Peak structure with a quadrupole mass filter operated in the first stability region

In the development of commercial quadrupole mass spectrometers, there is an interest in understanding the factors that affect the transmission and peak shape of ions passing through the mass filter. These include the ion collection effects of the ion optics that lead to the presence of a peak structure.

The peak structure can be observed by increasing the ion's axial kinetic energy and reducing the mass step size in a mass spectrum. The calculation of the maxima in the peak structure can be achieved by first obtaining the Mathieu characteristic parameter β_y from the scan lines for each mass. The ion's secular frequency, ω_0,y , along with the ion's transit time through the mass filter can then be used to calculate the location of the maxima in the peak structure.

Experimentally, a peak structure has been observed for m/z 42, m/z 118 and m/z 622 in the Q1 scan mode of a triple quadrupole mass spectrometer. The peak structure has shown a dependency on the ion's axial kinetic energy, but, a mass independency with regard to the number of maxima is observed in the peak structure. In the Q3 scan mode, there is an absence of the peak structure due to the ion collection efficiency of the HED detection system.

Under the right conditions, the peak structure due to ion collection effects can be observed with a quadrupole mass filter operated in the Mathieu first stability region. The presence of the peak structure has been shown to depend upon the ion collection effects at the exit of the mass filter.