High-resolution continuum source graphite furnace molecular absorption spectrometry for the monitoring of Sr isotopes via SrF formation: a case study

Abstract

High-resolution continuum source graphite furnace molecular absorption spectrometry (HR CS GFMAS) can provide isotopic information under certain conditions, thus broadening its field of application. However, to date, only elements with two major stable isotopes have been monitored via this technique. In this work, the possibilities of HR CS GFMAS to determine isotope ratios of elements with more than two stable isotopes are evaluated for the first time. For this purpose, Sr was chosen as the analyte and SrF as the target species, so four different signals corresponding to four stable Sr isotopes (⁸⁸Sr, ⁸⁷Sr, ⁸⁶Sr and ⁸⁴Sr) should be distinguished. Nevertheless, due to the number of strontium isotopes, the shape of the peaks, and the resolution that the instrument exhibits in the spectral window, isotopic signals overlap, thus leading to potentially biased results. To circumvent this issue, a deconvolution protocol, consisting of measuring and correcting for the contribution of each isotope on the signals of the rest, was developed. These contributions were calculated as the signal ratio between the absorbance of the monoisotopic profile at the wavelengths where the maxima of other isotopes are expected and at its own maximum. Therefore, the interference can be simply subtracted from the net signal registered for the interfered isotope. The performance of this method was demonstrated for both naturally abundant and isotope-enriched Sr standards, paving the way for future applications in this field. Analysis of a real sample (tap water) spiked with a ⁸⁴Sr solution is also demonstrated.