Identification of terpenoids from Ephedra combining with accurate mass and in-silico retention indices

Terpenoids in Ephedra were analyzed by gas chromatography-mass spectrometry (GC-MS), and their pure mass spectra (MS) were obtained using chemometric methods. NIST library search and retention indices (RI) were applied in further identification. The major bottleneck is the ambiguous NIST MS library search results, therefore, we used a simple external calibration method to obtain the accurate mass of molecular ion or key fragment, which included overlapped isotopes structures resolution and Gaussian fitting using Origin software. The calibration method was able to distinguish different molecular weights among a large number of known NIST MS library search results. In addition, a quantitative structure-retention relationship (QSRR) model with good predictive ability was established and the in-silico RI was applied in qualitative identification combined with NIST MS library search results. Two candidate compounds were found to have a moderate matching between the predicted RI values against the experimentally determined values, and incorrect formulas were excluded. Therefore, these auxiliary confirmation methods were able to reflect the analytical chemistry development trend.