QUANTITATIVE LI ANALYSIS BY LASER-INDUCED BREAKDOWN SPECTROSCOPY (LIBS)
LIBS is a laser ablation technique that collects and diffracts photons emitted from electron transitions that occur as the laser plasma cools. Fabre et al. (2002; Geochim. Cosmochim, 66:1401-1407) were the first to quantitatively measure Li concentrations in minerals with LIBS. They made fused glass standards with Ca-, Na-, and K-carbonates, Li2O, and SiO2 and constructed a univariate calibration using the 670.71 nm resonance Li line. They measured Li2O in spodumene and petalite with accuracy in the range 1-40% at 21% precision.
The goals of this study are to replicate quantitative analysis of Li by LIBS by developing a method using materials found in many geochemistry labs. Pressed pellet standards from 0% to 15% Li2O at 1% increments were made from a low-Li trachyte rock reference material with lithium tetraborate as a source of Li. One hundred LIBS spectra were acquired from each pellet and averaged into a single spectrum to minimize the effects of shot-to-shot variability and pellet heterogeneity. The multivariate technique Partial Least Squares Regression created a calibration curve with r2 = 997. Pellets with 0.5, 3.5, 6.5, and 9.5% Li2O were analyzed to validate the calibration curve, with accuracy of 30%, 9%, 3%, and -1% and 1σ precision of 48%, 7%, 3%, and 4%, respectively .
One spodumene and two lepidolite samples were also analyzed. Li2O in lepidolite 1 was measured at 5.25%, compared to 7.70 for ideal lepidolite and 6.56% by ICP-MS. Similarly, lepidolite 2 contains 7.40% Li2O (LIBS), compared to 7.70% (ideal) and 6.78% (ICP-MS). Li2O in the spodumene was measured at 8.03%, compared to 8.03% (ideal) and 8.09% (ICP-MS). Note that ICP-MS samples were prepared by acid digestion, and solid residues had detectable Li; thus, the ICP-MS values are minimum values. These results demonstrate that quantitative Li LIBS analysis is possible.