QUANTITATIVE LI ANALYSIS BY LASER-INDUCED BREAKDOWN SPECTROSCOPY (LIBS)

Li is a difficult element to analyze. Li-borate compounds are commonly used to flux samples for XRF, AA, or ICP-MS analysis. Also, Li is volatile during fusion and acid digestion sample preparation techniques. Li is currently analyzed by LA-ICP-MS, SIMS, and SHRIMP; we report here on success of quantitative analysis of Li by Laser Induced Breakdown Spectroscopy (LIBS). The advantages of LIBS analysis are low cost of purchase and operation, no sample preparation, and the ease and speed of analysis.

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.