Laboratory Analytical Capabilities
Each whole air sample that our group collects is analyzed at our UC-Irvine laboratory for more than 100 trace gases including C2-C10 NMHCs, C1-C2 halocarbons, C1-C5 alkyl nitrates, and selected sulfur compounds (Table 1).
|Methyl nitrate||CH3ONO2||1 mo||0.02|
|Ethyl nitrate||C2H5ONO2||2-4 wk||0.02|
|1-Propyl nitrate||C3H7ONO2||1-2 wk||0.02|
|2-Propyl nitrate||C3H7ONO2||1-3 wk||0.02|
|2-Butyl nitrate||C4H9ONO2||1-2 wk||0.02|
|2-Pentyl nitrate||C5H11ONO2||4-5 d||0.02|
|3-Pentyl nitrate||C5H11ONO2||4-5 d||0.02|
|Methyl chloroform||CH3CCl3||4.9 yr||1|
|Carbon tetrachloride||CCl4||35 yr||0.1|
|Methyl bromide||CH3Br||9-10 mo||0.1|
|Methyl chloride||CH3Cl||1.3 yr||5|
|Methyl iodide||CH3I||4 d||0.01|
|Methylene bromide||CH2Br2||3-4 mo||0.05|
|Methylene chloride||CH2Cl2||3-5 mo||1|
|Carbonyl sulfide||OCS||16 yr||10|
|Dimethyl sulfide||CH3SCH3||1-2 d||10|
|Dimethyl disulfide||CH3SSCH3||1 hr||10||Carbon disulfide||CS2||2-3 d||10|
The analysis uses gas chromatography (GC) with flame ionization detection (FID), electron capture detection (ECD), and mass spectrometric detection (MSD). We also perform exploratory measurements of other species such as oxygenated compounds and nitrogen and phosphorus species. To analyze a whole air samples, a 1520 cm3 sample aliquot (at ambient temperature and pressure) from an individual air sampling canister is introduced into the analytical system’s manifold and then passed over glass beads contained in a loop and maintained at liquid nitrogen temperature.The flow is regulated by a Brooks Instrument mass flow controller (model 5850E), and is kept below 500 cm3/min to ensure complete trapping of the relevant components. This procedure pre- concentrates the relatively less volatile components of the sample (such as halocarbons and hydrocarbons) while allowing more volatile components (such as N2, O2, and Ar) to be pumped away.
The less volatile compounds are next re-volatilized by immersing the loop containing the beads in hot water (80°C), and then flushed into a helium carrier flow (head pressure 48 psi). This sample flow is then split into six streams. We have found that the split ratios are highly reproducible as long as the specific humidity of the air is above 0.3 kPa at 298 K, which we ensure by the addition of 10 Torr (1.3 kPa) of water into each canister. Each stream is chromatographically separated on an individual column and sensed by a single detector. Each of the six column-detector combinations has unique separation and detection characteristics, and together they provide a comprehensive suite of quantified trace gases.
Three gas chromatographs (GC, each HP 6890) form the core of the analytical system. We use two electron capture detectors (ECD, sensitive to halocarbons and alkyl nitrates), two flame ionization detectors (FID, sensitive to hydrocarbons), and one quadrupole mass spectrometer detector (MSD, for unambiguous compound identification and selected ion monitoring). We have also purchased a nitrogen phosphorus detector (NPD, for detection of nitrogen species). The GC/column/detector details are given in Table 2.
The J&W Scientific DB-1 is a cross-linked surface bonded dimethyl-polysiloxane column and is the most non-polar siloxane stationary phase available. It separates gases based roughly on their boiling point. The J&W DB-5 is similar to the DB-1 but with 5% of the methyl side groups replaced with phenyl groups. It is slightly more polar than the DB-1. The DB-5ms is simply a lower bleed version of the DB-5 column. The GS-Alumina Porous Layer Open Tubular (PLOT) column is useful for complete resolution of C2-C5 hydrocarbon mixtures. Its separation properties are based on a gas-solid interaction (rather than gas-liquid). The Restek-1701 is a cross-linked surface bonded 14% cyanopropylphenyl - / 86% dimethyl -polysiloxane column whose “unique” polarity helps in the separation of halogenated compounds. We output the signal from each detector to a Spectra Physics 4400 integrator, which produces hardcopies of the analog response, and to a personal computer where the signal is recorded digitally using Labnet software (Spectra Physics, San Jose, CA). Each resulting chromatogram is manually modified, and each peak shape individually checked. This type of quality control is very important, especially for large data sets, because a slight change in retention time or peak shape can cause problems for automated quantification.
