Research Progress and Application of Ion Funnel Technique

Boosting the sensitivity of single photon ionization time-of-flight mass spectrometry using a segmented focus quadrupole-ion guide

Single photon ionization time-of-flight mass spectrometry (SPI-TOF-MS) is a powerful analytical technique for real-time detection of trace VOCs. However, efficient ion transmission within the ionization chamber has always been a challenging issue in SPI-TOF-MS. In this study, a novel ion guide termed the Segmented Focus Quadrupole Ion Guide (SFQ-IG) was introduced for SPI-TOF-MS. The SFQ-IG device consists of 12 printed circuit boards (PCB), each containing four quarter-ring electrodes with inner diameters progressively decreasing from 26 to 4 mm. The simulation results demonstrated that SFQ-IG exhibited superior ion transmission efficiency than both ion funnel (IF) field and direct current-only (DC-only) field. By integrating into a SPI-TOF-MS, this ion guide was optimized in terms of the ionization source pressure, direct current gradient, and radio frequency amplitude. Further comparative experiments demonstrated that the SPI-TOF-MS with the SFQ-IG exhibited higher sensitivity than both the IF field (1.3-7.4 times) and DC-only field (3.5-8.8 times) for the test VOCs. The improvements in limit of detection (LOD) with the SFQ-IG ranged from 1.6 to 5.3 times compared to the DC-only field for the test VOCs. Fabricated using PCB technology, the SFQ-IG is characterized by its cost-effectiveness, compact size, and high transmission efficiency, facilitating its integration into other mass spectrometers.

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.