Abstract
Abstract. We installed the new Isotopx ATONA Faraday cup detector amplifiers on an Isotopx NGX mass spectrometer at Lamont-Doherty Earth Observatory in early 2018. The ATONA is a capacitive transimpedance amplifier, which differs from the traditional resistive transimpedance amplifier used on most Faraday detectors for mass spectrometry. Instead of a high-gain resistor, a capacitor is used to accumulate and measure charge. The advantages of this architecture are a very low noise floor, rapid response time, stable baselines, and very high dynamic range. We show baseline noise measurements and measurements of argon from air and cocktail gas standards to demonstrate the capabilities of these amplifiers. The ATONA exhibits a noise floor better than a traditional 1013 Ω amplifier in normal noble gas mass spectrometer usage, superior gain and baseline stability, and an unrivaled dynamic range that makes it practical to measure beams ranging in size from below 10−16 to above 10−9 A using a single amplifier.
Highlights
The design of analog ion collectors for mass spectrometry has changed strikingly little for 70 years
Between the 1950s and 1980s, as the field of isotope geochemistry shifted from home-brewed instruments to commercial ones, available noble gas mass spectrometers consolidated around a design based on the Reynolds mass spectrometer using a “Nier-type” ion source, a fixed accelerating voltage, a variable magnetic field, and a single pair of collectors consisting of an analog electron multiplier and a Faraday cup, intended to be used separately for signals of different sizes (e.g., Reynolds, 1956; Bayer et al, 1989; Renne et al, 1998; Burnard and Farley, 2000)
There is a trade-off in noble gas mass spectrometry because of the evolution of the signal with time, it is important to mention that the signal from the production version of the ATONA can be subsampled without sacrificing the gain of the longer sampling time
Summary
The design of analog ion collectors for mass spectrometry has changed strikingly little for 70 years. The settling time problem is less severe on multicollectors that do not need to peak hop but can still affect signal stability during the start of a measurement These problems have limited the use of such collectors for decades, but the cost–benefit calculation has shifted due to improving electronic stability and new techniques for dealing with the tau correction (Zhang et al, 2016), as well as a cultural shift in the priorities of noble gas geochemistry labs toward young, small samples and higher precision (e.g., Wijbrans et al, 2011; Jicha et al, 2012; Mark et al, 2017; Rose and Koppers, 2019). The advent of multicollection means that isotope ratios could be computed directly at each time point and themselves extrapolated to time zero, but so far noble gas geochemists have largely used multicollection as a means to ensure that the maximum amount of data can be collected simultaneously for each isotope
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