Micropore analysis of microporous materials (e.g., activated carbon and zeolites) has mainly been performed by nitrogen adsorption at 77 K, but this is not satisfactory with regard to a quantitative assessment of the microporosity, especially in the range of ultra micro pores (pore widths < 0.7 nm). A pore width of 0.7 nm corresponds to the bilayer thickness of the Nz molecule. In addition, preadsorbed Nz molecules near the entry of an ultramicropore may block further adsorption. The pore filling of such narrow pores occurs at relative pressures of 10-7 to 10-5, where the rate of diffusion and adsorption equilibration is very slow. This leads to timeconsuming measurements and may cause under-equilibration of measured adsorption isotherms, which will give erroneous results of the analysis.

For many microporous systems (in particular zeolites) the use of argon as adsorptive at its boiling temperature (87.27 K) appears to be helpful. Argon fills micropores of dimensions 0.4 nm - 0.8 nm in most cases at much higher relative pressures, (i.e., at least 1.5 decades higher in relative pressures) as compared to nitrogen, which leads to accelerated diffusion and equilibration processes and thus also to a reduction in analysis time.

The different pore filling ranges of argon adsorption at 87 K and nitrogen adsorption at 77 K is illustrated in below figure based on sorption data obtained on a faujasite-type zeolite. The much lower pore filling pressure for nitrogen compared to argon is still not completely understood, but clearly indicates that the attractive interactions of nitrogen molecules with the pore walls of the zeolite are much stronger as compared to argon. The possibility that this enhancement of the adsorption potential is related to specific quadrupole interactions is under discussion.

Nitrogen and argon adsorption at 77 K and 87 K, respectively, on a faujasite -type zeolite

However, the micropores of some zeolites are too small to be characterized by nitrogen and argon adsorption at cryogenic temperatures. Here, the so-called molecular probe method offers a possible way of direct determination of the effective pore size. This method is based on the measurement of sorption rates and capacities using a series of sorbates of progressively increasing molecular diameter. There will be a very sharp sorption cutoff, when the molecule cannot enter anymore the micropore, and a good estimate of the effective pore size can be obtained in this way.

The problem of assessing ultramicroporosity appears in particular for microporous carbons, which often exhibit a wide distribution of pore sizes including ultramicropores. It has long been recognized that using CO2 adsorption analysis at 273.15 K can eliminate problems of this type. At 273.15 K, CO2 is still ca. 32 K below its critical temperature, and because the saturation pressure is very high (26200 torr), the relative pressure measurements necessary for the micropore analysis are achieved in the range of moderate absolute pressures (1 - 760 torr). Hence, due to the relatively high absolute temperatures and pressures compared with nitrogen and argon adsorption at cryogenic temperatures, diffusion problems can be eliminated. Because of the higher diffusion rate, adsorption equilibrium is achieved faster, which allows completion of the adsorption isotherm in a significantly shorter time compared to nitrogen adsorption at 77 K. In addition, the range of analysis can be extended to pores of smaller sizes that are accessible to CO2 molecules, but not to nitrogen and argon. However, if the analysis is performed with a conventional volumetric sorption analyzer, which can be used to pressures up to ca. 1 atm, the measurable pore size range is limited to pore sizes up to 1.5 nm. 

Gregg and Langford suggested assessing microporosity by making use of the strong retention of n-nonane in narrow pores (i.e., the method of "nonane preadsorption"). The method was often applied to microporous carbon. The aim is to fill all micropores with nonane while leaving the wider pores open. Because of the high physisorption energy, the preadsorbed nonane can only be removed at elevated temperatures. A possible procedure for the nonane preadsorption goes as follows: The sample is outgassed and a first nitrogen sorption isotherm is obtained. The sample is then exposed to nonane vapor. The sample, saturated with nonane is then outgassed again at room temperature, prior to the re-determination of the nitrogen sorption isotherm. In the following steps the sample is outgassed at increasingly higher temperatures. After each stage of outgassing, a nitrogen sorption isotherm is measured until the nonane has been completely removed. The difference in pore volumes and surface areas before and after nonane preadsorption is attributed to the fact that narrow micropores were completely blocked by preadsorbed nonane. However, the conclusions, which can be drawn from nonane preadsorption are not always straightforward (with regard to assessing the complete microporosity), mainly because of the pore size dependency of the adsorption potential, i.e. nonane molecules are more firmly trapped in the ultramicropores than in supermicropores. Another problem is associated with the blocking of wider pores due to the adsorption of nonane in the more narrow pores; this problem is in particular important in case the adsorbent consists of networks of pores with different sizes.

Kaneko et suggested helium as a good probe molecule for the assessment of ultramicropores, because helium is the smallest inert molecule and can possibly penetrate a narrow neck of a micropore. Within this context, Kaneko et al. determined the adsorption isotherms of He on activated carbon fibers at 4.2 K by a gravimetric method. The He adsorption isotherms were of typical type I and an analysis by the Dubinin Radushkevich method gave micropore volumes greater than those obtained from N2 adsorption at 77 K by 20 - 50%. The excess amount of He adsorption was ascribed to the presence of ultramicropores, which cannot be assessed by N2 molecules. There are a couple of reasons that could explain the large difference in micropore volumes obtained by He and N2 adsorption. The first reason is due to the fact that the He molecule is smaller and can enter narrower micropores than N2. The second reason is associated with the packing efficiency; smaller molecules can fill more effectively the restricted micropore space. Helium adsorption at 4.2 K is considered to be a suitable tool to assess ultramicroporosity.