Supporting Information for Micro-Collection of Gases in a Capillary Tube: Preservation of Spatial and Temporal Resolution AUTHOR INFORMATION Kristin A. Herrmann Favela, 1 * Pieter Tans, 2 Thomas Jaeckle, 1 William Williamson 1. 1. Southwest Research Institute, 6220 Culebra Road, San Antonio, TX 78238 2. National Oceanic and Atmospheric Administration / Earth System Research Laboratory, 325 Broadway, Boulder, CO 80305 Supporting information that follows includes: A description and schematic of the laboratory-based delivery system Experimental description and results for moisture tests A picture of the underside of the SkyWisp wing showing two micro-aircores installed for flight A figure illustrating the operational concept for in situ measurement of collected gas Results for testing with other volatile organic molecules This material is available free of charge via the Internet at http://pubs.acs.org. 1
Description of Laboratory-Based Delivery System The custom gas delivery system (SI-Figure 1) consists of two separate gas cylinders each connected to a mass flow controller (MFC) capable of maintaining flow from 0.1 ml/min to 9.0 ml/min (MKS). The mass flow controllers are fed into a six-port switching valve (Valco) which allows the flow to be directed to one of two outputs. Both outputs were constructed to be identical, consisting of a low-dead volume cross and a long capillary tube (30 m, 0.53 mm ID Restek Hydroguard fused silica). The dual construction was designed to balance the pressure so that upon switching gases, there is not a disruption in flow. The result is that both tubes become filled as mirror images of one another. During typical experiments presented here, the tube on the right was analyzed and the contents of the tube on the left ( dummy ) were discarded; an alternative set-up would be to fill the dummy tube with internal standard and alternate between the tubes during analysis. This is expected to provide improved accuracy but was not performed as part of this study; a six-port switching valve was used but only four ports were needed. The extra two ports needed to be connected using a short length of capillary thereby creating a small amount of dead volume for the dummy not present on the analytical side. If the dummy side is to be used, a four-port switching valve is ideal. All tubing after the switching valve consists of 0.53 mm capillary (Restek Hydroguard) in order to minimize dead volume. The MFCs and switching valve are fully programmable allowing precise intervals of gas to be delivered into the micro-aircore. A vacuum line provides evacuation of the apparatus to approximately 0.2-0.5 psia. During typical operation, the tubes are purged with test gas and evacuated. The tubes are filled with the desired pattern of gases with the valves at the end of the tubes closed; fill time is approximately 75 minutes at 0.1 ml/min to reach ambient pressure. The pressure on the dummy side is monitored throughout the experiment, with the micro-aircore side closed off to the vacuum gauge to prevent cross-contamination. After the micro-aircore tube reaches 2
atmospheric pressure, the micro-aircore inlet valve is closed and the gas flow stopped. The cross region just before the inlet valve is evacuated for 4 minutes to remove any residual gas that did not reach the tube. The flow path was such that the last gas delivered to the tube was the first gas detected (flow path reversed for detection). Analysis occurs by opening the micro AirCore tube to the Mass Spectrometer (MS) detector. The valve at the opposite end of the micro AirCore is also opened causing that end to remain at room pressure. The contents of the micro AirCore flow through a 3 m, 0.1 mm capillary (Restek Hydroguard fused silica) prior to reaching the MS source; this additional capillary provides restriction to lower the flow rate into the MS and control the ion source pressure. The analysis is passive, meaning that the vacuum of the MS source draws the contents through the tube, eventually ending with room air. Laboratory-Based Moisture Tests Eight cylinders were prepared to target high (600 ppm) and low (300 ppm) concentrations of 12 CO 2 using nitrogen fill gas containing 3515 ppm argon and 410 ppm CO as internal standards for microaircore/ms and GC/PDHID, respectively). Four of these cylinders were humidified (two low concentration and two high concentration, labeled hum 12 CO 2 std6 through std9). This was accomplished by adding approximately 400 µl of water to the evacuated cylinder and heating slightly to ensure saturation (100% relative humidity). The presence of an elevated level of water was confirmed by mass spectrometry. The dry cylinders are labeled dry 12 CO 2 std10 through std13. Actual concentration of 12 CO 2 in each cylinder was validated by GC/PDHID as described above. For the mass spectrometric measurement, the micro-aircore tube was filled with the test gas then analyzed as described above using argon as the internal standard. Standard deviation was calculated from triplicate measurements. 3
Results of the 12 CO 2 concentration measurement by GC/PDHID as compared to the measurement of the same cylinder by mass spectrometry are shown in SI-Figure 2. As observed for the 13 CO 2 measurements (Figure 6), the concentration as measured by GC/PDHID and micro-aircore/ms correlated well. The average percent deviation between the two measurement techniques was 2.0%. GC/PDHID and micro-aircore/ms each yielded average percent relative deviations of 1.5% and 1.3%, respectively, for replicate measurements. The humidified air did not affect the measurement of 12 CO 2 by micro-aircore/ms because the observed concentration agreed with that measured by GC/PDHID. GC/PDHID is known to not be sensitive to the presence of water vapor. Applications to Other Volatile Chemicals The micro-aircore was demonstrated for select EPA TO-15 compounds (1 ppm each of chloroform, benzene, hexane, and toluene). Using the laboratory-based validation system, 2 minute intervals of test gas separated by 10 minutes of nitrogen were delivered to the micro- AirCore device. Raw results are shown in Figure 8. An internal standard was not used to correct for detector drift. Note the tube was filled in one direction and then the contents analyzed in the opposite direction; the gas delivered to the device first was analyzed last. This is contributing to the band broadening observed as length along the tube increases. The results also demonstrate that a small chromatographic effect is occurring (separation of toluene from chloroform); in practice, results could be corrected by application of a retention factor. 4
Supporting Information Figures SI-Figure 1. Laboratory-based validation system 5
SI-Figure 2. GC/PDHID and micro-aircore/ms results for humidified and dry 12 CO 2 standards 6
SI-Figure 3. Underside of SkyWisp wing showing two micro-aircores installed for flight 7
SI-Figure 4. Raw mass spectrometric data demonstrating the operational concept for in situ measurement of collected gas with one valve (top) and both valves (bottom) open 8
SI-Figure 5. Volatile organic compounds are recovered from the micro-aircore tube at a concentration of 1 ppm (2 min intervals separated by 10 min of nitrogen 9