A Planar Quadrupole device for transmitting and trapping ions in high vacuum
To enable improved ion transport and trapping within a FT-ICR mass spectrometer, a new ion guide was developed using two parallel printed circuit boards, herein referred to as a planar quadrupole (PQ) ion guide. The PQ device allows RF and DC voltages to be combined to enable both axial transport as well as trapping of ion populations in the ultrahigh vacuum region of the mass spectrometer. Additionally, since the dimension orthogonal to the PCB boards and ion transmission axis is entirely open, the PQ device enables unprecedented optical access to the ions along the entire path of ion transport, unlike any other quadrupole ion guide or quadrupole ion trap.
All experiments were performed using a modified hybrid linear ion trap Fourier transform ion cyclotron resonance mass spectrometer (LTQ FT-ICR MS; Thermo Scientific, Bremen, Germany) equipped with a cylindrical Ultra ICR cell and 7 T actively shielded superconducting magnet (Japan Superconductor Technology, Tokyo, Japan). The original ion optics of the LTQ-FT include an octupole ion guide for the final segment of ion transmission immediately prior to the FT-ICR cell. After the removal of this Thermo stock octupole 3 and the installation of the PCB-based PQ and a custom multi-pin feedthrough flange on the source side of the vacuum system, the system was pumped and baked out overnight. A ribbon cable and Kapton-coated wires (22 AWG, Accu-Glass Products, Inc. Valencia, CA) were used to apply DC and RF voltages, respectively, to RIPT. Electrospray ionization (ESI) was used to generate ions with a syringe pump and infusion of samples at a rate of 3.0 μL/min. The sample used for evaluation included MRFA and Ultramark 1621 standard solution. ESI spray voltage of 4.5 kV was applied to a sample solution through a metal union for ionization. The ions were accumulated in the LTQ and then transferred to an ICR cell through the RIPT. Figure 1 shows a schematic diagram of the PQ components and assembly used for this study. The PQ consisted of two PCB boards, top and bottom, as shown in Fig. 1A. The top/bottom boards included segments to allow application of 11 independent DC voltages in the present case to serve as ion trapping electrodes to create axial trapping well. The 4 electrodes within each segment were also coupled to + and – RF voltages to establish a pseudo-quadrupolar radial RF trapping field about the central axis. The length and width of electrodes used for generating axial trapping well and RF trapping field were 25.91mm and 7.62mm, respectively. RF voltages in opposite phase were connected to adjacent segments via 0.72nF mounted capacitors. DC voltages were connected to each segment via 500K mounted resistors. The gaps between each electrode were 0.25mm. The RF and DC inputs were connected to an external RF power supply and a multi-channel programmable DC power supply (Modular Intelligent Power Source (MIPS), GAA Custom Electronics LLC, Kennewick, WA, USA) using Kapton-coated wires and a ribbon cable, respectively, through the multi-pin feedthrough flange. The distance between top and bottom boards was 6.5mm generated with aluminum spacers, and copper nuts and bolts as shown in Fig. 1B. The assembled PQ was installed with two brackets as shown in Fig. 1C in the place of octupole 3 after its removal.
Figure. 2. shows simulation results for pseudo-quadrupolar radial trapping field about the central axis with 400Vpp RF voltages, ion beam trajectories in the PD and DC trapping well with +6V DC voltages applied to 9 to 11 segments to trap ions in the 10 segment. The simulations were generated with SIMION 8.1. The ion beam was efficiently focused on the center of the PQ and transferred to the end of the PQ without ion loss as shown in Fig. 2B.
Figure 3 illustrates mass spectra acquired using either the Thermo stock octupole or the PQ. To obtain the spectrum with RIPT, 943kHz resonant frequency, 300Vpp RF voltage and -24V bias voltages were applied to it for ion transmission to an ICR Cell. The resulting spectrum acquired with the PCB-RIPT illustrated compatible performance with no obvious discrimination in ion m/z values, S/N or intensity as compared with the spectrum obtained with the Thermo stock octupole.
Figure 4 shows another benefit of the PQ that can reduce or eliminate mass discrimination associated with time-of-flight separation. Fig. 4A shows pulse peak area as a function of the number of trapping segments obtained with an ion current meter installed behind an ICR cell back lens. To trap ions in one segment (10th segment), for example, +6V trapping voltages were applied to 11th and 9th segments. After trapping ions in the 10th segment, -3V was applied to 11th segment to transfer the trapped ions to an ICR cell or ion current meter. To trap ions in two segments (9th and 10th segments), +6V trapping voltages were applied to 11th and 8th segments and then transferred the ions to the ICR cell or ion current meter by applying -3V to 11th segment. As shown in Fig. 4A, pulse peak area was linearly increased with the increased number of segments. Fig. 4B shows mass spectra acquired with variable number of segments used to trap ions in the PQ prior to transmission to the FT-ICR cell. Operating PQ with 1 or 3 segments to trap ions prior to transferring the ions into the ICR cell. The use of longer trapping region with 3 segments yielded a 69% increase intensity of low mass ions (m/z = 524) without a decrease in intensity of high mass ion (m/z 1822) as compared to trapping in a single segment. This benefit is observed because the use of a longer trapping well enable trapping of higher velocity ions (lower m/z) together with lower velocity ions (higher m/z), effectively reducing discrimination commonly observed due to time-of-flight separation of ions pulsed out of the LTQ.