El Positivismo: que reduce la realidad a lo meramente fenoménico, a lo

The beam preparation system is designed to transmit the wanted ion species, to filter unwanted species, to cool the ions and to form compact ion bunches matched for the time-of-flight analyzer (see figure 3.3). In addition, it may serve for first MS/MS experiments and as activation stage of mass-selected species in future experiments [Lippert, 2016]. It comprises an RFQ ion guide, an RFQ mass filter, a cooler RFQ and an RFQ trap. Further, diaphragms and encapsulation provide vacuum separation for the single compartments.

The radio-frequency quadrupole dimension is derived from the ratio rc= 1.14511r0and desired q-

values (see section 2.2.1). This depends on the diameter, the applied RF amplitudes and the mass to be transmitted. Since this instrument aims for bioanalytical application, higher masses m > 200 u have to be considered. A dimension of rc= 8 mm and r0= 7 mm is found to be favorable for high

transmission of ions in the desired mass range and compatible to technical boundary conditions, such as discharge in moderate vacuum. Figure 3.4 demonstrates the peak-to-peak voltage for the given geometry (see equation 2.21) and the resulting effective pseudopotential (see equation 2.22) for the two different frequencies of the system in order to obtain a stability parameter of

q = 0.15. The pseudopotentials are similar and both RFQ systems provide transmission of masses m > 2000 u.

The ion guide RFQ is operated in a standard transmission mode with stable ion trajectories for optimal transmission. A shallow linear potential gradient can be established by conductive RFQ material (compare figure 3.3). The length is determined to have sufficient cooling effect for atmo- spherically created ions and large gas flow conductance for the pressure reduction.

The mass filter is equipped with two Brubaker lenses to stabilize ion trajectories prior and after the optional mass filtering by an additional DC offset (refer to section 2.2.1.5). The length, the frequency of the RF field, the mass and the kinetic energy limit the maximal achievable mass re- solving power (see equation 2.33). The realistic mass resolving power is estimated to RRFQm ≈ 100

and expected to be dominantly restricted by geometric deviations. A characterization of the mass filter is part of a different work [Lippert, 2016]. An arrangement of two subsequent diaphragms prior the filter section is designed to focus the radial distribution of the desired mass and thus provide best initial conditions for filter operation [Dickel and Yavor, 2012]. Simulation studies presented in figure 3.5 show the advantage of this double aperture arrangement [Lippert, 2012a]. A focusing effect is achieved by a more negative potential on the second diaphragm. A slightly enhanced pressure reduction with same transmission values is an additional beneficial side effect for mass filtering, where residual or buffer gas collisions needs to be prohibited. No axial gradient is applied along the mass filter and ions travel unhindered with their kinetic energy obtained by applied DC potentials.

3.2. Mass Spectrometer

Figure 3.3.: Schematic view of RFQ beam preparation system with illustrated electric axial poten- tials; (a) to (d) describes individual states in the trap region of one cycle of operation: (a) simultaneous accumulation of ions passing the cooler RFQ and final cooling and bunching in the injection trap, (b) injection of compact ion bunch into the analyzer, (c) formation of accumulated ion packet in pre-trap before transport to trap, (d) transport from pre-trap to trap. The color indicates the level of cooling.

0 200 400 600 800 1000 1200 1400 1600 P e a k - t o - P e a k R F V o l t a g e / V Mass-to-Charge / u/e q=0,15 (1,2MHz) q=0,15 (1,4MHz) 0 200 400 600 800 1000 1200 1400 1600 1800 2000 0 5 10 15 20 25 30 P s e u d o p o t e n t i a l / V

Figure 3.4.: Peak-to-peak voltage and effective pseudopotential to obtain a stability parameter of

q = 0.15 with an RFQ of r0 = 7 mm; the frequencies are chosen according to the

resonance frequencies of the real RF voltage setup.

Figure 3.5.: Radial emission into mass filter with an arrangement of two apertures (2.2 mm and 1.8 mm open diameter) arranged in 1 mm distance and with single aperture 1.8 mm [Lippert, 2012a]; the radial emission is clearly reduced as highly favorable condition for mass filtering operation.

3.2. Mass Spectrometer

designed to provide sufficient buffer gas collisions to reduce ion momenta such, that ions lose their kinetic energy and are trapped within the linear potential gradient and accumulated for further processing. This accumulation ensures a duty cycle of 100% for the entire operation cycle. A short RFQ prior the injection trap (in the following named pre-trap) is a dedicated segment of the cooler RFQ with an individual DC potential (see figure 3.3). The potential of the pre-trap is first used as a potential barrier for continuous ion accumulation and shielding from switched electric fields of the injection trap potential. It is then switched to a lower potential to accumulate ions close to the next aperture and to form an ion packet that is finally transmitted into the trap. The length is long enough to provide shielding and short enough to transmit a broad mass range during the transfer from pre-trap to trap. This pulsed extraction of a quasi mono-energetic ion bunch causes a mass-time-dispersion up to the moment of applying the axial trap confinement. This causes some restrictions in mass range and has been considered in the design and has been extensively investigated by simulation studies shown in figure 3.6 [Wohlfahrt, 2011]. A transmission close to 100% for a mass range of a factor 4 at the transfer from pre-trap to trap is observed.

Figure 3.6.: Simulated relative transmission from pre-trap to trap with fixed operation parameters; a mass range as broad as 4 is possible to be transmitted simultaneously [Wohlfahrt, 2011].

The RFQ injection trap is a single linear quadrupole segment between two diaphragms. Such a linear Paul trap arrangement has been used in previous instruments [Plass et al., 2008] to serve as cooling and bunching instance for the delivery of a compact ion cloud into the analyzer. Confine- ment is provided by axial potentials on the neighboring diaphragms (see figure 3.3) and the radial pseudopotential field. The confinement in axial and radial direction are to some extent competitive. Strong confinement is principally desired, but whenever confinement in one direction is too strong, ions are pushed into the direction of weaker confinement and the ion cloud is formed to a disad-

vantageous phase space. Since the field of axially confining potential penetrates into the mid of the trap, higher voltages also create an additional DC offset of the trapped ions, which determines the kinetic energy in the drift region. The RF voltage which is applied for the cooling process can be switched off during the moment of injection as demonstrated in previous work [Haettner, 2011]. This prevents ions to experience changing electric fields when leaving the trap region, which may lead to errors in the mass calibration.