Potential direct mass spectral elemental analysis of solids by microwave plasma torch surface release and ionization

Abstract

Direct and rapid analysis of various elements from solid surfaces under ambient conditions remains a great challenge. In this work, in situ release and ionization of volatile vapor generation elements including As, Sb, Se, Ag, Te, Pb, Cu and Bi from solids by direct microwave plasma torch-mass spectrometry (MPT-MS) were studied. The results showed that MPT could induce in situ surface release and ionization of various volatile species forming elements directly from solid samples under ambient conditions without any reagent consumption and sample pretreatment. The method features high throughput and high selectivity, with the overall online data acquisition time for a single solid sample being within 5 seconds. Quantification analysis of As and Sb was also performed in soil samples with GBW07405 as the standard. The obtained detection limit was 1.4 μg g⁻¹ for As and 0.35 μg g⁻¹ for Sb. The high throughput, high sensitivity and simplified manipulation of MPT-MS was expected to provide a useful tool for the direct analysis of solid samples in the field of geochemistry, food safety, and environmental protection.

---

1. Introduction

In situ rapid analysis of various elements from solid samples plays an important role in the fields of food chemistry, geochemistry, mineral mining, environmental protection, national defense and the semiconductor industry, as it avoids tedious sample pretreatments such as digestion, extraction, filtration and dilution. Also, direct analysis of solid samples minimizes the use of chemical reagents and energy. To date, elemental analysis in solid samples has been achieved by X-ray fluorescence spectroscopy (XFS), glow discharge mass spectrometry (GD-MS),¹ secondary ion mass spectrometry (SIMS),² laser ionization mass spectrometry (LIMS)^3,4 and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS).⁵ Generally, elemental analysis is mostly based on high power ionization techniques because the atomization and ionization of inorganic elements require extremely high energy under vacuum conditions. In comparison, organic components can be easily ionized under mild energy conditions. A wide variety of ambient ionization techniques can be used, such as nano-electrospray ionization (nanoESI),^6,7 extractive electrospray ionization (EESI),⁸ desorption electrospray ionization (DESI),^9,10 desorption atmospheric pressure chemical ionization (DAPCI),¹¹ direct analysis in real time (DART),¹² atmospheric solids analysis probe (ASAP) technique¹³ and dielectric barrier discharge ionization (DBDI)^14–16 and so on.

On the other hand, our previous study showed that the analysis of some metal elements could also be achieved by ambient ionization mass spectrometry after converting these elements into volatile hydrides.¹⁷ Hydride-forming elements are a group of special elements allowing the generation of volatile hydrides, providing high selectivity and multiple sample introduction options.^18,19 These elements mainly include As, Sb, Pb, Bi, Sn, Ge, Ag, Se, Te, Cu and so on. Some of the elements are widely distributed in various environmental matrices and pose a big threat to environmental protection and human health.^17,20 The World Health Organization (WHO) announced the intake limits of 0.01, 0.005, 0.01 and 0.01 mg L⁻¹ for As, Sb, Se and Pb in drinking water, respectively. However, traditional hydride generation (HG) requires reduction reagents such as NaBH₄ and HCl to initiate the formation of hydrides. Although related studies have shown that some of the elemental ions in solution can be converted to volatile species by low temperature plasma sources, such as DBD,^21–25 direct desorption and ionization of these elements from solids under ambient condition have been rarely reported.

In this work, a novel strategy was proposed for rapid and direct analysis of hydride-forming elements from solid samples based on microwave plasma torch mass spectrometry. The method can achieve simultaneous ionization of various inorganic elements from solids under ambient conditions without any sample pretreatment and chemical reagents. The data acquisition time for an individual sample was within 5 seconds, facilitating high throughput analysis of various solid samples.

