Effects of Magnetic and Electric Field Uniformity on Coded Aperture Imaging Quality in a Cycloidal Mass Analyzer

Abstract

Cycloidal mass analyzers are unique sector mass analyzers as they exhibit perfect double focusing, making them ideal for incorporating spatial aperture coding, which can increase the throughput of a mass analyzer without affecting the resolving power. However, the focusing properties of the cycloidal mass analyzer depend on the uniformity of the electric and magnetic fields. In this paper, finite element simulation and charged particle tracing were used to investigate the effect of field uniformity on imaging performance of a cycloidal mass analyzer. For the magnetic field, we evaluate a new permanent magnet geometry by comparing it to a traditional geometry. Results indicate that creating an aperture image in a cycloidal mass spectrometer with the same FWHM as the slit requires less than 1% variation in magnetic field strength along the ion trajectories. The new magnet design, called the opposed dipole magnet, has less than 1% field variation over an area approximately 62 × 65 mm; nearly twice the area available in a traditional design of similar size and weight. This allows ion imaging across larger detector arrays without loss of resolving power. In addition, we compare the aperture imaging quality of a traditionally used cycloidal mass spectrometer electric design with a new optimized design with improved field uniformity. Graphical abstract ᅟ.

Improving the Performance of a Cycloidal Coded-Aperture Miniature Mass Spectrometer

“…The magnetic sector (flux density, 0.3 T) has the same opposed dipole design as described in previous publications. 20,41 Figure 1c,d shows CAD images of the electric sectors, and Figure 1e,f shows photographs of the assembled electric sectors from the previous C-CAMMS and the new C-CAMMS-MP instruments, respectively. Figures S1 and S5 show additional CAD images of the two instruments.…”

“…As depicted in Figures S2 and S3, the ion source in the new C-CAMMS-MP is placed in a cavity between electrodes 12 and 13 in the electric sector, enabling a more homogeneous electric field surrounding the embedded ion source, as described by Landry et al 41 In addition, the new C-CAMMS-MP mass analyzer also resolves the detector and mass analyzer focal plane alignment issue of the previous C-CAMMS mass analyzer by creating a cavity for the detector to be placed within the electric sector electrode 13, which is electrically grounded (Figure S3). This ensures that the detector is aligned with the mass analyzer focal plane and with the ion source aperture plane.…”

“…The cycloidal mass analyzer design for both the previous C-CAMMS and the new C-CAMMS-MP instruments consists of a magnetic sector constructed using a permanent magnet and an array of electrodes for the electric sector. The magnetic sector (flux density, 0.3 T) has the same opposed dipole design as described in previous publications. , Figure c,d shows CAD images of the electric sectors, and Figure e,f shows photographs of the assembled electric sectors from the previous C-CAMMS and the new C-CAMMS-MP instruments, respectively. Figures S1 and S5 show additional CAD images of the two instruments.…”

“…Third, as mentioned above (and shown in Figure S3), the ion source for the previous C-CAMMS was placed in-line with the electrically grounded electrode 13. Landry et al determined that this configuration resulted in a nonuniform electric field near the ion source. , The new C-CAMMS-MP electric sector in Figure f resolves the alignment issues by embedding both the ion source and the detector within its scaffolding. The following paragraphs describe the design changes to the electric sector in the new C-CAMMS-MP to fix these issues.…”

“…As depicted in Figures S2 and S3, the ion source in the new C-CAMMS-MP is placed in a cavity between electrodes 12 and 13 in the electric sector, enabling a more homogeneous electric field surrounding the embedded ion source, as described by Landry et al…”