6 Aug 2014 | 13:39
By Dr Jürgen Dienstmaier

The Dynamic Vapour Sorption Instrument (or DVS for short) was developed as a response to the pharmaceutical and industrial researcher’s need for fast and effective analytical methods to determine a material’s moisture content and related sorption isotherms.

Before the invention of the DVS, water sorption isotherms were obtained from a process known as the “Jar Method” or desiccator method. It was a slow and tedious process that took weeks, if not months, to achieve results. In short, the older method created a fixed water activity/relative humidity within a sealed container or jar by use of a salt slurry beneath a sample platform while maintaining the container at constant temperature. The sample would then be taken out of the container on a regular basis, weighed and returned to the container. When the change in mass was almost invariable between successive weighing, then the sample had achieved equilibrium (isotherm point) at that relative humidity (RH). If a complete isotherm, from 0% up to ~95% RH was required, many containers were needed. Further complicating the complete isotherm method, only experimental relative humidity values of known salt slurries were available thereby limiting the amount of equilibrium points used to construct the isotherm. This lengthy work was extended even further if experiments at several different temperatures were required. Considering the extreme work involved in preparation and measurement along with the chance to lose or contaminate samples during the weighing transfer process, it is understandable that a faster, simpler and more cost-effective method was urgently required.

First DVS

First DVS currently in Pfizer UK

In the early 1990s, this problem was tackled by Dr. Daryl Williams (Imperial College, London, UK) working together with Pete Marshall and Pat Basford (nee’ Cooke) from Pfizer (Sandwich, UK) and in 1993, the first DVS was produced and delivered to Pfizer. Development of the product was continued at Surface Measurement Systems but in close conjunction with our UK agent at the time, Scientific and Medical Products and specifically John Booth, who collaborated with us allowing us to develop the world’s best instrument at the first attempt! That very same first DVS is still in operation today at Pfizer demonstrating the robustness and durability of SMS instruments built in the course of these 21 years.

The DVS is an instrument that provides continuous information on a sample’s weight as a steady stream of carrier gas with a well-known and strictly controlled RH flows around it. It is also dynamic, because as stated above, the gases are constantly flowing and not static as in the Jar Method. It is also dynamic because the RH of these gases can be modified in a programmed manner. The DVS is housed inside a controlled temperature environment assuring very stable and closely controlled temperature conditions throughout experiments. Operation of the DVS is straight forward. Basically, a few milligrams of sample (typically between 5 and 15 mg), are placed in a pan which hangs from a very sensitive microbalance. The control software is programmed with the desired variations of RH or temperature (step-wise or ramped), and finally the method runs automatically to perform the experiment. With the DVS, 10 to 100 times less mass are needed for a water sorption experiment as compared to the Jar Method, the technique is 10 – 100x more accurate, and 10 – 100x faster. Hence, increased productivity and reliability can be gained from these technological advances. For these reasons DVS is now in used in more than 1000 laboratories throughout the world.

All DVS instruments operate using dedicated control and analysis software. These features allow analysis of the data in real time and programming of the experiments at almost any given relative humidity value. This was not possible using the Jar Method due to the limitation imposed by the slurry of salts described previously. In addition, not only the sorption part of a sample’s isotherm can be obtained but also it’s desorption part as well –a process that is usually neglected with the old Jar Method. Both isotherms can now be easily acquired and evaluated with the DVS, obtaining valuable information regarding not only the sorption and desorption mechanisms and hysteresis, but also the kinetics of vapour sorption, stability, hydration, diffusion, phase transitions, etc. of a material.

Before the advent of the DVS, some characteristics of materials related to sorption processes were very difficult to assess, if at all. Take for instance phase transitions mentioned before. With the DVS, it is possible to determine accurately at which RH a pharmaceutical sample changes from a partially amorphous phase to a crystalline one, i.e. its humidity-induced crystallization point

[1]. This is done by continuously increasing the RH of the DVS sample chamber and observing the RH values corresponding to changes in the rate of vapour uptake/loss. This information has practical uses, like determining the critical relative humidity values and temperatures for storage of sensitive pharmaceutical materials.

Over the past 20 years, the DVS product family has been continuously expanded and improved in pace with the needs of the industry and research laboratories worldwide. One of the most important advances is the ability to use organic vapours. Hence experiments can now be carried out to evaluate a sample’s response to humidity as well as to different organic vapours. This last feature has opened a whole new range of possibilities for materials’ evaluation and characterization using the DVS, such as solvate formation, surface area, and surface energy.

Although the first DVS users were pharmaceutical and food companies, they have now expanded to numerous other sectors. These users need to know rapidly and reliably how materials behave in the presence of moisture or organic vapours. Examples of these sectors are mining, personal care, nanomaterial research, polymers, construction and packaging.

References[1] Dan Burnett, Frank Thielmann. Determining the Moisture-Induced Glass Transition in an Amophous Pharmaceutical Material. DVS Application Note 35. Surface Measurement Systems.

Acknowledgment

We would also like to acknowledge the contribution of Mr John Booth to the first DVS system.

About the author:
Dr Jürgen Dienstmaier studied Chemistry at the Pontifical Catholic University of Peru, followed by a Master in Material Science at the Technische Universität Ilmenau in Germany. Thereafter a Ph. D. at the Ludwig-Maximilians- Universität in Munich concluded in 2013. He is currently Surface Measurement Systems Lead Application Scientist for DVS products, working on various DVS research studies and applications.

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