Exhaust Gas Analysis

Much can be learned from the exchange of gases in the metabolic process such as 02, C02, N2, NH3, and ethanol. In fact, most of the predictive analysis is based upon such calculations as oxygen uptake rate, carbon dioxide exchange rate or respiratory quotient. This information is best obtained by a component material balance across the reactor. A key factor in determining this is the analysis of the bioreactor offgas and the best method for measuring this is with a mass spectrometer because of its high resolution. Two methods of operation are utilized. These are magnetic deflection and quadrapole. The quadrapole has become the primary commercial system because of its enhanced sensitivity and its ability to filter out all gases but the one being analyzed.

Magnetic deflection mass spectrometers inject a gaseous sample into an inlet port, bombard the sample with an electron beam to ionize the particles and pass the sample through a magnetic separator. The charged particles are deflected by the magnet in accordance with its mass-to-energy (or charge) ratio—the greater this ratio, the less the deflection. Detectors are located on the opposing wall of the chamber and are located to correspond to the trajectory of specific components as shown in Fig. 3. As the ionized particles strike the detectors, they generate a voltage proportional to their charge. This information is used to determine the percent concentration of each of the gasses.

Deflection Ionised Co2
Figure 3. Magnetic deflection principle.

The quadrapole mass spectrometer also employs an electron beam to ionize the particles using the quadrapole instead of a magnet to deflect the path of the particles and filter out all but the specific component to be analyzed. The quadrapole is a set of four similar and parallel rods (see Fig. 4) with opposite rods electrically connected. A radio frequency and dc charge of equal potential, but opposite charge, is applied to each set of the rods. By varying the absolute potential applied to the rods, it is possible to eliminate all ions except those of a specific mass-to-energy ratio. Those ions which successfully travel the length of the rods strike a Faraday plate which releases electrons to the ions thereby generating a measurable change in EMF. For a given component the strength of the signal can be compared to references to determine the concentration.

The quadrapole, when used in conjunction with a gas chromatograph to separate the components, can measure a wide range of gases, typically from 50 to 1000 atomic mass units (amu).

As mass spectrometers are relatively expensive, the exhaust gas of three or more bioreactors is typically directed to a single analyzer. This is possible because the ofifgas analysis is done outside the bioreactors themselves. However, the multiplexing of the streams results in added complexity with regard to sample handling and routing, particularly if concerns of cross contamination need be addressed. The contamination issue is usually handled by placing ultrafilters in the exhaust lines. Care, however, must be taken to ensure that these filters don't plug resulting in excessive backpressure. Periodic measurement calibration utilizing reference standards must be sent to the spectrometer to check its calibration.

Figure 4. Quadrapole principle.

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