Lenses or No Lenses? A Study of Ion Transfer Efficiency at Interfaces in a Lens-Free Triple Quadrupole MS

The efficiency of ion transfer between the quadrupole mass analyzers and the Q₂ collision cell in a triple quadrupole mass spectrometry (MS) is critical for instrument robustness and sensitivity. Q₂ is a radio frequency (RF) only non-mass filtering quadrupole containing only an inert collision gas such as Ar, He, or N₂ to provide collision induced dissociation of the precursor ion selected in Q₁.

The traditional lens approach to MS involves a collision cell around Q₂ with narrow apertures in and out to limit gas flow. The electrostatic lenses are used to refocus ions in and out. Lens-based design TQ MS is common but it requires complex tuning and is prone to contamination so it requires periodic cleaning and is subject to undesired noding effects. The lens-free TQ MS (Figure 1) has a lens-free Q₀ ion guide and a collision area defined with seals around and outside of the Q₂ ion guide path (1). The RF-only ion guides and no lenses are used to guide ions in and out of Q₂. This approach provides high transmission, and has a simple and robust instrument design, also allowing more instrument up-time, without cleaning and re-tuning downtime. The lens-free approach is used in the Bruker SCION™ TQ GC–MS-MS platform.

Figure 1: Geometry details of the quadrupole interface models.

Experimental Methods

Two computational models are considered in this study. The first model is the lens-free quadrupole interface marked Q₁Q₂ and Q₂Q₃ in Figure 2a. Gas cell boundaries are marked with dashed lines. Ion transfer is achieved through close proximity RF-only ion guides: Q₁ post-filters, Q₂ RF-only input and output sections, and Q₃ pre-filters (2).

Figure 2: Details of ion trajectories in and out of Q2 for the lens-free (a) and three-lens (b) quadrupole interfaces with and without gas in Q2.

A second model is the three-lens quadrupole interface marked Q₁LLLQ₂ and Q₂LLLQ₃ in Figure 2b. Gas is contained in Q₂ with narrow apertures and three ion lenses are located at each end for refocusing the ion beam. The model also includes Q₁ post-filters and Q₃ pre-filters.

Ion trajectory simulations were undertaken using software SIMION 8 (3) and LUA programming.

Results

As observed in Figure 2a, smooth ion transfer occurred between RF-only quadrupole sections (pre- and post-filters). There were no losses due to collisions when Q₂ gas is on. Collisions occurred with the RF-only filters focus ion beam via "collisional cooling", reducing the ion loss in transmission. The three-lens based ion guide, on the other hand, showed significant ion transmission loss when the Q₂ gas is on (Figure 3b), demonstrating that the lens cannot effectively focus the ions in the presence of the collision gas.

Figure 3: Transmission efficiency in and out of Q2 for the lens-free and the three-lens interfaces as a function of Q2 pressure (collision energy 25 eV and m/z 264), collision energy (pressure 0.266 Pa and m/z 264) and mass (collision energy 25 eV and pressure 0.266 Pa).

Ion transmission studies were also carried out by varying the collision gas pressure and collision energy and masses, and the results are summarized in Figure 3:

• For both lens-free and lens-based systems, the ion transmission from Q₂ to Q₃ (Q₂...Q₃) is less efficient than that from Q₁ to Q₂ (Q₁...Q₂). This is due to the different resolution settings of Q₂ (wide open) and Q₃ (unit to 3 Da as a mass filter) in normal triple quadrupole operation.

• The lens-free transmission is relatively unaffected by collision pressure, energy, or mass, except at low masses < m/z 200. However, the three-lens transmission is significantly affected by collision pressure, energy, and mass.

• The three-lens transmission improves with collision energy due to reduced scattering loses.

• As collision pressure increases, the lens-free interface is significantly more efficient than the three-lens interface.

Another interesting observation of this study is that when Q₃ resolution is reduced, which is a common setting in triple quad MRM scanning, the lens-free interface maintains nearly 100% efficiency, but the efficiency of the three-lens interface is still significantly limited due to collision losses in the lens regions (Figure 4).

Figure 4: Transmission efficiency Q2...Q3 when Q3 is set to lower resolution.

Conclusions

A lens-free, RF-only interface between a quadrupole and collision cell can significantly improve the ion transmission in the presence of the collision gas, compared to the traditional lens based ion guides. Also unlike the lens-based ion guides, the high ion transmission efficiency of lens-free ion guides is not affected by the collision gas pressure, energy, and the mass. Additional benefit of the lens-free pre- and post-filters is that the tuning is much simpler and required less frequently, resulting in a more robust system for long operation.

References

(1) US Pat 6576897.

(2) "Lenses or No Lenses?" poster presented at the 59th ASMS Conference on Mass Spectrometry and Allied Topics, Denver, Colorado, 2011, ThP04 83.

(3) SIS Inc., Ringoes, NJ 08551; www.simion.com.

For research use only. Not for use in diagnostic procedures.

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