Ion traps operation

Areas in which ions are stable in the axial and radial directions are shown in Fig. 13. Areas on this figure where the two regions of stability overlap show where ions are stable in both directions, and may thus be stored in the trap. All commercial ion traps operate using the area of stability nearest the origin of the graph. 

A vitally important aspect of ion trap operation is the ability to impart translational energy selectively to ions via resonance absorption of alternating current (ac) voltages (10-450 kHz) applied to the endcap. Unlike for linear quadrupole (or other multipole) collision cells, the absorption of energy is m/z specific as each m/z in the trap precesses at a specific set of frequencies, the most important of which for MS/MS is ooz, the fundamental frequency of motion in the z dimension, which is defined by... 

Dumping gas flow 0.3mL/min excitation voltage 225 mV. b.p. base peak of fragmentation spectrum. MS conditions Ion trap operating in positive-ion chemical ionization (PICl) mode, reagent gas methane at flow 0.8mL/min, ion source temperature 200°C, scan range m/z 40-550 (Flamini ... 

Ion trap operates in a pulsed mode so that ions are accumulated mass selectively over time. Collision-induced dissociation in the ion trap is produced by several hundred collisions of a mass-selected ion with helium buffer gas atoms. An advantage of ion-trap instruments is the ability to perform MS . 

FIGURE 5.3 Prototype of FAIMS ion trap operating at atmospheric pressure (a) the scheme and (b) pseudopotential for gramicidin S (2+) ions computed along the axial direction marked in (a) by the dashed hne. (From Guevremont, R., Ding, L., EUis, B., Barnett, D.A., Purves, R.W., J. Am. Soc. Mass Spectrom., 12, 1320, 2001.)... 

Like most successful clever ideas, the mass selective instability scan mode seems rather simple once someone had conceived it. However, it turned out (Stafford 1984) that full experimental reahzation of the potential of the method required another innovation, the exploitation of the collisional coohng/focusing effect of a low pressure of helium, discussed above. It was found (Stafford 1984) that the presence of helimn at a relatively high pressure of approximately 10 torr significantly enhanced the mass resolution, sensitivity and detection limit of a Paul ion trap operated as a mass spectrometer using the mass selective instability scan to obtain the mass spectrum. 

Figure 10.6 Kinetic study of the dissociation of H H20. (a) Decay of the cluster ions with increasing trapping time in a 22-pole ion trap operated at various temperatures and (b) the dissociation rate constant of cluster ions at various temperatures [43]. Reprinted with permission from Wang, Y.-S., Tsai, C.-H., Lee, Y.T., Chang, H.-C., Jiang, ].C., Asvany, O., Schlemmer, S., Cerlich, D. (2003) Investigations of Protonated and Deprotonated Water Clusters Using a Low-temperature 22-pole Ion Trap. j. Phys. Chem. A 107 4217-4225. Copyright (2003) American Chemical Society...

IRMPD is a valuable tool for gas phase ion studies and analytical applications which often do not necessarily require the full power of an extremely expensive FT-ICR instmment. Therefore, attempts have been made to apply IRMPD on three-dimensional (Fig. 9.33) [140,141] and linear quadrupole ion traps [142-144]. Due to the slow heating of ions, collisional cooling by the buffer gas required for quadmpole ion trap operation is counteracting the accumulation of ion internal energy. To circumvent this problem a dynamic pressure operation mode has been developed for IRMPD in QITs where initial ion storage and precursor ion selection are mn at standard QIT pressure (3 x 10" mbar) while the buffer gas flow is... 

The basic theory of operation of quadrupole devices was enunciated almost 100 years before the quadrupole ion trap was invented by Paul and Steinwedel ( ]. The pioneering work of the inventors was recognized by the award of a 1989 Nobel Prize in Physics to Wolfgang Paul [2]. The quadrapole ion trap functions both as an ion store for gaseous ions and as a mass spectrometer. The mass-selective axial instability mode of ion trap operation developed by Stafford et al. [3] made possible the commercialization of the quadrupole ion trap. It should be recalled that the trapping parameter is given by... 

Since U = 0, a, and a are equal to zero and the common mode of ion trap operation corresponds to... 

Quadrupole ion trap operation is concerned with the criteria that govern the stability of an ion trajectory in both r and z directions within the field, that is, the experimental conditions that determine whether or not an ion is stored. 

The above operation can be expressed succinctly in a scan function, which shows the temporal variation of the potentials applied to the electrodes such that a scan function is a visual representation of the sequence of program segments in the software that controls ion trap operation. The scan function for the above mass spectrometric operation is shown in Figure 12. 

Ion trap mass spectrometry is a versatile technique of high sensitivity and high specificity. The relatively low cost of commercial instrumentation has permitted a substantial growth in the practice of mass spectrometry. The theory of ion trap operation differs from those of other mass spectrometers and presents an exciting challenge to the mass spectrometry community. 

Figure 6.1 compares two electron ionization mass spectra of diazepam, one recorded with a qnadrupole, the other with an ion trap operated with internal ionization. The mass spectra are very similar bnt the base peak is at m/z 256 in external source operation and at m/z 257 in internal sonrce. In an internal ionization ion trap, the danghter ions F + (m/z 256) react with the molecule M to produce the FH+ ion (m/z 257). The radical (M-H)- issued from the grasping of a hydrogen radical from the molecule does not, of course, appear because it is electrically neutral. 

The limit of detection of a method (or limit of quantification for a dosage method) is a very important parameter. To compare this parameter in quadmpole and ion trap operations, the acquisition mode of the mass spectrometer must be made precise. 

The curve resulting from plotting the chromatographic response of a given compound as a function of its concentration in analyte is called the response function (see Chapter 7). Figure 6.2 compares the response functions obtained with a quadru-pole and an ion trap operated in full scan mode for the same molecule. 

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