A method for increasing the precision of isotope ratio analysis on a Quadrupole ICP-MS based on measurements of lead and strontium

Abstract

In the present article a measurement and data evaluation method for high precision isotope ratio determination with the Quadrupole ICP-MS is presented and applied on Pb and Sr samples. The developed algorithm requires a modification of the measurement procedure. Instead of measuring one well defined concentration, the measurement of a concentration series of the sample has to be performed. The resulting signal intensities of each isotope are plotted against each other in an x,y-diagram. The different concentrations of the samples correlate by a linear model. The slope of the linear line is per definition the isotope ratio. This type of determination is called the Concentration-Gradient-Method, which is abbreviated as CGM. One particularity of this kind of data evaluation is a significant increase of precision and accuracy of the isotope ratio determination performed with Quadrupole ICP-MS. Moreover, a significant intercept is expressed by linear regression. It could be demonstrated that this intercept is a consequence of constant interference overlapping the isotope signal. Constant values that may cause such interference are signal noise and polyatomic interference of the measuring system as well as isobaric interference of the dilution medium. Last is proved in the present work. The potential of the presented method is shown by application examples of liquid and liquid digested Pb and Sr samples. In comparison the Concentration-Gradient-Method (CGM) increases the isotope ratio determination to a factor of approximately 3 and an increase of the accuracy of 10% with respect to the ordinary data acquisition and evaluation procedure.