Significant changes over time have been observed in surface and near surface phosphorus concentration for low dose phosphorus implants measured using secondary ion mass spectrometry (SIMS) with cesium bombardment. These variations in concentration affect the ability to make reproducible dose measurements. Phosphorus measurements have been documented for samples from wafers kept at ambient conditions and also for those stored in a range of other conditions including heat, high humidity, low humidity, and liquid nitrogen. An initial study of wafers ion implanted over a range of doses showed that the change in the surface phosphorus concentration was most apparent for the lowest phosphorus dose (1x10(12) atoms/cm(2)) and that heating the sample resulted in the most significant change (increase in almost three orders of magnitude) in apparent surface P concentration. Only the specimens stored at liquid nitrogen temperature showed no change in P surface concentration. SIMS analysis conditions were optimized and a second set of analyses were performed on wafers that included different wafer processing (thicker surface oxide, preamorphization, and anneal) in an effort to reduce the change in surface phosphorus. Significant improvement in P dose reproducibility was noted for the wafer that had been implanted through a 5 nm oxide and stored in dry conditions. (C) 2010 American Vacuum Society. [DOI: 10.1116/1.3406141]