16 Instrumentation and Control Systems John p King 1.0 INTRODUCTION The widespread use of advanced control and process automation for biochemical applications has been lagging as compared with industries such as refining and petrochemicals whose feedstocks are relatively easy to characterize and whose chemistry is well understood and whose measure- ments are relatively straightforward Biological processes are extraordinarily complex and subject to con siderable variability. The reaction kinetics cannot be completely determined in advance in a fermentation process because of variations in the biological properties of the inoculant. Therefore, information regarding the activity of the process must be gathered as the fermentation progresses. Directly measuring all the necessary variables which characterize and govern the competing biochemical reactions, even under optimum laboratory condi ions, is not yet achievable. Developing mathematical models which can be utilized to infer the biological processes underway from the measurements available, although useful, is still not sufficiently accurate. Add to this the constraints and compromises imposed by the manufacturing process and the task of accurately predicting and controlling the behavior of biological production processes is formidable indeed
16 Instrumentation and Control Systems John R King 1.0 INTRODUCTION The widespread use of advanced control and process automation for biochemical applications has been lagging as compared with industries such as refining and petrochemicals whose feedstocks are relatively easy to characterize and whose chemistry is well understood and whose measurements are relatively straightforward. Biological processes are extraordinarily complex and subject to considerable variability. The reaction kinetics cannot be completely determined in advance in a fermentation process because of variations in the biological properties of the inoculant. Therefore, information regarding the activity of the process must be gathered as the fermentation progresses. Directly measuring all the necessary variables which characterize and govern the competing biochemical reactions, even under optimum laboratory conditions, is not yet achievable. Developing mathematical models which can be utilized to infer the biological processes underway from the measurements available, although useful, is still not sufficiently accurate. Add to this the constraints and compromises imposed by the manufacturing process and the task of accurately predicting and controlling the behavior of biological production processes is formidable indeed. 675
676 Fermentation and Biochemical Engineering Handbook The knowledge base in fermentation and biotechnology has expanded at an explosive rate in the past twenty-five years aided in part by the development of sophisticated measurement, analysis and control technology Much of this research and technology development has progressed to the oint where commercialization of many of these products is currently The intent of this chapter is to survey some of the more innovative measurement and control instrumentation and systems available as well as to review the more traditional measurement, control and information analys technologies currently in use 2.0 MEASUREMENT TECHNOLOGY Measurements are the key to understanding and therefore controlling any process. As it relates to biochemical engineering, measurement technol ogy can be separated into three broad categories. These are biological, such as cell growth rate, florescence, and protein synthesis rate; chemical, such as glucose concentration, dissolved oxygen, pH and offgas concentrations of CO2, O2, N2, ethanol, ammonia and various other organic substances; and physical, such as temperature, level, pressure, flow rate and mass. The most prevalent are the physical sensors while the most promising for the field of biotechnology are the biological sensors biological processes is the maintenance of a sterile environment. This is necessary to prevent foreign organisms from contaminating the process. In-line measurement devices must conform to the AAA Sanitary Standards specifying the exterior st and materials of construction for the"wetted parts. "Instruments must also be able to withstand steam sterilization which is needed periodically to prevent bacterial buildup. Devices located in process lines should be fitted with sanitary connections to facilitate their removal during extensive clean- in-place and sterilize-in-place operations. Sample ports, used for the removal of a small portion of the contents from the bioreactor for analysis in a laboratory, must be equipped with sterilization systems to ensure organisms are not inadvertently introduced during the removal of a sample 3.0 BIOSENSORS Biosensors are literally the fusing of biological substrates onto electri circuits. These have long been envisioned as the next generation of analytical
