An increasing number of modern, highly effective pharmaceuticals are produced in advanced biotechnological fermentation processes. For optimised process control, a good understanding of the processes involved and fast monitoring of analyte concentrations are valuable assets. Hence, in-situ chemical sensors replacing the prevalently used at-line I offline analytics are of high interest. This is all the more true as the US FDA calls for exactly such monitoring tools in their PAT initiative, aiming at better process understanding and control in chemical and pharmaceutical production.
The key problem of using chemical sensors in industrial microorganism fermentations is the composition of the fermentation broths. These are comparable to full blood in complexity, turbid and frequently heavily aerated. Only a few single-analyte broth sensors (pH, dissolved oxygen) can be regarded as industrially established, while many attempts to build chemical sensors for further broth constituents have been thwarted by the reality of an industrial bioreactor. Yet, one approach using mid-infrared ATR probes has shown good potential.
Sensing in the mid-IR has a range of advantages. First, the method is capable of multianalyte sensing in complex mixtures and at relevant concentration levels. Sensitivity and selectivity of the sensor are inherent properties of the transducer itself, so no chemo selective layers are required. This helps in maintaining signals stable over longer periods of time. Second, when using ATR probes, varying turbidity or suspended gas bubbles do not interfere with the measurement. Third, with the MettlerToledo ReactiR System, a sensor platform meeting industrial requirements is commercially available.
The investigated sensor system consisted of a compact FT-IR spectrometer equipped with a specially adapted ATR probe. In a first step, the basic suitability was assayed. It was found that the sensor is capable of accurately detecting a wide range of different – organic and inorganic – analytes at concentration levels down to less than 0.5g/L. Temperature deviations, resulting in spectral variations of water absorptions, do not present a significant obstacle. Furthermore, the technology is fully compatible with the most stringent requirements of pharmaceutical bioprocess technology, e.g. withstands standard heat sterilisation processes at temperatures> 120•C without any problems.
In a second phase of the development, the sensor was used to monitor real bioprocesses insitu and in real time. It proved possible to measure the most relevant analytes glucose, acetate, immonia and phosphate with prediction errors less than 0.5g/L over the full length of a micro-organism fermentation process. Using a multivariate calibration model based on reference analysed samples of a prior run, the process could be continuously monitored at a time resolution of down to one minute. Standard process parameter variations do not interfere with the sensor predictions. A significant variation of the pH value however may bear a strong influence on the readings for certain analytes.
Based on these results, it is possible to name mid-IR ATR sensors a practically applicable,
highly interesting tool for in-situ real-time process monitoring of industrial micro-organism