# Experimental Setup and Graphical User Interface for Zero-Length Column Chromatography
## 3 Design and Operation of the ZLC  
A semi-automatic ZLC system was built to perform desorption experiments under kinetic and equilibrium conditions of pure components and mixtures. The system is composed of a first stage that supplies high purity gases to a gas mixer through drying columns. The mixture obtained is dosed to two 4-way and 6-way electrical switching valves located inside a gas chromatograph, in which the ZLC and the semi-capillary column are placed. In addition, two 3-way ball valves with L-shaped holes (P1 and P2) are required, which operate in positions A and B, respectively. The devices and the chromatograph are controlled by a personal computer via a data acquisition card and LabView version 14.0 from National Instrument (Austin, TX, USA). Once the operator has set up the experiment, the system can run and acquire data automatically. A complete schematic diagram of the experimental setup is shown in Figure 1.  
Figure 1: Schematic drawing of the semi-automatic experimental apparatus to determine: diffusion coefficients and adsorption isotherms of pure gases and mixtures. The main component of the system is the zero-length column, which consists of a Swagelok joint of \({}^{1}\)/s inches, in which a quantity of molecular sieve is packed between two sintered disks of 100 um mesh (see Figure 2). This configuration allows performing different experiments after degassing without exposing the sample to the environment. For this purpose, a mini electric oven with a PID controller was incorporated, which heats exclusively the ZLC and reaches temperatures of over 623 K. This reduces the experimental times, the amount of sample used, ensures the isothermal conditions in the column and allows degassing of materials demanding higher temperatures than those supported by other elements inside the chromatograph furnace with low working temperature limits. The small sample size used in the ZLC offers the possibility to determine particle size distribution in an accurate and quick way. Therefore, the complexity in processing the experimental data is reduced, as the determination of particle size distributions is often time-consuming, costly, and can become very complex when crystals are much smaller than 1 um[23]. The system also includes a Shimadzu GC-8A gas chromatograph, with two flame ionization detectors (FID-1 and FID-2 in Figure 1). The chromatograph consists of an oven that reaches a maximum temperature of 673.15 K and a signal conditioning stage consisting of a high impedance amplifier with selectable gain (\(G=1\), \(1\times 10^{2}\), \(1\times 10^{3}\), \(1\times 10^{4}\), and \(1\times 10^{5}\)) and level shifting. The three-way valve P2 allows redirecting the flow at the outlet of the ZLC to the FID-1 or to the flow 6-way selector valve (FSV-6). A standard Restek micro-packed semi-capillary separation column (SCSC) of 1 m length and 0.75 mm inside diameter was attached to FSV-6 to determine the effluent composition by mean FID-2. Both the flow 4-way selector valve (FSV-4), model EUH-6C4 WT, and the FSV-6, model EUH-6C4 WT, were purchased from Vici Valco [24]. They operate in two positions, A and B. Position A is represented by solid black lines and position B by dashed red lines in Figure 1. According to the specifications, the maximum operating temperature indicated is 523.15 K.  
The flow rates were controlled by four Horiba Z-500 Mass Flow Controllers (MFCs) (2 for the feed and 2 for the purge) [25]. They operate from 0 to 100 standard cubic centimeter per minute (sccm). Equilibrium and kinetic properties can be then investigated in the same series of experiments, simply using both sets of mass flow controllers. The pressure at each gas line was followed by ConST 211 analog pressure gauge. The operation of the ZLC starts with the degassing of the sample in the sample oven controlled by PID, this is achieved by switching both selector valves to position B.