Gravimetric system linearity

In both gravimetric and piezometric experiments it is generally desirable to make the measurement over a small differential concentration change in order to ensure that the assumption of system linearity is fulfilled. Under these conditions the transient sorption curve (expressed as fractional approach to equilibrium) should be independent of either the step size or direction (adsorption or desorption). Varying the step size and direction thus provides a simple and sensitive experimental test for system linearity. 

The pore diameters of MFI-type zeolites are comparable to the size of many commercially important molecules, such as aromatics or linear or branched hydrocarbons [1]. Thus, the study of the difiusion of reactive molecules in the channel system of zeolite catalysts is of considerable interest for the understanding of the catalyst performance. A variety of methods has been developed and applied to the measurement of diffii-sion coefficients, amongst others gravimetric techniques [2], neutron scattering [3], NMR [4] and Frequency Response [5]. The FTIR technique offers the possibility to study sorption and sorption kinetics under conditions close to those of catalytic experiments. By the use of an IR microscope, single crystals have become accessible to the FTIR technique. 

Autosampler precision can be checked by replicate injections of a control sample, with wash injection intervals between every two sample injections. The repeatability of peak areas, mathematically expressed as RSD, is used as a criterion for autosampler precision. For example, when 10 consecutive injections of 10 pL of a solution are performed, the expected RSD for peak area precision ranges from 0.5% to 1.0%. Single injections of different volumes, such as 5, 10, 50, or 80 pL, can also be used, simultaneously checking the linearity of the injector, the detector, and the data system. Another approach to qualify the autosampler involves the gravimetric determination of the average volume of water per injection withdrawn from a tared vial after six 50-pL injections. The procedure... 

Another source of error involves cases in which both the anodic and cathodic reactions are not charge transfer controlled processes, as required for the derivation of Eq 25. Modifications to Eq 25 exist for cases in which pure activation control is not maintained, such as in the case of partial diffusion control or passivation [35]. Other researchers have attempted to calibrate the polarization resist2ince method with gravimetrically determined mass loss [36], In fact, polarization resistance data for a number of alloy-electroljrte systems have been compared to the observed average corrosion currents determined from meiss loss via Faraday s law [28], A linear correspondence was obtained over six orders of magnitude in corrosion rates. 

The impedance measurements of this system have been combined with oscillometric-gravimetric measurements leading to both the volume of the swelling polymer and the mass of CO2 sorbed at a given gas pressure, cp. Chap. 5. These combined measurements show that there are nearly linear relations between the dielectric permittivity and the gas pressure as well as the volume of the polymer in the sorption state and the gas pressure, cp. data and Figures given in Sect. 3.2. 

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