After sensitivity and carrier gas flow tuning, the ICP was switched to MS/MS mode. To resolve the isobaric interference of 87 Rb and 87 Sr, Sr isotopes were measured as SrO + molecular ions in mass shift mode using an N 2 O-filled reaction cell. Throughout this paper we use the notation "~ y X" with y being the atomic mass of element X to refer to a +16 oxygen mass shift of that particular isotope (e.g. ~ 88 Sr, refers to 88 Sr 16 O). Flow rates of N 2 O were tuned to yield maximum counts on m/z = 104 (~ 88 Sr) and were between 15-19 %. Typically, the ~ 88 Sr/ ( 88 Sr + ~ 88 Sr) ratio was 0.97. Counts on m/z = 101 (~ 85 Rb) where checked and suggested negligible formation of RbO + (typically 1 cps). After a second sensitivity tune in MS/MS mode the single pulse response (SPR), i.e., the combined wash-in and wash-out time, was determined by (1) ablating NIST 610 at 1 Hz and dosage 1 in line scan mode and recording m/z = 85 (Rb) and m/z = 104 (~ 88 Sr) at a dwell time of 5 ms and (2) averaging the full width at 10 % of the maximum of 1200 single pulses. Usually, 85 Rb and ~88 Sr yielded similar SPR (Fig. , Table ). The last tuning step was tuning of pulse-analog (P/A) factors to correct for the non-linear response across the pulse to analog switch of the detector, which appears between 1-2 million counts per second (cps) or, more precisely, at count rates of 10 million cps multipled by the respective P/A factor (J. Hansmann, Agilent, personal communication). P/A tuning is done by switching the ICP from time-resolved analysis (TRA) to spectrum mode, opt-in "independent P/A factor" in the mass hunter configuration and "P/A factor adjustment" and then running several line scans with different spot diameter as calibration standards on NIST610. Resulting P/A factors were between ~ 0.15-0.17, varied between sessions and also displayed a certain relative difference between Sr isotopes (typically 1-2 % difference). It should be noted that for all mappings all ~ y Sr nuclides are acquired in pulse mode, whereas 85 Rb is acquired in pulse mode in NIST610, Phalaborwa biotite, GER2B, WIL1A and GÄD08 and in analog mode in SagaB biotite and Harvard 98973 muscovite.