2. Materials and methods

2.1 Materials and reagents

All chemicals and reagents were of analytical grade. Standard solutions of As(III), Sb(III), Bi(III), Ag(I) and Te(IV) (1000 mg L⁻¹) were purchased from the China National Reference Materials Center. NaOH (≥96%) and NaBH₄ (98%) were bought from Beijing Innochem Technology Co., Ltd HCl (37%) was supplied by Sinopharm Chemical Reagents Co., Ltd. All solutions were prepared daily from their stock solutions and kept at 4 °C before use. 18.2 MΩ cm deionized water was obtained from a Milli-Q equipment (Millipore, Milford, MA). Soil samples of GBW07405 and GBW07406 (Institute of Geophysical Chemistry, Chinese Academy of Sciences) were used as real samples. Argon and hydrogen gas (99.99%) were purchased from Jiangxi Guoteng Gas Co., Ltd Te powder (99.99%), Bi powder (99.999%), SeO₂ (AR, 99%), Sn powder (99.99%), GeO (99.99%) were purchased from Beijing Innochem Technology Co., Ltd.

2.2 Instrumentation

The microwave plasma torch (MPT) was provided by Jin's group from Zhejiang University (Zhejiang, China) and was connected with a 2450 MHz microwave generator through a coaxial radio frequency (RF) cable (TESCOM, RG400S). Similar to ICP, MPT has three coaxial tubes. The outer tube is made of brass while both the inner and intermediate tubes are made of highly conductive copper.

The structure of MPT is detailed in the ESI (Fig. S1 and S2†) and other reports.^26,27 The output power of the MPT device ranged from 30 to 200 W. The MPT was then coupled to a LTQ-XL mass spectrometer (Thermo Fisher, San Jose, CA). The capillary temperature was set at 250 °C. The capillary voltage and tube lens voltage of MS for both positive and negative ion modes were optimized depending on experimental settings. For MPT, the flow rate of supporting gas (Ar) was 200 mL min⁻¹, and the flow rate of carrier gas (Ar) was 800 mL min⁻¹.

2.3 Setup of direct MPT-MS

As illustrated in Fig. 1, the angle between the MPT and the ion transport capillary of MS is about 30°. The horizontal distance between the sample and MS inlet is about 5–8 mm. The solid sample was loaded onto a stainless plate which was placed between the MPT and MS inlet. The vertical distance between the sample and MS inlet was about 5 mm. During analysis, the solid sample was placed on the stainless plate using tweezers, ensuring that the sample was in contact with the plasma tip of MPT. An acrylic box was placed over the MPT plasma and MS inlet to avoid signal instability caused by surrounding airflow. With the ignition of the MPT plasma, the volatile species forming elements were desorbed and ionized. Noteworthily, during the whole procedure, the plasma is always on and different samples are simply placed in contact with the tip of the MPT plasma.

Fig. 1 Schematic of the experimental setup of direct MPT-MS for solid sample analysis.

2.4 Preparation of samples

Ferrihydrite was synthesized according to the method of Schwertmann and Cornell,²⁸ by adding KOH solution (1 M) dropwise to 500 mL of 0.2 M Fe(NO₃)₃·9H₂O until the pH increased to 7.5. The precipitate was then washed with deionized water, freeze-dried, and stored in a vacuum dryer.

The solid samples were first ground and then sieved using a 200-mesh filter in order to maintain particle size as uniform as possible (the scanning electron microscope (SEM) images are shown in Fig. S3†). The weighed sample powders (0.01 g) were then pressed into thin tablets (4.0 mm in diameter, and about 0.5 mm in thickness) under 15 MPa pressure for 1 minute using a tablet press machine (YP-12, Tianjin Hengchuang Co., Ltd). For quantification of As, Sb and Pb in soil samples, a soil powder without target analytes was used as the blank. Different concentrations of As, Sb and Pb standard solutions were added to 5 duplicate blanks (0.1 g), respectively, generating a series of mix standards (1, 2, 5, 10, and 20 μg g⁻¹ for As; 1, 2, 5, 8, and 10 μg g⁻¹ for Sb and 10, 20, 50, 80, and 100 μg g⁻¹ for Pb, respectively). These standards were then dried at 65 °C for 3 hours and the obtained powder were ground and pressed into tablets under the same conditions.

