How to detect the virtually
undetectable in the atmosphere.
Prof. Dudley Shallcross
School of Chemistry
3 most abundant gases in each planetary atmosphere
Jupiter H2 (93%) He (7%) CH4 (0.3 %)
Saturn H2 (96%) He (3%) CH4 (0.45 %)
Uranus H2 (82%) He (15%) CH4 (2.3 %)
NeptuneH2 (80%) He (19%) CH4 (1-2 %)
Venus CO2 (96%) N2 (3.5%) SO2 (0.015 %)
Mars CO2 (95%) N2 (2.7%) Ar (1.6 %)
Earth N2 (78%) O2 (21%) Ar (0.93 %)
90% of mass
9% of mass
0.9% of mass
The ozone layer (stratosphere 10-50 km)
O O O O
O O
O
O O
O
UV-A
UV-B
The Chapman Mechanism
In the 1930s Sidney Chapman devised a mechanism that accounted
for the ozone layer and the temperature structure.
O2 + h O + O
O + O2 + M O3 + M
O3 + h O2 + O
O3 + O 2O2
The Chapman Mechanism
In the 1930s Sidney Chapman devised a mechanism that accounted
for the ozone layer and the temperature structure.
O2 + h O + O
O + O2 + M O3 + M
O3 + h O2 + O
Very exothermic releases a lot of energy into the atmosphere
O3 + O 2O2
Challenge: ozone
• 1930s suspected ozone was important
• 10-50 km above our heads
• Present in ppm (parts per million level)
• In situ measurements or remote sensing
Ozonesondes (in situ)
Electrochemical concentration cell (ECC), ozone reacts with a dilute solution of potassium iodide and causes a change in electrical signal which is converted to a concentration. Data (pressure, humidity and temperature and ozone) telemetered back to the surface receiving station.
Measures up to about 35 km, higher at the equator
British Antarctic Survey data
• Ozonesonde data since 1950s, every day a sonde is launched
• Started to observe a decline in ozone over the South Pole in austral spring
• Paper published by Farman et al. in 1985 (25 year anniversary in 2010) showing ‘ozone hole’
British Antarctic Survey data
• In spring at certain altitudes, ozone disappears totally.
• Chlorofluorocarbons (CFCs), fully halogenated hydrocarbons, e.g. CF2Cl2, were shown to be the source of Cl that was responsible in the main for the loss.
Satellite observations
• UV backscatter technique (more about this in a minute) to measure ozone remotely
• Showed ozone hole …
Backscatter u.v.UV (blue) and visible (green) radiation from the Sun passes through the Earth’s atmosphere and the UV is absorbed by ozone.
Some of the light is reflected (scattered) back to space by clouds etc.
The satellite observes the backscattered light and compares with the direct beam from the Sun.
Satellite
Other geometries are also used
Satellite
Basic absorption spectroscopy
Regions of the spectrum used for remote sensing
Remote sensing
• Now satellites can observe numerous species by their characteristic absorption spectrum
• Potential global coverage• Still limitations with respect to altitude –
clouds and water vapour interfere with retrievals and so it is often restricted to the stratosphere and above, though some data for the upper troposphere is possible
Troposphere10 km
NO, NO2, VOC
VOCs
?
0 kmCompounds of both biogenic and anthropogenic origin
1 km
The Tropopause
The Boundary Layer
Troposphere - issues
• Lots of species
• Particles
• Clouds, aerosol, mists etc.
• Air quality
• Climate (e.g. CO2)
CO2 by infra red spectroscopy
High concentration easy to observe CO2 by absorption spectroscopy
Limitations of absorption spectroscopy
Long path-length absorption spectroscopy
Long path-lengths intra-cavity absorption
Cavity ring down spectroscopy - CRDS
CRDS sensor
Troposphere - issues
• Spectroscopy suffers from the fact that many species are present that absorb in the same part of the spectrum, e.g. all hydrocarbons contain a C-H bond. If we could separate out the species we could measure them individually.
• Gas chromatography coupled with a suitable detector will allow species to be measured
Gas chromatography (GC)
Gas chromatography is so-called because the mobile phase is a gas
Comprises both:
Gas-liquid chromatography (GLC) where the stationary phase is a liquid and the sorption process is mainly partition.
and
Gas-solid chromatography (GSC) where the stationary phase is a solid and adsorption is the major sorption process.
