Adsorption measuring methods with highest resolution are required for the characterization of new and rare materials. Also, measurement of light gas adsorption on small samples – for example investigating the H2-storage capacity in new materials – needs highest resolution. Commercially available high-end volumetric instruments provide resolutions down to the nano-mol level. But these instruments are limited to sub-atmospheric pressures. For accurate adsorption measurements at higher pressures usually gravimetric instruments are applied. The detection limit of sample mass changes in gravimetric measurements is defined by the resolution of the balance. Up to now commercially available gravimetric instruments for high pressure adsorption measurements are limited to resolutions in the mg range. For some cases – for example measuring adsorption on small samples – this resolution is too low.
Therefore, we present a new type of magnetic suspension balance (MSB). This new MSB weighs samples with up to 200 mg mass with a resolution of 5 ng. Its compact design allows performing measurements from vacuum up to high pressures. The working principle of the new MSB was developed and patented by Baur and Bourdon  at the Swiss Federal Institute of Technology. In collaboration with them the new MSB is further developed as weighing device in high resolution gravimetric adsorption measuring instruments.
Figure 1: Schematic setup and circuit diagram of new MSB
We will briefly present the working principle of the new balance and its specifications. Further the performance of the instrument will be demonstrated by presenting results of measurements performed with two prototypes. So far measurements for physical characterization (BET & pore size) of very small samples by N2 adsorption at 77 K and H2 adsorption measurements at higher pressures on small quantities of samples have been performed.
Figure 2: BET isotherm on mineral fraction of ice core EDC4734, sample mass 0.304 mg, specific surface area 33.01 ± 0.13 m²/g
These results prove the outstanding resolution, reproducibility and accuracy of the new MSB instrument, both for low and high pressure adsorption measurements. The new MSB provides the same level of resolution as the best available volumetric instruments. However, it measures with this outstanding high resolution at all pressures and is not limited to very low pressures as the volumetric instruments are.
Finally a perspective and the scheduled specifications of adsorption measuring instruments with the new MSB will be given.
1. Patents EP2006/011570, US2009/0160279 Patent application “Magnetic levitation system”, No. US 60/748,475, 8.12.2005; WO2007/065608, 1.12.2006