3. Results and discussion

3.1 In situ analysis of geological samples

First, in situ analysis of solid geological samples was conducted. A bulk stibnite solid sample (0.01 g) was placed under the MPT and directly examined using MPT without any sample pretreatment or additional gases (e.g., H₂). It can be seen that the dominant peaks appear at m/z 137–139 in the form of SbO⁺ in positive ion detection mode. On the other hand, Sb was detected mainly as SbO₂⁻ at m/z 155–157 in negative ion detection mode. The mass spectrum behavior of Sb was also further investigated by tuning the capillary voltage and tube lens voltages (Fig. 2, S4 and S5†). Clearly, characteristic Sb mass spectra were detected at m/z 121 (Sb⁺), 137 (SbO⁺), 155 (SbO⁺ + H₂O), 173 (SbO⁺ + 2H₂O), 189 (SbO₂⁺ + 2H₂O) and 207 (SbO₂⁺ + 3H₂O) in positive ion mode (Fig. S4a†). Basically, the signal at m/z 137–139 was higher than that of other species, and the signal intensity of various Sb species generally increased and then decreased with the elevation of tube lens and capillary voltages.²⁹ For instance, the dominant m/z 137–139 was detected under the tube lens and capillary voltage of 200 and 100 V, respectively. The results suggested that the Sb from solid stibnite was successfully desorbed and ionized by the MPT. In addition to stibnite, ferrihydrite was also examined as the sample matrix. The adsorption experiment was conducted in solution with an initial concentration of 1.0 mg L⁻¹ for As, Sb and Bi, and the adsorbent used was 50 mg of ferrihydrite (see ESI, Table S1†). Then the obtained precipitate was filtered and dried for direct MPT-MS analysis. The results are shown in Fig. 2b. Clearly, the three elements were all detected from solid ferrihydrite by direct MPT-MS in positive ion mode. As and Sb were mainly detected in the form of AsO⁺ and SbO⁺, while Bi was mainly detected in the form of Bi⁺. The results suggested that As, Sb and Bi can be directly detected from solid samples of stibnite or ferrihydrite by MPT-MS. It is worth noting that the solid samples were examined directly by MPT-MS without any reagents (such as NaBH₄ and HCl) or sample pretreatment, and the online data acquisition was completed within 5 seconds.

Fig. 2 (a) Mass spectrum of Sb by direct MPT-MS analysis of stibnite in positive ion mode; (b) mass spectrum of As, Sb and Bi by direct MPT-MS analysis of pre-adsorbed ferrihydrite precipitate in positive ion mode; (c) the influence of tube lens voltage on the signal intensity of Sb in positive ion mode; (d) the influence of capillary voltage on the signal intensity of Sb in positive ion mode. The signal of m/z 121, 137, 155 and 173 were assigned to Sb⁺, SbO⁺, SbO⁺ + H₂O and SbO⁺ + 2H₂O, respectively. The NL value means the normalization level, indicating the intensity of base peak.