We generate our own zero-air and nitrogen for use in our FIDs and ECDs. House air is passed through a CUNO Inc. model AP101T aqua pure water filter filled with glass wool, then through a Whatman 64-02 air dryer equipped with a 100-12 BX prefilter. This removes oil, water, and particulates from the air stream, which is then split and directed into a Domnick Hunter nitrox-nitrogen generator (NG7-0) and a Praxair zero-air generator (model Airlab WHA 76803). The output from these devices are split further and directed into gas regulators for head pressure regulation. Before entering our analytical system, all gases employed are passed through homemade graphite/molecular sieve traps to remove any remaining contaminants. These traps are preconditioned (and regenerated) by flowing hydrogen gas through them at a temperature of 5˚C for at least 5 hours. Both of our FIDs operate at a detector temperature of 250˚C with a zero-air flow of 450 mL/min, an H2(g) flow of 40 mL/min, and a detector makeup gas flow of 20 mL/min N2(g). Our ECDs operate at a detector temperature of 250˚C with a detector makeup flow of 50 mL/min N2(g).
Calibration and Measurement Intercomparisons.
The relative flow passing through an individual column depends primarily on its inner diameter. We split the flow among the channels in such a way as to facilitate detection of a variety of halocarbon and hydrocarbon species. The majority of the flow is directed to the PLOT column, due in part to the lower per-molecule sensitivity of the FID and the low ambient levels of many nonmethane hydrocarbons in remote locations, as we expect to encounter during the ARCTAS mission. The split ratios are found to be highly reproducible as long as the specific humidity of the injected air is above 2 g-H2O/kg-air. For this reason (as well as to increase the stability of certain compounds in the canisters) 10 Torr of water is added to each preconditioned, evacuated canister before it is sent into the field.
Calibration is an ongoing process, whereby new standards are referenced to older certified standards, with appropriate checks for stability, and also with occasional inter-laboratory comparisons. Multiple standards are employed, including working standards that are analyzed every four samples and absolute standards that are analyzed twice daily. Our laboratory technicians regularly collect and calibrate pressurized cylinders of air from different environments for use as working standards. Our primary reference standard for halocarbons was previously calibrated from static dilutions of standards prepared in this laboratory. Its absolute accuracy is tied to a manometer measurement and how accurately the appropriate volume ratios for the dilution line used are known. For hydrocarbons, we use a propane standard purchased from the National Bureau of Standards (SRM 1660A) to calculate a Per-Carbon-Response-Factor (PCRF) for our FIDs. This is compared to PCRFs calculated from more readily available commercial standards to check the absolute accuracy of the commercial standard, as well as the appropriateness of using the same PCRF for different compounds. From analysis of the commercial standard we assign a different PCRF for each alkane, from ethane to octane. This PCRF is then used for any compound with an equivalent number of carbons. For example, the PCRF determined for butane is employed during quantification of the butenes. We have cross-checked our calibration scheme against absolute standards from other groups for both hydrocarbons and halocarbons. Additionally, we have participated in the Non-Methane Hydrocarbon Intercomparison Experiment (NOMHICE). The results of this experiment demonstrate that our analytical procedures consistently yield accurate identification of a wide range of unknown hydrocarbons and produce excellent quantitative results. We estimate our typical absolute accuracy as 2-10%, increasing as we approach our detection limits [Colman et al., 2001]. We impose a conservative limit of detection (LOD) of 3 pptv on the NMHCs. The halocarbon LOD varies by compound, from 0.01 pptv for chlorobrominated species (e.g. CHBrCl2, CHBr2Cl, CH2BrCl) to 10 pptv for CFC-12 (Table 1).
|GC-1||DB-5a / Restek-1701b||ECD||6|
a DB-5 column (30 m, I.D. 0.25 mm, film 1 mm)
b Restek-1701 column (5 m, I.D. 0.25 mm, film 0.5 mm)
c DB-5ms column (60 m, I.D. 0.25 mm, film 0.5 mm)
d DB-1 column (60 m, I.D. 0.32 mm, film 1 mm)
e DB-5ms column (30 m, I.D. 0.25 mm, film 0.5 mm)
f Restek-1701 (60 m, I.D. 0.25 mm, film 0.50 mm)
g GS-Alumina PLOT column (30 m, I.D. 0.53 mm)
h DB-1 column (5 m, I.D. 0.53 mm, film 1 mm)
Although we go to great lengths to carefully condition and re-condition our stainless steel sampling canisters, they are known to be subject to slight alkene growth during whole air storage, to a maximum of about 0.1-0.2 pptv per day. Therefore during NASA field missions the samples are analyzed within 10 days of collection, and often sooner, which limits the size of any artifact to 2 pptv or less. Because of their short atmospheric lifetimes, many alkene measurements are often below the detection limit of 3 pptv.
