Absolute non-invasive measurement of CO2 mole fraction emitted by E. coli and S. aureus using calibration-free 2f WMS applied to a 2004 nm VCSEL

We report the first demonstration of accurate real-time

non-invasive measurement of the absolute cumulative

mole fraction of metabolic carbon dioxide emitted by

Escherichia coli and Staphylococcus aureus over a period

of several hours of their life cycles using a recentlydeveloped

calibration-free wavelength modulation

spectroscopy technique. A 1mW vertical cavity surface

emitting laser is used to interrogate a single rotational

vibrational absorption line of carbon dioxide at 2003.5

nm. The measurements are immune to laser intensity

fluctuations and variable optical coupling that is

inevitable in such free-space coupled experiments that

run over 10-18 hours. The cumulative carbon dioxide

mole fraction follows the characteristic modified

Gompertz model that is typical of bacterial growth in

batch cultures. The characteristics growth parameters

are extracted from this curve. The technique can be

readily extended to study multiple volatile organic

compounds that bacteria are known to emit.