Transient signal isotope analysis using multicollection of ion beams with Faraday cups equipped with 1012 Ω and 1011 Ω feedback resistors

Abstract

To improve the precision of isotope analyses of low ion intensities using the Faraday detection system, amplifiers equipped with 10¹² Ω resistors (hereafter 10¹² Ω amplifiers) have been developed. While the behavior of these amplifiers for steady signals has been well investigated, there is no ample evidence regarding the use of 10¹² Ω amplifiers for transient signal acquisition. In this work, we investigated the simultaneous use of amplifiers equipped with 10¹² Ω and 10¹¹ Ω resistors for transient signal acquisition. Using the equation describing the relationship between the input ion current and the output voltage in the amplifiers, we showed how the transient signal duration influences the accuracy of the isotope ratio measurements. In particular, lead transient signals were investigated using a Neptune Plus MC-ICPMS and ²⁰⁴Pb and ²⁰⁶Pb isotopes were measured using 10¹² Ω and 10¹¹ Ω amplifiers, respectively. The ²⁰⁴Pb/²⁰⁶Pb isotope ratio showed an important drift due to a large time lag between 10¹² Ω and 10¹¹ Ω amplifiers. The time lag was quantified (0.175(3) s) and the isotopic drift was corrected using a method of internal signal synchronization. The ²⁰⁴Pb/²⁰⁶Pb drift corrected data obtained from the 10¹²–10¹¹ Ω amplifier configuration were compared to the data obtained from 10¹¹–10¹¹ Ω amplifiers. Our results point out that for low transient signal intensities (<10⁻¹³ A), the use of 10¹²–10¹¹ Ω amplifiers is more beneficial in terms of isotope ratio uncertainty, repeatability and trueness, compared to the 10¹¹–10¹¹ Ω amplifier configuration.