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Chemical profiling of bulk alloys using micro electrochemical probe mass spectrometry Jiaquan Xu, Dacai Zhong, Konstantin Chingin, Lili Song, and Huanwen Chen Anal. Chem., Just Accepted Manuscript • Publication Date (Web): 06 May 2019 Downloaded from http://pubs.acs.org on May 6, 2019

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Analytical Chemistry

Chemical profiling of bulk alloys using micro electrochemical probe mass spectrometry Jiaquan Xu, Dacai Zhong, Konstantin Chingin, Lili Song, Huanwen Chen* Jiangxi Key Laboratory for Mass Spectrometry and Instrumentation, East China University of Technology, Nanchang, P. R. China. *Corresponding Author: [email protected] ABSTRACT: Micro electrochemical probe mass spectrometry (μECP-MS) is demonstrated as a method for the direct profiling of chemical composition of bulk alloy samples without tedious sample pretreatment. The spatial distribution of Zn and Cu components of a Cu/Zn alloy sample was successively identified by scanning the electrolysis potential from -0.6 to 0.4 V. The lateral resolution of alloy chemical profiling was ≤ 10 μm, and the depth resolution was ≤ 0.5 nm. Besides metal components, the method also allows the simultaneous detection of organic molecules on the sample surface. The limit-of-detection for rhodamine B, Zn and Cu depositions was 4.47 ag, 9.58 ag and 24.25 ag per μm2, respectively. The method is particularly useful for high-throughput (< 2 min per single run) quality monitoring of industrial parts and conductive materials of irregular geometries, such as alloy, microchips, weld spots, etc.

The worldwide consumption and applications of metal materials are steadily increasing.1-3 The alloy properties vary dramatically with regard to their elemental composition and spatial distribution.4-5 Therefore, the rapid characterization of alloy materials and industrial parts with high chemical sensitivity, minimal destruction, and spatial resolution is increasingly demanded in many fields of modern industry and research. Currently, analytical techniques including energy spectrometry, spectrometry and mass spectrometry are available for the chemical characterization of metallic materials. For example, electron probe microanalysis (EPMA),6-7 X-ray fluorescence spectroscopy (XRFS),8-9 proton induced X-ray emission spectroscopy (PIXRES),10-11 secondary ion mass spectroscopy (SIMS),12-13 and laser ablation-inductively coupled plasma-mass spectrometry (LAICP-MS)14-15 have the advantages of high spatial resolution, high sensitivity, and high speed of analysis. However, most of these methods provide only elemental information or require operation in vacuum environment, which greatly limits the throughput of analysis. Ideally, soft ionization is preferable for mass spectrometry analysis of alloys, particularly in the cases where the sample is of irregular geometries and the organic deposited on the alloy surface need to be molecularly characterized.16 Herein, a novel method combining micro electrochemical probe (μECP) with nano-electrospray ionization mass spectrometry (nano-ESI MS) is proposed for the simultaneous characterization of metallic and organic components in microarea of alloy surface (Figure 1). Localized electrolysis and extraction allows high spatial resolution of analysis for the chemical components of an alloy, including the organic and metal species. The formed ions are directly detected by online mass spectrometry analysis. The current method provides low LODs of 4.47 ag, 9.58 ag and 24.25 ag per μm2 for RhB, Zn and Cu, high lateral and vertical resolutions for a Cu/Zn alloy covered by a thin layer of rhodamine B (RhB). The method has been also demonstrated for the quality monitoring of alloys and industrial products, showing potential for the chemical analysis of micro-areas.

Figure 1 Schematic illustration of the μECP mass spectrometry for micro-area analysis of metal materials. a) Electrolyte loading by a micro injector; b) Micro-area chemical sampling by electrolysis; c) Chemical detection by nano-ESI MS.

