The development of a novel double-focusing thermal ionization mass spectrometer

Abstract

Unlike traditional Thermal Ionization Mass Spectrometers (TIMS) with single-focusing magnetic mass analyzers (such as Triton, Phoenix, Nu TIMS, etc.), a newly developed Double-Focusing Thermal Ionization Mass Spectrometer (DF-TIMS) enhances the system stability by mitigating accelerating high-voltage noise and drift. Featuring a Nier-Johnson type double-focusing mass analyzer, the instrument includes a laminated magnet with a 250 mm radius and a cylindrical Electrostatic Analyzer (ESA) with a 350 mm radius, achieving a mass dispersion of 560 mm. It is equipped with of 16 Faraday cups and 4 full-size discrete dynode secondary electron multipliers (SEM), combined with variable dispersion double quadrupole zoom optics, allowing for multi-collection of isotopes with up to 20 % mass dispersion, such as isotopes of Lithium, Boron and Calcium. Additionally, a compact and advanced Retarding Filter enhances abundance sensitivity from < 2 ppm to < 5 ppb. Automated tuning and measurement improve efficiency for both positive and negative ions. With a 0.2 mm source slit and a 1 mm receiving slit, DF-TIMS achieves a resolution over 470 and a peak shape factor below 0.3. System stability is less than 15 ppm/30 min credit to the double-focusing design. Each Faraday cup operates with a dynamic range of 0–50 V, featuring Root Mean Square (RMS) noise (4s integration, 10¹¹ Ω high resistance) under 20 μV and baseline drift below 1 × 10⁻¹⁶ A/h. The instrument has been applied extensively, delivering internal precision and external precision for Strontium and Neodymium measurements under 5 ppm, meeting stringent isotopic ratio analysis requirements in nuclear science and geoscience.