Dr Brady Carter reports on the issues caused by volatiles in water activity testing
Water activity measurements can be impacted by volatiles and must be accounted for when conducting water activity tests. Volatiles can interfere with all three of the most common sensors used to measure water activity, including: dewpoint, capacitance and resistive electrolytic. Volatiles can condense on the mirror in a chilled mirror dewpoint instrument causing incorrect readings or be absorbed by the capacitance or resistive electrolytic sensors causing the calibration to shift. Therefore, when volatiles are present, some type of intervention is necessary to obtain correct readings and prevent degradation of the sensor.
The resistive electrolytic and capacitance sensors will still give correct readings even in the presence of volatiles, but they are being degraded as the sensor absorbs the volatiles. If the sensor is not protected in some way from contact with the volatiles, it will eventually need to be recalibrated or replaced. Thankfully, resistive electrolytic water activity instruments provide a way to avoid the impact of volatiles by using filters that keep volatiles away from the sensor. These filters prevent degradation of the filter and allow for continued use in the presence of volatiles while still providing accurate water activity values. Other than through the use of expensive tunable lasers, a resistive electrolytic system equipped with an appropriate filter is the only way to obtain accurate water activity values for some volatile containing samples while avoiding frequent recalibration or sensor replacement.
The filters that are used to protect the sensor are easily installed and replaced. They do not need to be removed during verification or calibration, but will slightly increase the test time due to the water vapor passing through the filter. The filters do need to be routinely replaced as they will saturate after repeated exposure. Although there is no hard rule for how frequently the filters should be changed, a good rule of thumb is the following: if you do fewer or equal than 10 measurements a week, change filter every 6-12 months; if you do more than 10 measurements a week, change filter every 3-6 months; if instrument is in use day and night, change filter every 2 months.
If the sensor is exposed to volatiles, it is possible to avoid replacement and restore performance using a regeneration kit. An absorptive material is placed in the instrument at elevated temperatures and is exposed to the headspace to degas volatiles from the sensor. This approach can often fully restore a resistive electrolytic sensor.
Unfortunately, it is not always possible to know when volatiles are causing inaccurate water activity readings or damaging the sensor. Some systems have the capability to warn the user when the sensor health is being impacted by volatiles, but other systems will just give readings that are incorrect with no indication that volatiles are present. To help with this, Table 1 provides a list of the most common volatiles that can impact water activity measurements and the types of products that are likely to contain those volatiles. Anyone conducting water activity testing is encouraged to refer to Table 1 and consider if their samples contain volatiles and if so, to make sure they are using a system that can give correct readings and to use an appropriate filter to protect the sensor.
Dr Brady Carter is an application scientist with Novasina