EXPERIMENTAL SECTION Equipment, Materials, and Reagents: Chemical characterization of a micro-area on the surface of metal material samples were carried out using a commercial mass spectrometer (Orbitrap Fusion MS, Thermo Fisher Scientific) equipped with a home-made μECP device (Figure 1). The μECP device consisted of two parts, including micro-area sampling probe and nano-ESI MS. A μECP, which was prepared by inserting a platinum wire of 100 μm diameter into a micropipette with a sharp tip of 5 μm. The diameter of the micropipette is a crucial parameter for the sampling resolution. A micromanipulator (eppendorf transferMan 4r) and microscope (AmScope) were used for precise control of μECP position. After micro-area sampling, the μECP was positioned in front of the MS (Orbitrap Fusion MS, Thermo Fisher) inlet for nano-ESI MS analysis. Metal materials Cu, Fe, Zn, Al and brass (purity > 99.9%) were purchased from JianYi Company (Wenzhou, China). Platinum wire of 100 μm diameter was purchased from Junlilai company (Jiangsu, China). Quartz capillary (B100-7510, 0.50 mm i.d., 1.00 mm o.d., Sutter Instrument Company, American) was used to prepare micropipette with defined tip size by laser puller (P2000, Sutter Instrument Company, American). Ethylenediaminetetraacetic acid (EDTA), phenanthroline (Phen), rhodamine B (RhB), FeCl2 were purchased from Tianjin Guang Fu Technology development Co., Ltd. Cu(NO3)2, FeCl3, Zn(NO3)2, Al(NO3)3 were purchased from Sinopharm Chemical Reagent Co., Ltd.

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extraction of organic species thereon. Before the micro sampling, a nanoliter electrolyte was sucked into the capillary through the tip of μECP. The volume of the electrolyte was about 2.5 nL (Figure S2) which was controlled by fixed the liquid height “h” (Figure 1a) by an micro injector under microscope. Then, the μECP was quickly moved forward to the alloy surface by micromanipulator. After the μECP was gently touched with the alloy and a liquid junction formed between the μECP and the alloy, the extraction of organics occurred, and then the metal electrolysis would be triggered when appropriate voltage was applied between the μECP (used as cathode) and alloy (used as anode) (Figure 1b). After electrolysis/extraction for a short time (e.g., 20 s) which was determined by the intensity of MS signal, the electrolyte was enriched with the organics and metal ions. Then, the μECP was moved away from the alloy and placed in front of the MS inlet used as a nano-ESI emitter. Upon applying a high voltage (±2 kV) to the μECP, analyte ions were generated at the tip of μECP for MS analysis (Figure 1c).

CH3COONH4 and acetonitrile were both HPLC grade and purchased from Fisher Scientific and Merck, respectively. All chemicals unless specified were reagent grade and were used as received. Electrochemical experiment was performed on a CHI 660D electrochemical workstation (CH instruments Inc.). Scanning electron microscope and energy dispersive spectra (EDS) results were obtained on Oxford Inca Energy X-Max20 (Oxford instruments). Micro-area analysis: As shown in Figure S1a, a piece of alloy was placed on a three-axis translation stage. The travel range of the translation stage is 20 mm and the minimal increment is 0.5 μm. To prevent the tip of µECP from crashing to the alloy surface, the μECP was insert into a glass tube (ID=1.3 mm) which was fixed on the micormanipulator (Figure S1a). The travel range of 50 mm across three dimensions and a minimum incremental motion of 0.1 μm. Microscope was used to view the analytical area and the movement of alloy sample and μECP. The image of the sampling was displayed on the computer screen with a scale plate. For a typical micro-area analysis experiment (Figure 1), electrolyte (10 ppm Phen in v/v=1:1 CH3CN/H2O) with nano liter level was first sucked into the μECP (5 μm tip) by micro injector (CellTram 4r Air, Eppendorf, American) (Figure 2b). Then, the μECP was accurately moved to the metal sample surface by micromanipulator with the assistance of a microscope. Because of the hydrophobicity of metal, a liquid junction was formed between the μECP and metal sample when the μECP touched with metal surface. The transformation of metal into metal ions occurred upon applying +0.6 V (CHI 660D) voltage between the metal sample (anode) and the μECP (cathode). For imaging of the alloy, (1) the µECP was moved from the left to right (Y-axis) to sample the alloy along the scale plate (Figure S1b). (2) After a complete sampling process from left to right, the alloy sample was moved forward (X axis) 25 μm. (3) Then, the µECP was moved from the left to right again to sample the alloy. By repeating the processes (1) - (3), the 2D alloy sampling in XY domain was completed. After electrolysis, the μECP was placed at the front of MS entrance. The distance between the MS inlet and the tip of the μECP was 1 cm. Then, a high voltage (+2 kV) was applied on the μECP to generate ions for MS (Orbitrap Fusion MS) analysis. A similar procedure was used for other metal materials or biochemical molecule analysis with a little change of electrolyte and electrolysis potential. Calibration curve preparation: Analytical grade of Zn(NO3)2 and Cu(NO3)2 was used as the source of Zn2+ and Cu2+. Working solutions of Zn2+ and Cu2+ with different concentration (1 ppb, 10 ppb, 50 ppb, 100 ppb, 500 ppb, 1000 ppb) were prepared by dissolving Zn(NO3)2 and Cu(NO3)2 in 10 ppm Phen CH3CN/H2O solution. RhB working solutions (1 ppb, 10 ppb, 50 ppb, 100 ppb, 500 ppb, 1000 ppb) were prepared by dissolving RhB in CH3CN/H2O solution. Then, the working solution was analyzed by μECP-MS to prepare the calibration curve.

