Laser-ablation vs. bulk tissue ICP-MS for conifer tissue elemental analysis

Laser ablation inductively coupled plasma mass spectrometry (LA ICP-MS) has emerged as a robust tool for directly measuring trace elements in solid, intact samples. Laser ablation requires minimal sample preparation, whereas more conventional bulk sample analysis entails preliminary size reduction and acid digestion, and hence risks sample contamination and volatilization losses. LA ICP-MS may allow for rapid determination of elemental constitution in plants at low detection limits; however, application of LA ICP-MS on plant tissues is challenged by sample heterogeneity, as well as the lack of recognized standards for calibration and criteria for sample preparation. We analyzed needle samples from an adult jack pine (Pinus banksiana L.) tree through LA ICP-MS using a widely available NIST SRM 610 quartz calibration standard, with ⁴³Ca as an internal standard element measured by electron probe microanalysis. LA ICP-MS analyses were run on intact needle samples and on pulverized, homogenized and pelletized samples (n = 21), and compared to needles dried, ground, and analyzed in triplicate with aqua regia acid digestion and conventional ICP-MS. Overall, the LA ICP-MS measures on intact pine samples accurately predicted the chemical composition of pine needle tissue obtained by the bulk sample acid-digestion and ICP-MS method for most elements, while preliminary pulverizing and pelletizing did not result in greater accuracy or reduced bias. LA ICP-MS analysis of many plant nutrients (³¹P, ²⁴Mg, ⁶⁶Zn) from both intact and pelletized tree needle samples matched the values obtained from acid-digestion ICP-MS most closely, while measures of less-stable metals (such as ¹⁴⁰Ce, ¹³⁹La, and ⁵⁷Fe) deviated more from acid-digested samples. Values of ³⁹K estimates varied between methods, as storage of ³⁹K is preferentially located in needle mesophyll layers beyond laser depths. Large deviations were found for volatile elements, with significantly lower ²⁰⁸Pb concentrations measured by LA ICP-MS than acid-digested samples. We conclude that LA ICP-MS of intact tissues is a viable tool for accurate, non-destructive analysis for most elements and is particularly suitable for analysis of small samples and for volatile elements not amenable to conventional methods.