Assessing Matrix and Nonmatrix, Single, and Multipoint Calibration of Trace Elements Using LA-ICP-MS on a Tropical...

Suites of trace elements are routinely used in speleothems as proxies to understand periods of past climate change. Laser ablation techniques are regularly implemented to acquire high resolution (50-μm) trace element concentrations in carbonate archives for paleoclimatology. There exists limited research investigating Laser Ablation-Inductively Coupled Plasma-Mass Spectrometry (LA-ICP-MS) protocols using speleothem samples. This study investigates the difference between using matrix (carbonate) and nonmatrix (silicate) matched reference materials and the utility of 1-point versus multiple point calibration curves.

Following an extensive review of published literature on speleothem LA-ICP-MS analyses, we conducted two laser ablation experimental runs 8 months apart on a 2.7-cm section of a natural speleothem using matrix and nonmatrix matched reference materials. We used a 193-nm wavelength Analyte G2 laser attached to a X-Series-2 ICP-MS, a silicate reference material, and three carbonate reference materials. Next, we calculated concentrations using a 1-point calibration curve, a 2-point calibration curve, and a 3-point calibration curve.

The analysis of matrix and nonmatrix matched reference materials demonstrates that the trends of trace elements/Ca are minimally impacted by the matrix material of the standard. We also show that 2- and 3-point calibration curves bracket the range of sample concentrations compared to a 1-point (silicate) calibration curve. The calculated cave-air temperatures using Mg/Ca concentrations fall within error of each other regardless of the calibration curve approach applied.

Our experiments provide a proof of concept on the conventional setup of standards during LA-ICP-MS speleothem analysis. We suggest the use of at minimum a 2-point (silicate plus carbonate or carbonates) calibration curve that crucially bracket the range of sample concentrations rather than relying on a 1-point silicate standard that does not bracket the sample concentration. Finally, our results have implications for both speleothem studies that use LA-ICP-MS analytical techniques and additional carbonate archives.