June 2018 – High precision isotope ratio measurements on MC-ICP-MS using new amplifier technology

Dynamic time correction for high precision isotope ratio measurements on Neptune Plus MC-ICP-MS with 1013 Ohm amplifier technology

With multicollector mass spectrometers, low intensity ion beams are typically collected on ion counters rather than with Faraday cup collectors, as the precision of Faraday cup measurements would be limited by the electrical noise of the standard 1011 Ω amplifier.  A recent development in the evolution of multicollector inductively coupled plasma mass spectrometry (MC-ICP-MS) has been the introduction of Faraday cup amplifiers incorporating 1013 Ω resistors. Compared to the standard 1011 Ω amplifier, the signal-to-noise ratio of the 1013 Ω amplifier is improved by 4 to 5 fold; which should correspond to a similar improvement in isotope ratio precision for analyses with small ion beams.  Consequently many of the applications that previously required ion counters can now be measured with 1013 Ω amplifiers. Large variations in response time between different types of amplifiers reduces the effectiveness of high gain 1013 Ω amplifiers for many applications.

Grant Craig and his colleagues have demonstrated the utility of a dynamic tau correction to perform transient isotope ratio analysis by LA-MC-ICP-MS using 1013 Ω amplifiers. Once the tau constant has been determined for each amplifier, the correction can be applied over a series of runs, as it is not depended on factors such as integration time or signal intensity. Up to 4x improvement in precision were achieved for Pb isotope ratio analysis. With this and related dynamic time corrections, the 1013 Ω amplifier technology can offer even greater benefits to multicollector mass spectrometry.

Read more in Technical Note TN30396 in the Knowledge Base.