Transient signal isotope analysis using multicollection of ion beams with Faraday cups equipped with 10 12 Ω and 10 11 Ω feedback resistors
Résumé
To improve the precision of isotope analyses of low ion intensities using the Faraday detection system, amplifiers equipped with 10(12) Omega resistors (hereafter 10(12) Omega 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 10(12) Omega amplifiers for transient signal acquisition. In this work, we investigated the simultaneous use of amplifiers equipped with 10(12) Omega and 10(11) Omega 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 Pb-204 and Pb-206 isotopes were measured using 10(12) Omega and 10(11) Omega amplifiers, respectively. The Pb-204/Pb-206 isotope ratio showed an important drift due to a large time lag between 10(12) Omega and 10(11) Omega amplifiers. The time lag was quantified (0.175(3) s) and the isotopic drift was corrected using a method of internal signal synchronization. The Pb-204/Pb-206 drift corrected data obtained from the 10(12)-10(11) Omega amplifier configuration were compared to the data obtained from 10(11)-10(11) Omega amplifiers. Our results point out that for low transient signal intensities (<10(-13) A), the use of 10(12)-10(11) Omega amplifiers is more beneficial in terms of isotope ratio uncertainty, repeatability and trueness, compared to the 10(11)-10(11) Omega amplifier configuration.