Evaporation dynamics of sessile droplets in the ablation cell for LA-ICP-MS analysis of liquids

Abstract

Background

Inductively coupled plasma mass spectrometry (ICP-MS) is a powerful method for elemental analysis. However, it faces challenges when analysing organic-rich samples due to plasma instability. Laser ablation coupled with ICP-MS (LA-ICP-MS) offers a suitable alternative by introducing minimal sample amounts into the plasma, maintaining its stability. In the case of liquids, rapid evaporation of liquid droplets limits the applicability of LA-ICP-MS, as it impacts analyte concentration. Understanding droplet evaporation dynamics within the ablation cell, where helium accelerates droplet evaporation, is essential to improve reliability of the analysis. This study explores the evaporation behaviour of organic solvent droplets in LA-ICP-MS. In addition, it aims to enhance liquid sample analysis by modelling observed evaporation processes.

Results

Experiments were conducted with organic solvent droplets evaporating on various sample-holder surfaces, focusing on evaporation within helium and air environments. Polytetrafluoroethylene (PTFE) was identified as the optimal surface material for sustaining high contact angles of sessile droplets, thus prolonging droplet lifetime. A mathematical model was developed to describe evaporation, incorporating contact angle, droplet geometry, and mass transfer dynamics. The model demonstrates that most droplets initially evaporate in a constant contact radius mode, then transition to a constant contact angle mode. The experimental data align closely with the model predictions, affirming the impact of initial contact angle and droplet volume on evaporation behaviour. Additionally, we established correlations to predict droplet lifetime based on saturated vapour pressure and initial droplet height, contributing a practical reference for management of organic solvents in LA-ICP-MS applications.

Significance and novelty

This research provides a straightforward approach to understanding and controlling evaporation dynamics of liquid samples in LA-ICP-MS, which is essential for reliable elemental analysis of organic-rich liquid samples. By modelling sessile droplet behaviour and validating it with experimental data, this study lays the groundwork for a wide range of applications, such as the analysis of metals in oils and other low-volatility organic solvents or analysis of microliter-scale liquid samples.