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New Approaches to Measuring Sticky Molecules: Improvement of Instrumental Response Times Using Active Passivation J. R. Roscioli,* M. S. Zahniser, D. D. Nelson, S. C. Herndon, and C. E. Kolb Center for Atmospheric and Environmental Chemistry, Aerodyne Research, Inc., 45 Manning Road, Billerica, Massachusetts 01821, United States

Downloaded by UNIV OF NEBRASKA-LINCOLN on August 31, 2015 | http://pubs.acs.org Publication Date (Web): July 7, 2015 | doi: 10.1021/acs.jpca.5b04395

S Supporting Information *

ABSTRACT: A novel method has been developed to improve sampling system response times for nominally “sticky” molecules such as HNO3 and NH3. The method reported here makes use of active, continuous passivation, where the instrument interfaces are continuously exposed to 0.01−1 ppm of fluorinated acidic or basic surfactants. To reduce HNO3 response times, perfluoroheptanoic acid and perfluorobutanesulfonic acid vapors are evaluated as passivation species. 1H,1H-perfluorooctylamine is used to improve NH3 response times. The resulting time responses using the perfluoroalkanoic acids are on the order of 0.4−0.7 s for a 75% quantitative recovery of HNO3, and 1−5 s for 90% recovery. Similar response time improvements are seen in detection of NH3 using perfluorooctylamine (10 measurements of each data point.

demonstrating that the improved performance is largely independent of humidity. The presence of water at the interfaces of the inlet is likely a primary influence on HNO3 “stickiness”. Saliba et al. have shown that at room temperature, anywhere between 1 and 12 monolayers of water (where one monolayer correspond to ∼1 × 1015 #/cm2) adsorbs to the surface of silica at room temperature, depending upon relative humidity.33 Given that HNO3 is a strong acid, it would be reasonable that the presence of multiple layers of water at an interface serves to bind and solvate HNO3 at intermediate humidity, enhancing the binding efficiency. In light of this, a primary goal of any passivation should therefore be to remove water to prevent further HNO3 binding. Although moderate inlet heating can be very helpful in this regard, it may not successfully remove all water because of (a) the possibly strong binding between water and the surface, which could be larger than the water−water interaction and (b) the presence of small crevices and pockets in all but the most pristine surfaces, which could be difficult to evacuate with heat. That PFHpA and PFBSA improve the response time of the inlet suggests that they are not only ejecting HNO3 but also efficiently removing or mitigating the effects of water at the interface. Although a typical passivation level of 50−500 ppb is large compared to the measured HNO3 concentration (∼5 ppb), it is a factor of 103−105 smaller than typical ambient H2O concentrations. Thus, the efficiency of the continuous passivation method is high considering its requirement to

response exponentially decreases with concentration before leveling off. At 200 ppb PFHpA, where the 75% and 90% response times approach their lowest values, the PFHpA consumption rate is approximately 3 mg/h. Similarly, at 60 ppb PFBSA, the consumption rate is 0.6 mg/h. As can be seen in Figure 5, passivation with PFBSA can provide faster response times than with PFHpA. However, at PFBSA concentrations above ∼100 ppb the time response of the instrument begins to increase again, in addition to the decrease of instrument multipass mirror reflectivity (as discussed above). The position of this minimum value may depend upon ambient concentrations of NH3 and H2O. An interesting aspect of the concentration dependence is the asymptotic values of 0.4−0.6 s for 75% and 2−4 s for 90% response. As discussed above, the fastest possible time scale based on the cell volume (625 cm3) and flow rate through the cell (300 LPM) is a 1/e time of 0.1−0.2 s, with observed τ75 and τ90 values of 0.18 and 0.23 s. Although the 75% and 90% points do not achieve this, the 1/e time associated with the faster of the two exponentials in the fit is 0.1−0.5 s, indicating F

