Noise identification and sampling frequency determination for precise Pb isotopic measurements by quadrupole-based Inductively Coupled Plasma Mass Spectrometry

Analytical precision of the isotope ratios measured by quadrupole-based ICP -MS is drastically controlled by the low-frequency noises which originate f rom nebulisation and vaporisation processes, and from sample introduction s ystems. The undesirable influence of these latter can be however reduced by choosing efficiently the operating parameters. In the present study, the c hoice of the stabilisation time necessary in peak jump mode, and the one of the number of sweeps are discussed in the light of noise power spectra obt ained with various speeds of the peristaltic pump used as sample introducti on system. Obviously, the settings proposed are probably efficient only on our own ICP-MS, but they can be determined without any difficulty on any ot her equipment, only by following the methodology detailed here. With such g uidelines, experimental within-run RSD % were observed following closely th ose predicted by the counting statistics (similar to 110 %). The isotopic m easurements of natural samples (overbank sediments, airborne particulate ma tter, lichens and rainwater) exhibited fair accuracy and good reproducibili ty, making highly convenient the use of the ICP-MS, at least in an environm ental purpose.