676 Fermentation and Biochemical Engineering Handbook The knowledge base in fermentation and biotechnology has expanded at an explosive rate in the past twenty-five years aided in part by the development of sophisticated measurement, analysis and control technology. Much of this research and technology development has progressed to the point where commercialization of many of these products is currently underway. The intent of this chapter is to survey some of the more innovative measurement and control instrumentation and systems available as well as to review the more traditional measurement, control and information analysis technologies currently in use. 2.0 MEASUREMENT TECHNOLOGY Measurements are the key to understanding and therefore controlling any process. As it relates to biochemical engineering, measurement technology can be separated into three broad categories. These are biological, such as cell growth rate, florescence, and protein synthesis rate; chemical, such as glucose concentration, dissolved oxygen, pH and offgas concentrations of CO,, O,, N,, ethanol, ammonia and various other organic substances; and physical, such as temperature, level, pressure, flow rate and mass. The most prevalent are the physical sensors while the most promising for the field of biotechnology are the biological sensors. One concern when considering measuring biological processes is the maintenance of a sterile environment. This is necessary to prevent foreign organisms from contaminating the process. In-line measurement devices must conform to the AAA Sanitary Standards specifying the exterior surface and materials of construction for the “wetted parts.” Instruments must also be able to withstand steam sterilization which is needed periodically to prevent bacterial buildup. Devices located in process lines should be fitted with sanitary connections to facilitate their removal during extensive cleanin-place and sterilize-in-place operations, Sample ports, used for the removal of a small portion of the contents from the bioreactor for analysis in a laboratory, must be equipped with sterilization systems to ensure organisms are not inadvertently introduced during the removal of a sample. 3.0 BIOSENSORS Biosensors are literally the fusing of biological substrates onto electric circuits. These have long been envisioned as the next generation of analytical
Instrumentation and Control Systems 677 sensors measuring specific biomolecular interactions. The basic principle is first to immobilize one of the interacting molecules, the ligand, onto an inert substrate such as a dextran matrix which is bonded (covalently bound) to a metal surface such as gold or platinum. This reaction must then be converted into a measurable signal typically by taking advantage of some transducing phenomenon. Four popular transducing techniques are Potentiometric or amperometric, where a chemical biological reaction produces a potential difference or current flow across a pair of electrodes Enzyme thermistors, where the thermal effect of chemical or biological reaction is transduced into an electrical resistance change Optoelectronic, where a chemical or biological reaction evokes a change in light transmission Electrochemically sensitive transistors whose signal de- One example is the research(I to produce a biomedical device which can be implanted into a diabetic to control the flow of insulin by monitoring the glucose level in the blood via an electrochemical reaction. One implant able glucose sensor, designed by Leland Clark of the Childrens Hospital Research Center in Cleveland, utilizes a microprobe where the outside wall is constructed of glucose-permeable membrane such as cuprophan. Inside, an enzyme which breaks the glucose down to hydrogen peroxide is affixed to an inert substrate. The hydorgen peroxide then passes through an inner membrane, constructed of a material such as cellulose acetate, where it reacts with platinum producing a current which is used to monitor the glucose A commercial example of a biosensor, introduced by pharmacia Biosensor AB2, is utilizing a photoelectric principle called surface plasmon resonance(SPR)for detection of changes in concentration of macromolecu r reactants. This principle relates the energy transferred from photons bombarding a thin gold film at the resonant angle of incidence to electrons in the surface of the gold. This loss of energy results in a loss of reflected light at the resonant angle The resonant angle is affected by changes in the mass concentration in the vicinity of the metal's surface which is directly correlated to the binding and dissociation of interacting molecules
Instrumentation and Control Systems 677 sensors measuring specific biomolecular interactions. The basic principle is first to immobilize one of the interacting molecules, the ligand, onto an inert substrate such as a dextran matrix which is bonded (covalently bound) to a metal surface such as gold or platinum. This reaction must then be converted into a measurable signal typically by taking advantage of some transducing phenomenon. Four popular transducing techniques are: Potentiometric or amperometric, where a chemical or biological reaction produces a potential difference or current flow across a pair of electrodes. Enzyme thermistors, where the thermal effect of the chemical or biological reaction is transduced into an electrical resistance change. Optoelectronic, where a chemical or biological reaction evokes a change in light transmission. Electrochemically sensitive transistors whose signal depends upon the chemical reactions underway. One example is the research['] to produce a biomedical device which can be implanted into a diabetic to control the flow of insulin by monitoring the glucose