Increased Sensitivity in Proton Transfer Reaction Mass Spectrometry by Using a Novel Focusing Quadrupole Ion Funnel

Abstract

Sensitivity enhancement in proton transfer reaction mass spectrometry (PTR-MS) is an important development direction. We developed a novel drift tube called a focusing quadrupole ion funnel (FQ-IF) for use in PTR-MS to improve the sensitivity. The FQ-IF consists of 20 layers of stainless steel electrodes, and each layer has 4 quarter rings. The first 6 layers have a constant inner hole diameter of 22 mm; the latter 14 layers taper the inner diameter down to 8 mm. The FQ-IF drift tube can also operate in the direct current (DC) mode (similar to a conventional drift tube) and ion funnel (IF) mode (similar to a conventional ion funnel drift tube) by changing the voltage loading method. The simulation results show that the transmission efficiency of the FQ-IF is significantly improved compared to that of the other two modes. Further experiments show that the product ions of limonene tend to convert into smaller m/z fragment ions at higher voltages for the DC and IF modes. However, unlike the DC and IF modes, the distribution of product ions is stable at higher voltages for the FQ-IF. In other words, a higher RF voltage for the FQ-IF will not increase the collision energy of ions. In addition, the improvements in sensitivity for the FQ-IF range from 13.8 to 87.9 times compared to the DC mode and from 1.7 to 4.8 times compared to the IF mode for the 12 test compounds. The improvements in the limit of detection (LOD) for the FQ-IF range from 2.7 to 35.7 times compared to the DC mode. The FQ-IF provides a valuable reference for improving the sensitivity of PTR-MS and other mass spectrometers.

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

“…44−47 Recently, a novel focusing quadrupole ion funnel (FQ-IF) was reported to improve the sensitivity of proton transfer reaction mass spectrometry (PTR-MS). 48 With stacked layers of four-quarter rings and RF application method similar to that of a typical quadrupole, the FQ-IF achieved not only enhanced sensitivity but also decreased low-mass discrimination compared with conventional ion funnels and direct current (DC) drift tubes. In addition, the hybrid of ion funnel with other ion guides 49,50 was also reported as another potential approach to improve the transmission of low-mass ions.…”

“…The integration of these ion funnels has favored major gains in sensitivity of MS in alone or coupling ion mobility separation (IMS). − Moreover, the miniaturized ion funnels have also been reported to boost the sensitivity of miniature mass spectrometers (mini-MS). − However, a ion funnel generally suffers from the decreased ion transmission for low-mass ions compared with larger ones, namely, low-mass discrimination effect, which results from the potential barrier along the central axis . To reduce this effect, few approaches have been explored by optimizing either the electrode structure − or electric field. − Recently, a novel focusing quadrupole ion funnel (FQ-IF) was reported to improve the sensitivity of proton transfer reaction mass spectrometry (PTR-MS) . With stacked layers of four-quarter rings and RF application method similar to that of a typical quadrupole, the FQ-IF achieved not only enhanced sensitivity but also decreased low-mass discrimination compared with conventional ion funnels and direct current (DC) drift tubes.…”

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

“…Du et al proposed a field‐gradient‐focusing ion guide whose internal electric field gradually changes from a larger‐range dodecapole to a smaller‐range quadrupole as ion transport occurs, which can improve the sensitivity of arginine by a factor of 10 23 . Bao et al proposed a QIF based on experiments and simulations and demonstrated the potential of the QIF approach in improving the detection limit and sensitivity for some small molecules 24 . However, to date, there has been a lack of further research into QIF configurations.…”

“…23 Bao et al proposed a QIF based on experiments and simulations and demonstrated the potential of the QIF approach in improving the detection limit and sensitivity for some small molecules. 24 However, to date, there has…”

Dipole and quadrupole resonance excitation in linear quadrupoles

“…For this purpose, the field of circular electrodes is described by a complex potential. 11 The problem of input fringing fields is solved by means of a Brubaker prefilter (delayed DC ramp), 12 by using electrodynamic ion funnels 13 or by gas damping in quadrupole or hexapole RF fields. 14…”