82 Fermentation and Biochemical Engineering Handbook In the amperometric(polarographic)approach, oxygen again perme ates a diffusion barrier and encounters an electrochemical cell immersed in basic aqueous solution. a potential difference of approximately 1.3 V is maintained between the anode and cathode as the oxygen encounters the cathode. an electrochemical reaction occurs 02+ 2H20+ 4e+40H(at cathode) The hydroxyl ion then travels to the anode where it completes the electro- chemical reaction process 40H+>O2+ 2H20+ 4e(at anode) The concentration of oxygen is directly proportional to the amount of current passed through the cell 7.0 EXHAUST GAS ANALYSIS Much can be learned from the exchange of gases in the metabolic process such o2, CO2, N2, NH3, andethanol. In fact, most ofthe 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 particle 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. Thi information is used to determine the percent concentration of each of the gass682 Fermentation and Biochemical Engineering Handbook In the amperometric (polarographic) approach, oxygen again perme￾ates a diffusion barrier and encounters an electrochemical cell immersed in basic aqueous solution. A potential difference of approximately 1.3 V is maintained between the anode and cathode. As the oxygen encounters the cathode, an electrochemical reaction occurs: 0, + 2H,O + 4e- + 40H- (at cathode) The hydroxyl ion then travels to the anode where it completes the electro￾chemical reaction process: 40H- + 0, + 2H20 + 4e- (at anode) The concentration of oxygen is directly proportional to the amount of current passed through the cell. 7.0 EXHAUST GAS ANALYSIS Much can be learned from the exchange of gases in the metabolic processsuchas O,, CO,,N,,NH,,andethanol. Infact, mostofthepredictive 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 ofthe 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 avoltage proportional to their charge. This information is used to determine the percent concentration of each of the gasses