Automated Measurement and Monitoring of Bioprocesses: Key Elements of the M3C Strategy - Measurement, Monitoring, Modelling, and Control of Bioprocesses - page 8

cultivation at a glance: if the fully growth-associated CO 2 production happens to
be exponential, the raw (logarithmic) signal of the pCO 2 electrode develops lin-
early with time; from its slope, one can directly derive the specific growth rate.
The response time of pCO 2 electrodes is usually longer than that of pO 2 or pH
electrodes, since the equilibration of the inner electrolyte buffer is slow. With time,
this buffer becomes saturated, thus limiting the useful lifetime of the electrode.
However, pH measurement is not selective for CO 2 . This implies that all acidic or
alkaline components that can diffuse through the membrane, e.g., H 2 S, formic,
acetic acid etc. or NH 3 in alkalophilic cultivations, must also contribute to the final
signal; i.e., they exhibit cross-talk.
Of course, optical variants have also been described, even with low cost, for
instance by Ge et al. [ 22 ]. The correlation between the signal and the dissolved
CO 2 is not linear, but stability of at least 10 days is claimed.
2.2.5 Composition of the Gas Phase
Oxygen and carbon dioxide are the noninert components of the gas phase in a great
majority of bioprocesses. Various analyzers of exhaust gas are on the market from
different suppliers. Depending on the construction of the analyzer (and its price),
signals represent the particle density per volume of the measuring cuvette and not
a percentage, i.e., the molar ratio times 100: n/V = p/R/T.
If the analyzer cuvette (with constant volume) is ventilated against atmosphere,
then the signal is directly affected by pressure changes, which must be accounted for.
The measuring cuvettes are usually thermostated, so temperature variations are less
critical.
Carbon dioxide is a greenhouse gas, and this effect is often exploited to quantify
it via its infrared absorption. Water (vapor) is a greenhouse gas and absorbs IR
radiation as well. Therefore, water should be eliminated prior to measurement (by
a gas cooler and/or dryer) because water is always present in the exhaust gas of a
bioprocess.
Oxygen is one of the few paramagnetic gases and certainly the only one which
can be tolerated in bioprocesses. The respective analyzers are usually mechani-
cally sensitive to water vapor, and therefore the gas should also be dried before
measurement.
The sound velocity in a gas depends on its composition; it is proportional to
M -0.5 , where M is the molar mass of the gas [ 23 ]. So, electroacoustic measure-
ments are also valuable alternatives to determine CO 2 . Magnetoacoustic instru-
ments are the alternatives for oxygen.
If other gases such as H 2 or CH 4 or volatiles such as alcohols and aldehydes
need to be determined too, mass spectrometry is obviously the most usual choice
because it is so versatile. A relatively inexpensive instrument with electron impact
ionization and a quadrupole mass filter can cover all the relevant gases. Magnetic-
field mass filters are probably more stable. If greater sensitivity and excellent
dynamics are required, chemical ionization techniques, e.g., PTR-MS instruments,
are used [ 6 , 24 , and Chapter by Striedener and Bayer].