Figure 2 Characterization of μECP and micro-area sampling process. a) SEM image of the μECP. The inset is the enlargement of the area mark by red rectangle; b) Cyclic voltammetry of 0.01 M K3[Fe(SCN)6] in 0.01 M KCl with 100 mV/S scan rate; c) Cyclic voltammetry of 0.01 M K3[Fe(SCN)6] in 0.01 M KCl/ acetonitrile (v/v=1:1) with 100 mV/S scan rate; d) Electrochemical characterization of the micro-area electrochemical sampling process. Step 1: μECP is moved forward to metal sample. Step 2: μECP is brought in contact with metal sample. Step 3: μECP is moved away from the metal sample; e) A photo of μECP filled with electrolyte solution in contact with metal sample surface; f) Upper panel corresponds to the SEM images of a Cu substrate sampled by different size probe. Lower panel corresponds to the fluorescence pictures of an RhB substrate sampled by different size μECP. The black area is the sampling area.

RESULTS AND DISCUSSION Concept of μECP-MS for molecular profiling. The μECP-MS combined micro electrochemical sampling with nano-ESI MS for trace analysis (Figure 1). The essential feature of the method was to sample the chemicals of a microarea by electrolysis of metal components and simultaneous

Characterization of micro-area sampling process by μECP. The present configuration of μECP device was prepared by inserting a platinum wire (100 μm) into a micropipette (Figure 1a). Depending on the required spatial resolution, the tip size of the μECP was varied by controlling


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Analytical Chemistry EDTA/CH3CN/H2O as electrolytes. This is because Al3+ cannot form complex with Phen, while [Al+EDTA-4H]- can be formed in EDTA/CH3CN/H2O (Figure S3b). Predominant signals of [Cu+EDTA-4H]2- (Figure S3c), [Zn+EDTA-4H]2(Figure S3d), and [Fe+EDTA-4H]- (Figure S3e) can also be observed in the analytical results of Cu, Zn and Fe using EDTA/CH3CN/H2O as electrolytes (Figure S3c-e). Metal has characteristic electrolysis potential under the defined condition. Figure 3c illustrates the relationship between the electrolysis potential and the signal intensity of Zn and Cu. Electrolysis of Zn was triggered at -0.4 V and enhanced along the potential increasing, while electrolysis Cu was initiated at about +0.1 V and enhanced with the increase of potential. Therefore, in the range from -0.4 V to +0.1 V, electrolysis of Zn rather than Cu was started. Thus by tuning the electrolysis potential one can achieve successive detection of components in Zn/Cu alloy. Both of the electrolysis reached a plateau at above +0.2 V and +0.4 V for Zn and Cu, respectively. Thus, the potential for the best signals of Zn and Cu was above 0.4 V.

