MEASURING CLUMPED ISOTOPES WITH A HIGH-RESOLUTION MASS SPECTROMETER

John M. Ferry is celebrated for pioneering work on fluid flow during metamorphism. He also pioneered the application of clumped isotope geochemistry to fluid flow during dolomitization. This paper is dedicated to his forward-looking career.

Panorama is a gas source, electron-impact, double-focusing, multi-collector, isotope ratio mass spectrometer with a mass resolving power of 40,000 to 80,000. High mass resolution is achieved by a large geometry analyzer with radii of its cylindrical electrostatic analyzer and magnetic sector of 1017.6 mm and 800 mm, respectively. Mass resolving power (MRP) is increased by (1) image demagnification of 0.448; (2) a continuously adjustable source slit; and (3) continuously and independently adjustable slits controlling ion beam entry into each Faraday bucket and ion counter. The impact of these factors on mass resolution is given by:

MRP = mass dispersion/peak width; where dispersion = 630 mm for Panorama

and peak width =

((image magnification*width source slit) + width collector slit + width image aberration)

A continuously adjustable source slit makes it possible to precisely control the width of ion beams entering the collectors. Continuously and independently adjustable collector slits control the width of each of the flat-topped peaks needed for stable measurements. Panorama is thus capable of an exquisitely optimal tuning of mass resolution vs. stability for each of the working gases and each of their isotopologues to be analyzed. Mass spectra show baseline resolution of ¹⁵N¹⁵N  from ¹⁴N¹⁶O, ¹⁸O¹⁸O from ³⁶Ar and H³⁵Cl, and ¹³CDH₃ and ¹²CD₂H₂ from interfering ¹³CH₅ and ¹²CDH₄. Transmission of ions from source to collectors is enhanced by electrostatic lenses including 4 quadrupole, 2 hexapole, and one octopole lens while reducing ion optical aberrations. Multi-collection of ion beams is accomplished by 1 ion counter and 9 Faraday buckets each with independently adjustable slits. Operating at an accelerating voltage of 16 Kv, measured standard errors are 0.006 ‰ for δ¹³CH₄, 0.04 ‰ for δ¹²CDH₃, 0.1‰ for Δ¹³CDH₃, and 0.5‰ for Δ¹²CD₂H₂.

Experiments to calibrate the equilibrium relationship between Δ¹³CDH₃ and Δ¹²CD₂H₂ based on hydrothermal decomposition of Si₅C₁₂H₃₆ show an approach to equilibrium at both 500°C and 600°C for Δ¹³CDH₃ and at 600°C for Δ¹²CD₂H₂.