AquaLab Sample Preparation
done to decrease the reading times of these types of samples. For faster reading, it is important to have the water activity of the chamber at or below the water activity of these type of samples. This causes the sample to release water to the vapor phase and equilibrate with the chamber. If the water activity of the headspace is greater than this type of sample, a long period of time will be required to reach equilibrium and the aw of the sample may be affected. Propylene Glycol AquaLab will give accurate readings on most alcohols. However, samples with propylene glycol in concentrations higher than 10% will give inaccurate aw values for consecutive samples. This is because propylene glycol condenses on the mirror during the reading process, but it does not evaporate from the mirror as water does. As a result, the first reading will be accurate, but subsequent readings will not unless the propylene glycol is cleared out of the chamber after each reading. You can clean the propylene glycol out of the chamber by running a sample of the activated charcoal (included as part of the AquaLab cleaning kit) after each propylene glycol-bearing sample. Once you have finished sampling, clean the chamber to make sure all residue is eliminated (see Chapter 10).
As another solution, Decagon offers a special sample block designed for measuring volatiles such as propylene 36