DOI: 10.1021/acs.jpca.5b04395 J. Phys. Chem. A XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry A eliminate the effects of water at the interface. In principle, passivation may not eject water at all, but instead simply bind the surface water molecules to its highly polar acid group, preventing them from solvating HNO3. In that case, however, any acid-functionalized species, such as heptanoic acid, should provide the same improved time response. As described above, heptanoic acid ejects the HNO3 from the surface but does not improve time response. As in the case of the increase in HNO3 mixing ratio when PFHpA is injected into the inlet there may similarly be a concomitant rise and fall in H2O signal upon passivation. For a typical H2O monolayer density of ∼1 × 1015 molecules/cm2, such a rise would be expected to be on the order of ∼1 ppm, below the H2O sensitivity of the spectrometer of ∼20 ppm. Even in the limit of air saturated with water, where ∼10 ML of H2O might be present at the interface,33 the H2O ejection event would likely be below the detection limit. Such a measurement is complicated by the fact that H2O may be present in the passivation material and would enter the inlet upon passivation. Future work will be aimed at increasing sensitivity toward H2O to further understand the interactions between HNO3 and H2O at the interface, and how passivation affects this interaction. Eddy Flux Measurements Using Active Passivation. As a demonstration of the utility of active passivation, the instrument described above was deployed to measure HNO3 fluxes in a central U.S. prairie using eddy covariance. The HNO3 mixing ratio was measured at 5 Hz using a QCL spectrometer with a 204 m multipass cell, providing a 1 s HNO3 sensitivity of 60 pptv. The wind was measured at 5 Hz with an R. M. Young 3D sonic anemometer (model 81000RE). The eddy fluxes were measured on a tripod at 2.5 m height, with a heated (60 °C), 25 m sample transfer line from the inertial inlet to the spectrometer. Active passivation was performed using PFBSA at concentrations of ∼100 ppbv and then turned off before the flux measurement. The observed 75% response time in the field was 0.75 s with active passivation, and 0.9−1.0 s after passivation was turned off. The increase in response from 0.45 s in the laboratory (Figure 5) to 0.75 s in the field during passivation could be due to the 25 m transfer line causing axial dispersion of the air sample, the effects of sampling from ambient (rather than scrubbed) air, or the introduction of the larger-volume (1.5 L), 204 m multipass. Figure 7 depicts the flux of HNO3 over the prairie environment on March 15th, 2015, with the flux becoming more negative (i.e., depositing) as the sensible heat flux and friction velocity increase after sunrise (solar elevation shown in orange). The maximum measured deposition flux was found to be −0.11 (nmol/m2)/s, with an average value of −0.064 (nmol/m2)/s over the peak. As a measure of the role of instrument time response upon the flux determination, the correlations between wind and temperature, HNO3 mixing ratio and temperature, and HNO3 mixing ratio and wind are presented as a function of time lag in Supporting Information (Figure S2). Importantly, the characteristic time scale of the wind-temperature correlation (1/e time constant of 3.6(1) s) is within uncertainty of that of the HNO3-temperature correlation (4.0(5) s), indicating that the HNO3 flux measurement was not limited by instrumental response time. The ∼4 s time scale of the correlation is instead due to the time scale of the eddy fluctuations at the sampling site. We note that this peak flux value is generally low compared to other HNO3 flux measurements based upon inferential,4,34,35

Figure 7. Measurement of HNO3 flux on March 15, 2015, in a central U.S. prairie. Top traces: sensible heat flux (red) and friction velocity (light blue), both increasing between sunrise and sunset. Bottom traces: HNO3 flux (black) calculated in 30 min intervals, and solar elevation (orange) over the course of the day. Error bars correspond to the measured flux value when the lag time is ±100 s from its optimal value.

relaxed eddy accumulation,36,37 and eddy covariance.38,39 The origin of this difference may be due to the unique environment of the measurement (cold/wet winter prairie), or from measurement variation due to a single day of sampling. Future studies will extend the deployment to several weeks, in an environment similar to that of other studies, for a more direct comparison with previous work. NH3 Time Response Improvement. Although the use of an acidic passivating agent reduces the HNO3 time response, any NH3 sensitivity is effectively suppressed, likely due to the reaction R‐COOH(g) + NH3(g) → NH4 +·RCOO−(s)