the parameters of laser puller. Figure 2a shows a typical micropipette with tip of about 5 μm diameter. Cyclic voltammetry method was used to characterize the electrochemical properties of μECP. As shown in Figure 2b-c, the typical “S” curve of K3[Fe(SCN)6] was obtained in both 0.01 M KCl (Figure 2b) and KCl/acetonitrile (Figure 2c), indicating that the μECP have ordinary electrochemical performance.17 The sampling process was also characterized by electrochemical method. In a typical analysis process, nano liter electrolyte was first sucked into the μECP tip by a micro injector under microscope. Then, a voltage above the metal electrolysis potential was applied between μECP (cathode) and alloy sample (anode). Under the microscope and with the help of micromanipulator, the μECP was accurately positioned toward the sample (step 1 in Figure 2d). When the μECP probe touched the sample (Figure 2e), a liquid junction formed between the tip of μECP and sample, thus making the whole setup a conductive circuit. The generation of current (step 2 in Figure 2d) indicated that the electrolysis of metal component had started. After electrolysis, the μECP was removed away from the sample (step three in Figure 2b), and the current was stopped to terminate the electrolysis/sampling process. Both the SEM images (the upper panel) of a Cu substrate after sampling with μECP of different tip sizes (20 μm, 10 μm and 5 μm) and the microscope pictures (the lower panel) of an RhB-covered substrate after sampling by CH3CN/H2O with μECP of different tip sizes (20 μm, 10 μm and 5 μm) are shown in Figure 2f. The black areas with diameter of 25 μm, 16 μm and 10 μm in Figure 2f correspond to the sampling spots. Note that due to the liquid spreading during the sampling, the size of the sampling area is larger than the μECP tip. A minimal spatial resolution of 10 μm was obtained by using μECP with 5 μm tip. Characterization of analytical performance of μECPMS. Several metal samples were used to investigate the analytical performance of μECP-MS, such as Cu, Zn, Fe, Al. For example, in Cu analysis, 10 ppm Phen in CH3CN/H2O was used as electrolyte, and 0.6 V potential was used to sample the Cu with a spatial resolution of 10 μm. After electrolytic sampling for 60 s, the μECP was employed as nano-ESI for MS analysis. Typical analytical results for Cu are displayed in Figure 3a. Signals at m/z 181.0758, 211.5327 and 301.5668 correspond to [Phen+H]+, [Cu+2(Phen)]2+ and [Cu+3(Phen)]2+, respectively. The strong signal for Cu-Phen complexes (Cu2+ and Phen) indicates that the μECP-MS method can used for high-sensitivity and high spatial resolution analysis of Cu. Notably, only Cu2+ was detected in this experiment, while no Cu+ signal was detected, probably because the Cu2+ ions were formed before the analytes were nanoelectrosprayed under the experimental conditions, without the notable transient state of Cu+.18 In addition, the micro-area analytical results of Zn (Figure 3b) and Fe (Figure S3a) were also successfully obtained by μECP-MS employed Phen/CH3CN/H2O as electrolytes. Notable signal of [Zn+2(Phen)]2+ (m/z 212.0325), [Zn+3(Phen)]2+ (m/z 302.0665), [Fe+3(Phen)]2+ (m/z 298.0693), [Fe+3(Phen)]2+ (m/z 208.0354) can be observed in the mass spectra. Furthermore, our results indicate the importance of organic ligand, which is consistent with previous reports.19-20 There were no characteristic signals in the analytical results of Al employed Phen/CH3CN/H2O as electrolytes, while obvious signal can be observed using

Figure 3 Qualitative performance of μECP-MS for the analysis of metal materials. a) Mass spectrum of Cu sampled with +0.6 V electrolysis potential; b) Mass spectrum of Zn sampled with +0.6 V electrolysis potential; c) The relationship between the mass spectrum signal intensity of metal and the electrolysis potential; d) Mass spectrum of Rhodamine B sampled without applying electrolysis potential; e) Mass spectrum of RhB/Zn/Cu sampled with -0.2 V electrolysis potential; f) Mass spectrum of RhB/Zn/Cu sampled with 0.6 V electrolysis potential. The tip of nano-ESI was 1 cm away from the entrance of MS. The spray voltage was +2 kV. The dominant peak assignments: m/z 181.0764 [Phen+H]+; m/z 211.5330 [Cu+2(Phen)]2+; m/z 301.5666 [Cu+3(Phen)]2+; m/z 212.0330 [Zn+2(Phen)]2+; m/z 302.0674 [Zn+3(Phen)]2+; m/z 443.2329 [RhB-Cl]+.

Without applying potential on the μECP, molecular analytes such as RhB was extracted as neutral molecules and then analyzed by nano-ESI MS. Figure 3d shows the analytical results of RhB on a Cu foil using CH3CH/H2O as the extraction solvent without applying potential on the μECP. Characteristic signal of RhB ([RhB-Cl]+, m/z 443.2329) which was confirmed by the MSn spectra with the fragment of m/z 399 and m/z 355 (Figure S4),21 was dominantly observed in the mass spectrum, showing that the method could be easily



adapted for micro-area analysis of either metal materials and/or organic species. As shown in Figure S5, a high extraction efficiency ( ≥ 99%) was obtained for RhB after extraction about 30s (the experimtal process was detailed in Figure S5), suggesting that quantification of trace organic analytes could be implemented. Using a sample of Zn/Cu alloy covered by RhB, a proof-ofprinciple experiment was performed to use μECP for sequential micro-area analysis of organic species and metal materials. The characteristic signals [RhB-Cl]+ (m/z 443.2327) was obviously observed in the mass spectrum (Figure S6), while no metal ion signal appeared, indicating that neutral RhB was selectively sampled without applying a voltage for electrolysis. When a potential of -0.2 V was applied to the μECP for extraction/sampling, the signals of [Zn+2(Phen)]2+ (m/z 212.0330), [Zn+3(Phen)]2+ (m/z 302.0674) were detected in the mass spectrum (Figure 3e), indicating that Zn was detectable with the appearance of [RhB-Cl]+ (m/z 443.2327). By altering the electrolysis potential to +0.4 V, Cu signal was successively detected after Zn component and RhB component (Figure 3f). The results confirmed that sequential analysis of the RhB/Zn/Cu sample was achieved by controlling the electrolysis potentials using the μECP-MS.

