Article pubs.acs.org/ac
Microcollection of Gases in a Capillary Tube: Preservation of Spatial and Temporal Resolution Kristin Herrmann Favela,*,† Pieter Tans,‡ Thomas H. Jaeckle,† and William S. Williamson† †
Southwest Research Institute, 6220 Culebra Road, San Antonio, Texas 78238, United States National Oceanic and Atmospheric Administration/Earth System Research Laboratory, 325 Broadway, Boulder, Colorado 80305, United States
‡
S Supporting Information *
ABSTRACT: Researchers at the National Oceanic and Atmospheric Administration (NOAA) have recently developed and reported a novel air collection device for measuring vertical profiles of trace gases in the atmosphere, which has been applied to carbon dioxide and methane so far. The device consists of a long stainless steel tube that is prefilled with calibrated gas and allowed to ascend on a weather balloon. During ascent, the device is evacuated as it equilibrates with the decreasing atmospheric pressure. During descent, the tube is filled with atmospheric gas in an ordered manner. The diffusion rate inside the tube is slow enough that the collected gas remains ordered. Here is reported a miniaturization of the device described above, in which the stainless steel tube is replaced with Hydroguard fused silica tubing (0.53 mm × 30 m) and two lightweight valves having a total mass of less than 28 g. This micro-AirCore device was deployed on the SwRI-developed unmanned SkyWisp glider. Profiling of carbon dioxide in the atmosphere proceeded via mass spectrometric detection. A laboratory-based validation system was used for evaluating the micro-AirCore function, including accuracy and precision, for carbon dioxide. The diffusion profiles of carbon dioxide, argon, oxygen, and methane were also investigated. Overall, the microAirCore demonstrated an accuracy error of 2% (12CO2) and minimal diffusion over a period of 16 h (peak width increased by a factor of 1.6). Even after 63 h, mixing of the gases inside the tube was not complete. A triplet of micro-AirCores was deployed on the SkyWisp glider yielding a relative standard deviation of 0.08%, or 0.3 ppm, for CO2. The profile collected resulted in observation of the boundary layer with elevated CO2 levels, a region in the free troposphere with relatively constant CO2 mole fraction, and a gradual decrease in CO2 above 10 000 m. This microdevice has broad applications extending beyond vertical profiling. Fitting the device with a metering device could enable horizontal collection of gases. with the influence of recent air quality fluctuations being relatively small. The AirCore system has been successful in capturing greenhouse gas profiles in the atmosphere, but suffers from a few drawbacks. It is currently carried aloft using a balloon, which requires FAA approval because of the total size and weight. It descends with a parachute therefore the landing site cannot be precisely predicted. Miniaturization of the AirCore device will alleviate these logistical challenges because a smaller, more lightweight device could be deployed on a lightweight Unmanned Aerial System (UAS). One such UAS, owned by NOAA, is the SwRI-developed SkyWisp. SkyWisp is carried aloft on a weather balloon, but its size and weight is low enough that it is not subject to the same launch restrictions as the larger AirCore device. SkyWisp has been carried to altitudes as high as 30 000 m. Once released from the balloon, it autonomously glides downward around a set goal-point. Recovery of the glider is
A
irCore is a NOAA-developed, innovative sampling system that is capable of collecting trace gas profiles in the atmosphere.1,2 This sampling system is important for a number of reasons, including the validation of spectroscopic and satellite measurements, the observation of greenhouse gas profiles above the altitudes that can be reached by aircraft, and the validation/ refinement of atmospheric chemical transport models.3−12 AirCore is a remarkable technology in terms of its simplicity and cost-effectiveness; it is essentially a long tube sealed on one end. During ascent the tube is evacuated because of ambient pressure change and upon descent the tube fills reflecting the vertical atmospheric profile of targeted gases. Diffusion inside the tube is slow enough that the “memory” of greenhouse gas concentration as a function of altitude is retained. Once the device is recovered, its contents are currently analyzed via cavity ring down-spectroscopy (CRDS). Cavity Ring Down has provided, in practice, an exceptional accuracy level of about one part in 2000 or better.13,14 High accuracy (e.g., ideally ±0.1 ppm CO2 in an atmospheric concentration of approximately 400 ppm) is desirable for the greenhouse gases (CO2 and CH4) because they are very well mixed throughout the atmosphere, © 2012 American Chemical Society
