Applications and mechanistic studies of direct ionization mass spectrometry

Abstract

Direct ionization mass spectrometry (DI-MS) is a one-step technique that could directly analyze raw solid samples, including Chinese herbal medicines (CHMs), within minutes. This technique has the advantages of simplicity, reliability and short analysis time, compared to conventional methods that typically involve homogenization, extraction, separation and centrifugation, which are labor intensive and time-consuming. The aims of this study are to develop DI-MS for rapid characterization of CHMs and to investigate its extraction and ionization mechanisms that were still unclear. In this study, DI-MS has been developed for rapid characterization of popular and valuable CHMs, i.e. Gastrodiae rhizoma (known as Tianma in Chinese) and Ganoderma (known as Lingzhi in Chinese). Characteristic mass spectra of these CHMs could be generated by DI-MS directly from the raw herbal medicines with the application of a high voltage and solvents. Rapid authentication of Gastrodiae rhizoma could be achieved based on this method, as the acquired DI-MS spectra showed that the major active components of Gastrodiae rhizoma could be found only in genuine Gastrodiae rhizoma samples but not in counterfeit samples. Similarly, rapid differentiation of the Ganoderma species that are officially stated in the Chinese pharmacopoeia from easily confused Ganoderma species could also be achieved. In addition, classification of wild and cultivated Gastrodiae rhizoma and Ganoderma and potential differentiation of Gastrodiae rhizoma and Ganoderma from different geographical locations could be accomplished based on their different intensity ratios of characteristic ions or principal component analysis (PCA) or hierarchical clustering analysis (HCA). Our experimental results confirmed that DI-MS is rapid, simple and reproducible, and can be further extended to the analysis of other herbal medicines. Apart from the applications of DI-MS, the extraction and ionization mechanisms for DI-MS of CHMs were also investigated. CHMs with different classes of active ingredients were investigated with solvents of various properties, and the products generated at different extraction and ionization stages were collected, analyzed and compared, in order to understand the process and factors governing the signal responses of this direct analysis technique and to enhance the detection of active ingredients of CHMs. The experimental results revealed that the solvent loaded onto the surface of CHMs was crucial for the observation of desired ion signals as it served simultaneous extraction of compounds from the sample and ionization of the extracted compounds in DI-MS. The detailed extraction and ionization mechanisms for DI-MS of CHMs were further investigated by comparing the physical properties of the analytes such as the surface activity, polarity and acidity of the analytes. The experimental results demonstrated that the extraction and ionization mechanisms for DI-MS of CHMs were related to the physical properties of the analytes, which are useful parameters for the prediction of DI-MS signal responses of analytes. These results allowed further insight into the extraction and ionization mechanisms of DI-MS in order to explore the ways to enhance the sensitivity and selectivity of DI-MS.