X.Y. Zhou, Z.Q. Zhu, C.Q. Xiong, R. Chen, W.J. Xu, H.W. Qiao, W.P. Peng, Z. X. Nie*, and Y. Chen*, Characteristics of stability boundary and frequency in nonlinear ion trap mass spectrometer, J. Am. Soc. Mass Spectrom. 21, 1588 (2010).