Mass Spectroscopy essentially involves ionization of neutral molecules and fragmentation followed by grouping of the ionized species on the basis of their charge to mass (m/z) ratios for detection and quantitation. The present article discusses the common available options for discrimination of the ionized mass fragments and their analysis.
Quadrupole Mass Analyzers
A quadrupole mass analyzer comprises of four parallel rods assembled in a square shaped arrangement. The ionized molecules are directed through the centre of the square formed by the rods. A combination of DC and RF potentials is applied to the diagonally opposite rods thereby permitting only selected m/z ratio ions to pass and reach the detector. The other ions strike the rods get destroyed and are unable to reach the detector.
Quadrupole systems offer a cost-effective mass discrimination option. The reproducibility is good but other mass analyzers would be required for applications requiring higher resolutions and pulsed ionization sources. Due to their lower cost and simplicity quadrupole systems are found in majority of bench top GC/ MS and LC/MS systems.
Magnetic Sector Analyzers
Magnetic sector analyzers make use of the magnetic field of a permanent magnet or electromagnet to resolve the ionized mass fragments. The magnetic field is applied in a perpendicular direction to the flow path of molecular ions. The field deflects the ions in a circular arc thereby resolving different masses. Higher resolution is achieved by subjecting the ionized molecules additionally to magnetic fields. In ‘reverse geometry’ arrangement magnetic sector precedes the electric sector. The electric field is generally held at a fixed value and the magnetic field is varied to scan the different mass to charge ionized species. Ions of larger or smaller m/z values strike the tube and get destroyed.
Magnetic sector systems offer high degree of reproducibility and resolution along with a high dynamic mass range. However, such systems are large in size and also higher in cost. Ideally such analyzers are useful for accurate mass measurements and determination of isotopic ratio composition of the analytes.
Time of Flight Mass Analyzers
A uniform field is applied to all ions at the same time permitting them to accelerate down the length of the flight tube. Lighter ions travel faster so as to reach the detector before the heavier ones. The mass to charge ratios are discriminated by the time taken to reach the detector. Such arrangements also help detect mass accurately and are well suited for pulsed ionization sources such as MALDI. The systems provide high mass range among different available mass analyzers.
Quadrupole Ion Traps
An ion trap comprises of a circular ring electrode in addition to two end caps which together form a confined space where ions get trapped with the help of electromagnetic fields .The RF and DC potentials are then scanned to eject successively different mass to charge ratio fragments to reach the detector in a sequence.
Ion trap systems serve as compact size bench top mass analyzers. However, dynamic range is limited and quantitation of ionic species is poor.
The mass analyzers provide their own characteristic features which can be utilized for different mass spectroscopic analysis applications. Some typical applications will be taken up subsequently.