level in the blood via an electrochemical reaction. One implantable glucose sensor, designed by Leland Clark of the Childrens Hospital Research Center in Cleveland, utilizes a microprobe where the outside wall is constructed of glucose-permeable membrane such as cuprophan. Inside, an enzyme which breaks the glucose down to hydrogen peroxide is affixed to an inert substrate. The hydorgen peroxide then passes through an inner membrane, constructed of amaterial such as cellulose acetate, where it reacts with platinum producing a current which is used to monitor the glucose concentration. A commercial example of a biosensor, introduced by Pharmacia Biosensor AB2, is utilizing a photoelectric principle called sufluceplusmon resonance (SPR) for detection of changes in concentration of macromolecular reactants. This principle relates the energy transferred from photons bombarding a thin gold film at the resonant angle of incidence to electrons in the surface of the gold. This loss of energy results in a loss of reflected light at the resonant angle. The resonant angle is affected by changes in the mass concentration in the vicinity of the metal's surface which is directly correlated to the binding and dissociation of interacting molecules
678 Fermentation and Biochemical Engineering Handbook Pharmacia claims its BLAcore system can provide information on the affinity, specificity, kinetics, multiple binding patterns, and cooperativity of a biochemical interaction on line without the need of washing, sample dilution or labeling of a secondary interactant. Their scientists have mapped the epitope specificity pattens of thirty monoclonal antibodies(Mabs) against recombinant core HIV-I core protein 4.0 CELL MASS MEASUREMENT The on-line direct measurement of cell mass concentration by using optical density principles promises to dramatically improve the knowledge of the metabolic processes underway within a bioreactor. This measurement is most effective on spherical cells such as E. Coli, The measurement technology is packaged in a sterilizable stainless steel probe which is inserted directly into the bioreactor itself via a flange or quick-disconnect mounting(Fig. 1) By comparing the mass over time, cell growth rate can be determined This measurement can be used in conjunction with metabolic models which employ such physiological parameters as oxygen uptake rate(oUR), carbon dioxide evolution rate(CER)and respiratory quotient(RQ)along with direct measurements such as dissolved oxygen concentration, pH, temperature, and ffgas analysis to more precisely control nutrient addition, aeration rate and agitation. Harvest time can be directly determined as can shifts in metabolic pathways possibly indicating the production of an undesirable by-product Cell mass concentrations of up to 100 grams per liter are directly measured using the optical density probe. In this probe, light of a specific wavelength, created by laser diode or passing normal light through a sapphire crystal, enters a sample chamber containing a representative sample of the bioreactor broth and then passes to optical detection electronics. The density is determined by measuring the amount of light absorbed, compensating for backscatter. Commercial versions such as those manufactured by Cerex Wedgewood, and Monitec are packaged as stainless steel probes that can be mounted directly into bioreactors ten liters or greater, and offer features such s sample debubblers to eliminate interference from entrained air Another technique used to determine cell density is spectrophotometric titration which is a laboratory procedure which employs the same basic principles as the probes discussed above. this requires a sample to be withdrawn from the broth during reaction and therefore exposes the batch to contamination
678 Fermentation and Biochemical Engineering Handbook Pharmacia claims its BIAcore system can provide information on the affinity, specificity, kinetics, multiple binding patterns, and cooperativity of a biochemical interaction on line without the need ofwashing, sample dilution or labeling of a secondary interactant. Their scientists have mapped the epitope specificity patterns of thirty monoclonal antibodies (Mabs) against recombinant core HIV-1 core protein. 4.0 CELL MASS MEASUREMENT The on-line direct measurement of cell mass concentration by using optical density principles promises to dramatically improve the knowledge ofthe metabolic processes underway within a bioreactor. This measurement is most effective on spherical cells such as E. Coli. The measurement technology is packagedin a sterilizable stainless steel probe which is inserted directly into the bioreactor itself via a flange or quick-disconnect mounting (Fig. 1). By comparing the mass over time, cell growth rate can be determined. This measurement can be used in conjunction with metabolic models which employ such physiological parameters as oxygen uptake rate (OUR), carbon dioxide evolution rate (CER) and respiratory quotient (RQ) along with direct measurements such as dissolved oxygen concentration, pH, temperature, and offgas analysis to more precisely control nutrient addition, aeration rate and agitation. Harvest time can be directly determined as can shifts in metabolic pathways possibly indicating the production of an undesirable by-product. Cell mass