Blake, N. J., D. R. Blake, B. C. Sive, T.-Y. Chen, F. S. Rowland, J. E. Collins Jr., G. W. Sachse, and B. E. Anderson,
Biomass burning emissions and vertical distribution of atmospheric methyl halides and other reduced carbon gases in the South Atlantic region,
J. Geophys. Res., 101, 24151-24164, 1996.
Blake, N. J., D. R. Blake, O. W. Wingenter, B. C. Sive, C. H. Kang, D. C. Thornton, A. R. Bandy, E. Atlas, F. Flocke, J. M. Harris, and F. S. Rowland,
Aircraft measurements of latitudinal, vertical, and seasonal variations of NMHC, methyl nitrate, and selected halocarbons during ACE-1,
J. Geophys. Res., 104, 21803-21817, 1999.
Blake, N. J., D. R. Blake, I. J. Simpson, J. P. Lopez, N. A. C. Johnston, A. L. Swanson, A. S. Katzenstein, S. Meinardi, B. C. Sive, J. J. Colman, E. Atlas, F. Flocke, S. A. Vay, M. A. Avery and F. S. Rowland,
Large scale latitudinal and vertical distributions of NMHCs and selected halocarbons in the troposphere over the Pacific Ocean during the March-April 1999 Pacific Exploratory Expedition (PEM-Tropics B),
J. Geophys. Res., 106, D23, 32,627-32,644, 2001. [download pdf]
Blake, N. J., D. R. Blake, A. L. Swanson, E. Atlas, F. Flocke, and F. S. Rowland,
Latitudinal, vertical, and seasonal variations of C1-C4 alkyl nitrates in the troposphere over the Pacific Ocean during PEM-Tropics A and B: Oceanic and continental sources,
J. Geophys. Res. 108, D2, 10.1029/2001JD001444, 2003.
Blake, N. J., Streets, D., J.-H. Woo, I. J. Simpson, J. Green, S. Meinardi, K. Kita, E. Atlas, H. E. Fuelberg, G. Sachse, M. A. Avery, S. Vay, R. W. Talbot, J. E. Dibb, A. R. Bandy, D. C. Thornton, F. S. Rowland, and D. R. Blake,
Carbonyl sulfide (OCS) and carbon disulfide (CS2): Large scale distributions and emissions from Asia during TRACE-P,
J. Geophys. Res., 10.1029/2003JD004259, 2003.
Colman, J. J., A. L. Swanson, S. Meinardi, B. C. Sive, D. R. Blake, and F. S. Rowland,
Description of the analysis of a wide range of volatile organic compounds in whole air samples collected during PEM-Tropics A and B,
Anal. Chem., 73, 3723-3731, 2001.
S. Meinardi, I. J. Simpson, N. J. Blake, D. R. Blake, F. S. Rowland,
Dimethyl disulfide (DMDS) and dimethyl sulfide (DMS) emissions from biomass burning in Australia,
Geophys. Res. Lett., 30, 10.1029/2003GL016967, 2003.
Simpson, I. J., J. J. Colman, A. L. Swanson, A. R. Bandy, D. C. Thornton, D. R. Blake and F. S. Rowland,
Aircraft measurements of dimethyl sulfide (DMS) using a whole air sampling technique,
J. Atmos. Chem. 39, 191-213, 2001.
Simpson, I. J., N. J. Blake, D. R. Blake, E. Atlas, F. Flocke, J. H. Crawford, H. E. Fuelberg, C. M. Kiley, S. Meinardi, and F. S. Rowland,
Production and evolution of selected C2-C5 alkyl nitrates in tropospheric air influenced by Asian outflow,
J. Geophys. Res. 108 (D20), 8808, doi:10.1029/2002JD002830, 2003.
Simpson, I. J., N. J. Blake, D. R. Blake, S. Meinardi, M. P. Sulbaek Andersen, and F. S. Rowland,
Strong evidence against methyl chloroform emissions from biomass burning,
Geophys. Res. Lett., 34, L10805, doi:10.1029/2007GL029383, 2007.
[Breath Analysis Research]
[Laboratory Analytical Capabilities]