Near‐field laser ablation inductively coupled plasma mass spectrometry: a novel elemental analytical technique at the nanometer scale

Abstract

An analytical technique utilizing a near-field effect (to enhance the incident light energy on the thin tip of an Ag needle) in a laser ablation inductively coupled plasma mass spectrometry (NF-LA-ICP-MS) procedure was developed. To produce the thin needles with a tip diameter in the hundreds of nm range a robust needle etching procedure was established. The 'sample-to-tip' distance was controlled via the measurement of a tunnel current between the needle and sample surface. The NF-LA-ICP-MS technique thus developed was applied for the analysis of copper isotopic standard reference material NIST SRM 976 and tungsten-molybdenum alloy NIST SRM 480 in the nm resolution range. The observed craters ranged from 200 nm to about 2 microm in diameter and were dependent on the needle used as well as on the 'sample-to-tip' distance. The mass spectrometric measurements of (63)Cu(+) ion intensity on NIST SRM 976 showed that using near-field enhancement in laser ablation allowed a roughly 6-fold increase in the ion intensity of the analyte when the needle was about 100 nm (and below) from the surface, in contrast to when it was far away (e.g. 10 microm) from the sample. The relative standard deviation (RSD) of the (65)Cu(+)/(63)Cu(+) isotopic ratio measurements by NF-LA-ICP-MS was 3.9% (n = 9). The detection efficiencies obtained for the compared LA-ICP-MS and NF-LA-ICP-MS methods were found to be 4.6 x 10(-3) counts per second (cps)/ablated atom and 2.7 x 10(-5) cps/ablated atom, respectively.