Laser Ablation Molecular Isotopic Spectrometry: Strontium and its isotopes Xianglei Mao a , Alexander A. Bol'shakov b , Inhee Choi a , Christopher P. McKay c , Dale L. Perry a , Osman Sorkhabi a , Richard E. Russo a, b, ⁎ a Lawrence Berkeley National Laboratory, University of California, Berkeley, CA 94720, USA b Applied Spectra, Inc., 46661 Fremont Boulevard, Fremont, CA 94538, USA c NASA-Ames Research Center, Moffett Field, CA 94035, USA abstract article info Article history: Received 31 July 2011 Accepted 3 December 2011 Available online 13 December 2011 Keywords: Optical isotopic measurement Laser ablation plasma Molecular emission spectrum LIBS analysis LAMIS of strontium The experimental details are reported of Laser Ablation Molecular Isotopic Spectrometry (LAMIS) and its ap- plication for performing optical isotopic analysis of solid strontium-containing samples in ambient atmo- spheric air at normal pressure. The LAMIS detection method is described for strontium isotopes from samples of various chemical and isotopic compositions. The results demonstrate spectrally resolved measure- ments of the three individual 86 Sr, 87 Sr, and 88 Sr isotopes that are quantified using multivariate calibration of spectra. The observed isotopic shifts are consistent with those calculated theoretically. The measured spectra of diatomic oxide and halides of strontium generated in laser ablation plasmas demonstrate the isotopic res- olution and capability of LAMIS. In particular, emission spectra of SrO and SrF molecular radicals provided clean and well resolved spectral signatures for the naturally occurring strontium isotopes. A possibility is dis- cussed of using LAMIS of strontium isotopes for radiogenic age determination. © 2011 Elsevier B.V. All rights reserved. 1. Introduction Laser Ablation Molecular Isotopic Spectrometry (LAMIS) is a new approach to rapid optical isotopic analysis of condensed samples in ambient atmospheric air [1,2]. The technique exploits laser ablation of the sample and measurement of optical spectra from molecular species that are produced during plasma expansion into the air. A well established and closely related technique known as Laser In- duced Breakdown Spectroscopy (LIBS) uses optical emission spectra of atoms and atomic ions to analyze solid, liquid or gaseous samples, including aerosols and suspended particles [3–6]. LAMIS expands the capabilities of LIBS by adding isotope measurements. The recognized advantages of LIBS include elemental analysis and material classification at atmospheric pressure in real time; no sam- ple dissolution or other sample preparation, and no acidic consum- ables that would have to be disposed of after the analysis. As only a microgram-sized amount of ablated material is required for analysis, the sample can be preserved for archiving. Both laboratory and field environments are appropriate to perform analyses, and in the latter case open-path standoff measurements can be carried out [7,8]. LIBS is generally not utilized for isotopic detection because of two factors: very small isotope splitting in spectra of the majority of atomic spe- cies and significant broadening of these spectral lines in laser ablation plasmas at atmospheric pressure. For example, a characteristic isotope shift in atomic emission spectra of strontium isotopes 86 Sr and 88 Sr is only 0.25 pm (165 MHz = 0.005 cm -1 ) [9,10] but the spectral lines of metals in LIBS plasma are broadened up to several nanometers at early stages [11] and remain broadened to the order of 100 pm at typical acquisition delays of ~1 μs and temporal gate widths of several microseconds [12,13]. In earlier studies of LIBS at atmospheric pressure, the isotopic spectra of atoms or atomic ions were resolved or at least partially resolved only for three elements (H, Li, and U) [14–16]. Hydrogen and lithium are very light atoms in which nuclei move about the centroid common with their electron shells, thus resulting in relatively large changes in atomic ener- gy dependant on the number of protons and neutrons. In contrast, the uranium nucleus is so large that its volumetric charge causes deviations from the intra-atomic Coulomb field distribution, which varies depend- ing on the number of protons and neutrons included in the nucleus. There are other subtle effects on the isotopic shift, such as the collective multi-electron interaction with the nucleus recoil, momentum coupling perturbations and an influence of the nuclear binding energy. However in general, very light and very heavy elements represent the two oppo- site ends of relatively large splitting in atomic isotope spectra (15 pm between 6 Li and 7 Li atomic lines at 670.8 nm; and 25 pm between 235 U and 238 U ionic lines at 424.4 nm [16]). For intermediate elements, interactions of electron shells with their nuclei are weak, yielding gener- ally very small isotopic differences in optical spectra [17]. As a result, the isotopic shifts for the majority of elements are no more than a few pic- ometers [18]. Broadening of spectral lines in laser ablation plasmas decreases with the reduction of pressure, due to the decrease in collision Spectrochimica Acta Part B 66 (2011) 767–775 ⁎ Corresponding author at: Lawrence Berkeley National Laboratory, University of California, Berkeley, CA 94720, USA. E-mail address: rerusso@lbl.gov (R.E. Russo). 0584-8547/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.sab.2011.12.002 Contents lists available at SciVerse ScienceDirect Spectrochimica Acta Part B journal homepage: www.elsevier.com/locate/sab