Abstract
We have optimized techniques to measure uranium and thorium isotopic compositions using secondary electron multiplier (SEM) and peak-jumping protocols on a multi-collector inductively coupled plasma mass spectrometer (MC-ICP-MS, Thermo Scientific Neptune Plus). This instrument is equipped with an advanced interface pump, X- skimmer cone, Jet sample cone, and desolvating nebulizer system (CETAC Aridus II) that enables overall ioni- zation and transmission efficiency up to 3% for both U and Th. Approximately 50 fg 234U or 230Th can routinely yield a precision of 1–2‰ (2σ) for abundance and isotopic measurements. Non-linear behavior was not observed in the MasCom SEM with a retarding potential quadrupole in our MC-ICP-MS system when the beam intensity was limited to < ∼1="" ×="" 106="" cps.="" we="" performed="" a="" series="" of="" tests="" to="" examine="" the="" influence="" of="" tuning="" parameters,="" peak="" shift,="" and="" operating="" voltage="" of="" the="" sem="" on="" isotopic="" measurements.="" results="" for="" u/th="" standards="" are="" consistent="" with="" published="" values="" and="" demonstrate="" the="" reliability="" of="" our="" protocols.="" lastly,="" the="" precision="" and="" accuracy="" of="" 230th="" ages="" based="" on="" u="" and="" th="" isotope="" ratios="" is="" presented="" for="" a="" variety="" of="" carbonates,="" including="" both="" modern="" and="" secular-equilibrium="" samples.="" for="" corals="" as="" young="" as="" ∼10-yr="" old,="" we="" have="" significantly="" reduced="" the="" procedural="" chemistry="" blanks="" so="" that="" only="" 40="" mg="" is="" required="" to="" yield="" an="" analytical="" precision="" of="" ±="" 1.4="" yr="" (2σ).="" all="" 230th="" ages="" from="" modern="" and="" fossil="" corals="" and="" speleothems="" are="" in="" agreement="" with="" band="" counting="">