Determination of lead isotope ratios in crude oils with Q-ICP/MS

Abstract

A method was developed to directly measure lead isotope ratios in crude oils by ICP-MS. Sample introduction strategy was based on the mixing of a USN produced dry aerosol together with an organic aerosol. Two introduction systems, a USN and a Scott-type spray chamber, were fitted to a specifically designed dual inlet torch. This specific configuration allowed simultaneous introduction of organic solutions and aqueous solutions in the plasma, and permitted mass bias correction of lead isotope ratios using the certified Pb isotope standard NIST SRM 981. Crude oils diluted to total Pb concentrations ranging from 0.6 to 10 ng g⁻¹ were analysed for their Pb isotope ratios. A careful optimisation of instrumental settings was performed for the measurement of lead isotope ratios. In order to determine the best operating conditions, isotope ratio precision, different dwell times, number of sweeps/replicate, and settle times were investigated while keeping the same total acquisition time/run of 43.2 seconds distributed between ²⁰²Hg, ²⁰⁴Pb, ²⁰⁶Pb, ²⁰⁷Pb, and ²⁰⁸Pb isotopes. The use of 1.5 ms as a settle time and 1000 sweeps for 43.2 s of total acquisition time/run and 7.5 s of stabilisation time/run appeared to be a good compromise between high precision and fair time management. Routine measurements of 10 replicate (i.e. run) organic samples containing 10 ng g⁻¹ of lead provided precision better than 0.2% for ²⁰⁸Pb/²⁰⁷Pb, ²⁰⁸Pb/²⁰⁶Pb, ²⁰⁶Pb/²⁰⁷Pb ratios, and better than 0.5% for ²⁰⁶Pb/²⁰⁴Pb, ²⁰⁷Pb/²⁰⁴Pb, and ²⁰⁸Pb/²⁰⁴Pb ratios. The isotopic measurements of crude oils of specific geochemical interest exhibited good accuracy and reproducibility, making highly convenient the use of this technique, at least for petroleum geochemistry. This study brings new isotopic information for the reconstruction of petroleum system history.