Mass selection of ions from beams using waveform isolation in radiofrequency quadrupoles.

A waveform isolation method is described for the mass-selective transmission of ions through quadrupole mass filters, and it is implemented on a new tandem mass analyzer instrument. The method features the application of broad-band waveforms comprising appropriate frequencies to cause mass-selective instability in ions of particular mass-to-charge (m/z) and to transmit all others. The experiment is implemented in a tandem quadrupole system in which the first mass filter is a rectilinear ion trap (RIT) operated in a continuous mass-selective mode to transmit ions of ions of one or more arbitrarily selected m/z value(s). The second analyzer was used to verify the quality of the mass selection achieved using the first analyzer via conventional quadrupole ion trap mass-selective instability scanning. A new subtype of product ion tandem mass spectrometry (MS/MS) scan, termed the summed product ion scan, is demonstrated with a mixture of biological compounds. It is used to characterize product ions arising after simultaneous isolation and collisional activation of multiple precursor species, in this case ions of the same analyte generated in different charge states. The summed product ion scan can be useful for enhancing sensitivity for the analyte of interest or for providing more comprehensive information about an analyte than is available by monitoring a single ionized form of the analyte. The analytical performance of the waveform isolation method is tested using simple drug mixtures, and its potential for increasing overall yields in preparative mass spectrometry is explored briefly. It is shown that efficiencies of ca. 70% of ion transfer to a surface for ion soft landing surface can be achieved. The upper mass range is limited by axial acceleration arising from the stretched geometry, and one solution to this problem is provided.