Quadrupole mass spectrometer

This disclosure presents an alternative, improved approach to the RF only quadrupole mass spectrometers. The ions whose masses place them near the stability limit for a given operating voltage and RF frequency, can be strongly influenced by the application of a very small dc voltage to the quadrupole rods. If this voltage is modulated at a low frequency (typically a few hundred hertz), the (a,q) values will pass alternately through the stability boundary and ions will be transmitted with the imposed frequency. The advantages of the new approach are two-fold (a) lock-in amplifier synchronous detection schemes can be used. These give improved signal/noise ratios. Background noise due to photons, soft X-rays or excited neutrals-often a problem in quadrupole mass filters-will not be modulated and will not be detected. (b) Higher resolution can be achieved.