3.2 Direct analysis of hydride-forming elements from soil samples

Direct MPT-MS analysis can also be applied for the analysis of other hydride forming elements in solid samples such as soils. In this work two soil standards of GBW 7405 and GBW 7406 were examined. Since the commercial soil standard consists of fine powders (Fig. S3†), the sample was simply pressed into tablets (4.0 mm in diameter, and 0.5 mm in thickness) before analysis. The results showed that characteristic mass spectra for various hydride-forming elements such as As, Sb, Pb and Bi were clearly detected in the soil sample (Fig. 3). Under the capillary voltage, tube lens voltage and microwave power of 100 V, 200 V and 80 W, respectively, these elements were detected with distinct selectivity in the form of M⁺ or MO⁺ in positive ion mode. For instance, As was detected at m/z 91 (AsO⁺); Sb was detected at m/z 137–139 in the form of SbO⁺, and Ag was detected as Ag⁺ at m/z 107–109. Pb was detected predominantly as Pb⁺ with the highest abundance at m/z 208, and a much less abundant signal of PbO⁺ was also detected at m/z 224. Similarly, Bi was detected as both Bi⁺ and BiO⁺ at m/z 209 and 225, respectively. Confirmation of these peaks was achieved by conducting a spike experiment, analysis of individual standards and collision induced dissociation (CID) analysis. For instance, in order to verify that the peaks at m/z 107–109 were assigned to Ag, a spike experiment was performed by addition of 50 μg g⁻¹ Ag standard to the soil sample, and the results demonstrated that the signal intensity of Ag was remarkably enhanced after spiking (Fig. 3b). The signal of Pb was confirmed by comparing the detected peaks with its isotope abundance ratio. The detected isotope abundance ratio of m/z 204: m/z 206: m/z 207: m/z 208 was 0.03 : 0.49 : 0.42 : 1, which was in agreement with the natural isotope abundance ratio of 0.027 : 0.46 : 0.42 : 1 for Pb. Therefore, m/z 204, 206, 207 and 208 were confidently assigned to Pb (Fig. 3c). Similarly, Se was also detected as Se⁺ (the highest abundance was observed at m/z 80) (Fig. 3d). As for Bi, the analysis of a Bi standard was carried out and the results showed that characteristic peaks at m/z 209 and 225 were detected, and further CID analysis showed that the m/z 225 peak fragmented into ions at m/z 209 by losing mass of 16. Thus, m/z 209 was assigned to Bi⁺, and m/z 225 was assigned to BiO⁺ (Fig. 3e). In addition, the signals of Te and Sn were also confirmed by spike experiments with the soil sample of GBW 7405 (Fig. 3f and g). Te was detected in the form of Te⁺ and TeO⁺, and Sn was detected in the form of Sn⁺ and SnOH⁺, respectively. Noteworthily, the characteristic mass spectra for Ag, Te and Pb could be successfully detected from solids by direct MPT-MS, whereas they were difficult to detect by HG-MPT-MS analysis of solution samples. Particularly, the element Cu which has rarely been reported as detectable by conventional HG techniques, was also successfully observed in GBW07406 by direct MPT-MS analysis (Fig. S6†). Generally, Cu was monitored in the form of Cu⁺ in positive ion mode at m/z 63 and 65, corresponding to its two isotopes. The detection of Ag and Cu might be ascribed to their ability to generate volatile vapors and the thermal effects of MPT. The temperature of the plasma tip was measured to be about 180 °C with an MPT power of 80 W.

Fig. 3 (a) Direct MPT-MS analysis of a standard soil sample of GBW07405; (b) mass spectrum of GBW07405 with a Ag spike of 50 μg g⁻¹; (c) mass spectrum of Pb in comparison with its abundance ratio; (d) detected mass spectrum of Se (in the form of HSe⁺) in comparison with its abundance ratio; (e) mass spectrum of Bi (detected as Bi⁺ and BiO⁺); the inset shows the CID analysis of m/z 225 which was fragmented into m/z 209; (f) detected mass spectrum of Te (detected as Te⁺ and TeO⁺) in comparison with its abundance ratio; (g) mass spectrum of Sn (detected as Sn⁺ and SnOH⁺) in comparison with its abundance ratio.

The results suggested that the method could be preferably used for high throughput analysis and rapid screening of hydride-forming elements from solid samples, since the non-hydride forming elements were effectively eliminated. In order to further confirm the capability of direct MPT-MS in the analysis of these elements, pure metal (Te, Sn, and Bi) or metal oxides (SeO₂ and GeO₂) (Fig. S7–S11†) were examined by MPT-MS individually. Again, consistent signals for these elements were monitored as those detected in the soil samples.

