCGC unit is similar to the one described in Eglinton et al.29 Figure 1 illustrates the major components of the system. It is comprised of a conventional Hewlett-Packard 6890 Series Plus gas chromatograph, programmable cooled injection system (CIS), and a preparative fraction collector (PFC). The HP 6890 GC was equipped with a FID and a HP 7673 autoinjector. A Rtx-1 60 m × 530 µm × 1.5 µm fused silica capillary column coated with a crossbonded 100% dimethyl polysiloxane (Restek) was used to separate thiazolidine from other compounds. The hydrogen carrier gas flow rate was 4.5 mL/min. The GC oven temperature cycle was programmed as follows: isothermal at 50 °C for 1 min, increased to 150 °C at a rate of 8 °C/min, increased to 250 °C at a rate of 20 °C/min, and isothermal at 250 °C for 2 min. The cooled injection system, CIS, integrated with liquid nitrogen cooling, permits septumless sample introduction and rapid change of temperature at its injection head. The CIS injector was operated under “pulsed splitless” mode to allow a greater amount of the injected sample to be deposited on the column. The initial and final inlet temperatures during an injection were set at 50 and 200 °C, respectively, with a heating rate of +12 °C/s. At the end of the GC column, a zero-dead-volume effluent splitter delivers ∼1% of the flow to the FID and the remaining fraction to the preparative fraction collector, PFC. The PFC consists of an 8-port zero-dead-volume splitting unit in a heated interface (200 °C) and seven traps (six sample traps and a waste trap). The PFC switching device and transfer line temperatures were kept at 200 °C during all injections. GC effluent was delivered to the trapping tube containing ∼0.28 mL of 1% H3PO4 solution during thiazolidine elution. The trapping time window for thiazolidine was set to collect its whole peak. The rest of the chromatographic run was sent to a waste trap although additional traps could have been used to collect thiazolidines produced from other filter collected carbonyl compounds like acetaldehyde. Each air sample required about 50 GC injections to process 0.2-0.3 mL of the thiazolidine extract and to isolate the thiazolidine in the trap. The collected thiazolidine salt solution was transferred into a precombusted 9 mm o.d. quartz tube. An aliquot of ∼30 µL was set aside to test its purity and the remainder in the quartz tube was prepared for AMS measurement.
Sample Preparation for AMS. The quartz tube containing the thiazolidine salt solution was connected to a vacuum line to slowly remove water. After drying, about 100 mg of copper oxide and 5 mg of silver were added to the tube. The tube was reconnected to the vacuum line, evacuated, and flame-sealed. It was combusted at 850 °C for 5 h to completely convert thiazolidine salt to CO2. Carbon dioxide was purified through a series of cold traps and quantified by manometry on the vacuum line. About 10% of the CO2 was kept for δ13C analysis by isotope ratio mass spectrometry, and the remaining 90% was reduced to graphite using NOSAMS established procedures.20,40 All 14C measurements were made with AMS at NOSAMS/WHOI. Method Development and Evaluation Tests. Auxiliary Analytical Systems. GC-MS: A Hewlett-Packard 6890 Series Plus gas chromatograph, equipped with a 5973 mass spectrometer, and a HP 7673 autoinjector, was used to assay purchased 95% thiazolidine and CH2O derivatization product thiazolidine. The oven temperature was programmed at 35 °C (45 °C, if chloroform was used as a solvent) for 2 min, increased to 80 °C at a rate of 5 °C/min, then 20 °C/min to 250 °C and isothermal for 2 min. An HP-5 ms column (30 m × 320 µm × 0.25 µm Agilent) was used for separation. Helium carrier gas flow rate was 1.8 mL/min. Injector and detector temperatures were set at 200 and 250 °C, respectively. GC-FID: An Agilent 6890N gas chromatograph equipped with a FID and a 7683B series injector was used to evaluate thiazolidine separation from other aldehyde derivatives and to perform air sample thiazolidine salt purity tests. An Agilent HP-5 column (30 m × 320 µm × 1.00 µm) was used in this GC-FID. The oven temperature was programmed the same as the GC-MS, and helium carrier gas flow rate was 1.8 mL/min as well. Injector and detector temperatures were set at 200 and 250 °C, respectively. Process Blank. Eight unexposed NaHSO3-coated glass fiber filters were individually processed and derivatized to thiazolidine. The amount of thiazolidine from each unexposed NaHSO3coated glass fiber filter was evaluated by GC-FID. In order to have sufficient carbon for radiocarbon analysis, it was necessary to combine all eight filter extracts into a single sample for PCGC and AMS measurement as one procedural/process blank. PCGC Separation of Thiazolidine and Its Efficiency. The derivatization and PCGC separation of thiazolidines were investigated. (40) McNichol, A. P.; Gagnon, A. R.; Jones, G. A.; Osborne, E. A. Radiocarbon 1992, 34, 321–329.