(4a)

R‐SO3H(g) + NH3(s) → NH4 +·RSO3−(s)

(4b)

To improve the NH3 time response, a perfluorinated compound with a basic functional group is required instead. Specifically, the basic amine group of 1H,1H-perfluorooctylamine (PFOAm, Figure 1c) does not react with NH3, yet it can provide a surface passivation effect similar to that from the −COOH group of PFHpA. Figure 8 shows the change in response time upon injecting PFOAm into the inlet at a concentration of approximately 1 ppm. The 75% and 90% recovery times decrease by factors of 6 and 1.5, respectively. Upon injection of PFOAm, the NH3 mixing ratio exhibits a fast rise and double-exponential fall similar to that observed for HNO3, shown in Figure 8c. Assuming that all of the surfacebound NH3 is ejected by active passivation, integration over the G


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The Journal of Physical Chemistry A

Figure 8. Time response of the NH3 signal (red line) using inertial inlet (a) before and (b) after injection of 1H,1H-perfluorooctylamine at a concentration of ∼1 ppm for active passivation. (c) Change in NH3 mixing ratio upon injection of 15 ppm PFOAm at t = 10 s, indicating ejection of NH3 upon passivation. Blue and gray dashed lines represent single and double exponential fits to the observed decay. Inset in (c): data presented on a logarithmic scale emphasizing the double exponential nature of the decay.

observed rise and fall of NH3 provides a quantification of the surface coverage. In this case, for an uncoated inlet that has been exposed to room air for several weeks, these data indicate a NH3 surface coverage of 8 × 1014 molecules/cm2. This value is similar to the monolayer coverage of NH3 on MgO (a similar insulating surface) of ∼7 × 1014 molecules/cm2,40 although the coverage observed here is expected to be heavily dependent upon surface roughness. As in the case of the HNO3 analysis above, this reduced response time can be probed as a function of PFOAm passivation concentration. This dependence, shown in Figure 9, is qualitatively similar to that in Figure 5, but with the response continuing to decrease at higher concentrations. The pKb value for PFOAm is predicted to be ∼6,41 indicating that it serves as a relatively weak base. As opposed to PFHpA, such a large pKb value indicates that a small fraction of PFOAm should be ionized at the water-coated interface. This may be the primary reason why a higher concentration of PFOAm is required for a similar time response as compared to PFHpA. Future work will include investigating the humidity dependence of the NH3 response time, as well as exploring other basic functional groups that may have lower pKb values. Applications to Other Inlet Systems. The virtual impactor property of the inertial inlet described above is optimized to remove particulates from the incoming gas stream, and prevent those particulates from aggregating and releasing volatiles into the sample. However, other separation methods that are frequently employed in field applications, such as cyclone-based systems and simple PTFE membrane filters, may also be subject to active passivation. The mechanism of a cyclone system relies upon the vortex motion of the gas sample to deposit particulates onto the walls of the inlet.42 The gas sample (and particles that are smaller than the cyclone’s design cutoff) are routed out the top of the cyclone to the instrument.

Figure 9. Dependence of NH3 response time upon passivation concentration of 1H,1H-perfluorooctylamine (PFOAm). Dashed lines correspond to instrument time response of a “non-sticky” molecule. Error bars correspond to standard deviation of the mean over >10 measurements of each data point.

