1. Direct detection of C2H2 in air and human breath by cw-CRDS
Florian Schmidt, Olavi Vaittinen, Markus Metsälä and Lauri Halonen
Laboratory of Physical Chemistry, University of Helsinki, Finland

2. Cavity ring-down spectroscopy (CRDS)
Laser
Scan
Detector
Absorption enhancement due to cavity: 103 to 105
CRDS independent of laser power fluctuations
No calibration needed

3. Experimental CRDS setup - schematic
Tuning range: nm ( cm-1)
Cavity Finesse:
Sensitivity: 7×10-11 cm-1
Acquisition time: 1 s - 20 min
Detected at ppt level:
C2H2, NH3, HCN
Other detectable species:
CH4, N2O, CO, CO2, H2O
ECDL = external cavity diode laser
OI = optical isolator
AOM = acousto-optic modulator
MFC = mass flow controller

4. Experimental CRDS setup

5. Motivation to measure acetylene
One of the most common hydrocarbons in the troposphere
Mostly anthropogenic sources (fossil- and biofuels, biomass burning)
Correlates with other combustions gases and VOCs
Trace pollution and exposure to combustion gases (exhausts, smoking)
Baseline of breath C2H2 in the healthy population? Is C2H2 a biomarker for a medical condition (e.g. Proteus mirabilis bacterial infection)?
Not previously detected in ambient air with high time resolution and without pre-concentration.
Not previously quantified in breath
GC-MS slow and expensive
Not measurable with PTR-MS and SIFT-MS with standard pre-cursors

6. Spectra from breath and ambient air
Air acetylene concentration 0.93 ppbv; Scan time 20 min; Detection limit 120 ppt

7. System response to C2H2 absorption

8. Outdoor air – continuous flow
F. M. Schmidt, O. Vaittinen, M. Metsälä, P. Kraus, L. Halonen, Appl. Phys. B (2010)

9. Day – Night comparison
F. M. Schmidt, O. Vaittinen, M. Metsälä, P. Kraus, L. Halonen, Appl. Phys. B (2010)

10. Indoor – outdoor air comparison
F. M. Schmidt, O. Vaittinen, M. Metsälä, P. Kraus, L. Halonen, Appl. Phys. B (2010)

11. Breath collection and sample handling
No adsorption/desorption phenomena in the bags, the gas system tubing and the quartz-coated ring-down cavity
Acetylene can be collected and stored in aluminum breath bags (statistical error for 10 bags: 0.4 ppb)
Acetylene concentration stays within 0.4 ppb for 5 days in the bag
No quantitative difference between alveolar/full breath samples and nose/mouth breathing (non-smokers)
Thus: static measurements were possible
Spectrometer temperature: 22 degrees (room temperature)
Sample pressure in the cavity: 0.1 bar

12. Study - Background level of C2H2 in breath
40 healthy volunteers
22 male, 18 female, 20 to 63 years old, 31 non- smokers, 9 smokers
Subjects requested to have stayed at sampling location for 30 min prior to breath collection
One full breath sample collected to a breath bag
At the same time: one ambient air sample collected to a second bag
Samples were analyzed on the same day or at least within 48 hours.

13. Results – Breath acetylene minus air acetylene
Breath C2H2 baseline = Ambient air C2H2 level
Smokers could be identified with a sensitivity and specifity of 100 %

14. Timer after smoking

15. Exposure studies Subject smokes a cigarette or takes a puff
Continuous breathing into CRDS system for up to 30 min
C2H2 decay recorded standing on peak of absorption profile (3 points per second)
For long-term decay: bag samples or shorter intervals of continuous breathing
Fit exponential functions to decay
Simple 3-compartment model (respiratory tract, blood, tissue)

16. Breath cycles

17. Acetylene residence times

18. Summary Mean C2H2 mixing ratio in Helsinki outdoor air of ~1 ppbv
CRDS suitable for fast and direct detection of C2H2 in air and breath
Mean C2H2 mixing ratio in Helsinki outdoor air of ~1 ppbv
C2H2 correlates with CO, human activity and temperature
Strong fluctuations (up to 60 ppbv on a minute time scale) in urban outdoor air during daytime. Single bag measurements unreliable.
Breath C2H2 baseline is equal to ambient air C2H2 level
Smokers were identified with 100 % sensitivity and specifity
Breath acetylene returns to baseline level around 3 hours after exposure. Urban air pollution could impair smoking status assessment
High-time resolution decay measurements could improve knowledge about residence times of gases and related physiological processes

19. Acknowledgements Dr Olavi Vaittinen Dr Markus Metsälä Peter Kraus
Mirva Skyttä
Prof Lauri Halonen
Academy of Finland