3.3 Mechanism of desorption and ionization by MPT

The mechanism of in situ release and ionization by MPT was then disclosed based on the above results and the properties of the MPT plasma. As is well known, conventional hydride generation involves the consumption of reagents such as NaBH₄ and HCl, and gaseous hydrides are formed through the sequential formation of borane complexes, initiated by the reaction between NaBH₄ and HCl.³⁰ However, the MPT alone also enabled the detection of these elements directly from solid samples. Therefore, the speculated analytical mechanism by MPT involves a complex process including the generation of active hydrogen species (e.g., hydrogen radicals) in the MPT; selective generation of volatile elemental surface species (e.g., metal hydrides); elemental surface release and ionization (Fig. 4a). Indeed, microwave plasma has been widely investigated as an excitation source.^31,32 In this work, microwave plasma was also investigated for elemental surface release and efficient ionization. On the other hand, previous studies suggested that some plasma sources, such as cathode glow discharge (SCGD)^33,34 and dielectric barrier discharge (DBD),³⁵ can facilitate chemical vapor generation of some elements from solutions. In this work we found that MPT plasma can be directly used for in situ release and ionization of these elements from solid samples. The unique structure and properties of MPT provided a variety of active components, such as NO⁺, O₂⁺, and ArH⁺ (Fig. S12†). In the MPT plasma, Ar˙⁺ (or Ar₂˙⁺) was generated due to the impact of high energy electrons on Ar (eqn (1)), which then collided with H₂O from ambient air. As a result, H₂O was ionized into H₂O˙⁺ (2) which was not stable and thus underwent hydration with another H₂O, resulting in the production of H₃O⁺ (3) and even clusters of ((H₂O)_nH₃O⁺), n = 1–2 due to successive hydration.^36–38 On the other hand, the abundant high-energy electrons reacted with H₃O⁺, generating active hydrogen species (4) which act as reductants for the formation of gaseous hydrides. Consequently, these volatile species were then ionized by the MPT plasma and recorded in the mass spectrometer.

Ar + e → Ar˙⁺ + 2e (1)

Ar˙⁺ + H₂O → H₂O˙⁺ + Ar (2)

H₂O˙⁺ + H₂O → H₃O⁺ + OH˙ (3)

e + H₃O⁺ → H˙ + H₂O (4)

Fig. 4 (a) Illustration of in situ release and ionization of analytes from solid surfaces; (b) mass spectrum of a mixed standard solution containing As, Sb, Ag, Te, Pb and Bi by conventional HG-MPT-MS; (c) total ion chromatography of ferrihydrite by MPT-MS analysis; (d) direct MPT-MS analysis of solid ferrihydrite pre-adsorbed with As, Sb, Ag, Te, Pb and Bi; (e) calibration curves of As and Sb in soil samples by MPT-MS, which were plotted based on GBW07405 diluted by different factors using synthesized ferrihydrite.

Noteworthily, the results also suggested that MPT served as a source of active hydroxyl radicals, which was in agreement with the observation of strong OH emission bands in electrolyte cathode atmospheric glow discharges.³⁶ This work also confirmed that the introduction of H₂ into the MPT further promoted the signal intensity of analytes (Fig. S13 and S14†), probably due to the fact that H₂ is excited and ionized into various active hydrogen species by the Ar plasma, such as H₃O⁺, H˙, H⁺ and H⁻, which contribute to the enhancement of signal intensity³⁹ (Table S2†). However, obvious tailing of the extracted ion chromatogram was observed after the introduction of H₂ (Fig. S15†), probably because the application of H₂ led to some continuous reactions between the active hydrogen species and residual analytes in the ion trap of mass spectrometer. Generally, the MPT combined the advantages of thermal effects, selective release and ionization without using of any reducing reagents and sample pretreatments. Therefore, the MPT (or other types of plasma) may be used as an efficient in situ selective release and ionization method for the direct detection of volatile species forming elements from various solid samples.

3.4 Characterization of analytical performance of MPT-MS

The results suggested that direct desorption and ionization by MPT were also suitable for the analysis of some of the hydride-forming elements which were hardly detected by the conventional hydride generation technique in solution samples (such as Pb, Te and Ag) (Fig. 4b and d). To confirm this, comparison experiments were conducted between the solution sample (500 μg L⁻¹ multi-element standard containing As, Ag, Sb, Te, Pb and Bi) and solid sample (multi-element pre-adsorbed ferrihydrite). The solution sample was analyzed via conventional HG-MPT-MS using NaBH₄ and HCl as reduction reagents as described in a previous report.²⁹ The solid ferrihydrite sample pre-adsorbed with these elements was pressed into a tablet and the mass spectra were examined and compared. Fig. 4b and d show the analytical results of the two samples. Clearly, only three of the elements (As, Sb and Bi) were detected from the solution sample (Fig. 4b). However, three additional elements (Ag, Te and Pb) were also detected from solid samples (Fig. 4d and S16†). The comparison verified that MPT-MS showed significant advantages in the direct analysis of these elements from solid samples. Moreover, the data acquisition for a single sample could be completed within 5 seconds (Fig. 4c).

