Alloys contain at least one metal element and are ubiquitous materials available in virtually any type of modern products including the fillings in teeth, (1,2) the wheels on cars, (3,4) and the space satellites, (5,6) etc. The property of alloys varies dramatically along with their elemental compositions, (7) particularly for the alloys which are made up of multiple metal elements. (8) In material science and engineering, tuning the elemental composition may control the function of alloys, leading to new functional materials. (7,8) In the mechanical industry, alloy particles in the lubricating oil or exhaust gas are useful indictors of the engine conditions. (9,10) In the manufacturing industry, the elemental composition of an alloy is a useful quality index. (7,8) Thus, the rapid characterization of alloys to obtain the elemental composition is of sustainable interest in multiple disciplines, including life science, (11) material science, (12) mechanical industry, (13) and biological engineering. (14)

To date, alloys are normally dissolved using various chemical reagents to form analyte solutions prior to elemental analysis. The sample solutions are usually offline analyzed by either atomic emission spectrometry (AES), (15) atomic absorption spectrometry (AAS), (16) or inductively coupled plasma mass spectrometry (ICPMS). (17) The offline procedures render a significantly long time for sample manipulation and pretreatment, (18) resulting in a low analytical throughput. Laser ablation-inductively coupled plasma mass spectrometry (LA-ICPMS) or laser ablation electrospray ionization mass spectrometry has been attempted for online alloy analysis (19) but require expensive high-energy laser generator and hard-to-reach standards for quantitative analysis. No matter if the alloy is online or offline treated, ICPMS provides only elemental information due to the highly energetic ionization process occurring inside the ICP torch. (17) Ideally, soft ionization is preferable for mass spectrometry analysis of alloys, particularly in the cases where the organics on the alloy surface are of analytical interest. (20)

Herein, electrochemical ionization mass spectrometry (ECI-MS) was developed to directly characterize alloys at the molecular level without tedious sample pretreatment, for which sequential formation of metal ions of bulk alloys via electrolysis was a crucial step. As schematically shown in Figure 1, charged electrolytic solution (e.g., H2O/CH3CN, 1:1, v/v) was injected at 2.0 μL/min by a syringe pump to flow through a 50 μL fluid tight microelectrolytic cell (MEC), inside which the organic chemical species on the alloy surface took precedence over the rest of the material to be extracted into the solution. Once the metal was exposed to the solution, a potential (e.g., 1.0 V) was applied to the working electrode inside the MEC. Consequentially, the elemental components were successively converted into the corresponding metal ions by tuning the electrolysis potentials, ensuring the high selectivity and specificity for metal ion generation. (21) Note that the electrolysis potential was floated on the ESI voltage (±3.0 kV), such that the ESI high voltage brought no interference to the electrolysis. (22,23) Once the metal ions were formed, they reacted immediately with specific organic ligands added in excess into the electrolyte solvent to form metal–organic complexes, which were carried by the solvent flow toward the outlet of the MEC to produce ambient ions, with the assistance of the pneumatic system and the electric field, for mass analysis using a LTQ-Orbitrap-MS instrument...

Construction of ECI-MS Device
The analysis device consisted of a MEC (about 50 μL), a nebulization system, and a mass spectrometer. The MEC employed two platinum wires (diameter = 500 μm) as the working electrode and auxiliary electrode and Ag/AgCl as the reference electrode, respectively. In order to ensure the reproducibility of experimental behavior and to avoid the need for the cleaning of the electrode surface, a new disposable MEC unit was assembled and used in every experiment. Metal was connected to the anode or acted as a bipolar electrode placed between two platinum wires. (24) Electrolyte was injected into the MEC by a pump. During the detection of metal materials, a potential (E) floated on +3 kV high voltage was applied to the working electrode by CHI 660D electrochemical workstation. When potential E is high enough to meet the requirement of metal electrolysis, metal would transform into metallic ions. Then, the electrolyte contained metal ions was sprayed into the MS by pneumatic nebulization for analysis. The distance between the spray emitter and the entrance of the MS is 1.0 cm.

Figure 1. Schematic illustration of the ECI-MS method for analysis of alloys