concentrations of up to 100 grams per liter are directly measured using the optical density probe. In this probe, light of a specific wavelength, created by laser diode or passing normal light through a sapphire crystal, enters a sample chamber containing a representative sample of the bioreactor broth and then passes to optical detection electronics. The density is determined by measuring the amount of light absorbed, compensating for backscatter. Commercial versions such as those manufactured by Cerex, Wedgewood, and Monitec are packaged as stainless steel probes that can be mounted directly into bioreactors ten liters or greater, and offer features such as sample debubblers to eliminate interference from entrained air. Another technique used to determine cell density is spectrophotometric titration which is a laboratory procedure which employs the same basic principles as the probes discussed above. This requires a sample to be withdrawn from the broth during reaction and therefore exposes the batch to contamination
se Re3457 0x Figure 1. Photo of MAX Cell Mass Sensor. (Courtesy ofCEREX jjamsville, Maryland
Instrumentation and Control Systems 679 ~ C:- .s ~ ~ 'I). .- ::: § c, "5- u ~ ;::;, r/) 5 "' ~ ~ ~ f "' "' ~ ] 0 ~ u ~ - ~ ~ = ~ rz
680 Fermentation and biochemical engineering Handbook 5.0 CHEMICAL COMPOSITION The most widely used method for determining chemical composition is chromatography. Several categories have been developed depending upon the species being separated. These include gas chromatography and several varieties of liquid chromatography including low pressure(gel permeation) and high pressure liquid chromatography and thin layer chromatography The basic principle behind these is the separation of the constituents traveling through a porous, sorptive material such The degree of retardation of each molecular species is based on its particular affinity for the sorbent. Proper selection of the sorbent is the most critical factor in determining separation. Other environmental factors such as temperature and pressure also play a key role The chemical basis for separation may include adsorption, covalent bonding or pore size of the material Gas chromatography is used for gases and for liquids with relatively low boiling points. Since many of the constituents in a biochemical reaction are of considerable molecular weight, high pressure liquid chromatography is the most commonly used. Specialized apparatus is needed for performing his analysis since chromatograph pressures can range as high as 10, 000 psi Thin layer chromatography requires no pressure but instead relies on the capillary action of a solvent through a paper-like sheet of sorbent. Each constituent travels a different distance and the constituents are thus separated Analysis is done manually, typically using various coloring or fluorescing Gel permeation chromatography utilizes a sorbent bed and depends on gravity to provide the driving force but usually requires a considerable time to effect a separation All of these analyses are typically performed in a laboratory; therefore hey require the removal of samples. As the reaction is conducted in a sterile environment, special precautions and sample removal procedures must be utilized to prevent contaminating the contents of the reactor 6.0 DISSOLVED OXYGEN Dissolved oxygen is one of the most important indicators in a fermen ss. It determines the potential for growth. The measurement of dissolved oxygen is made by a sterilizable probe inserted directly into the aqueous solution of the reactor. Two principles of operatic
680 Fermentation and Biochemical Engineering Handbook 5.0 CHEMICAL COMPOSITION The most widely used method for determining chemical composition is chromatography. Several categories have been developed depending upon the species being separated. These include gas chromatography and several varieties of liquid chromatography including low pressure (gel permeation) and high pressure liquid chromatography and thin layer chromatography. The basic principle behind these is the separation ofthe constituents traveling through a porous, sorptive material such as a silica gel. The degree of retardation of each molecular species is based on its particular affinity for the sorbent. Proper selection of the sorbent is the most critical factor in determining separation. Other environmental factors such as temperature and pressure also play a key role. The chemical basis for separation may include adsorption, covalent bonding or pore size of the material. Gas chromatography is used for gases and for liquids with relatively low boiling points. Since many of the constituents in a biochemical reaction are of considerable molecular weight, high pressure liquid chromatography is the most commonly used. Specialized apparatus is needed for performing this analysis since chromatograph pressures can range as high as 10,000 psi. Thin layer chromatography requires no pressure but instead relies on the capillary action of a solvent through a paper-like sheet of sorbent. Each constituent travels a different distance and the constituents are thus separated. Analysis is done manually, typically using various coloring or fluorescing reagents. Gel permeation chromatography utilizes a sorbent bed and depends on gravity to provide the driving force but usually requires a considerable time to effect a separation. All of these analyses are typically performed in a laboratory; therefore they require the removal of samples. As the reaction is conducted in a sterile environment, special precautions and sample removal procedures must be utilized to prevent contaminating the contents of the reactor. 6.0 DISSOLVED OXYGEN Dissolved oxygen is one of the most important indicators in a fermentation or bioreactor process. It determines the potential for growth. The measurement of dissolved oxygen is made by a sterilizable probe inserted directly into the aqueous solution ofthe reactor. Two principles of operation
