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In biopharma applications there are two technology options for continuous pH measurement. Glass membrane pH sensors are the prevalent technology due to their long history and adoption into many different applications. Optical pH sensors are a relatively new technology that is gaining interest in the marketplace. This article will explore both technologies along with their benefits and limitations.

Understanding glass membrane pH sensors

How do Glass Membrane pH sensors work?

Glass membrane pH sensors are electrochemical sensors that generate a millivolt output corresponding to changing pH values. As the name suggests, the measurement electrode portion of the sensor relies upon a specially formulated pH sensitive glass that reacts with the hydrogen ion concentration in the media.

A reference electrode provides a stable voltage potential to complete the pH measurement circuit and allow for current flow through the sensor. A pH electronics (transmitter or analyzer) measures the millivolt signal to display the proper pH value. A more in-depth article on sensor reference design can be found here.

What are the benefits of glass membrane pH sensors in cell culture applications?

Since the glass membrane pH sensor technology has been available for many years it is well known to most users. The existing bioreactor controller inputs have been designed to accept the sensor’s millivolt input so compatibility is not an issue. The logic for sensor configuration and calibration is normally part of the controller software and written into existing SOP procedures.

Glass membrane pH sensors can withstand gamma sterilization, repeated autoclave and SIP (Steam In Place) cycles, as well as CIP (Clean In Place). They are compatible with most aqueous processes however some applications with proteins, sulfides, or extreme pH levels may require more frequent cleaning and calibration.

What are the limitations of glass membrane pH sensors?

As these sensors are electrochemical they will lose accuracy and responsiveness over time. This effect can be corrected with calibration however eventually the sensor will need to be replaced. Care in handling during installation, cleaning, and calibration is also critical since glass is the main component for the sensor and breakage is always a concern.

Understanding Optical pH Sensors

How do optical pH sensors work?

As the name implies, optical pH sensors use spectroscopy for their measurement principle. The sensing element is composed of a fluorescent dye embedded in a hydrophilic gel compound. This fluorescent dye is sensitive to the hydrogen ion concentration in the media. A light source (typically blue light) is directed on the sensing element which emits light at a corresponding wavelength. Changes in pH will result in a wavelength shift that can be measured through an optical detector. The ratio of change of the wavelength will correspond to the change in pH value. The sensing element is normally produced in a flat sheet where each sensor is die-cut from the sheet and affixed to a fiber optic cable or to a window that allows light to pass through without distortion. Because of the design, these sensing elements may be referred to as spots or patches.

What are the benefits of optical pH sensors in cell culture applications?

The sensing element for optical pH sensors is the only portion of the measurement system in contact with the process. The design is very compact, low cost and disposable which makes them ideal for single use applications. The spots have no shelf life restrictions and need no calibration prior to use (there may be lot-specific wavelength information that must be entered into the spectrometer electronics for best accuracy).

What are the limitations of optical pH sensors?

Optical pH sensors have some cross-sensitivity to certain chemicals. Of greatest concern are those chemicals that could attack the gel matrix and damage fluorescent dye. Organic solvents should be avoided. In other cases, changing conductivity (osmolarity) can impact the accuracy of the pH measurement. This can be compensated for by performing a one-point offset calibration during the run. Finally the full pH measurement range can not be measured with optical technology thus applications outside of approximately 5 to 8pH should be avoided.

The technology for optical pH has been on the market for approximately 20 years however it is not widely known by the market so integration into existing bioreactor controllers and transmitters can be difficult.

Technology Comparison

Since each application has different requirements the comparison chart below should be helpful in deciding which technology is best suited for the user’s measurement.

PARAMETERGLASS MEMBRANEOPTICAL

Measurement Range

0 to 14 pH

4.5 to 8pH

Temperature Range

0 to 140C (32 to 284F)

5 to 50C (41 to 122F)

Pressure Range

0 to 6 bar (sensor dependent)

Unknown

Response Time (t90)

30 Seconds

30 to 120 Seconds

Autoclavable

Yes

Yes (avoid condensed water)

SIP

Yes

No

CIP

Yes

No

Gamma Irradiation

Yes, to 45 kGy (OneFerm)

Yes

Integrated temperature Compensation

Yes

No

Requires specialized equipment?

No

Yes

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