The combination of an electrospray ion source and an electrostatic storage ring for lifetime and spectroscopy experiments on biomolecules

An electrospray ion source has been coupled to an accelerator that injects ions into an electrostatic heavy-ion storage ring. Since the dc ion current produced by electrospray ionization is low ${\text{(∼10}}^6\hspace{0.17em}\mathrm{ions/s}\mathrm{),}$ ions are accumulated in a cylindrical ion trap filled with a helium buffer gas. The ions are collisionally damped in the buffer gas and confined to the central trap region by a rf field. Extraction from the trap occurs within a few microseconds and after acceleration through 22 kV, the ions of interest are selected by a magnet according to their mass to charge ratio. The ion bunch is subsequently injected into the ring. Both positive and negative ions have been stored, with masses ranging over 3 orders of magnitude ${\text{(∼10}}^2{\text{–10}}^4\hspace{0.17em}\mathrm{Da}\mathrm{).}$ From a pickup signal in the ring, the number of ions in a bunch is estimated to be of the order of ${10}^3{\text{–10}}^4$ when the accumulation time is 0.1 s. Our first measurements show that we can store a sufficient number of ions to study the decay of metastable ions and to determine relative destruction cross sections. The technique could be useful to probe conformers differing only in size. Furthermore, our setup can be used for spectroscopic measurements of the ion-photon interaction such as the excitation of [Cytochrome $\mathrm{c}\text{+17}\mathrm{H}{]}^{\text{17+}}$ protein ions with 532 nm photons.