Obtaining High Resolution, Low Pressure Isotherms with a Constant Volume / VectorDose Method
VectorDose, or constant volume dosing, can be used as an alternative to the standard Target Pressure Mode to measure high resolution isotherms in the low pressure range. Care must be taken in choosing the parameters in order to obtain equilibrated data and these parameters and their effect on the isotherm are discussed.
There are two primary ways to measure the data for an adsorption isotherm with manometric adsorption analyzers: target pressure mode and constant volume dosing (vector dosing) mode. In the target pressure method, the relative pressure for each data point is specified and repetitive doses of adsorptive are applied and equilibrated until the target pressure is achieved. An advantage of the target pressure data acquisition method is that, compared to constant volume dosing, equilibrated adsorption data are more easily achieved, even at very low relative pressures (e.g., < 100 mtorr, which corresponds to relative pressure (P/P0) less than 1 x 10-4 for N2 and Ar adsorption at 77 K and 87 K, respectively). However, because each point is specified, high resolution isotherms may take longer to measure compared to isotherms measured using other methods such as constant volume dosing.
In contrast, in the constant volume dosing method, dose size (volume) and equilibration times are specified. The user-defined dose size is applied to the sample and allowed to equilibrate for the indicated amount of time, following which the isotherm data point is measured. This is repeated until the full isotherm has been obtained. Contrary to the target pressure method, an advantage of the constant volume dosing method is that it allows one to obtain extremely high resolution adsorption isotherms, but unlike target pressure mode, care must be taken in selecting the correct experimental parameters (dose size, equilibration time) as changes in each experimental parameter can easily cause deviations from the correct, equilibrated isotherm. In addition, optimal dose size and equilibration times are, in this case, dependent on the sample amount.
Because of this, the target pressure method can be considered the standard method and great care must be applied if one chooses to use the constant volume dose method. Important aspects concerning the application of the constant volume dose method will be discussed. For the following systematic study, sample mass was held constant to eliminate the effect on experimental parameters and the resulting isotherms.
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