Received: July 2, 2012 Accepted: August 30, 2012 Published: August 30, 2012 8310
dx.doi.org/10.1021/ac301707w | Anal. Chem. 2012, 84, 8310−8316
Analytical Chemistry
Article
Laboratory-Based Precision and Accuracy Tests. A series of five gas standards (labeled dry 13CO2, std1 through std5) were prepared by spiking an evacuated cylinder with the pure gas (13CO2) using a gastight syringe. The fill gas consisted of dry nitrogen containing 3515 ppm argon and 410 ppm CO (Praxair certified custom blend). The argon was intended as an internal standard for mass spectrometric measurements, and the CO is an internal standard for validation of standard concentrations by Gas Chromatography/Pulsed Discharge Helium Ionization Detection (GC/PDHID). Actual concentrations were measured by GC/PDHID (Hewitt Packard 5890A; Valco PD-D2-IS). The column (Alltech CTR1, part no. 8700) consists of an inner column (6 ft × 1/8 in. packed with porous polymer mixture) surrounded by an outer column (6 ft × 1/4 in. packed with activated molecular sieve). The carrier gas was helium; 250 μL samples were injected through a sample loop. The initial temperature was 35 °C held for 2.0 min then increased to 50 °C at a rate of 35 °C per minute. The total analysis time was 10.0 min and the flow rate was 35 mL/min. All GC/PDHID measurements were performed in triplicate. An external calibration curve was prepared using commercially available, validated gas standards. Deviations from targeted concentrations are due to inaccuracies associated with small spiking volumes and pressurizing cylinders to exact volumes. The deviation for std5 (target concentration was 400 ppm) is due to a gross preparation error (likely 13CO2 spike volume); however, this deviation is useful as a further confirmation that mass spectrometry can accurately measure these concentrations since the preparation error should be reflected in the instrument response for both GC/PDHID and micro-AirCore/MS. A small amount of CO2 (5 ppm) was found in a blank preparation and is likely due to atmospheric contamination from dead volumes in the apparatus used to prepare the standards. A calibration curve was made by measuring the instrument response for the blank and five validated gas standards by direct flow into the MS. This experiment was performed in triplicate for each standard and the blank; the MS signal was measured at three points for each replicate and results averaged. Instrument drift was accounted for by using the argon-38 signal. Pairs of standards were tested by alternating the selected gas in 10 min intervals. The response was measured at three peaks and three valleys for each experiment, and normalized results averaged across all observations of each test standard. The response was then compared to the calibration curve to calculate a concentration. In Situ Collection of Atmospheric Gases. The SwRIdeveloped stratospheric glider platform is known as Skywisp (Figure 1). The platform consists of a glider, weather balloon, science payload, and autonomous control system. The GPSbased system allows the user to set a goal point and release altitude. Upon separation from the balloon, the nonmotorized glider will aim toward and eventually pass the goal-point, causing the glider to automatically turn around. The resultant flight path approximates a spiral. The goal point can be reset at any time during the flight. The rate of descent is controlled by adjusting the pitch. Once the glider is sighted from the ground, it is remotecontrolled for landing at a user-designated location. The microAirCore devices were formed into an oval-shaped coil and installed on the underside of the wings (Supporting Information Figure 3). Two capillaries (0.53 mm × 30 m Restek Hydroguard fused silica) were installed on each wing. A short length of polyethylene tubing was installed to the inlet of each capillary such that gas collection was perpendicular to the slipstream.
therefore much easier and safer than for the large AirCore device because the landing site is precisely controlled. Miniaturization of the AirCore system is described here using fused silica tubing with an internal diameter of 0.53 mm and a length of 30 m; this type of tubing is typically used for mass spectrometry (MS). The resulting tube volume is only 6.6 mL; conventional high precision CO2 measurement systems have a cell size of typically several milliliters or more, and they are therefore not practical for the miniaturized system. MS, however, is highly sensitive and is capable of measuring