MSI methods are unsuitable for metal element imaging. On the other hand, LA-ICP-MS is suitable for the imaging of element distribution,33-34 but it is unsuitable for biomoleuclar imaging. To date, it is extremely challenging to achieve MSI of metal and biomolecule components at the same time. Because of the high lateral resolution for metal and organic component analysis, μECP-MS is of great potential in molecular imaging of alloys. As a proof demonstration, an artificial sample was first prepared by chemical deposition of a “Cu” symbol on a Zn surface (Figure 5a). Then, μECP-MS was used to analysis the component point by point. The characteristic peak [Cu+3(Phen)]2+ was used for Cu imaging. The time duration for single pixel was about 2 min and the pixel number of the image was about 400. Figure 5b was a result of Cu component profile. Out of the “Cu” symbol area, there was no Cu component, while obvious signals were obtained in the area of “Cu” symbol. As shown in Figure 5a and Figure 5b, the molecular imaging of Cu obtained by μECP-MS well coincided with the optical image, indicating that μECP-MS was able to generate the metal compound profile of alloys. Because the imaging of organic compounds have been demonstrated in many earlier works using liquid extraction mass spectrometry, e.g. LESA-MS35 and nanoDESI-MS,36 the imaging of orgincs were not pursued in this study.

Figure 4 The intensity levels of the characteristic ions against the analyte concentrations. a) Intensity of [Zn+3(Phen)]2+ vs. concentration of Zn2+; b) Intensity of [Cu+3(Phen)]2+ vs. concentration of Cu2+; c) Intensity of [RhB-Cl]+ vs. concentration of RhB. The Zn2+, Cu2+ and RhB were all dissolved in acetonitrile/ultrapure water (v:v=1:1) solution and detected by nano-ESI MS.

The detection limit of μECP-MS was experimentally evaluated by detecting different concentration Zn2+, Cu2+ and RhB working solutions (1.0-1000 ppb). As shown in Figure 4, the signal intensities of [Zn+3(Phen)]+, [Cu+3(Phen)]+, [RhBCl]+ linearly responded to Zn2+, Cu2+, RhB concentration levels (Figure 4, R2≥0.99) over the range of 10-1000 ppb, 10500 ppb, 5-1000 ppb, respectively. Based on the calibration curve (Figure 4), the LOD (solution) of the μECP-MS for Zn2+, Cu2+ and RhB solution were calculated to be 1.59 ppb, 4.03 ppb and 1.24 ppb according to LOD (solution)=3σ/a, where “σ” was the standard deviation of blank solution and the “a” was the slope of calibration curve. Because the volume (V) of working solution was about 2.5 nL (Figure S1), the LOD (Mass) of the μECP-MS for Zn2+, Cu2+ and RhB were calculate to be 3.98 fg, 10.07 fg and 3.1 fg by the formula LOD (Mass)= LOD (solution) ×V. Thus the LOD of μECPMS for Zn, Cu and RhB on alloy surface were 9.58ag, 24.25ag and 4.47 ag per μm2 by a formula LOD=LOD (Mass)/S, where “S” is the sampling area. Spatial profiling of Cu/Zn alloy by μECP-MS. Mass spectrometry imaging (MSI) is widely used in many areas, including biomedical science,22-23 forensics,24-25 food science26. By now, a lot of MSI methods have been developed. Most of the MSI methods are developed for biomolecular imaging, such as MALDI,27-28 DESI-MS,29-30 LAESI-MS.31-32 These

Figure 5 Molecular profiling of Cu deposited on Zn using μECP-MS with a tip size of 20 μm. a) Optical image of Cu/Zn alloy. The black symbol “Cu” was chemical deposited Cu; b) Ion image of [Cu+3(Phen)]2+; c) SEM images of Cu membrane (thickness = 50 nm) etched at 0.6V for different time; d) Mass spectrum of the Cu membrane after micro-area sampling 30 s at 0.6 V; e) Mass spectrum of the Cu membrane after micro-area sampling 100 s at 0.6 V

