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Total Cell Density Measurement Principle

Real-Time Optical Density Measurement in Biopharma

Optical density (OD, sometimes called turbidity) is commonly used to estimate the total biomass or concentration of cells within a suspension. Cell concentrations are important to monitor in fermentation processes to detect and correct for process deviations. When measuring OD, light of a specified wavelength passes through a portion of the cell suspension. Components in the cell broth absorb and reflect some of the light from the emission source, with the residual light reaching the detector. The amount of light measured (transmitted) at the detector is directly related to the absorbance of the sample. This method of absorbance measurement is called transmission.

Offline optical density measurements (performed in cuvettes using visible 600 nm wavelength = OD600) are affected by media color, which can absorb some of the visible light and alter the amount of light transmitted to the detector. Offline sampling requires excessive time and energy, and detection of process deviations may take hours, missing critical events.

Optical Cell Density Measurement principle

In-Line Near-Infrared Measurement Principle

The Dencytee Arc sensor performs in-line measurement of Total Cell Density (TCD) in solution, based upon OD measurements using near-infrared (NIR) light at 860 nm (OD860), a wavelength that is insensitive to media color changes. The OD860 signal correlates with the total cell density in a bioprocess: living and dead cells, cellular debris, and all particles or molecules which absorb or scatter the light. This measurement is effective after inoculation while cells are quickly expanding but total concentrations are lower than the detection threshold for capacitance-based sensors.

Dencytee Measurement Principle

Combination of Transmission and Reflection in One TCD Sensor

Dencytee Arc features an LED light source at 860 nm, a back detector next to the light source, and a front detector across from the light source. The emitted light passes through a 5 mm gap before transmitting light onto the front detector. Any reflected light is measured on the back detector. When cells are present in the gap, they will absorb and scatter light. This results in a decreased amount of light reaching the front detector (transmission) and an increased amount of light reaching the back detector (reflection). The unique multi-detector absorbance measurement enables Dencytee Arc to precisely measure total cell density at both low and high cell concentrations.

This optical density signal is easily converted into your current offline units using the ArcAir software, providing a real-time in-line total cell density sensor with a wide measurement range.

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