Frequency modulated selected ion species isolation in a quadrupole ion trap

A method of creating a supplemental voltage waveform for use in conjunction with modulation of a nominal trapping field applied to a quadrupole ion trap mass spectrometer to resonantly eliminate unwanted masses from the ion trap, such that ions in a mass range comprising one or more masses are selectively stored in the ion trap, comprising the steps of:

• (a) determining the masses at the high and low ends of the range of masses that are held in the ion trap by the nominal trapping field,
• (b) determining the secular frequencies of the highest and lowest masses, thereby defining a frequency range spanning said secular frequencies,
• (c) dividing said frequency range into a plurality of subranges,
• (d) establishing a set of frequency components for each subrange, each frequency component within a subrange being spaced apart from adjacent frequency components by substantially the same amount,
• (e) determining a continuous range of masses m.sub.1 -m.sub.2 that are to be selectively stored in the ion trap, said range comprising at least one mass value,
• (f) determining the secular frequencies f.sub.1 and f.sub.2 of m.sub.1 and m.sub.2 in the unmodulated trapping field,
• (g) calculating a set of edge frequencies by adjusting the values of f.sub.1 and f.sub.2 to compensate for the effects of modulating the trapping field,
• (h) repeating steps (e)-(g) for each additional range of masses that are also to be selectively stored in the ion trap,
• (i) generating a voltage waveform incorporating each of the edge frequencies and each of the frequency components in each set of frequency components other than those which lie between respective sets of edge frequencies.

A method of constructing a supplemental dipole voltage waveform from a master set of frequency components for use in conjunction with a modulated trapping field in an ion trap to eliminate any selected range of ions in the mass range held by the trap under nominal trapping conditions, comprising the steps of:

• determining the mass range that is effectively held in the ion trap under the nominal trapping conditions,
• determining the secular frequencies of the end points of the mass range to define a frequency range,
• dividing said frequency range into a plurality of contiguous subranges,
• for each frequency subrange, generating a plurality of evenly space apart voltage frequency components spanning the subrange and combining said frequency components into the master set of frequency components, wherein the spacing of the frequency components is different in the different subranges, whereby said supplemental
• voltage waveform is realized.

A method of obtaining a waveform having edge frequencies for defining a gap in the set of frequency components included in a supplemental voltage waveform used to selectively retain a range of masses in an ion trap in combination with a trapping voltage that is modulated about a nominal value, comprising the steps of:

• determining the respective secular frequencies of the mass values at the extremes of the mass range to be selectively retained in the ion trap,
• adjusting the secular frequencies by a scaling factor to define preliminary edge frequencies,
• adjusting the value of preliminary edge frequencies to compensate for modulation of the trapping field to define edge frequencies,
• rounding the values of adjusted preliminary edge frequencies to frequencies exhibiting a selected common factor, and
• generating said supplemental waveform from said frequency components of said set comprising frequency components other than frequency components having values between said adjusted secular frequencies.