With the use of 18 keV Cs+ mass-filtered primary beam of 500 nA focused into a spot similar to30 mum, rastered over a nominal area of 50 mum x 50 mum and a pressure in the analysis chamber lower than 2 x 10(-8) Pa during analysis we were able to achieve an oxygen background (bkg) equivalent to 8 x 10(15) cm(-3). This means that, provided adequate reproducibility of comparative measurements of blank and O-implanted standards, the bkg subtraction allows to reliably determine O-16 concentration down to similar to5 x 10(15) cm(-3) - a value close to the natural abundance of oxygen in floating zone (FZ) silicon. However, for depths exceeding similar to50 mum, the depth resolution degrades; also the removal rate decreases. In order to explore these effects more systematically, a special test structure consisting of phosphorus markers 20 mum apart embedded in undoped Si was manufactured with a chemical vapour deposition technique. A precise rate vs. depth function was derived employing a polynomial fit. When a dynamic range of at least two orders of magnitude is required for large depths, beveling of the sample followed by lateral line-scan analysis is necessary. (C) 2002 Elsevier Science B.V. All rights reserved.