Online micro solid phase extraction coupled with ultra‐performance liquid chromatography–tandem mass spectrometry for trace analysis of endogenous plant hormones in Ulva linza

Objective Ulva linza (L.) is a species of green algae widely distributed in China. We aimed to establish a sensitive online analytical method for quantification of endogenous phytohormones in fresh minute seaweed samples. Method The method for quantification of endogenous plant hormones in fresh minute samples was developed based on a homemade online micro solid phase extraction (m‐SPE) system coupled with an ultra‐performance liquid chromatography–tandem mass spectrometry (UPLC‐MS/MS) platform. The online m‐SPE instrument injected the eluent of m‐SPE directly onto the LC separation column, improving the utilization of samples and saving time. The m‐SPE column, of which the effective size was 9.6 × 2 mm i.d., was filled with 19 mg of C18 (10 μm). Results Under optimized conditions, the limits of detection were 0.002–0.060 ng ml−1 for five plant hormones. The actual sample recoveries of phytohormones were 76.4–103.4% and the coefficients of variance were below 14.1%. The temporal distribution of these endogenous plant hormones of U. linza during different growth periods is described. Conclusion The proposed online m‐SPE method was successfully applied to quantification of endogenous acidic and alkaline plant hormones in U. linza. It provides important information for the further study of the physiological and ecological effects of plant hormones in lower algal species. A sensitive online analytical method was established for quantification of endogenous phytohormones in fresh minute seaweed samples based on a homemade online m‐SPE instrument coupled with UPLC‐MS/MS. The limits of detection were 0.002–0.060 ng mL−1 for five plant hormones. The actual sample recoveries of phytohormones were 76.4%–103.4%, and the coefficient of variance were below 14.1%. The method is used to determine the content of endogenous plant hormones in Ulva linza (L.) and its temporal distribution in different growing period.