Quadrupole Mass Filter: Circular Rods and Peak Shapes

Abstract

A frequently observed characteristic of quadrupole mass filters is the appearance, at high resolution, of a separate “precursor” peak on the low mass side of each mass peak. The present report shows that such false precursor peaks can result from the use of round rods (rather than hyperbolic rods) that are common in all commercial instruments. Round rods introdnce sixth- and tenth-order terms to the ideal quadrupole field. We have computed the amount of sixth-order distortion introduced by the use of circular rods. The equipotential line produced at the surface of a round rod can be represented quite well by a quadrupole field with a large amount of sixth-order distortion. This distortion introduces a resonance line in the Mathieu stability diagram that at high resolution is near the edge of the peak on the low mass side. The resonance line can distort the peak to produce an apparent precursor peak.