“…Manually probing the cells with SIMS revealed that the 7-h biofilm mass spectrum exhibits prominent peaks at m/z 244.18, 260.18, 270.20, 272.21, 286.19, 288.21, and 298.23, which have been previously assigned to 2-alkyl-4-(1H) quinolines (AQ) MH + ions and are confirmed here with tandem MS (Table 1 and Figure S1, Supplemental Information). 10,23,24 The observed ions fall into three molecular classes, specifically 2-heptyl-4-quinoline (HHQ)-derived ( m/z 244.18, 270.20, and 272.21), 2-heptyl-3-hydroxyquinolone (PQS)-derived ( m/z 260.18 and 288.21), and 4-hydroxy-2-heptylquinoline-N-oxide (HQNO)-derived ( m/z 260.18, 286.19 and 288.21) ions. Among these compounds, the isomeric pairs PQS/HQNO and C 9 -PQS/NQNO both appear in the 7-h biofilm based upon tandem-MS fragmentation patterns (Figure S1, Supplemental Information).…”