# Stationary Phases for Green Liquid Chromatography
## 3 Stationary Phases Used in Pure Water Conditions
### Instability of Stationary Phases--A Motivation to Search for Solutions
#### 3.1.1 Silica-Based Packing Materials
Subsequent detailed studies proved that PALC could obtain retention factors as good as HILIC for separating polar compounds. Modifying silica particles with the functionalized carbon nanoparticles (CNPs) obtained from corn stalk soot allows polar stationary phases to be obtained. Such materials can work with both water-rich and acetonitrile-rich mobile phases [73]. PALC also enables the use of hybrid silica materials such as a 1.7 \(\upmu\)m ethylene-bridged hybrid silica stationary phase (BEH HILIC); its application to the separation of 12 imidazole-based ionic liquids' cations showed that the PALC system could enable retention with both hydrophobic and ion-exchange mechanisms [74]. Another application of PALC was used by [PERSON] et al., combining it with size-exclusion chromatography (SEC) to determine the water-soluble organic matter (WSOM) content in atmospheric aerosols collected from urban areas during different seasons [75]. Due to its positive ecological and economic aspects, PALC is sometimes used as a complementary technique to RP-LC. Detection of four types of biogenic amines and five nucleic bases and nucleotides was performed on a silica-based column with Congo red molecules attached (Sil-CR) using PALC [76]. The results obtained showed better separation with PALC than using HILIC, which consumes large amounts of organic modifiers. In the RP-LC technique, applying pure water as a mobile phase requires surface modification of the silica-based stationary phase. These phases have embedded polar groups to provide a mixed retention mechanism and allow water to interact with the silica surface. The polar-embedded and polar-endcapped phases will be discussed in subsequent sections of this paper. Commercially available silica-based stationary phases and the groups of compounds separated on them are summarized in Table 1. \begin{table}
\begin{tabular}{c c c c c c c c c c c} \hline \hline  & & & & & & & & & & & \\ \hline Akzo Nobel Kronnal Bernuity-25-C18 & & & x & x & x & x & x & & & [48] \\ \hline Chovanatores C-18 & & & & & & & & & & & [33] \\ \hline Daiso (OOS-BP) & & & & x & & & & & & [77] \\ \hline Developmental CSO-UC5 & x & & & & & & & & & [78] \\ \hline Fuji Slipvia Chromatography 10 \(\mu\)m & & & & & x & & & & [79] \\ \hline Hyperper CDS & x & & & & & & & & & [80] \\ \hline Hyperper Bessel C18 & & & x & & & x & & & [66,81,82] \\ \hline Interchain Urtigsphere Strategy C18-2 and & & & x & x & x & x & x & & [68] \\ \hline C18-3 & & & x & & & & & & [33] \\ \hline Kronnal C18 & x & & & x & & & & & [33] \\ \hline L-column OOS-Chemoids Evaluations and & & & & & & & & [34] \\ Research Institute, Japan & & & & & & & & [84] \\ \hline Nanoporous glass modified with TFPS and & & x & & & & & & & [80] \\ ethyl acetate & & & & & & & & & [87,85] \\ \hline Novopack C18 & & & & & & & & & [83] \\ \hline Nucleonic C18 & x & & & & & & & & [66,63] \\ \hline Nucleonic C18 AB & x & x & & x & & & & & [87] \\ \hline OOS Cheropack & x & & & & & & & [87] \\ \hline Partial OOS ES & x & & & & & & x & x & [88] \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Literature dataset of silica-based chromatographic columns and groups of chemical compounds separated by them using pure water.