Angelicae Pubescentis Radix is the dried root of Angelica pubescens Maxim.f. biserrata Shan et Yuan, which shows the pharmacological functions of dispelling rheumatism, relieving pain and protecting cardiovascular system [1,2], mainly due to the coumarins such as osthole and columbianadin contained in it. It has been an important medicine since ancient times and remains a frequently used drug for treating rheumatoid arthritis. Although theories of Traditional Chinese Medicines are difficult to understand in contemporary scientific terms and lack scientific data to some extent, they particularly form the foundations of robust empirical clinical treatments for research into refractory rheumatoid arthritis cases and the discovery of ideal anti-arthritis and rheumatic drugs [3]. Furthermore, the modern pharmaceutical researches indicate that osthole is a promising therapeutic agent for anti-atherosclerotic [4], and columbianadin exhibits the potentially therapeutical effects on cardiovascular system [5], which chemical structures can be found in Supplementary Information file (SI) Fig. S1. However, coumarin compounds showing a certain degree of hepatotoxicity have been listed as one kind of the third-class carcinogens by the World Health Organization in 2000 [6]. In addition, osthole and columbianadin are the representative structures of simple coumarins and furanocoumarins, respectively. As a result, regarding on the instability of active ingredient content in Chinese herbal materials, a simple and efficient method with strong applicability for the quantitative detection of coumarins contained in Angelicae Pubescentis Radix is an urgent issue, which can provide supplemental data for the drug safety, and supply conditions for the pharmacological research of Angelicae Pubescentis Radix. So far, methods have been used for the quantitatively determination of coumarins in Angelicae Pubescentis Radix include high-performance thin layer chromatography (HPTLC) scanning [7], 1H-quantitative nuclear magnetic resonance spectroscopy (1H-qNMR) [8], ultra high-performance liquid chromatography (UPLC) [9] and high-performance liquid chromatography (HPLC) [10], [11], [12], etc. Compared to HPTLC and 1H-qNMR, HPLC method shows high efficiency, good resolution, fast speed and strong applicability. Furthermore, compared to UPLC, HPLC method is more general, more important, which shows better cost performance for the quantitative detection of active components in Chinese herbal medicines. However, Chinese herbal medicine is seriously complex, and the content of active components is generally low [13], which will seriously affect the precision, accuracy and sensitivity of a quantitative analytical method. Furthermore, the matrix of carbohydrates, proteins, tannins are not only easy to produce irreversible adsorption on the analytical column, but also easy to cause HPLC system blockage, which will shorten service life of the instrument and its attachment. Therefore, it is necessary to pretreat samples before analysis, to reduce the interference of matrix components and improve the sensitivity and accuracy of the quantitatively analytical method [14].

Generally used sample pretreatment methods mainly include liquid-liquid extraction, supercritical fluid extraction, ultrasound-assisted extraction and solid-phase extraction (SPE), etc. Liquid-liquid extraction is a classical extraction method, which is suitable for large-volume extraction showing advantage of simple operation, but it also has the disadvantage of consuming solvent [15]. The supercritical fluid extraction method does not require large amounts of organic solvents, but it needs to be carried out under a high pressure, and the equipment is expensive, which extraction efficiency is greatly affected by temperature and pressure [16]. Although ultrasound-assisted extraction is simple, it shows poor selectivity [17]. Regarding these problems mentioned above, a simple and solvent-saving sample-pretreatment method with high extraction efficiency and good selectivity is need urgently. Solid-phase extraction (SPE) has the advantages of organic solvent-saving, simple operation, good selectivity, and easy to perform with analytical instruments to achieve automate operations [18]. Furthermore, on-line SPE can be carried out for simultaneous extraction and determination of target components from complex samples, combing with an analytical column via HPLC instrument, which shows good accuracy, good precision and high selectivity due to the removal of complex matrix.

SPE adsorbent is a crucial factor affecting the extraction efficiency, matrix-removal degree and sample-load capacity in the sample pre-treatment procedure, which then affect the accuracy, precision and sensitivity of the following analytical method. Adsorbents generally used in SPE mainly include silica-bonded materials [19], carbon-based materials [20], organic polymer materials [21], and other new materials such as metal organic frameworks [22] and covalent organic framework [23], etc. Long-chain alkyl silica-bonded materials including octadecylsilane and octylsilane are the most generally used adsorbents, which have been developed in commercial SPE cartridge. Although silica-bonded materials exhibit good selectivity, they only have a good ability to identify components with polar differences due to their selection mechanism based on hydrophobic force. In addition, silica-bonded materials also suffered from the irreversible adsorption of the complex matrix, which gives it a short service life and is usually disposable [24]. Carbon-based materials mainly including graphitized carbon blacks, porous graphitic carbons, fullerenes, carbon nanotubes, graphene and hydrophilic carbon show better adsorption property than silica-bonded materials, but their eluting solvents are incompatible with the subsequent analytical methods due to them owning a strong retention ability for the analytes [25,26]. New materials used as adsorbents of metal organic frameworks and covalent organic framework also show well-ordered porous structure and high specific surface area, but their preparation is very complex and difficult [27,28]. Compared with these materials mentioned above, organic polymer monolithic materials occupy the characters of simple preparation, high porosity, good permeability, fast mass transfer, easy desorption with common washing solvent, tunable pore structure, as well as easy to be post modified, which also can be combined with other materials to give full play of multiple advantages [29], [30], [31]. For example, combing the biochar and organic polymer can achieve an adsorbent with increased surface area, enhanced functional groups and improved porous structure, due to the biochar owing characteristics of high specific surface area, rich functional groups and multi-dimensional pore structure [32].

Biochar can be obtained from multiple materials, such as wood, bamboo and some crop wastes, among which biochar derived from bamboo showing high specific surface area and high porosity present good property in the SPE application [33]. Therefore, combing bamboo biochar and polymer organic monolithic material to prepare a composite monolithic SPE adsorbent will fully play the advantages of the two materials, which can be expected to exhibit good selectivity for complex samples.

In this work, a composite adsorbent was prepared using diallyl phthalate as the monomer and ethylene dimethacrylate as the crosslinker with the addition of biochar, via polymerization in a stainless-steel tube, which was used for the extraction and determination of two coumarins in Angelicae Pubescentis Radix, combing with a C18 analytical column through an HPLC instrument. The present work provided a simple and efficient method for the extraction and determination of osthole and columbianadin from Angelicae Pubescentis Radix with a good accuracy and good precision, which also can be applied for the determination of simple cumarins and furanocoumarins contained in Chinese herbal medicines.