General gas analysis with a mass spectrometer is based on linear regression analysis. This generally involves the measurements of ion intensities and the manipulations of the fragmentation matrix such as matrix inversion and transpose matrix. Only in simple cases, where no ion interference exists and the fragmentation matrix of the analysis is a diagonal matrix, the linear regression analysis reduces to just ion intensity ratio measurements. In these occasions small and low cost RGAs have been widely used with success. However, if this simple method of intensity ratio measurements, instead of the full linear regression analysis, is applied to the cases where ion interferences are not negligible, severe errors will be introduced in the concentration measurements. Process control applications with mass spectrometers often involve gas streams with substantial ion interferences between the detection ions. High precision is required on the control parameters for the process feedback control loop, therefore the mass spectrometer must be precisely calibrated and have a perfect linearity over a wide range of concentrations. These control parameters are generally calculated through the measured component concentrations and are required to update with fast speed. With these online process control applications, the sensitivity, precision, and linearity of the instrument become critical performance parameters In this article, the linear regression analysis for general gas analysis will be briefly described and the importance of the instrument sensitivity, measurement precision, data repeatability and linearity of the mass spectrometer instrument will be demonstrated through some examples from t he process control applications.