Department of Chemistry, University of Utah Octopole Technology ---Sha Joshua Ye  Department of Chemistry, University of Utah Nov 6th, 2002

Outline Theory behind the octopole Some experimental applications Gerlich, D. Inhomogeneous rf Fields: A Versatile Tool for the Study of Processes with Slow Ions. In State-Selected and Sate-to State Ion-Molecule Reaction Dynamics, Part I: Experiment, Ng, D.-Y.; Baer, M., Eds.; 1992; pp 1-176 Ervin, K. M.; Armentrout, P. B. Translational Energy Dependence of Ar+ + XY  ArX+ + Y (XY = H2, D2, HD) from Thermal to 30 eV cm. J. Chem. Phys. 1985, 83, 166-189. Armentrout, P. B. Mass Spectrometry—Not Just a Structural Tool: The Use of Guided Ion Beam Tandem Mass Spectrometry to Determine Thermochemistry. J. Am. Soc. Mass Spectrom. 2002, 13, 419-434.

Development of the theory Thomson’s (1903) determination of the X-ray scattering cross section. Electron motion eq: If include a static field, the above eq can be: If E0 is homogeneous, we get the special solution: where E is the electronic field;  is the angular freq;

If E0 is inhomogeneous, using the 1st order approxation: Few steps later: = q2·E02/4m2 Also Es= Φs If we define an effective potential:

We get a simple expression: = -V(R0) The first integral of that is: Because Motion through an inhomogeneous field leads to an exchange between kinetic energies and electrostatic potential energy

Under adiabaticity approximations, we obtain the so-call adiabaticity parameter : =2q| E0|/m2 The empirical safe operation value of  is <0.3, derived from that the (r/r0)max<0.8 How to obtain the effective potential for the 2D multipole device?

By solving the Laplace’s Equation Ф(r)=0 assuming the equipotential surfaces of electrodes, we obtain the potential for long cylindrical conductors: For the 2D multipole, we adopt the following form for the applied potential: So by setting Ф(r=r0,φ)=Ф0cosnφ, Ф(r=0,φ)=0

A couple of steps later, where 2n is the number of poles, r0 is the inner radius to the poles,and the rf potential applied to alternate rods is V0cos(t) The r2n-2 term tells us why octopole is mainly used for confining ion(guiding ion beam). How is the trapping field of octopole looks like compared to that of quatrupole?

Collect more scattered reactant and product ions. Bring the usable energy region down to 0.04eV(lab). The relatively flat well renders much less pertubation in ions’ radial momentum.

Cross views of the effective potential: Ideal hyperbolic electrodes Some deformation of electrodes Total E

The total instaneous kinetic energy of an ion during one reflection from an rf wall.

The average kinetic energy distribution The most probable average Ekin The fluctuations of the mean Ekin

Some applications The first octopole technique was used to by Gerlich in 1971 to make the first version of the guided-ion-beam apparatus. The improved one by Gerlich in 1984 extend the energy range to below 0.01eV(lab). Commercially, it has also been widely used.

Guided-ion-beam apparatus 26cm 14cm Crossed-beam arrangement 46cm

Agilent 7500c octopole reaction system in Inductively Coupled Plasma MS The octopole is mounted off-axis to prevent photons reaching the detector

New concept of ion traps having field of octopole or hexopole components. 200cm 3.5cm 14cm FT-ICR The coulombic interactin of ions in the octople ion trap casue a strong non-linearity trapping efficiency in the octopole

The Trick About Life is Too Make It Look Easy

In reality, the inscribed circle radius r0=(n-1)d/2 This figure shows the potential disturbtion from the cylinder surrounding an octopole. where a = qE0/m2