5.5 Common Mass Filters (Mass Analyzers)

Mass analyzers separate the molecular ion and its fragments by ion velocity, mass, or mass to charge ratio. A number of mass filters/analyzers are available for GC, LC and CE interfaces, but not all are commercially available. These can be used individually or coupled in a series of mass analyzers to improve mass resolution and provide more conclusive analyte identification. This text will only discuss the most common ones.

The measure of “power” of a mass analyzer is resolution, the ratio of the average mass (m) of the two adjacent peaks being separated to the mass difference (Dm) of the adjacent peaks, represented by

Rs = m/Dm

Resolution (Rs) is achieved when the midpoint between two adjacent peaks is within 10 percent of the baseline just before and after the peaks of interest (the valley between the two peaks). Resolution requirements can range from high resolution instruments that may require discrimination of a few ten thousands (1/10 000) of a gram molecular weight (0.0001) to low resolution instruments that only require unit resolution (28 versus 29 Daltons). Resolution values for commonly available instruments can range from 500 to 500 000.

Before introducing the various types of mass analyzers, remember our current location of the mass analyzer in the overall MS system. The analyte has been ionized, underwent fragmentation, been accelerated, and in some cases focused to a focal point with a velocity towards the mass analyzer. Now the packet of ion fragments needs to be separated based on their momentum, kinetic energy, or mass-to-charge ratio (m/z). Often the terms mass filter and mass analyzer are used interchangeable, as is done in this text. But, first a controversy in the literature needed to be addressed with respect to how a mass filter actually separates ion fragments.

Some resources state that all mass analyzers separate ions with respect to their mass to charge ratio while others are more specific and contend that only quadrupoles separate ions by mass to charge ratios. The disagreement in textbooks lies in what components of the MS are being discussed. If one is discussing the affect of the accelerator plates and the mass filter, then all mass filters separate based on mass to charge ratios. This occurs because the charge of an ion will be a factor that determines the velocity a particle of a given mass has after interacting with the accelerator plate in the electronic, magnetic sector, and time of flight mass analyzers. But after the ion has been accelerated, a magnetic section mass filter actually separates different ions based momentums and kinetic energies while the time of flight instrument separates different ions based on ion velocities (arrival times at the detector after traveling a fixed length). In the other case, no matter what the momentum or velocity of an ion, the quadrupole mass analyzer separates different ions based solely on mass to charge ratios (or the ability of the ion to establish a stable oscillation in an oscillating electrical field). These differences may seem semantic but some MS users insist on their clarification. For the discussions below, in most cases, mass to charge will be used for all mass analyzers.

©Dunnivant & Ginsbach, 2008