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

By AZoM

Table of Contents

Introduction

The robustness and sensitivity, that make triple quadrupole reliable work-horses in quantitative mass spectrometry (MS) is due to the efficient transfer of ions between the Q₂ collision cell and quadrupole mass analyzers in the triple quadrupole MS.

Challenging Requirements

Collision-induced dissociation of the precursor ion selected in Q₁ is provided by Q₂, a radio-frequency-only (RF-only) non-mass filtering quadrupole that contains only an inert collision gas such as Ar or He. The challenges in the design requirements are:

• Though Q₁ and Q₃ are in same vacuum region there must not be any collision.
• To achieve best results, the ion beam wide and divergent at the exit of the previous quadrupole, should be narrow and centered at the entrance of each quadrupole, hence must be collimated.

Common Implementations Today

According to the conventional lens approach to MS, a collision cell is formed around Q₂ having narrow apertures in and out for limiting gas flow/ Even though the lens-based design TQ MS is common it requires complex tuning and also results in periodic cleaning and undesired noding effects as it is prone to contamination. The lens-free TQ MS as shown in 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. The RF-only ion guides and no lenses are used to guide ions in and out of Q₂. The design is simple and robust, provides high transmission and also allows more instrument up-time, without cleaning and retuning downtime. The Bruker SCION TQ GC-MS/MS platform uses the lens-free approach. The auto-focusing q0 ion guide curved at 90° for maximum robustness (Figure 2) is an additional design feature.

Figure 1. Lens-free design features of SCION TQ GC-MS.

Figure 2. Auto-focusing q0 ion optics: 90° for maximum robustness, heated q0, and auto-focusing option.

Experimental Methods: Quadrupole Interface Models

Two computational models are considered in this study, which include the following:

• The lens-free quadrupole interface marked Q₁Q₂ and Q₂Q₃ in Figure 3a. The dashed lines show the gas cell boundaries. Ion-transfer takes place via close proximity RF-only guides, Q₁ post filters, Q₂ RF-only input and output sections and Q₃ pre-filters.
• The three-lens quadrupole interface marked Q₁LLL Q₂ and Q₂ LLL Q₃ in Figure 3b. Q₂ contains gas with narrow apertures and each end comprises three ion lenses for refocusing the ion beam. Q₁ post filters and Q₃ pre-filters are included in the model.

Figure 3. Geometry details of the quadrupole interface models.

Software SIMION 8 and LUA programming are used to undertake ion trajectory simulations.

Results

Ion Trajectory Analysis. Lens-free Quadrupole Interface

Collisions with smooth ion transfer occur within RF-only guides, help to collimate the ion-beam and improve transmission efficiency, as seen in Figure 4a. There are no losses due to collisions when Q₂ gas is on. Ion loss occurs in Q₃ due to limited mass filter acceptance. No significant electrostatic focusing could be achieved with the pre-post filter ion guides.

Three-lens Quad Interface

Efficient ion focusing and transfer occurs when Q₂ gas is off and when no collision occurs, as in Figure 4b. But there were significant losses when the Q₂ collision gas is on, the common mode of operation for a triple quad. Most ion losses occurred within the electrostatic lenses. Hence, the conclusion, that lenses cannot efficiently focus ions in a collision environment.

Figure 4. 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.

Ion Transmission Analysis

An ion transmission study by tuning the voltages vs. mass for maximum efficiency in the absence of collisions was performed. Since they are present in a common triple quad instrument, voltages following Q₂ are also adjusted to account for ion energy loss when the collision is on. The results are presented in Figure 5.

• For both systems, the ion transfer out of Q₂ is less efficient than into Q₂ due to the smaller acceptance of analyzing quadrupole Q₃ as compared to the RF-only quadrupole Q₂.
• The lens-free transmission is relatively unaffected by collision pressure, energy or mass, except at low masses. But the three-lens transmission is significantly affected by collision pressure, energy, and mass.
• Though the three-lens transmission improves with collision energy due to reduced scattering loses, the lens-free interface is significantly more efficient than the three-lens interface (Figure 5a) as collision pressure increases,

Figure 5. Transmission efficiency in and out of Q₂ for the lens-free and the 3-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 25eV and pressure 0.266Pa).

Another interesting observation is that the lens-free interface is almost 100% efficient when Q₃ acceptance is increased by reducing Q₃ resolution but three-lens interface is significantly limited due to collision losses in the lens regions (Figure 6).

Figure 6. Transmission efficiency Q₂…Q₃ when Q₃ is set to lower resolution.

Conclusions

A lens-free, RF-only interface is not only simpler to tune and more robust than a lens-based interface but it also provides significantly higher ion transfer efficiency and overall sensitivity.

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