Exactive esi orbitrap mass spectrometer

All mass spectra were obtained in negative ion mode with injection concentrations of 5 μM in G-quadruplexes and 5 μM internal standard dT₆. Native electrospray mass spectra of oligonucleotides were obtained using several instruments, with similar results. (1) On an LCT Premier mass spectrometer (Waters, Manchester, UK), the ESI source voltage was set to 2.2 kV with a desolvation temperature of 60°C. Unless otherwise mentioned, the ion guide 1 voltage is 50 V. The source pressure was increased to 35 mbar. This pressure is measured using a Center Two probe (Oerlikon Leybold Vacuum, Cologne, Germany). The syringe injection flow was fixed to 200 μL/h. In all experiment 5 μM dT₆ (monoisotopic mass 1762.318 Da) was used as internal standard. (2) An Exactive ESI-Orbitrap mass spectrometer (Thermo Scientific, Bremen, Germany) was also used to monitor the titration of TG₄T by KCl. In this case, ESI spray voltage and capillary voltage are 2.75 kV and –20 V, respectively. Capillary temperature is set to 150°C. Tube lens and skimmer voltage are fixed to –180 V and –10 V. In order to help the desolvation process to occur, the HCD cell voltage is set to 10 eV and the cell pressure is 2.5 × 10^–5 mbar.

All ESI-MS experiments were carried out in the negative ion mode on an Exactive ESI-Orbitrap mass spectrometer (Thermo Scientific, Bremen, Germany). The ESI spray voltage and capillary voltage used were −2.75 kV and −20 V, respectively. The capillary temperature was set to 150 °C. Tube lens and skimmer voltage were fixed to 180 V and −10 V, respectively. Samples were injected at a flow rate of 4 μL min⁻¹.