GC columns
• GC column is the heart of the gas chromatograph. Consists of a coil of stainless steel, glass or fused silica (quartz) tubing between 1 and 100 m long and having an
internal diameter of between 0.1 and 3 mm.
• Column is enclosed in thermostatically controlled oven whose temperatures can be held constant to within +0.1oC.
• The operating temperature of the GC oven may remain constant during an analysis – isothermal or automatically increased at a predetermined rate to speed the elution process – temperature programming. More on this later.
GC columns
Capillary columnPacked column
ADS
Tedlar Bag or direct Sample
detector
Adsorption / Desorption System &
Microtrap
1000 fold increase
Adsorption / Desorption System &
Microtrap
1000 fold increase
Gas Chromatography
Column
Gas Chromatography
ColumnChromatographChromatograph
Experimental Setup at Bristol
Flame ionisation detector (FID): universal detector
• The FID is the most widely used GC detector.
• Effluent gas from the column is mixed with H2 and air and burned at a small jet.
• The jet forms the –ve electrode of an electrolytic cell. The +ve collector electrode is positioned above the flame.
• The potential across the two electrodes being about. 200 V.
• As organic compounds emerge from the column they burn in the flame creating ions which create a current between the electrodes.
• The FID responds to virtually all organic compounds with a very high sensitivity and widest linear range (107) of any detector (ng – mg)
Electron capture detector (ECD): selective detector
• Based on the use of a -ray ionizing source.
• As carrier gas flows through the source the 63Ni or 3H ionise the gas forming ‘slow’ electrons which migrate towards the anode giving a standing current when only carrier gas is present.
• If an analyte with a high electron affinity elutes from the GC column some of the electrons will be ‘captured’ thereby reducing the current in proportion to its concentration.
• The detector is very sensitive to compounds
containing halogens, S, anhyrides, peroxides,
congugated carbonyls, nitrites, nitrates and organometallics.
•Linear range only 102 to 103.
AB + e- AB- non-dissociative A. + B- dissociative
Compound identification by GC
Unknowns
Standards
• Most common means of identifying analytes in GC is by direct comparisons of retention times with an authentic sample analysed under the same conditions.
•Comparing retention times on two GC phases of contrasting properties improves confidence in the identification.
•Best way is to connect GC to MS and obtain mass spectra.
Replacement CFC observations at Mace HeadReplacement CFC observations at Mace Head
• HCFC-142b (CFCl2CH3)• Foam plastics• Significant use in 1970s & 80s• Growth 1.1 ppt/yr
• HFC-134a (CF3CH2F)• Major HFC replacement for CFC-12 (CF2Cl2) in domestic & auto applications• Growth 3 ppt/yr (25%/yr, 1999)
Surface Acoustic Wave sensors (SAWs)
The Sauerbrey equation (1957)
f = - 2.26 x 10-6 f2 m/A
As the mass goes upthe frequency comes down
The higher the resonant frequencyof the crystal the greater the sensitivity
SAWs are NOT selective
Recognition Elements
SAW
Recognition element (MIP)
Recognition ElementRecognition Element
Coat with selective polymer
e.g. Molecular Imprinted Polymers
Nandrolonea, Terpenesb, Amino Acidsc
Self Assembled Monolayers
PAHsd
Biosensing
Enzymese
Poly-Butadiene
Ozonea. Percival et al., 2002; b Percival et al., 2001;
c Stanley et al., 2003a; d Stanley et al., 2003b;
e. Evans et al., 2006
0
10
20
30
40
50
60
70
80
00:00 04:48 09:36 14:24 19:12 00:00
Time / GMT
[ozo
ne] /
ppb UV
QCM
Comparison with UV measurementsComparison with UV measurements
QCM spin coated dilute mixture of poly-butadiene in cyclohexane
DQCM system utilised
5MHz unpolished AT-cut, 25 mm
diameter, with Cr/Au contacts
L.O.D. = 3 ppb
Measurements made indoors
10 second averages
Conclusions
• Optical techniques can be used to detect very low concentrations if a long pathlength can be used (expensive)
• GC based techniques good for stable species but bulky and low frequency measurement (relatively expensive)
• SAWs etc. small, low cost, specific sensors?
Thanks
Prof. Andrew Orr-Ewing
Prof. Richard Evershed
Dr. Simon O’ Doherty
Dr. Carl Percival