As we all know, direct analysis of various analytes in solid samples has always been a big challenge due to the lack of matrix-matching standards and the non-uniformity of surface particle sizes of solid samples. Therefore, in this work, various approaches were implemented to simplify the procedure and reduce errors. For one thing, the solid powders were carefully ground and sieved using a 200-mesh filter in order to maintain a uniform particle size as much as possible (the SEM images are listed in Fig. S3†). The powders were then pressed into tablets under controlled pressure and time. For another, samples without the desired analytes were used as blanks. First, the linearity of As and Sb was checked through successive dilution of the solid standard of GBW07405 with pure synthesized ferrihydrite by various factors (e.g., 1–100 times). A vortex mixer was used to make the powder as “homogeneous” as possible. Considering that the certified concentrations of As and Sb in GBW07405 were 412 ± 16 μg g⁻¹ and 35 ± 5 μg g⁻¹ respectively, the resulting concentration ranges were 4.12–412 μg g⁻¹ for As and 0.35–35 μg g⁻¹ for Sb. The linear curves were plotted based on the intensity of As and Sb as a function of the mass percentages of GBW07405. It can be seen that both As and Sb showed good linearity within the studied mass ranges, with correlation coefficients of 0.94 and 0.98, respectively (Fig. 4e). The detection limits were also investigated based on the definition of IUPAC (International Union of Pure and Applied Chemistry). That is, the corresponding concentration was calculated as 3 times the standard deviation of the signal/noise (S/N) ratio divided by the slope of the calibration curve for each analyte. Consequently, the detection limits for As and Sb were 1.4 μg g⁻¹ and 0.35 μg g⁻¹ respectively. Finally, the quantification of As, Sb and Pb in two soil samples was conducted by both MPT-MS and HG-AFS. For HG-AFS analysis, the samples were first digested into solution (Table S3†). For MPT-MS analysis, a soil sample with undetectable As, Sb and Pb was used as the blank. A series of mix standards containing As (1, 2, 5, 10, 20 μg g⁻¹), Sb (1, 2, 5, 8, 10 μg g⁻¹) and Pb (10, 20, 50, 80, 100 μg g⁻¹) were prepared by adding different concentrations of As, Sb and Pb aqueous standards to the soil blanks. These standards were then dried, ground and pressed into tablets under the same conditions. The results showed that the concentrations obtained by MPT-MS were generally in good agreement with those obtained from HG-AFS analysis (Table S3†). The quantification further suggested that MPT-MS is a potentially useful tool for the direct analysis of various solid samples.

3.5 Analytical figures of merit

Unlike conventional hydride generation devices, MPT-MS avoids the waste of HCl and NaBH₄. Also, the MPT allowed ambient ionization with extremely low argon (about 1 L min⁻¹) and power consumption (less than 100 W). Direct MPT-MS analysis features minimized manipulation and short online data acquisition time (less than 5 s per sample). The detection limits for As and Sb were calculated to be 1.4 μg g⁻¹ and 0.35 μg g⁻¹, respectively. The linear behavior of As and Sb based on successive dilution of the soil standard GBW07405 with pure synthesized ferrihydrite was also demonstrated. The results suggest that As and Sb showed good linearity (0.94 for As and 0.98 for Sb) in the range of 4–400 μg g⁻¹ and 0.3–30 μg g⁻¹ respectively. Quantification of As, Sb and Pb in soil samples showed that direct MPT-MS analysis was in good agreement with HG-AFS analysis, with the relative standard deviation (RSD) ranging from 7.8 to 29.4%. However, greater effort is still needed to ensure an accurate quantitative analysis of analytes in solid samples.