Instrumentation and Control Systems 681 re used for this measurement the first is an electrochemical reaction while the second employs an amperometric(polarographic) principle The electrochemical approach uses a sterilizable stainless steelprobe with a cell face constructed of a material which will enable oxygen to permeate across it and enter the electrochemical chamber which contains two elec- trodes of dissimilar reactants(forming the anode and cathode)immersed in a basic aqueous solution(Fig. 2). The entering oxygen initiates an oxidation reduction reaction which in turn produces an EMF which is amplified into a signal representing the concentration of oxygen in the solution Figure 2. Sterilizable polarimetric dissolved oxygen probe. (Courtesy of ingold Elec- trodes, Inc, wilmington, Mass)
Instrumentation and Control Systems 681 are used for this measurement: the first is an electrochemical reaction while the second employs an amperometric (polarographic) principle. The electrochemical approach uses a sterilizable stainless steel probe with a cell face constructed of a material which will enable oxygen to permeate across it and enter the electrochemical chamber which contains two electrodes of dissimilar reactants (forming the anode and cathode) immersed in a basic aqueous solution (Fig. 2). The entering oxygen initiates an oxidation reduction reaction which in turn produces an EMF which is amplified into a signal representing the concentration of oxygen in the solution. Figure 2. Sterilizable polarimetric dissolved oxygen probe. (Courtesy of Ingold Electrades, Inc., Wilmington,Mass.)
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 gass
682 Fermentation and Biochemical Engineering Handbook In the amperometric (polarographic) approach, oxygen again permeates 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 electrochemical 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
Instrumentation and Control Systems 683 MAGNETIC SEPARATOR FOCUSING DETECTOR SOURCE 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 fror 50 to 1000 atomic mass units(amu) 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 offgas analysis is done outside the bioreactors them- selves. However, the multiplexing of the streams results in added complexity with regard to sample handling and routing, particularly if concerns of cross contaminationneed be addressed The contamination issue is usually handled by placing ultrafilters in the exhaust lines. Care, however, must be taken to
Instrumentation and Control Systems 683 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 successfidly travel the length ofthe 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 ofgases, 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 offgas analysis is done outside the bioreactors themselves. However, the multiplexing ofthe 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
68 Fermentation and Biochemical Engineering handbook 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 NONRESONANT ION ELECTI ION COLLECTOR RESONANT ION dc AND r VOLTAGES ELECTRON BEAM 8.0 MEASUREMENT OF pH Metabolic processes are typically highly susceptible to even slight changes in pH, and therefore, proper control of this parameter is critical Precise manipulation of pH can determine the relative yield of the desired species over competing by-products. Deviations of as little as 0. 2 to 0.3 may adversely affect a batch in some cases. Like the cell mass probe and dissolved oxygen probes described earlier, the pH probe(see Fig. 5)is packaged in a sterilizible inert casing with permeable electrode facings for direct insertion into the bioreactor. The measurement principle is the oxidation reduction potential of the hydrogen ion and the electrode materials are selected for that purpose
684 Fermentation and Biochemical Engineering Handbook 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. NONRESONANT ION m W IONIZING ELECTRON BUM dc AND rl VOLTAGES Figure 4. Quadrapole principle 8.0 MEASUREMENT OF pH Metabolic processes are typically highly susceptible to even slight changes in pH, and therefore, proper control of this parameter is critical. Precise manipulation of pH can determine the relative yield of the desired species over competing by-products. Deviations ofas little as 0.2 to 0.3 may adversely affect a batch in some cases. Like the cell mass probe and dissolved oxygen probes described earlier, the pH probe (see Fig. 5) is packaged in a sterilizible inert casing with permeable electrode facings for direct insertion into the bioreactor. The measurement principle is the oxidation reduction potential of the hydrogen ion and the electrode materials are selected for that purpose
