# Metal-Organic Frameworks Characterization via Inverse Pulse Gas Chromatography
## 1 Introduction
The advantage of the frontal technique is that, due to its continuous operation, equilibrium can be achieved, whereas the pulse technique relies on the assumption of rapid equilibrium between the probe and the solid surface. In the case of the continuous flow method, which is a classical form of frontal technique [23], the column can be detached from the gas flow if the concentration is changed and afterwards attached again when the flow is balanced. Then, a positive peak will appear for a higher concentration and adsorption process, while a negative peak will indicate a lower concentration and desorption process. The peak area is directly proportional to the amount of adsorption or desorption. This technique is utilized in many instruments conceived for the dynamic estimation of Brunauer-Emmett-Teller (BET) surface area [15]. However, this approach cannot be considered an IGC technique since retention time is not a parameter in the experimental calculations [15]. In comparison, classical frontal experiments evaluate the nature of retention and, therefore, are considered an IGC technique. A significant proportion of publications utilized pulse instead of frontal approaches because they are quicker, more convenient, and provide higher accuracy, especially if there are poor interactions between the tested material and probe. The frontal approach is a suitable alternative if a slow equilibrium is the topic of examination. Some research groups tend to prefer the term \"pulse gas chromatography\" over the term \"inverse gas chromatography,\" while others used the full expression \"inverse pulse gas chromatography.\" We have noticed that these different terminologies were sometimes misleading, indicated by few cross citations. Furthermore, subsequent studies claim that \"_there have been almost no studies using the IGC with MOFs_\" [24]. After the IGC technique was recognized as a sensitive and simple tool for investigating physicochemical properties, it was applied for different types of materials, including non-volatile and solid materials with numerous morphologies. For instance, it was applied to evaluate the impact of grinding on the surface and bulk features of drug molecules [25], which is important for medicinal industry improvements. IGC has also been used to deconvolve the chemical and structural properties of carbon nanotube surfaces [26]. Additionally, IGC was used to characterize polymers, minerals, modified silicas, surfactants, and zeolites [22; 27]. However, in the case of metal-organic frameworks (MOFs), which are considered a recently introduced porous material, few studies have used IGC for the characterization of their different properties. Metal-organic frameworks (MOFs) are a subclass of porous crystalline materials constructed by linking metallic nodes like single-ion or clusters with organic linkers. They have been developed in the last twenty years as potential candidates for solving many of the challenges associated with different applications, such as gas storage, separation, catalysis, and sensing [28; 29; 30], by means of the anticipated on-demand control of their pore-system (i.e., pore-aperture, shape, size, and functionality). MOFs are considered organic-inorganic hybrid porous materials and have supreme chemical, structural, and topological adaptability [31; 32]. Their pore system is tunable via different design and engineering strategies [33]. MOFs can be efficiently designed with the proper properties in terms of pore aperture size, controlled by the organic linkers linked through metallic-nodes and/or shape (channels or cages), to achieve the targeted application. Successful exercising of reticular chemistry ensures the suitable inorganic and organic building blocks, encompassing defined geometrical information that ciphers a designated net, are constructed into a hybrid material framework with the desired topology. MOFs with a wide range of porosities and chemical functionalities have been synthesized and explored in many applications such as separation and catalysis [30; 34].