4. Conclusion

Ambient plasma-based techniques such as MPT, DART and DBDI are normally used for the direct analysis of organic components due to their good thermal effects and the presence of multi-active species in the plasma. However, the preliminary study results in this work suggested that these plasma-based techniques may also be used for selective analysis of some elements from solid surfaces. MPT-MS combines the advantages of selective surface release and tunable ionization. Different solid samples such as metal, metal oxides, soil, stibnite and synthesized ferrihydrite were examined by direct MPT-MS, and various volatile vapor generation elements including Ag, As, Sb, Te, Bi, Cu and Pb were successfully detected. Generally, the technique may provide a useful tool in different fields. For instance, the study of metal enrichment mechanisms and metallogenic research of Sb polymetallic deposits. In addition, atmospheric particulate pollutants often contain various heavy metals such as Pb and As.^40,41 Therefore, it is of great importance to develop a high throughput and robust analytical method for the direct analysis of various solid samples.

Data availability

The authors confirm that the data supporting the findings of this study are available within the article and its ESI.†

Conflicts of interest

The authors declare no competing financial interest.

Acknowledgements

This work was supported by the National Key R&D Program of China (2023YFC2906801), the National Natural Science Foundation of China (42367067), the Jiangxi Provincial Natural Science Foundation (20232BAB203009), and the Open fund project of Jiangxi Province Key Laboratory of the Causes and Control of Atmospheric Pollution (1412000026).

