Gas chromatography–mass spectrometry following microwave distillation and headspace solid-phase microextraction for fast analysis of essential oil in dry traditional Chinese medicine

Abstract

In this paper, a novel method based on gas chromatography–mass spectrometry (GC–MS) following microwave distillation-headspace solid-phase microextraction (MD-HS-SPME) was developed for the determination of essential oil in dry traditional Chinese medicine (TCM). TCM is dried before being preserved and used, there is too little water to absorb microwave energy and heat the TCM samples. In the work, carbonyl iron powders (CIP) was added and mixed with the dried TCM sample, which was used as microwave absorption solid medium for dry distillation of the TCM. At the same time, SPME was used for the extraction and concentration of essential oil after MD. The dry rhizomes of Atractylodes lancea DC was used as the model TCM, and used in the study. The MD-HS-SPME parameters including fiber coating, microwave power, irradiation time, and the amount of added CIP, were studied. To demonstrate the method feasibility, the conventional HS-SPME method was also used for the analysis of essential oil in the TCM. Experimental results show that more compounds were isolated and identified by MD-HS-SPME than those by HS-SPME. Compared to conventional HS-SPME, the advantages of the proposed method are: short extraction time and high extraction efficiency. All experimental results show that the proposed method is an alternative tool for fast analysis of essential oils in dry TCMs.

Introduction

Over the past years, headspace solid-phase microextraction (HS-SPME) with these advantages of simplicity, rapidness and need of no solvent, has widely been employed for the analysis of volatile organic compounds (VOCs) in a variety of complicated matrices such as environmental, food, and biomedical samples [1], [2], [3], [4], [5], [6], [7]. In our previous studies [8], [10], [11], [12], [13], [14], HS-SPME was successfully developed for the analysis of essential oils in traditional Chinese medicines (TCMs). To facilitate HS-SPME, it is essential to have target analytes transferred from the sample matrices into the headspace. Usually, for aqueous samples, adequate agitation allows good recovery of the analytes from aqueous samples, as does an increase in sample salt concentration or proper adjustment of pH [15]. However, when HS-SPME is used for samples for which a strong association between native analytes and sample matrix exists, heating may be required to enhance the release of analytes into the headspace phase. For solid samples such as TCMs, heating can enhance the analyte concentrations in the headspace [8], [10], [11], [12], [13], [14]. Moreover, an increase in the sample temperature is generally beneficial in speeding the achievement of extraction equilibrium. However, by using traditional heating, sample temperature increase, and headspace temperature also increased at the same time. High-temperature extraction can cause significant deterioration of the coating/sample distribution coefficient, resulting in a decrease in the equilibrium amount of analytes extracted.

Microwave heating involves internal heating based on conduction and dielectric polarization caused by microwave irradiation [16]. It is therefore not only more efficient when compared to traditional heating but also may result in an external temperature much lower than that of the sample with the control of the time and output power of microwave irradiation. Recently, microwave-assisted techniques have been described as time saving, energy saving, and highly efficient and have been widely used for the extraction of plant essential oils [17], [18], [19]. Microwave-assisted hydrodistillation (MAHD) technique was developed for the extraction of essential oils [20], [21], [22], [23], [24], [25]. More recently, Chemat and co-workers developed a microwave-distillation (MD) technique for the isolation of the essential oil from fresh plant tissue [26]. These microwave extraction techniques have the best advantages: much reducing the time of the isolation of essential oil from plant tissues. However, the methods still require large amount of organic solvent to further extract the isolated essential oil prior to the analysis.

Development of the novel technique combined the advantages of microwave extraction and SPME is very interesting. In 2003, Pawliszyn and co-workers developed microwave-assisted headspace solid-phase microextraction (MA-HS-SPME) as a simple and effective method for fast sampling of volatile compounds from Eucalyptus citriodora Hook (E. citriodora) leaves [27]. The in situ water in the fresh plant leaves can absorb microwave, and heat the leaves to evaporate VOCs to the headspace. In our previous study [28], microwave distillation coupled with HS-SPME, was developed for fast analysis of essential oil in fresh plant tissue. It has been demonstrated that MD-HS-SPME is a rapid, simple and effective method for the extraction of the essential oils in fresh plant tissues. However, traditional Chinese medicines are dried before being preserved and used, there is too little water to absorb microwave energy and heat the TCM samples. So essential oils in these TCMs cannot be evaporated and extracted simply by the previous MD methods [26], [28].

Carbonyl iron powders (CIP) are among the most popular conventional magnetic absorption materials that have good microwave absorption capacity [20], [29], [30]. In this work, a novel method, MD-HS-SPME followed by gas chromatography–mass spectrometry (GC–MS) was developed for the analysis of essential oil in dry TCM, which CIP added and mixed with TCM sample was used as the microwave absorption medium for microwave distillation. A common TCM, the dried rhizomes of Atractylodes Lancea DC was used in this study. The MD-HS-SPME parameters were studied, and the method precision was also investigated.