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First, commercially available 95% thiazolidine was used to calibrate a GC-MS and used to confirm the formaldehyde-cysteamine thiazolidine product. Second, the commercial thiazolidine and formaldehyde and acetaldehyde (10 mM each) derivatization products were used to evaluate GC separation of the aldehyde derivatives and establish the collection time window for the PCGC fraction collector. Third, the purity of the collected thiazolidineH3PO4 salt was determined by adding 1 N NaOH to the trap solution, thus returning the thiazolidine to solution, then reextracting the thiazolidine into dichloromethane and reanalyzing it on a GC-FID. Last, the overall efficiency of the PCGC process including solvent evaporation was evaluated by stoichiometrically comparing the amount of thiazolidine before PCGC processing to the amount of CO2 after the combustion of the trapped thiazolidine salt. Isotopic Fractionation Effects during the Sample Processing. Isotope fractionation41 could occur during several process steps: gaseous formaldehyde collection, water extraction and cysteamine reaction, PCGC separation, and collection in the salt trap. Three tests making use of 13C were performed to evaluate the potential for 14C isotopic fractionation during sample collection and processing. Isotopic fractionation during the conversion of CO2 to graphite is well studied and known40 and not re-examined here. Isotopic fractionation during the process from gaseous CH2O collection through to CO2 product was examined. A stack of two NaHSO3-coated filters were used to collect CH2O. The δ13C content of the ultimate CO2 produced from each filter was determined. These two filters will have identical 13C values and by implication the same 14C contents in the absence of fractionation. A total of six serial-filter collection tests were made. Isotopic fractionation from PCGC separation to CO2 was tested using a known δ13C thiazolidine. This thiazolidine was injected into the PCGC system and collected in the trapping tube as thiazolidine salt. The salt was then dried on the vacuum line, combusted to CO2 as described above, and the δ13C of the CO2 was determined. A known δ13C paraformaldehyde was used to evaluate isotopic fractionation during sample processing from aqueous formaldehyde to final CO2. A 12.2 µgC/ml CH2O solution was prepared by dissolving the paraformaldehyde into water at ∼60 °C. An aliquot of 10 mL was then processed as if it was an aqueous filter extract and the δ13C of final CO2 was determined. Safety Considerations. In this procedure, normal laboratory safety precautions associated with acids/bases, organic solvents, vacuum, compressed gases, and high temperatures are required. No exotic chemicals were used or developed that require special handling or disposal. We used aqueous phosphoric acid in the PCGC trap rather than hydrochloric acid as there is an explosion risk associated with chlorine in the subsequent thiazolidine to CO2 combustion step. Formaldehyde and para-formaldehyde are suspected carcinogens at high concentration. RESULTS AND DISCUSSION 14 C Content in Cysteamine Chloride. The 14C content of the cysteamine hydrochloride used to convert formaldehyde (41) Hoefs, J. Stable Isotope Geochemistry, 4th ed.; Springer: New York, 1998.
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to thiazolidine was 0.0020 ± 0.0003, expressed as fraction modern (Fm, Fm is defined to be the ratio of 14C to 12C of the sample to the ratio of 14C to 12C of the modern reference;9 Fm is also defined mathematically in the Supporting Information (SI)). Seven milligrams of cysteamine were analyzed. Process Blank. The thiazolidine process blank was determined from the eight filters and contained 9.47 ± 1.21 µg of carbon with 1/3 of this from CH2O (∼3 µgC). The blank was subtracted from all samples as described below and in the SI. The fraction modern of the 8-filter combined blank was 0.05. Potential sources of CH2O in the blank are (1) incomplete combustion of the filters, (2) the drying air, (3) the extraction water, and (4) the cysteamine reagent. To investigate the source of the carbon blank, two precombusted glass fiber filters (no NaHSO3 on the filter) were processed as samples. Milli-Q water, ∼35 mL, was added with the cysteamine reagent, processed as a sample, and its thiazolidine content determined. The thiazolidine content of the filter samples and water were the same indicating neither CH2O nor a contaminant was introduced by the filter. The total organic carbon levels in water used during the experiments were less than 10 ppb which is equivalent to ∼0.04 µgC in 35 mL water. We suspect the reactant cysteamine may contain trace amounts of impurities and contribute to the carbon found in the blanks. Paraformaldehyde Calibration. By comparing the δ13C values of paraformaldehyde and its processed product CO2, we examined isotope fractionation in all the sample preparation steps from aqueous CH2O to CO2: cysteamine derivatization, dichloromethane extraction of thiazolidine, PCGC isolation, evaporation, and conversion to CO2. The paraformaldehyde work indicated a small depletion effect (-4.66‰) during sample processing. Normalization of Fm to a constant δ13C value removes any impact this will have on the Fm (see SI S1). PCGC Isolation of Formaldehyde Derivative Thiazolidine. Conventional PCGC cryogenic trapping29 of a compound from the GC effluent by immersion of the trap tube in a temperature controlled cyro-unit and subsequent solvent removal by gentle N2 flow was unfavorable for the collection of thiazolidine. Attempts to trap thiazolidine cryogenically using temperatures between -196 and -20 °C had very low yields (
