Rationale, design and initial performance of a dual-wavelength (157 & 193 nm) cryo-LA-ICP-MS/MS system

Abstract

Owing to their wide applicability and relative ease of use, 193 nm ArF excimer lasers are commonly-used laser sources for laser ablation inductively-coupled plasma mass spectrometry (LA-ICP-MS). However, some materials, like quartz, sulphates, potassium salts, fused silica or water ice, often show poor ablation characteristics at 193 nm due to low absorption at deep-UV (DUV) wavelengths. Only very few LA-ICP-MS systems have utilized 157 nm F₂ excimer lasers, likely due to their low laser energy output in combination with the challenges that the transmission of vacuum-UV (VUV) radiation poses. Nevertheless, by using a 157 nm laser, some of the shortcomings of 193 nm LA can be overcome, because many of the “difficult to ablate” materials are opaque at 157 nm and the ∼20% higher photon energies at 157 nm. Here we describe a custom-built dual-wavelength (157 nm & 193 nm) cryo-LA-ICP-MS/MS system, built around the RESOlution-SE LA system with an S155 two-volume ablation cell, to which a separate 157 nm beam path was added. The system utilizes two distinct laser sources and beam paths for the two wavelengths, each optimized for the specific requirements and use-cases, and facilitates switching between the wavelengths within less than half a day. Furthermore, the system can be equipped with a newly-designed large cryo-sample holder for the S155 LA cell to analyze natural ice samples. Alongside the characterization of the 157 nm beam path, yielding on-sample fluences of up to 8 J cm⁻², we present comparative results of ablation characteristics for a range of materials at the two wavelengths, including threshold fluences of ablation and effective absorption depths. Our results show that ablation at 157 nm happens at low fluences (0.3–0.5 J cm⁻²) comparable with 193 nm for soda-lime glasses and calcites. For materials like calcium sulphates, quartz and fused-silica glasses, we demonstrate controlled, photochemical ablation at low fluences (0.3–1.1 J cm⁻²). To illustrate the applicability of 157 nm laser ablation for ICP-MS measurements, a trace element map of a quartz sample with variable composition is shown. Additionally, initial, qualitative results of the ablation of water ice are shown for both 193 nm and 157 nm, which demonstrate controlled ablation behaviour even in low impurity ice at 157 nm. Overall, our results indicate that LA-ICP-MS at 157 nm is a viable analytical method for sample matrices that are near-transparent at 193 nm and thus often difficult to ablate.