Functionalization using polymer or silane? A practical test method to characterize hydrophilic interaction chromatography phases in terms of their functionalization method

Herein, commercially available columns employed in hydrophilic interaction chromatography (HILIC) were characterized by determining their ability to selectively distinguish the minute structural differences between small molecules such as nucleosides and xanthines in complex sample matrices. Principal component analysis (PCA) was applied to the data obtained from structurally similar analytes, and the results showed that HILIC columns could generally be classified into two groups: (i) silane-modified columns that were prepared from either native silica particles or silica particles modified with low-molecular-weight silanes and (ii) polymer-modified columns obtained from silica particles functionalized with organic polymers. These two groups could be further subdivided based on the functionalities attached to the respective stationary phases. These results were confirmed via cluster analysis by preparing a dendrogram using the morphology-based selectivity parameters associated with the respective columns. We were able to determine the selectivity of columns for the OH groups, i.e., α(OH) and the prevailing pH conditions (cation- and anion-exchanging natures) on the surface of the respective stationary phases; α(theobromine/theophylline) was employed to obtain a similar two-dimensional plot. This test scheme, in which five compounds were analyze for each column, was helpful for understanding the impact of factors such as the hydrophilicity, degree of hydration, acidity/basicity, or the weak ion-exchange nature of the respective stationary phases on the separation characteristics of new HILIC stationary phases. The selectivity of columns for the CH₂ group was also examined. The cation-exchange nature of the HILIC columns significantly influenced native silica columns and some polymer-modified columns. Herein, 45 commercially available HILIC columns were classified according to this method, and the results proved useful for understanding distinct separation characteristics of each HILIC column, enabling improved column selection.

Development and Evaluation of HILIC-type Sorbents Modified with Hydrophilic Copolymers for Solid-phase Extraction

Hydrophilic interaction chromatography (HILIC) has attractive attention for the separation of water-soluble compounds via HPLC. There are, however, few studies on the pretreatment of the HILIC-type solid-phase extraction (SPE) due to the difficulty of obtaining the HILIC-type sorbent. Therefore, the development of HILIC-type sorbents for SPE is essential. In this study, four different hydrophilic copolymers, namely diallylamine-maleic acid copolymer (DAM), diallylamine-acrylamide copolymer (DAA), allylamine-maleic acid copolymer (MAM), and partly methylcarbonylated allylamine acetate copolymer (MAC), were immobilized on glycidyl methacrylate (GMA)-base resin, and their adsorptive properties were evaluated. The results of the physical and adsorptive properties indicated that a balance between the water content of the water-enriched layer on sorbent and the amount of hydrophilic copolymer immobilized on the GMA-base resin was vital for the adsorption in HILIC-type sorbent for SPE.

Evaluation of the adsorption properties of nucleobase-modified sorbents for a solid-phase extraction of water-soluble compounds

We developed hydrophilic interaction chromatography (HILIC)-type sorbents modified with nucleobases for solid phase extraction (SPE). The synthesized hydrophilic base resins were modified by each nucleobase (adenine, guanine, and cytosine). The measurement of the amount of water content indicated that each nucleobase-modified sorbent had a water layer. To evaluate the adsorption properties in the HILIC mode, we chose two nucleobases (uracil and adenine) and four nucleosides (uridine, adenosine, cytidine, guanosine) as water-soluble analytes, which were loaded into an SPE cartridge packed with the nucleobase-modified sorbent. Firstly, 95% acetonitrile (ACN) solutions were used in the process of conditioning and sample loading of the above polar analytes. High recoveries of the analytes were observed in each nucleobase-modified sorbent, and the Diol-type sorbent (no modification with any of the nucleobases) did not adsorb each water-soluble analyte. On the basis of this result, a 98% ACN solution was used during the process of conditioning and sample loading to decrease the concentration of water in the sample, which potentially inhibited the formation of hydrogen bonding between each analyte and the modified nucleobase. Considerable improvements of recoveries were observed in Adenine- and Cytosine-modified sorbents. These results were possibly attributed to the effective expression of hydrogen bonding by decreasing water concentration in the sample solution. Although a non-aqueous (100% ACN) sample solution can be expected to obtain higher recoveries compared with the 98% ACN solution, a decrease in recoveries was observed in Adenine-modified sorbent. From these results, the highest adsorption property was observed in Adenine-modified sorbent using 98% ACN as a sample condition, and the combination of this sample condition and sorbent is effective for high adsorption under HILIC condition. Moreover, we also revealed that a balance between the thickness of water layer and the modification amount of nucleobase is important for retention in the HILIC-type sorbent.