Comparison of Cu+, Ag+, and Au+ Ions as Ionization Agents of Volatile Organic Compounds at Subatmospheric Pressure

TL;DR

This study compares how Cu+, Ag+, and Au+ ions ionize volatile organic compounds under subatmospheric pressure, revealing differences in reactivity and detection capabilities.

Contribution

The paper introduces a novel comparison of Au+ ions' reactivity in ionizing VOCs, highlighting their unique detection advantages and challenges.

Findings

Au+ ions showed the highest reactivity but caused more spectral complexity due to side reactions.

Au+ ions enabled detection of saturated hydrocarbons, which Cu+ and Ag+ could not detect.

Detection limits for VOCs ranged from 0.1 to 1.4 nmol/L using the subatmospheric ion source.

Abstract

Ionization
of volatile organic compounds (VOCs) by coinage metal
ions (Cu+, Ag+, and Au+) generated
by laser desorption and ionization (LDI) of a metal nanolayer in subatmospheric
conditions is explored. The study was performed in a commercial subatmospheric
dual MALDI/ESI ion source. Five compounds representing different VOC
classes were chosen for a detailed study of the metal ionization mechanism:
ethanol, acetone, acetic acid, xylene, and cyclohexane. In the gas
phase, ion molecular complexes of all three metal ions were formed,
typically with two ligand molecules. The successful detection of the
metal complexes with VOCs strongly depended on the applied voltages
across the ion source, minimizing the in-source fragmentation. The
employed orbital trap with ultrahigh resolving power and sub-parts-per-million
mass accuracy allowed unambiguous identification of the formed complexes
based on their molecular formulas. The detection limits of the studied
compounds in the gas were in the range 0.1–1.4 nmol/L. Compared
to Cu+ and Ag+ ions, Au+ ions exhibited
the highest reactivity, often complicating spectra by side products
of reactions. On the other hand, they also allowed detecting saturated
hydrocarbons, which did not produce any signals with Ag+ and Cu+.

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