Blaum, K Bollen, G Herfurth, F Kellerbauer, A G Kluge, H J Kuckein, M Sauvan, E Scheidenberger, C Schweikhard, L Eur. Phys. J. A 10.1140/epja/i2001-10262-4 245-248 15 1-4 2002 2002 The cyclotron frequencies of singly charged carbon clusters C/sub n //sup +/ (n >or= 2) were measured with the Penning-trap mass spectrometer ISOLTRAP at ISOLDE/CERN. The present limit of mass accuracy delta m/m = 1.2.10/sup -8/ and the extent of the mass- dependent systematic shift ( delta m/m)/sub sys/ = 1.7(0.6).10/sup -10//u.(m - m/sub ref/) of the setup were investigated for the first time. In addition, absolute mass measurements by use of pure clusters of the most abundant carbon isotope /sup 12/C are now possible at ISOLTRAP. (15 refs).