Errors of isotope ratios acquired by double interpolation

Measurement of isotope ratios via double interpolation of cyclic peak jumping produces error correlations between time-adjacent ratios which, if ignored, result in underestimated errors of their means. Equations that incorporate the error correlations are derived, and an example given for a typical U-Pb/zircon analysis via ion microprobe showing that isotope-ratio uncertainties for a single spot are usually underestimated by factors of 1.2-1.3. (C) 2009 Elsevier B.V. All rights reserved.