Analyzers, Ion Detectors, and Tandem Mass Spectrometers

Mass Analyzers, Ion Detectors, and Tandem Mass Spectrometers 

The unit of m/z is sometimes referred to as the Thomson (Th) in honor of one of the pioneers of This nota- 

Quadrupoie. Quadrupoie mass spectrometers, sometimes known as quadrupoie mass filters (QMF), are currently the most widely used mass spectrometers, having displaced magnetic sector mass spectrometers as the standard instrument. Although these instruments lag behind magnetic sector instruments in terms of sensitivity, upper mass range, resolution, and mass accuracy, they offer an attractive and practical mix of features that accounts for their popularity, including ease of use, flexibility, adequate performance for most applications, relatively low cost, non-critical site requirements, and highly developed software systems. 

The device may be operated in either a SIM mode or a scannmg mode. In SIM mode, both the DC and RF voltages are fixed. Consequently, both the center of the passband and the width of the passband are fixed. For example, the mass spectrometer may be set to pass ions of m/z 363 0.5. Both the center m/z and the Am/z can be adjusted by the appropriate choice of DC and RF. 

The physical principles underlying the operation of a quadrupole mass spectrometer require the solving of a complicated differential equation, the Mathieu equation. In operation when an ion is subjected to a quadrupoiar RF field, its trajectory can be described qualitatively as a combination of fast and slow oscillatory motions. For descriptive purposes, the fast component will be ignored here and the slow component emphasized, which oscillates about the quadrupoiar axis and resembles the motion of a particle in a fictitious harmonic pseudopotential. The frequency of this oscillation is sometimes called the secular frequency. 

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