Quadrupole ion trap

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A quadrupole ion trap (also known as a Paul trap, QIT, twisted quadrupole ion trap or sometimes just ion trap) refers to an ion trap that uses DC and radio frequency (RF) ~1 MHz oscillating AC electric fields to trap ions as well a mass spectrometer that uses such a trap to analyze ions. The invention of the quadrupole ion trap itself is attributed to Wolfgang Paul who shared the Nobel Prize in Physics in 1989 for this work.

[edit] Theory

Scheme of a Quadrupole ion trap of classical setup with a particle of positive charge (red), surrounded by a cloud of similarly charged particles (red). The electric field E (blue) is generated by a quadrupole of endcaps (a, positive) and a ring electrode (b). Picture 1 and 2 show two states during an AC cycle.

The trap itself generally consists of two hyperbolic metal electrodes with their foci facing each other and a hyperbolic ring electrode halfway between the other two electrodes. The ions are trapped in the space between these three electrodes by AC ~1 MHz and DC (non-oscillating, static) electric fields. The AC radio frequency voltage oscillates between the two hyperbolic metal electrodes at the 'top' and 'bottom' of the trap ('top' and 'bottom' are in phase) and the hyperbolic ring electrode that forms the 'side' of the trap. The ions are first pulled up and down axially while being pushed in radially. The ions are then pulled out radially and pushed in axially (from the top and bottom). In this way the ions move in a complex motion that generally involves the cloud of ions being long and narrow and then short and wide, back and forth, oscillating between the two states.

The quadrupole ion trap has two configurations: the three dimensional form described above and the linear form made of 4 parallel electrodes. The advantage of this design is in its simplicity, but this leaves a particular constraint on its modeling. To understand how this originates, it is helpful to visualize the linear form. The Paul trap is designed to create a saddle-shaped field to trap a charged ion, but with a quadrupole, this saddle-shaped electric field cannot be rotated about an ion in the centre. It can only 'flap' the field up and down. For this reason, the motions of a single ion in the trap are described by the Mathieu Equations. These equations can only be solved numerically, or equivalently by computer simulations.

The linear form of the trap can be used as a selective mass filter, or as an actual trap by creating a potential well for the ions along the axis of the electrodes.

Linear Ion Trap at the University of Calgary

• Nobel Prize in Physics 1989

[edit] References

• W. Paul Electromagnetic Traps for Charged and Neutral Particles Taken from Proceedings of the International School of Physics <<Enrico Fermi>> Course CXVIII “Laser Manipulation of Atoms and Ions”, (North Holland, New York, 1992) p. 497-517

• R.I. Thompson, T.J. Harmon, and M.G. Ball, The rotating-saddle trap: a mechanical analogy to RF-electricquadrupole ion trapping? (Canadian Journal of Physics, 2002: 80) p. 1433–1448

• M. Welling, H.A. Schuessler, R.I. Thompson, H. Walther Ion/Molecule Reactions, Mass Spectrometry and Optical Spectroscopy in a Linear Ion Trap (International Journal of Mass Spectrometry and Ion Processes, 1998: 172) p. 95-114...