Towards multimodal HPLC separations on humic acid-bonded aminopropyl silica: RPLC and HILIC behavior

Abstract

The stationary phase characteristics of the material obtained through immobilization of humic acid (HA) to aminopropyl silica (APS) via amide-bond formation were investigated. The material was characterized in terms of elemental analysis, FTIR, thermogravimetric analyses, pH point of zero charge measurements, potentiometric titrations, and contact angle measurements. Amount of HA bonded to APS was determined from the elemental analysis results, and found as 170 mgHA/gAPS. Stability of the material was studied in aqueous media at different pH values, and amount of HA released at pH=8 did not exceed 2% of the total immobilized HA. Stationary phase characteristics of the well-characterized material were investigated in an HPLC system by using some low-molecular weight polar compounds (i.e. some nucleosides and nucleobases) as test solutes. Effect of some experimental variables such as column conditioning, composition of mobile phase, and temperature on the chromatographic behavior of the studied compounds was studied. Role of ammonium solutions at different pH values on retentive properties of the species was also studied. Retention factors (k') versus volume percentage of organic modifier exhibited a U-curve, which was evaluated as an indication for RPLC/HILIC mixed-mode behavior of the stationary phase. Orthogonality between RPLC and HILIC modes was analyzed through geometric approach, and found as 48.5%. Base-line separation for the studied groups of compounds was achieved under each studied mode, and some differentiations were observed in elution order of the compounds depending on the HPLC mode applied. Chromatograms recorded under RPLC and HILIC modes were compared with those recorded on APS under similar conditions, and thus the influence/importance of HA immobilization process was evaluated in detail. In light of the obtained results, immobilized HA is represented as a useful stationary phase for HPLC separations.

Evaluation of new silica‐based humic acid stationary phase for the separation of tocopherols in cold‐pressed oils by normal‐phase high‐performance liquid chromatography

“…This result stems from the use of a stationary phase that is in the multimode functionality. Hence, EC‐ImHA‐APS stationary phase used in this study has a feature that can act as an effective stationary phase in different modes . Therefore, these multimode stationary phases may not provide a good separation as those of in single‐mode stationary phases.…”

Fabrication and Characterisation of Novel Natural <i>Lycopodium clavatum</i> Sporopollenin Microcapsules Loaded <i>In-Situ</i> with Nano-Magnetic Humic Acid-Metal Complexes

“…Despite the multifunctional feature of HA, its high solubility at pH > 2 in water makes it difficult to separate from suspensions and also restricts the use of HA as a solid substrate [26]. Therefore, it was necessary to immobilise HA to certain solid materials having special properties to take advantages of its unique features [27]. Recently, different types of materials, such as resins [26], hematite [28], alginate [29], polymers [30] and silica particles [31] [32] [33] [34] [35] were used for HA capping.…”

Humic-makeup approach for simultaneous functionalization of polyacrylonitrile nanofibers during electrospinning process, and dye adsorption study

“…According to their increased solubility in water, they are usually classified as humin, humic acid (HA) and fulvic acid. To overcome this limitation, some treatments (such as immobilization (6)(7)(8)(9)(10) and A c c e p t e d M a n u s c r i p t 3 insolubilization (2,5,11,12)) have been suggested to turn HA into a material which is less soluble in water, and thus to use it as an adsorbent, safely. The presence of some functional groups (i.e.…”