WATER ACTIVITY UNDER PH. EUR. 2.9.39: CAN YOU USE A RESISTIVE ELECTROLYTIC SENSOR?
The chapter describes the dew point/chilled mirror method in detail, which can create the impression that chilled mirror technology is mandatory. However, the scope of the chapter is broader: it allows water activity to be determined using different suitable hygrometric technologies.
This distinction is particularly important for pharmaceutical laboratories evaluating water activity instruments for use according to Ph. Eur. requirements.
What does Ph. Eur. 2.9.39 actually say?
Chapter 2.9.39, Water-solid interactions: determination of sorption-desorption isotherms and of water activity, defines water activity through the equilibrium relative humidity generated by a sample in a closed system.
The chapter explains that relative humidity can be determined either from direct measurements such as partial vapor pressure or dew point, or indirectly using sensors whose physical or electrical characteristics change in response to relative humidity.
It further explains that, in principle, different types of hygrometers may be used for water activity measurement, provided that the measurement system is suitable for analytical purposes.
The dew point/chilled mirror method is then described as one possible measurement approach. Importantly, however, the chapter subsequently refers to commercially available instruments based on the chilled mirror method or other technologies.
What does this mean for Novasina instruments?
Novasina water activity instruments use a resistive electrolytic sensor to determine the equilibrium relative humidity above the sample. As relative humidity changes, the electrical resistance of the sensor changes. The instrument uses this response to determine equilibrium relative humidity and consequently water activity.
This measurement principle therefore falls within the type of approach described in Ph. Eur. 2.9.39, which specifically recognizes sensors whose electrical characteristics change as a function of relative humidity.
As with any analytical instrument used in a regulated pharmaceutical environment, the measurement system must be demonstrated to be suitable for its intended use and appropriately validated, calibrated and controlled within the laboratory’s pharmaceutical quality system.
What about alternative analytical procedures?
The European Pharmacopoeia General Notices also provide a framework for the use of alternative analytical procedures under defined conditions.
EDQM guidance confirms that alternative methods may be used for in-house control when they have been appropriately demonstrated to be suitable or equivalent in accordance with the applicable requirements. In situations of doubt or dispute involving a pharmacopoeial monograph, the official pharmacopoeial method remains authoritative.
This principle further supports the important distinction between a pharmacopoeial analytical requirement and a requirement to purchase one particular sensor technology.
Chilled mirror or resistive electrolytic: the regulatory question
The regulatory discussion should therefore not be reduced to a comparison of sensor names.
Ph. Eur. 2.9.39 recognizes water activity determination based on equilibrium relative humidity and allows different suitable measurement technologies. Chilled mirror is one established approach. Resistive electrolytic sensing is another approach for determining equilibrium relative humidity through an electrical response.
For a pharmaceutical laboratory, the appropriate instrument should be selected based on factors such as analytical performance, intended samples, suitability, validation requirements, calibration, data integrity and the laboratory’s quality system — rather than on the assumption that Ph. Eur. mandates chilled mirror technology.
The key takeaway
Novasina’s resistive electrolytic sensor determines relative humidity through changes in the electrical characteristics of the sensor and therefore represents a measurement principle consistent with the broader technological framework described in Chapter 2.9.39.
For pharmaceutical use, the decisive requirement is not that an instrument uses chilled mirror technology, but that the selected measurement system is suitable for its intended analytical use and appropriately validated, calibrated and controlled.
Does your laboratory need to evaluate water activity measurement according to Ph. Eur. 2.9.39?
Talk to a Novasina water activity specialist about instrument qualification, method suitability and pharmaceutical water activity measurement: sales@novasina.ch
