Design and performance evaluation of a novel ion funnel driven by a phase-modulated rectangular wave

Simulation study of three new quadrupole ion funnels to improve low-mass ion transmission

RATIONALE

By applying radio frequency (RF) and direct current (DC) voltages to corresponding ring electrodes, ion funnel (IF) can efficiently focus and transmit ions. However, IF has an inherent mass discrimination problem that will greatly limit low mass-to-charge (m/z) ion focusing and transmission. To improve the transmission efficiency (TE) of the IF, this paper explores three new profile quadrupole ion funnels (QIF).

METHODS

Computer simulations of the potential field distributions of QIFs and conventional IFs were performed to assess their focusing characteristics. To compare the TE, ion optics simulation programs SIMION and AXSIM were used to perform a series of simulations. Three QIF types (toroidal, cylindrical, and hyperbolic configurations) were used to improve ion TE, and their transmission and focus performance were also compared with conventional IF.

RESULTS

By simulating the trajectories of ions in the IF, the optimum electrical parameters for three new QIFs were obtained and compared with conventional IFs, with TE improvements recorded for m/z < 100 of 16%, 20%, and 13%.

CONCLUSIONS

The results indicate that studying these three new IF configurations has great research significance for improving sensitivity to low m/z ions in mass spectrometer instruments.

Field-Gradient-Focusing Ion Guide for Enhanced Transfer Efficiency of Low-Mass Ions

Efficient transmission of low-mass ions in a rough vacuum pressure region has always been a challenging issue in mass spectrometry (MS). In this study, a novel ion guide, namely, field-gradient-focusing ion guide (FGF-IG), was proposed to improve the transfer efficiency of ions, especially low-mass ions in a rough vacuum region. The FGF-IG has 12 electrodes whose surfaces gradually narrowed and tilted inward, and its electric field gradually varies from dodecapole (or multipole) to quadrupole along the ion transfer route. The field radius was gradually decreased from 6 to 2 mm in the multipole region (65 mm in length) and finally remained unchanged as 2 mm in the quadrupole region (20 mm in length). By integrating into a miniature mass spectrometer (mini-MS) with a continuous atmospheric pressure interface, this ion guide was optimized in terms of inlet capillary position, radio frequency amplitude, and direct current voltage applied on it. Results showed that a reduced low-mass discrimination effect and improved efficiency of simultaneously transferring mid and low m/z ions were achieved for FGF-IG compared with a conventional ion funnel. Under optimized conditions, a limit of detection of 1 ng/mL was obtained for both reserpine (m/z 609) and arginine (m/z 175) ions by integrating FGF-IG into the mini-MS. The sensitivity of smaller arginine ions using FGF-IG was enhanced by ∼10 times than that obtained using the conventional ion funnel (10 ng/mL) in comparative experiments. The idea of smooth transfer from dodecapole to quadrupole fields could be extended to other multipole fields, as well as in lab-scale MS instruments.

Performance optimization of an ion funnel driven by novel radiofrequency waveforms

RATIONALE

In mass spectrometry, ion transmission is usually achieved by driving an ion funnel with a reversed sine wave radiofrequency. However, the mass range of this conventional ion funnel is limited. In order to overcome this limitation, and to improve the transmission efficiency of the ion funnel, we explore the use of different radiofrequency waveforms for different m/z ranges.

METHODS

Right triangle, sawtooth, and variable phase sine (VPS) waves are used in different m/z ranges to improve ion transmission efficiency. We use SIMION-based numerics to simulate their potential field distributions and ion flight trajectories. We compare transmission and focusing performances with those of a conventional ion funnel.

RESULTS

Ions with high m/z values require a larger potential gradient to limit their flight trajectory. Right triangle waves can quickly adjust electrode potential through a step change. The equipotential line distribution of VPS waves is wider than that of a sine wave which improves focusing performance. At the same time, the local ion trap effect at the outlet of the ion funnel is improved because of the "snake potential". The maximum effective potential of the sawtooth wave is smaller than that of the sine wave, which is suitable for low m/z ion transmission.

CONCLUSIONS

Sawtooth and VPS waves may improve the transmission performance of the ion funnel in the low m/z regime whereas right triangle waves may improve the transmission performance in the high m/z regime.