Brominated 1-methylindoles in mackerel (Scomber scombrus) fillet from the North and Baltic Sea

GC/Orbitrap-HRMS measurements of purified mackerel (Scomber scombrus) fillet from the North and Baltic Sea indicated relevant levels of two tribrominated 1-methylindoles (triBMI a and triBMI b) and one tetrabrominated 1-methylindole (tetraBMI). To get more insights in this scarcely analyzed group of halogenated natural products (HNPs), a mixture of brominated 1-methylindoles (BMIs) was synthesized by bromination of 5-bromo-1-methylindole. Subsequently, the synthesized BMI mixture was fractionated by countercurrent chromatography (CCC). Based on GC/Orbitrap-HRMS measurements, three CCC fractions containing the target BMIs were selected and purified by high-performance liquid chromatography (HPLC). A quantitative mix standard was used to determine BMIs in twenty mackerel (Scomber scombrus) samples from two areas in both North and Baltic Seas. The triBMI levels (0.1 to 2.0 ng/g lipids) were slightly lower than the one of tetraBMI (0.2-17 ng/g lipids). Overall, BMIs contributed 1.5-8% to the sum of all halogenated natural products (HNPs) determined in the samples which in turn was on the same level as the sum of four anthropogenic persistent organic pollutants (PCB 153, PCB 138, PCB 180, and HCB). However, HNPs and POPs showed different distribution patterns and variation in individual levels of HNPs.