Vitamins are organic micronutrients that aid in the carrying out of various natural activities in the human organism. A well-balanced diet can supply the vitamins the body needs, but in some cases, nutritional supplements are also recommended. Vitamins can be classified into two categories based on their solubility, namely water-soluble vitamins (WSVs) and fat-soluble vitamins (FSVs). Vitamin C and B vitamins are considered as water-soluble vitamins [1]. Low intake of B vitamins can lead to various types of anemia, a weakened immune system, suppression of the nervous and metabolic system, and illnesses like pellagra and beriberi [2]. On the other hand, excessive intake of them can cause skin changes, intestinal ulcers, fatty liver, indigestion, and other health injuries [3]. The precise quantification of vitamins in foods, nutritional supplements, and pharmaceuticals is thus of great significance [4]. High-performance liquid chromatography (HPLC) is the most preferred method for detecting vitamins due to its high selectivity, sensitivity, and ease of use. This method can be coupled with different detectors, such as ultraviolet (UV), electrochemical, fluorescence, and mass spectrometry (MS) [5]. Among these, HPLC coupled with a UV detector is extensively utilized for the simultaneous detection of WSVs [[6], [7], [8]]. Usually, sample preparation is an essential step before HPLC analysis, as it eliminates potential interferences from the sample matrix and pre-concentrates the target analytes [9,10]. The sample preparation of B vitamins has been effectively carried out using various extraction techniques, including cloud point extraction (CPE) [5,11], ultrasound-assisted dispersive solid-phase extraction (USA-d-SPE) [12], packed-fiber solid phase extraction (PFSPE) [13], solid-phase membrane micro-tip extraction (SPMMTE) [9], and magnetic solid phase extraction (MSPE) [10]. In recent years, there has been a growing tendency towards utilizing miniaturization techniques, particularly miniaturized solid phase extraction (SPE) techniques. The pipette tip-micro solid phase extraction (PT-MSPE) technique is a form of miniaturized SPE that employs pipette tips as cartridges. PT-MSPE is more inexpensive, faster, and easier than SPE. Also, the amount of adsorbent and solvent used is less, and it has more throughput than SPE [14]. Adsorbent selection like other SPE methods is very important in the PT-MSPE. Silica-based C18 sorbents with a single retention mechanism (hydrophobic retention mechanism) are not suitable for very hydrophilic B vitamins [1]. Thus, adsorbents with several retention mechanisms have developed such as polypyrrole [15] and polystyrene [16] nanofibers, bio-metal-organic frameworks [17], silica-based strong anion exchanger [18] and molecularly imprinted biopolymers [12]. Recent studies suggest that porous organic polymers (POPs) are ideal adsorbents due to their unique properties such as variable structure, large specific surface area, low density, and excellent physicochemical stability.

The azo-liked porous organic polymers (AL-POPs) are POPs with functional hydroxyl groups and -N = N- bonds. These POPs are synthesized by diazo-coupling reactions between aryl di/triamines and polyphenols in aqueous solutions under mild conditions without the use of catalysts [19]. Polyphenols such as phloroglucinol [20], gallic acid [21], tannic acid [22], quercetin [23], and rutin [24] have been used in the synthesis of AL-POPs. The coating of AL-POP as an adsorbent on various substrates, such as stir bars [20], can effectively prevent the aggregation of adsorbent particles. As a result, analytes can more easily access and interact with the active sites on the adsorbent surface, leading to a significant improvement in extraction efficiency.

Melamine sponge is an environmentally friendly commercial material with an open pore structure and high porosity [25]. The open pore structure of the sponge enables it to trap and immobilize adsorbent particles within its network of interconnected pores. Based on these desirable characteristics, melamine sponges have been introduced as ideal substrates for various sorbents in SPE [[25], [26], [27], [28]].

The present study describes the synthesis and application of AL-POPs as an efficient sorbent for the extraction and analysis of six B vitamins from multivitamin syrup and non-alcoholic beer. New AL-POPs were synthesized using caffeic acid and benzidine as the polyphenol and aryl diamine, respectively. The synthesized POPs contained various functional groups, such as carboxylic acid and hydroxyl, located adjacent to the benzene rings, which enabled them to interact with B vitamins through different types of forces, including: electrostatic, hydrogen bonding, π-π, and van der Waals interactions. To utilize the properties of the synthesized AL-POPs, they were incorporated into a melamine sponge substrate and used as sorbents for PT-MSPE to extract six types of B vitamins. The extracted B vitamins were then analyzed by HPLC-DAD.