Reactive Paper Spray Mass Spectrometry Enables Speciation of Trace Levels of Mercuric Halides

Gaseous oxidized mercury (GOM) plays a key role in atmospheric mercury cycling, yet its specific molecular forms remain known mostly from theoretical calculations. Existing analytical methods relying on preconcentration and thermal desorption erase the original chemical speciation of this ultratrace pollutant, providing only indirect evidence. Here, we report a nonthermal, reagent-directed ionization strategy based on reactive paper spray mass spectrometry (RPS-MS), allowing us to recover the chemical identity of preconcentrated mercuric halides. The ionization is initiated by the addition of a halide reagent ion and proceeds through reversible halide addition and loss steps that drive ligand-exchange reactions characteristic of mercuric halides in solution and on surfaces. The mechanism is elucidated through kinetic modeling and experimentally validated by observing ionic products of halide addition and halide loss. By comparing iodide and chloride as ionizing reagents, we show that a higher degree of ionization can be achieved with iodide, using a significantly lower concentration and at the expense of only moderate overexchange. This reagent-controlled chemistry offers a generalizable framework that can be extended to atmospheric GOMs and other ultratrace oxidized metal systems, as demonstrated by speciating an ambient air sample and a CdCl₂ standard.