16 ANALYTICAL AND LABORATORY EQUIPMENT
Smart instrument engineering D
Darren Broom explains how to extend the operational range of automated dynamic vapour sorption analysers.
ynamic vapour sorption (DVS) typically involves the measurement
of moisture uptake at near ambient temperature in the relative humidity (RH) range 2 to 98%. Applications include the determination of transport rates in food and packaging, and amorphous phase detection and quantification, hydrate analysis and hygroscopicity studies of pharmaceuticals. In some fields, there are significant advantages to the measurement of moisture sorption outside the above regime; however, this adds additional complications.
In DVS, water vapour is usually delivered to the sample by mixing wet (saturated) and dry gas streams to a varying extent. At low RHs the problem is the control of the flow rate. DVS analysers invariably operate using thermal mass flow controllers (MFCs), which provide very stable control but
have limitations both in absolute accuracy and the control of low flow rates, typically below 2% of their full-scale range. Tis corresponds to 2% RH if the flow ranges are matched. MFCs of different ranges can be used but this in turn limits the achievable RH range unless additional flow streams are added, which increases instrument cost and complexity.
Te difficulties at high RH originate from the problem of bulk condensation. A high level of temperature control is required to prevent this occurring near 100% RH because relatively small changes in the temperature can result in significant shifts in the saturation vapour pressure – or the partial pressure at which bulk condensation will occur. See the inset of Fig. 1, which shows the large shift at 50°C caused by an error of ± 1°C. Tis sensitivity poses significant challenges to designers of DVS instruments.
With regard to temperature, the saturated stream is typically generated using a reservoir held at the same temperature as the sample. Tis helps maintain a stable, isothermal environment but places limitations on the achievable measurement range. Te minimum is typically 5°C because temperatures lower than 0°C will result in freezing while the boiling point of water defines the maximum.
Overcoming challenges Each of these difficulties, however, can be overcome by appropriate instrument engineering. For high temperature measurements, for example, the sample can be independently heated. Tis approach is used in the Hiden Isochema IGAsorp-HT, a DVS analyser that can perform measurements up to 300°C (see Fig. 1). Tis configuration also allows control below 1% RH at ambient temperature.
Fig. 1. Water sorption isotherms for chabazite at elevated temperatures and (inset) the sensitivity of RH to small changes in temperature, illustrated by the example of a ± 1°C error at high RH at 50°C.
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