Cooling of direct current beams of low mass ions V. Koslovsky a , K. Fuhrer b, *, A. Tolmachev a , A. Dodonov a , V. Raznikov a , H. Wollnik b a Institute of Chemical Physics, Chernogolovka, Russia b II. Physikalisches Institut, Universita ¨t Giessen, D-35392 Giessen, Germany Received 5 June 1998; accepted 14 August 1998 Abstract Collisional focusing of low mass ions of 1– 40 eV is investigated in a rf-only quadrupole. The final ion intensities and energies are monitored by an orthogonal-extracting time-of-flight mass spectrometer. It was found that in this energy range Na + and K + ions can be cooled and focused as well as Cs + ions if the residual gas pressure in this “quadrupole beam cooler” is high enough that each ion undergoes several collisions. (Int J Mass Spectrom 181 (1998) 27–30) © 1998 Elsevier Science B.V. Keywords: Collisional cooling; Elemental ions; rf quadrupole 1. Introduction The energy of an ion beam can be reduced by collisions of the ions with residual gas atoms and molecules. Such coolings have been achieved for a pulse of light ions in ion traps [1, 2] as well as in rf-only quadrupole arrangements [3, 4] for a dc beam of large molecular ions. In such a “quadrupole beam cooler” the ions are dragged through residual gas while oscillating between quadrupolar rods to which a radio frequency signal of several 100 V amplitude is applied. Large ions are cooled in such an arrange- ment. However, for small ions it is not obvious that such a cooling is achievable at all since the energy exchanges per collision and the changes in flight direction are large for ions whose masses are similar to the buffer gas molecules/atoms [5]. This effect can be diminished by using He as a buffer gas. However, our experiments were performed with air. The system used for the experiments was an existing electrospray-ionization time-of-flight mass spectrometer with an orthogonal ion acceleration [6]. In this system electrosprayed ions pass from a region of 1 mbar into the quadrupole beam cooler at 10 -3 mbar. From here they exit into a region of 10 -5 mbar. In this low pressure region the ions are accelerated orthogonally to their initial flight direction by 6000 V pulses (see Fig. 1.). Their flight times and intensities are recorded after a flight path L y  3 m. Due to the orthogonal extraction our detection sensi- tivity is strongly dependent on K 2x , the initial ion energy in x direction in the extraction region (see Sec. 2.2). Thus the energy distribution of the ions exiting the “cooler quadrupole” can be investigated. * Corresponding author. 1387-3806/98/$19.00 © 1998 Elsevier Science B.V. All rights reserved PII S1387-3806(98)14154-4 International Journal of Mass Spectrometry 181 (1998) 27–30