Notched broad-band excitation of ions in a bench-top ion trap mass spectrometer

Abstract

Simple ancillary equipment consisting of waveform generators, a balun and an amplifier are used to adapt the notched stored waveform inverse Fourier transform (SWIFT) broad-band excitation method to a commercial bench-top ion trap mass spectrometer. These changes extend the capabilities of the bench-top ion trap by allowing use of powerful mass spectrometric tools like selected ion monitoring (SIM), tandem mass spectrometry (MS-MS) and mass-selective ion/molecule reactions, thereby providing additional selectivity and enhanced sensitivity. The capabilities are demonstrated for analytes introduced into the ion trap by membrane introduction mass spectrometry (MIMS). Notched SWIFT waveforms are used to accumulate selected ions in the ion trap while a simple function generator is used to provide a supplementary AC signal of desired frequency, amplitude and duration, to resonantly excite ions for MS-MS experiments. The combination of MIMS with mass-selective ion storage enhances sensitivity such that parts per quadrillion (ppq) levels of some volatile compounds can be analyzed directly from aqueous solution. MS-MS of the selected parent ion allows characterization of solutes with minimal interference. Ion/molecule reactions, performed on mass-selected ions, provide an alternative route to molecular characterization, as illustrated by Diels-Alder reactions of acylium fragment ions.