References

1. A. A. Ganeev, A. R. Gubal, K. N. Uskov and S. V. Potapov, Russ. Chem. Bull., 2012, 61, 752–767.
2. Y. Chen, Z. Xie, S. Dong, Q. Lei and J. Gao, J. Anal. At. Spectrom., 2022, 37, 2529–2536.
3. Y. Meng, L. Hang and W. Hang, Spectrochim. Acta, Part B, 2021, 176, 106041.
4. L. Li, B. Zhang, R. Huang, W. Hang, J. He and B. Huang, Anal. Chem., 2010, 82, 1949–1953.
5. D. Zhang, J. Pan, J. Gao, T. Dai and R. C. Bayless, Ore Geol. Rev., 2021, 133, 104097.
6. M. Rahman, D. Wu and K. Chingin, J. Am. Soc. Mass Spectrom., 2019, 30, 814–823.
7. M. M. Rahman, D. Wu, K. Chingin, W. Xu and H. Chen, Talanta, 2019, 202, 59–66.
8. H. Chen, S. Yang, M. Li, B. Hu, J. Li and J. Wang, Angew. Chem., Int. Ed., 2010, 49, 3053–3056.
9. H. Chen, N. N. Talaty, Z. Takáts and R. G. Cooks, Anal. Chem., 2005, 77, 6915–6927.
10. R. G. Cooks, Z. Ouyang, Z. Takats and J. M. Wiseman, Science, 2006, 311, 1566–1570.
11. S. Yang, J. Ding, J. Zheng, B. Hu, J. Li, H. Chen, Z. Zhou and X. Qiao, Anal. Chem., 2009, 81, 2426–2436.
12. R. B. Cody, J. A. Laramée and H. D. Durst, Anal. Chem., 2005, 77, 2297–2302.
13. C. Wu, K. Qian, C. C. Walters and A. Mennito, Int. J. Mass Spectrom., 2015, 377, 728–735.
14. H. Hayen, A. Michels and J. Franzke, Anal. Chem., 2009, 81, 10239–10245.
15. S. Liu, Q.-Y. Yang, S. Chen, Y.-L. Yu and J.-H. Wang, Anal. Chem., 2022, 94, 16549–16554.
16. S. Liu, X.-X. Xue, Y.-L. Yu and J.-H. Wang, Anal. Chem., 2021, 93, 6262–6269.
17. D. Wu, D. Li, L. Dong, G. Li, L. Wang, Z. Tang, M. M. Rahman and S. Yang, J. Anal. At. Spectrom., 2022, 37, 2103–2110.
18. Z. Zhu, C. Yang, P. Yu, H. Zheng, Z. Liu, Z. Xing and S. Hu, J. Anal. At. Spectrom., 2019, 34, 331–337.
19. Z. Zhu, S. Zhang, J. Xue and X. Zhang, Spectrochim. Acta, Part B, 2006, 61, 916–921.
20. D. Wu, S. Yang, F. Li, T. Zhu and H. Chen, Anal. Chem., 2020, 92, 14309–14313.
21. X. Mao, Y. Qi, J. Huang, J. Liu, G. Chen, X. Na, M. Wang and Y. Qian, Anal. Chem., 2016, 88, 4147–4152.
22. Z. Zhu, J. Liu, S. Zhang, X. Na and X. Zhang, Spectrochim. Acta, Part B, 2008, 63, 431–436.
23. X. Gong, D. Zhang, I. B. Embile, Y. She, S. Shi and G. Gamez, J. Am. Soc. Mass Spectrom., 2020, 31, 1981–1993.
24. X. Gong, S. Shi and G. Gamez, J. Am. Soc. Mass Spectrom., 2017, 28, 678–687.
25. N. Na, Y. Xia, Z. Zhu, X. Zhang and R. G. Cooks, Angew. Chem., Int. Ed., 2009, 48, 2017–2019.
26. D. Zhu, W. Jin, B. Yu, Y. Ying, H. Yu, J. Shan, Y. Yan and Q. Jin, J. Anal. At. Spectrom., 2017, 32, 1595–1600.
27. B. Yu, W. Jin, Y. Ying, H. Yu, D. Zhu, J. Shan, W. Liu, C. Xu and Q. Jin, J. Anal. At. Spectrom., 2016, 31, 759–766.
28. U. Schwertmann and R. M. Cornell, in Iron Oxides in the Laboratory: Preparation and Characterization, Wiley-VCH, Weinheim, Germany, 2nd edn, 2008, pp. 103–112.
29. T. Shi, Z. Zhou, Z. Tang, Y. Guo, D. Wu, L. Wang and C. Leng, Talanta, 2024, 279, 126539.
30. A. D'Ulivo, J. Dědina, Z. Mester, R. E. Sturgeon, Q. Wang and B. Welz, Pure Appl. Chem., 2011, 83, 1283–1340.
31. R. C. Machado, C. D. B. Amaral, J. A. Nóbrega and A. R. Araujo Nogueira, J. Agric. Food Chem., 2017, 65, 4839–4842.
32. W. Yang, H. Zhang, A. Yu and Q. Jin, Microchem. J., 2000, 66, 147–170.
33. C. Huang, Q. Li, J. Mo and Z. Wang, Anal. Chem., 2016, 88, 11559–11567.
34. C. Yang, G. Cheng, S. Q. Cheng, X. Liu, Y. Liu, H. T. Zheng, S. H. Hu and Z. L. Zhu, Anal. Chem., 2021, 93, 16393–16400.
35. M. Z. Huang, S. C. Cheng, Y. T. Cho and J. Shiea, Anal. Chim. Acta, 2011, 702, 1–15.
36. P. Mezei and T. Cserfalvi, Appl. Spectrosc. Rev., 2007, 42, 573–604.
37. M. Pavlik and J. D. Skalny, Rapid Commun. Mass Spectrom., 1997, 11, 1757–1766.
38. T. Zhang, W. Zhou, W. Jin, J. Zhou, E. Handberg, Z. Zhu, H. Chen and Q. Jin, J. Mass Spectrom., 2013, 48, 669–676.
39. Q. He, Z. Zhu and S. Hu, Rev. Anal. Chem., 2014, 33, 111–121.
40. T. Tziaras, S. A. Pergantis and E. G. Stephanou, Environ. Sci. Technol., 2015, 49, 11640–11648.
41. N. Asante and V. Nischwitz, Spectrochim. Acta, Part B, 2020, 174, 105993.

---

Footnote

† Electronic supplementary information (ESI) available: Preparation of ferrihydrite sample; MPT structures; SEM images; influence of capillary and tube lens voltages; pure metal and metal oxides; influence of additional H₂; calibration curves; spiking experiment; and table of analysis of pre-adsorbed ferrihydrite by direct MPT-MS. See DOI: https://doi.org/10.1039/d4ja00362d

---