Rapid screening for cyclo-dopa and diketopiperazine alkaloids in crude extracts of Portulaca oleracea L. using liquid chromatography/tandem mass spectrometry

A simple and rapid qualitative liquid chromatography/tandem mass spectrometry (LC/MS/MS) method was developed and validated for screening cyclo‐dopa and diketopiperazine alkaloids in crude extracts of Portulaca oleracea L. at sub‐ppm levels. An electrospray ionization orbitrap mass spectrometer, which provides accurate full scan MS and MS/MS data, was used in this study. After simple extraction with ethanol and purification by AB‐8 resin, the extracts were subjected to LC/MS/MS analysis. A high mass tolerance (10 ppm) was used in the initial screening to filter the full scan MS data. The cyclo‐dopa and diketopiperazine alkaloid standards gave limits of detection (LODs) at or below 5 ng/mL. The results also indicated that the method had an acceptable precision for day‐to‐day use in the identification of compounds. The alkaloids could be identified based on their MS/MS data, elemental compositions, and retention behavior. This system was used to assay trace amounts of cyclo‐dopa and diketopiperazine alkaloids in crude extracts of Portulaca oleracea L., leading to the identification of 5/2 confirmed/unconfirmed cyclo‐dopa and 7/6 confirmed/unconfirmed diketopiperazine alkaloids, respectively. The screening method considerably reduces the time and cost involved in the identification of cyclo‐dopa and diketopiperazine alkaloids in Portulaca oleracea L., as well as being a simple and convenient approach to the identification of other structural families of natural products. Copyright © 2008 John Wiley & Sons, Ltd.