The quadrupole offset voltage applied to the quadrupole mass analyzer determines the kinetic energy (velocity) of the ions that are transmitted through the quadrupole mass analyzer, and this energy dictates the length of time that the ions spend transmitting. The quadrupole offset voltage is a DC potential applied to the quadrupole rods in addition to the resolving DC voltage. The resolving DC voltage is equal in amplitude and opposite in sign. Consequently, this offset voltage affects the mass filtering properties of the quadrupole mass analyzer.
NOTE
During system calibration, the MS determines the optimum mass-dependent quadrupole offset voltage for the quadrupole mass analyzer.
The quadrupole offset voltage setting is an important balance between several factors, therefore the voltage is calibrated in a mass-dependent manner. For example, if ions are transmitted through the quadrupole mass analyzer using a high voltage (for example, 20 V), they experience a limited number of RF cycles, which might prevent ions of adjacent m/z values from experiencing different flight paths. In this example, resolution would be lowered because both ions would pass through the quadrupole instead of one being filtered. If a low offset is used, the ions experience a high number of RF cycles, the flight paths of adjacent m/z values are the most different, and resolution is the highest. However, if the quadrupole offset voltage is too low, the ions may struggle to make it through the crucial interface regions at the entrance and exit of the analyzer, decreasing transmission efficiency.
Additionally, filtered ions can build-up at the entrance of the quadrupole mass analyzer and can charge as the quadrupole mass analyzer gets used overtime, so using more energy helps ions move past this region.