U-Pb dating of zircons by laser ablation inductively coupled plasma mass
spectrometry (LA-ICPMS) is a widely used analytical technique in Earth
Sciences. For U-Pb ages below 1 billion years (1 Ga), Pb-206/U-238 dates
are usually used, showing the least bias by external parameters such as
the presence of initial lead and its isotopic composition in the
analysed mineral. Precision and accuracy of the Pb/U ratio are thus of
highest importance in LA-ICPMS geochronology. We consider the evaluation
of the statistical distribution of the sweep intensities based on
goodness-of-fit tests in order to find a model probability distribution
fitting the data to apply an appropriate formulation for the standard
deviation. We then discuss three main methods to calculate the Pb/U
intensity ratio and its uncertainty in the LA-ICPMS: (1)
ratio-of-the-mean intensities method, (2) mean-of-the-intensity-ratios
method and (3) intercept method. These methods apply different functions
to the same raw intensity vs. time data to calculate the mean Pb/U
intensity ratio. Thus, the calculated intensity ratio and its
uncertainty depend on the method applied. We demonstrate that the
accuracy and, conditionally, the precision of the ratio-of-the-mean
intensities method are invariant to the intensity fluctuations and
averaging related to the dwell time selection and off-line data
transformation (averaging of several sweeps); we present a statistical
approach how to calculate the uncertainty of this method for transient
signals. We also show that the accuracy of methods (2) and (3) is
influenced by the intensity fluctuations and averaging, and the extent
of this influence can amount to tens of percentage points; we show that
the uncertainty of these methods also depends on how the signal is
averaged. Each of the above methods imposes requirements to the
instrumentation. The ratio-of-the-mean intensities method is
sufficiently accurate provided the laser induced fractionation between
the beginning and the end of the signal is kept low and linear. We show,
based on a comprehensive series of analyses with different ablation pit
sizes, energy densities and repetition rates for a 193 nm ns-ablation
system that such a fractionation behaviour requires using a low ablation
speed (low energy density and low repetition rate). Overall, we conclude
that the ratio-of-the-mean intensities method combined with low sampling
rates is the most mathematically accurate among the existing data
treatment methods for U-Pb zircon dating by sensitive sector field
ICPMS.