The transmission properties of an RF-only quadrupole mass filter

The performance of a quadrupole mass filter operated in the RF-only mode was investigated. Data are presented which demonstrate that the transmission properties of such devices deviate markedly from that which would be predicted from the Mathieu stability diagram. Discussion of the factors influencing the transmission efficiency of ions through an RF-only quadrupole is presented. The transmission efficiency of ionic species through an RF-only quadrupole is primarily influenced by three factors. The first of these is the formal stability of the ion trajectory as summarized by the coordinates of the particle in a, q space. The second factor is the acceptance aperture of the quadrupole in combination with the emittance of the ion source. The final factor is shown to be spatial focusing conditions which arise due to the nature of the frequency components that are contained within the trajectory of an ion as it traverses the quadrupole electrode structure. The theory of such spatial focusing conditions is reported. The measurement of naturally occurring isotope abundance ratios is used to demonstrate the magnitude of errors which may result due to the non-ideal transmission properties of the RF-only quadrupole. Suggestions are provided to minimize such mass discrimination effects.