Trapping of ions

Since ions show stronger interactions with EM fields than neutral atoms, which experience only a weak force because of their polarizability, they can be stored more effectively in EM traps. Therefore trapping of ions was achieved long before neutral particles were flapped [1215,1216]. Two different techniques have been developed to store ions within a small volume in the radio frequency (RF) quadrupole trap [1216,1217,1242] the ions are confined within a hyperbolic electric dc field superimposed by a RF field, while in the Penning trap [1220] a dc magnetic field with a superimposed electric field of hyperbolic geometry is used to flap the ions. 

The EM quadrapole trap (the Paul flap, which won the Nobel prize for Wolfgang Paul in 1989) is formed by a ring electrode with a hyperbolic surface and a ring 

With the voltage U between the caps and the ring electrode, the electric potential (f (r, z) is for points within the trap [1217], 

The applied voltage U = Uo + Vbcos( RFr) is a superposition of the dc voltage Uo and the RF voltage Vocos( yRFf)- In Fig- 9.41 the electric field lines and the extension of the trapped ion cloud are shown for two opposite phases of the voltage U. The equation of motion 

The equation of motion (9.49) is known as Mathieu s differential equation. It has stable solutions only for certain values of the parameters a and b [1221]. Charged particles that enter the trap from outside cannot be trapped. Therefore, the ions have to be produced inside the trap. This is generally achieved by electron-impact ionization of neutral atoms. 

The applied voltage U = Uo Vo cos(r/.)RpO superposition of the dc voltage Uo and the RF voltage Vocos(curfO- The equation of motion 

The EM quadrupole trap is formed by a ring electrode with a hyperbolic surface and a ring radius of rg as one pole, and two hyperbolic caps as the second pole (Fig. 14.17). The whole system has cylindrical symmetry around the z-axis. The distance 2zg between the two pole caps (which are at equal potential) is adjusted to be 2zq = rgv.  

The stable solutions of (14.29) can be described as a superposition of two components A periodic micro movement of the ions with the RF 

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The FT-ICR/MS is an ideal instrument for studying ion-molecule reactions over an extended time scale due to the excellent trapping of ions in the cell and the unmatched mass resolution and mass accuracy. Mass resolution is defined as the mass divided by the peak width at half height... 

A simple method for the production and cryogenic trapping of ion-radicals is mentioned. The technique, cold window radical discharge (CWRD), enables the isolation of short-lived species in rare gas matrices, such as p-dichlorobenzene cation-radical. These species are formed within discharge plasmas, close to the trapping surface (Kolos 1995). 

Figure 13. Magnetic trapping of ions in a magnetic bottle as a function of mass.

Ion-cyclotron resonance Trapping of ions in cubic cell under influence of trapping voltage and magnetic field. Orbital frequency related inversely to m/z value. 

Inclusion is associated with trapping of ions of similar charge and size to the analytes and reagent, which causes isomorphous inclusion in the precipitate and tends to occur in colloidal precipitates due to the large surface area. 

Ion trap analysers use a similar principle to quadrupole mass analysers but employ a system of entrance, exit and end-cap electrodes together with a ring electrode that surrounds the trap cavity (Figure 9.10). As with quadrupole so with ion trap, for each ion type with a given value of m/z there is a corresponding value of interactions between ion type and external quadrupole field are such as to enable the trapping of ion within the analyser prior to release for detection. Ion traps are relatively inexpensive, quite sensitive and robust, so are fairly widespread, despite being less accurate than TOF and quadrupole mass analysers. 

An alternative approach is plasma ECD [86], in which electrons (0.1-15 eV) are collided with pulsed nitrogen gas prior to the trapping of ions in the ICR cell. The induced plasma conditions result in significant increase in ECD efficiency. A single plasma ECD mass spectrum of carbonic anhydrase ca 29 kDa) showed peaks corresponding to cleavage of 183/253 N-Ca bonds c/116/258 by activated ion ECD. 

The trapping of ions generates a further fundamental motion of ions called magnetron motion. Magnetron frequencies are independent of m/z of the ions and are much lower frequencies (1—100 Hz) than cyclotron motion (5 kHz to 5 MHz). Cyclotron motion is characterized by its cyclotron frequency (f), which depends from (i) the magnetic field, (ii) charge of the ion, and (iii) the mass of the ion ... 

An important quantity in all electric quadrupole devices (linear and three-dimensional) is the low mass cutoff, the minimum value of m z that has stable trajectories in the device under the stated operating conditions, i.e., the ion that has q just less than 0.908 (see discussion of Eignres 6.10 and 6.18). Then for this trap operated at Vg = 757 V, m/z) = 60.27/0.908 = 66.4, i.e., in practice the low mass cutoff for trapping of ions would be m z 67. The converse question, (i.e. what change would have to be made to the operating conditions to permit a specified low mass cutoff) amounts to determination of the required value of Vg since the RF frequency is considerably more difficult to adjust once fixed. Then the required value of Vg (in volts) for this particular hypothetical trap (i.e., rg, Zg and w all fixed) is given by Equation [6.36] as [(0.908/0.0796).( v z)iiiio] = H-41.(ffV z)imo-... 

When used as a linear ion trap (2D trap) many of the same techniqnes nsed with Paul traps can be used with only minor modifications. Trapping of ions with selected mJz values can be achieved, as in 3D traps, by resonant excitation methods or hy methods that exploit the boundaries of the stahihty diagram. Selective ion trapping is necessary if the hap is to be used also to create fragment ions by in-hap collision induced dissociation for subsequent analysis by a Panl hap or by FTICR or time of flight analyzers. Also, particnlarly in the case of downsheam trapping analyzers, excess unwanted ions that cause space charge problems can be selectively ejected. Resonant excitation at the ions secnlar frequencies (Eqnation [6.33])... 

Here z and M are charge and mass (in Daltons) of the trapped ions, respectively, /= Q/2jt is the frequency of the RF field (in Hz), ro is the inscribed radius of the trap expressed in cm, and Ura is the maximum kinetic energy of the ions (in eV units) that can be trapped. For a particular ion trap geometry (i.e., n, ro), Eq. (6) determines the minimum RF amplitude F in (at t] 0.3) that can be used to trap ions with transverse kinetic energy (expressed in eV). Equations (2), (3) and (5) then allow an evaluation of the minimum frequency required for trapping of ions with a particular M/z. 

Trapping of ions in a potential well implies reflection of ions between the trapping plates that induces an oscillatory motion along the z-axis the frequency (Oz of which is given by... 

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