Study on the provision of traceable flow rate of water vapor for calibrating a time-of-flight mass spectrometer

Abstract

Forming a traceable flow rate is the key to mass spectrometer calibration, and the flow rate can be calculated through the conductance. The theoretical and experimental analysis of measuring the conductance of water vapor using the VΔp measurement method is presented, the measurement error of that method increases with the decrease of flow rate, which is due to the strong adsorption characteristic of water vapor. Based on the VΔp measurement method, a new method called the pΔt measurement method is proposed to give the relationship between the orifice conductance and the inlet pressure for water vapor, and the conductance can be calculated by measuring the time of pressure change, which can correct the adsorption influence of water vapor on the measurement of conductance. The relative standard uncertainty of the pΔt measurement method is 8%. The conductance of an orifice, whose diameter is about 22.2 µm, was measured by that method, and the measurement results agree well with the theoretical values. According to those two methods, an apparatus was developed to calibrate a miniature time-of-flight mass spectrometer (TOF-MS) for water vapor, and it can supply the flow rate of water vapor in the range from 3.67 × 10−9 to 2.19 × 10−5 Pa m3 s−1. Finally, there is a linear relationship between the signal and the flow rate for water vapor in the range from 1.39 × 10−8 to 1.29 × 10−5 Pa m3 s−1.