Besides surface analysis, depth profiling of metal materials is always required in industry. Classical methods used for vertical analysis of metal material include EDS, XPS, or LAICP-MS. However, EDS and XPS can only ablate a surface of several nanometers, while LA-ICP is a hard ionization


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Analytical Chemistry technique which cannot be used for the analysis of organic molecules. The μECP-MS allows simultaneous analysis of metal and organic components of a sample. Moreover, electrolysis can be used for etching the metal materials by transforming them into metal ions. To investigate the etching efficiency of electrolysis, a 50 nm thickness Cu foil was prepared by magnetron sputtering. A +0.6 V potential was applied between μECP and Cu foil to etch the Cu. Figure 5c displayed the SEM images of Cu foil after etching for 30 s 60 s and 100 s, which showed that the etching began at the edge of sampling area and then proceeded toward the center, which probably was caused by the non-uniform profile of electric field. After electrolysis for 100 s, 50 nm thickness Cu almost disappeared inside the sampling spot (inset in Figure 5c), resulting in an averaged 0.5 nm/s vertical resolution. The signal level for Cu increased with the etching time (Figure 5d and Figure 5e), showing that more Cu was sampled with lengthened etching time. The etching performance of the present method has great potential in depth profiling of metal materials (e.g. layered materials, core-shell particles). Applications of μECP-MS: Semiconductor chip or circuit board is widely used in information industry. There are many conductive spots on the chip or board. For example, the inset of Figure 6a displays a part of a computer chip. There are many conductive spots (the yellow spots) on the chip. The place between the yellow spots is insulated. The chip would go out of work if the gap between the yellow spot was conductive (e.g., covered by metal particles, conductive dusts). Thus, sensitive and nondestructive detection of the conductive species is essential to guarantee the performance of a chip. For a demonstration, a computer chip contaminated with Cu particles was used as sample. Note that the small area marked by a red circle (inset of Figure 6a) was the analysis area. No conductive current was measured using a sensitive ampere meter. Accordingly, strong single of Cu was observed in the spectrum (Figure 6a), indicating that μECP-MS was more sensitive than the ampere meter to screen potential conductive tiny spot on chips. Welding is an important metal material processing technology, which has been widely used in petrochemical industry, equipment manufacturing, power system, aerospace, aviation, transportation, construction, military, metallurgy, etc.37-38 However, the welding quality is affected by many factors, inapposite parameter will lead to the formation of pits, fissures, pores and other defects.39 These defects may cause enormous harm to the product. Thus, welding quality analysis is of great significance to guarantee the machining quality of metal materials. By now, physical methods are normally used to test the welding quality40 although the chemical information of the micro structure of weld defects is always desirable. As shown in the inset of Figure 6b, several black pits with diameter about 20 μm were observed on the welding surface. In the μECP-MS spectrum of the micro-area marked by the red circle, both signals of Pb and Cu were (Figure 6b) observed, while weak signals of Pb appeared in the white area. The results demonstrate that μECP-MS is a promising method for the chemical analysis of micro weld spots.

Figure 6 The application of μECP-MS for chip and welding quality testing. a) Application of μECP-MS in chip analysis; b) Application of μECP-MS in weld analysis. The dominant peak assignments: m/z 175.4934 [Cu+EDTA-4H]2-; m/z 248.0171 [Pb+EDTA-4H]2-.

CONCLUSION In summary, μECP-MS was developed for the simultaneous molecular profiling of metal and organic components in metal alloys. This method features high sensitivity (LOD=4.47 ag/μm2), high specificity, high lateral resolution (10 μm) and depth resolution (0.5 nm). Proof-of-concept application of the present method in the analysis of solder side and organics covered alloy indicates that the present method is a promising analytical method for the micro-area chemical profiling of alloys.

ASSOCIATED CONTENT Supporting Information: Schematic diagram of experimental set-up; Measurements of the electrolyte volume in μECP; Qualitative performance of μECP-MS for metallic materials analysis; MSn of RhB; RhB extraction efficiency of μECP; Mass spectra of RhB deposited on the surface of the alloy.

AUTHOR INFORMATION *Corresponding Author: [email protected] Notes: The authors declare no competing financial interest and no conflicts of interest.

ACKNOWLEDGMENT The work was supported by the National Natural Science Foundation of China (no. 21864001, 21727812), Program for Changjiang Scholars and Innovative Research Team in University (PCSIRT) (no. IRT_17R20), China Postdoctoral Science Foundation (no. 2017M622091).

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