# Stationary Phases for Green Liquid Chromatography
## 5 Summary  
The separation in liquid chromatography can be performed using pure water as a mobile phase. The application of pure water is environmentally friendly, and it is the best option for \"green\" chromatography.  
Separations in purely aqueous conditions may be carried out at room temperature using specific polar-embedded stationary phases; however, in such a case, the selectivity depends almost entirely on the stationary phase. The second option is separation at elevated temperatures--superheated water chromatography. The change in water temperature changes its polarity, dielectric constant, viscosity, surface tension, and many other properties; as a result, the elution strength of water increases. Unfortunately, another problem may arise in this case--the thermal stability of both separated substances and stationary phases. Thus, several stationary phases based on silica gel, carbon, polymers,  
Figure 4: Separation of seven nucleosides on a polar-embedded stationary phase (Ester-C18) using pure water at low temperatures as a mobile phase. Description of the compounds—IN: uridine; 2N: guanosine; 3N: 1-methylinosine; 4N: thymidine; 5N: 1-methylguanosine; 6N:N2-methylguanosine; 7N: adenosine. Adapted from ref. [14].  
or metal oxides were obtained for application in purely aqueous separation. More new stationary phases will likely be developed soon, and \"green\" liquid chromatography will gain popularity in the coming years.  
For completely \"green\" chromatography, it is necessary to use only nontoxic, environmentally friendly solvents; such solutions include carbon dioxide, ethanol, and water. In this review paper, we focus on water; its use as the sole component of the mobile phase presents many challenges to chromatography, including viscosity, high elution force, dielectric constant, degradation of stationary phases, hydrolysis, and decomposition of analytes. Thus, it is necessary to select either an appropriate analysis method--as we discussed in our previous work [9]--or an appropriate choice of stationary phase, as described in this work; often, the two can be combined. The most convenient option is to use a stationary phase that allows the use of water without increasing the temperature or using additives in the form of salts or buffers; such solutions include phases with silica, polymer, zirconium or aluminum oxide, or carbon supports. In recent years, silica-based phases with embedded polar groups have proven to be very promising; they are thermally stable and, through the mixed hydrophilic-hydrophobic nature of the surface, allow operation under pure water conditions without the use of additives or elevated temperatures. The goal is to find phases that allow us to simultaneously understand retention mechanisms, control selectivity (e.g., by selecting appropriate groups at the functionalization stage of the stationary phase), and obtain good resolution and efficiency. The phases mentioned above were found by attempting to change the support; in contrast, polar-embedded and polar-endcapped stationary phases open possibilities to manipulate the parameters via a good selection of polar and non-polar groups attached to the silica surface at the synthesis stage; this provides an extensive range of possibilities for the preparation of different phases. In our opinion, it is on these phases that research aimed at finding stationary phases to work under pure water conditions should focus.  
This research received no external funding.  
Not applicable.  
This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.  
The authors declare no conflict of interest.