Alternatively, the PTFE membrane filter restricts passage of particles above a certain size on the basis of pore size.42 To test the effectiveness of active passivation on these particle separation devices, they were each used as the inlet for the Aerodyne QCL spectrometer. In each case, approximately 25 ppm of PFHpA (i.e., a large excess) was used for passivation H


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Figure 10. HNO3 signal (red) before and after injection of ∼25 ppm of PFHpA using (a) a cyclone particle filter and (b) a PTFE membrane filter. The subsequent reduction in the time response of the instrument is shown by the blue points.

while the HNO3 concentration was modulated by ∼5 ppb every 5 min. As can be seen in Figure 10, in both cases the HNO3 signal (red trace) exhibited a large, rapid increase when PFHpA was injected, followed by a slower decay. The 75% recovery time (blue points) decreases by a factor of 5−40 after passivation, indicating that PFHpA injection can serve as an effective means to improve instrumental time response when these filter types are used. Further work will aim to determine the dependence upon passivation concentration when these filters are used. Also of interest is understanding to what extent the observed time response varies over time as the filter accumulates particulates (effectively increasing the active surface area of the inlet). Notably, when the cyclone is used (Figure 10b), the HNO3 modulation amplitude is significantly suppressed before passivation. This may be due to irreversible loss on stainless steel components at the entrance and exit ports of the cyclone. The fact that the HNO3 modulation amplitude increases significantly after passivation indicates that this loss mechanism is at least partly mitigated by the PFHpA. Finally, it should be considered whether such an approach could be useful for other analytical methods, such as chemical ionization mass spectrometry (CIMS). Although the passivation concentrations used in this study may yield daughter ions that could saturate the CIMS detector, it may still be useful if the passivation flow is shut off before measurements are made. As discussed above, the fast response times can be maintained for hours or days after flow is stopped. This possible application is currently being pursued in the laboratory.

acid groups and the nonpolar behavior of perfluoroalkyl groups. Continuously passivating an Aerodyne Research, Inc. QCL direct absorption spectrometer inlet with 550 ppb of PFHpA reduced the 75% HNO3 time response by a factor greater than 30. The dependence upon passivation concentration shows asymptotic behavior, where the response improves with increasing PFHpA and PFBSA concentrations, until a minimum time response is achieved. Variation of sample stream humidity reveals that the reduction is largely unaffected by water content. Likewise, to improve the NH3 response, continuous passivation using 1H,1H-perfluorooctylamine (PFOAm) yielded a factor of 6 reduction in the 75% recovery time for NH3 detection. As a demonstration of the utility of active passivation, the system was deployed to measure HNO3 fluxes in a central U.S. prairie using eddy covariance. The results show that these measurements were not limited by the instrumental response time of the inlet. Finally, this method was applied to two other inlet possibilities: a cyclone particle separator and a PTFE membrane filter. In both cases passivation using PFHpA reduced the 75% response times for HNO3 by a factor of 5−40. Future studies will be aimed at further elucidating the mechanisms underlying the improved response times, the use of other possible passivation compounds, the role of other atmospheric species in the response, and applications to detection of other species, such as sulfuric acid.

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CONCLUSIONS A new method to the reduce time responses of spectroscopic HNO3 and NH3 measurements is demonstrated. The technique utilizes continuous passivation to provide constantly refreshed instrument surfaces, preventing highly polar molecules from adsorbing at interfaces. To improve the time response of HNO3, perfluoroalkanoic acids and perfluorobutanesulfonic acid are used, leveraging both the surface binding capability of

ASSOCIATED CONTENT

S Supporting Information *

A table of the passivation materials tested and notable results is presented in the Supporting Information, in addition to an assessment of the lag-time dependence of the HNO3 eddy flux measurements. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/ acs.jpca.5b04395. I


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AUTHOR INFORMATION

Corresponding Author

*J. R. Roscioli. E-mail: [email protected]. Phone: 978663-9500. Notes

The authors declare no competing financial interest.


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ACKNOWLEDGMENTS Financial support for the development and time response improvement of the HNO3/NH3 dual QCL spectrometer was provided by the DOE SBIR program (Grant No. DESC0006193). The authors thank John Nowak, Michael Agnese, and Ryan McGovern for their contributions to the conceptual and technical development of this study. They also acknowledge Roisin Commane and Cody Floerchinger for their contributions to the field deployment and analysis of the eddy covariance measurements. Finally, the authors would like to express their appreciation to James G. Anderson for many years of collegial discussions, and his inspirational scientific and technological advancements in physical chemistry and the atmospheric sciences.

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