The rate coefficients of the ion-molecule reactions between H3O+, NO+, O2+, and phosphine were determined using a selected ion flow tube. Using these data, the selected ion flow tube mass spectrometry (SIFT-MS) method was applied to the real-time measurement of phosphine in nitrogen without sample preparation down to concentrations in the mid parts per trillion range. This is the first reported measurement using SIFT-MS in the parts per trillion range. Linear dependencies on concentration were found from 190 ppt to the ppm range, and the limit of detection for a 10-s scan was 190 ppt (0.27 pg/mL).