Charge detection quadrupole ion trap (QIT) mass spectrometry (MS) is an effective technique for high-speed mass analysis of micron-sized particles such as biological cells and aerosols. In this technique, the trap can be operated by scanning the frequency of the ac field under a low-vacuum condition. This paper describes a calibration method which involves determination of the points of ejection (qeject) of the individual particles to be analyzed how the ejection points vary with the pressure of buffer gas. Using NIST polystyrene size as the test samples, a mean ejection point of qeject =0.952 is attained at the trap driving voltage amplitude of 1617 V, with He buffer gas pressure of 5.33 Pa, and a frequency scan rate of 70 Hz/s over the scan range of 450～100 Hz. The mean value (0.947) of qeject is measured without the buffer gas. The result indicates the buffer gas has an effect on the ejection point of particle in the trap.