Temperature Dependent
Absorption
Cross-sections
of
PFTBA
1
1
2
2
1
P. J. Godin , S. Conway , A. Hong , S. Mabury , and K. Strong
1. Department of Physics, University of Toronto
2. Department of Chemistry, University of Toronto
Experimental Set-Up
Introduction
Temperature and Pressure Dependence
A recent paper by Hong et al. (2013) determined that perfluorotributylamine (PFTBA) has the highest radiative efficiency
of any compound detected in the atmosphere [1]. PFTBA is a fully-fluorinated liquid commonly used in electronic reliability and
quality testing. PFTBA vapour can be considered a potential
greenhouse gas due being radiatively active in the mid-IR spectral region and having a long atmospheric lifetime due to no
known sinks.
Presently the only published spectra of PFTBA are room temperature measurements taken in an N2 background [2]. Furthermore the purity the sample is unknown. This is a problem as
PFTBA can often exist with its congener, which has a different
structure as seen in Figures 1 and 2. Any effect of the congener,
if present, should be accounted for.
Fig. 9: 298.2±0.5K measurements. 1.21 Torr measurement is 200 co-adds at
0.02 cm-1 resolution. Other pressures are 700 co-adds at 0.1 cm-1 resolution.
Fig. 5: Schematic of experimental set-up.
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We present a preliminary analysis of temperature and pressure dependent absorption cross sections of PFTBA.
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Fig. 1: Chemical structure of PFTBA
Fig. 2: Chemical structure of PFTBA
congener.
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PFTBA was detected in
the atmosphere over Toronto from in situ measurements using ion chromatography.
Detected at a VMR of
0.18 ±0.01 pptv.
Using the HYSPLIT model, back trajectories of
PFTBA can be calculated.
No correlation between
air mass movement and
PFTBA VMR.
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1. Resolution limit of 0.004 cm-1.
2. Detectable range of 400-8500 cm-1.
3. Infrared Globar source.
MKS 10 Torr Baratron, resolution of ±0.005 Torr.
PFTBA sample from Supelco, certified 99.0±0.5% pure.
Fig. 10: 318.5±0.5 K measurements. All measurements are at 0.1 cm-1 resolution and 600 co-adds.
Procedure
Atmospheric Detection
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Active optical pathlength of 10 cm.
Thermocouple in-line and additional one outside of cell.
Heating Bands allow for temperatures upwards of 373 K.
DA8.002 FTS:
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Take a background measurement of the empty cell.
While background measurement is running preform
freeze-pump-thaw cycles on
sample to remove impurities.
Add sample to cell and wait
for pressure to equilibrate.
Run measurement with sample in cell.
Evacuate cell and take another background scan to insure
stability of background.
Fig. 11: 350.5±0.5 K measurements. All measurements are at 0.1 cm-1 resolution and 500 co-adds.
Integrated Cross-sections
Fig. 6: Laboratory set-up.
Theoretical Calculations
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Fig. 3: HYSPLIT back trajectories of
PFTBA. Image taken from [1].
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Partial Pressure Temperature Dependence
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Integration performed using trapezoidal.
Low level geometry optimization calculation followed by high
level frequency calculation.
The calculations have determined the optimized geometrical
configuration and IR intensities and wavenumbers of the
harmonic frequencies .
PFTBA: Gaussian B3LYP/6-311G(d,p) basis set.
Congener: Gaussian B3LYP/6-311++G(3df,3pd) basis set.
Standard pressure and temperature.
Fig. 12: Integrated band strength from
690-750 cm-1.
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Fig. 13: Integrated band strength from
750-900 cm-1.
PFTBA has a low vapour pressure limiting the range of accessible pressures for a given temperature.
Vapour pressures are calculated using the August Equation, which relates pressure and temperature using chemical specific constants:
Measurements in this poster were recorded at room temperature and above, for which the vapour pressure is within
our detection limits.
Fig. 7: Theoretical calculation of spectra for PFTBA at STP.
Fig. 14: Integrated band strength from 1100-1400 cm-1.
Conclusions and Future Work
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Fig. 4: Partial pressure of PFTBA as a function of temperature, calculated
using the August equation with constants A = 10.511 and B = 2453 K. [3].
Fig. 8: Theoretical calculation of spectra for PFTBA congener at STP.
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First known temperature and pressure dependent measurements of PFTBA.
Presence of congener appears to be minimal.
Integrated cross-section varies linearly with pressure.
Increasing temperature reduces the impact of pressure on integrated cross-sections.
Contamination appears to be water as seen by the lines in
the 1400 cm-1-1500 cm-1 region.
Improvements to the system required to reduce contamination of sample and leaking at higher temperatures.
Higher quality spectra for integrated band strengths and
cross-section calculations.
References: [1] A. C. Hong, C. J. Young, M. D. Hurley, T. J. Wallington, and S. A. Mabury, Geophys. Res. Lett., 40, 1-6 (2013). [2] C. J. Young, Ph. D. Thesis, University of Toronto (2010). [3] 3M Datasheet for PFTBA.