Negative ion mode mass spectrometry - Journal of Chemical ...

Negative ion...Somsubhra Ghosh et al.The resulting electrical field within the space defined by the four rods are used for resolve ions injectedinto the field. Ions entering this quadrupole electric field undergoing oscillatory trajectories, and at anygiven combination of applied fields, only one m/z ratio attains a stable trajectory that carries it through theanalyzer;and other ions are collected on the rods. By varying the dc or rf fields, different m/z ratios arebrought to focus.Fig. 5: Shows Schematic diagram of Quadrupole mass analyzer (13)(b)Time –of-flight analyzer: Ion separation is based on the principle that ions of different masses,possessing equal kinetic energy, have different velocities. If there is a fixed distance for the ions to travel,the time of travel will vary with their mass, the lighter ions travelling faster and reaching the detector in ashorter period of time. According to this concept, the kinetic energy of an ion accelerated through anelectrical potential V will beZv =mv 2 /2 (Equation 1)and the velocity of the ion is the length of the flight path L divided by the time t. It takes the ion to travelover that distance:substituting the equation 2 in equation 1Zv=mL 2 /2t 2Thus, it follows thatv=L/t (Equation 2)m/z=2Vt 2 /L 2(Equation3)The TOF mass analyzer requires very fast electronics to accurately measure ion flight times that may besubmicrosecond. Furthermore, the ions in a TOF system must be created in short, well defined pulses sothat the ions all start their journey toward the detector at the same moment.1467 J. Chem. Bio. Phy. Sci. Sec. C, 2012, Vol.2, No.3, 1462-1471.