The main modifications and extensions are described which were made to use the existing EN-tandem of ETH Zurich routinely as an ultrasensitive mass spectrometer. Some features of the system which are important for accurate routine measurements of isotope ratios in the range of 10^-12 to 10^-16 are discussed: a fast cycling system which allows the quasi-continuous measurement of different isotopes (e.g., ¹²C, ¹³C, ¹⁴C), a specially designed ion source with an automatic sample changing device, and a few different sample preparation methods. First tests with graphite samples have proven the usefulness of the chosen concept. At present, the reproducibility of the modified system under optimum conditions is better than one percent.