The use of MCs+ secondary ion mass spectrometry for quantitative analysis of nitrogen in oxynitride layers on silicon was studied using Cs+ primary ions at energies between 250 eV and 1 keV and impact angles between 0degrees (normal incidence) and 80degrees. To achieve constant signal levels in the SiO2 layer, the oxide was chosen to be comparatively thick (4.9 nm). Due to differences in cesium surface coverage, the yields of the reference species Cs+ and SiCs+ varied with depth by up to three orders of magnitude, depending on the beam energy and the angle of incidence. Large differences in ion formation probability by up to a factor of 10 became evident from angular dependent changes of the OCs+/SiCs+ signal ratio in the oxide. Based on the observation that the depth dependent variations of the Cs+ and the SiCs+ signals are quite similar, attempts were made to rationalize the SiCs+ and OCs+ yields by normalizing to the Cs+ yields and the apparent target current. By way of scaling the NCs+ signals to the normalized OCs+ level in SiO2, the angular dependent variations of the apparent nitrogen content could be reduced to about 30%-40%. The remaining uncertainty is attributed to differences in the matrix effect for OCs+. and NCs+ in oxynitrides. It is also shown that silicon bombardment with cesium above "critical" angles of about 50degrees at 250 eV, 58degrees at 500 eV, and 65degrees at 1 keV causes very rapid ripple formation. This surface roughening is responsible for long-term changes of the SiCs+ matrix reference signal observed here and in earlier work.