Analysis of trinitrotoluene in solutions using Aerodynamic Thermal Breakup Droplet Ionization (ATBDI) mass spectrometry

Analysis of trinitrotoluene in solutions using Aerodynamic Thermal Breakup Droplet Ionization (ATBDI) mass spectrometry

Talanta 212 (2020) 120770 Contents lists available at ScienceDirect Talanta journal homepage: www.elsevier.com/locate/talanta Analysis of trinitrot...

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Talanta 212 (2020) 120770

Contents lists available at ScienceDirect

Talanta journal homepage: www.elsevier.com/locate/talanta

Analysis of trinitrotoluene in solutions using Aerodynamic Thermal Breakup Droplet Ionization (ATBDI) mass spectrometry

T

Viktor V. Pervukhin, Dmitriy G. Sheven∗ Nikolaev Institute of Inorganic Chemistry of SB RAS, Acad. Lavrentieva Ave., 3, 630090, Novosibirsk, Russia

ARTICLE INFO

ABSTRACT

Keywords: Aerodynamic thermal breakup droplet ionization Mass spectrometry Trinitrotoluene

The droplets breakup and compounds ionization in the heated capillary were investigated; the ionization method was named Aerodynamic Thermal Breakup Droplet Ionization (ATBDI). The mass spectrometric analysis was performed for solutions of trinitrotoluene (TNT) in water, ethanol, and acetonitrile. Considering the total current and the target analyte peaks, the ionization efficiency in the heated capillary was correlated with the physical properties of the solvents. For aqueous and ethanol solutions of TNT, the limit of detection (LOD) was obtained when using the ATBDI mass spectrometry. It was estimated to be 10−7 g mL−1 and 10−6 g mL−1 for water and ethanol solutions, respectively.

1. Introduction Nowadays, electrospray ionization (ESI) [1] and atmospheric pressure chemical ionization (APCI) [2] are widespread ionization methods for mass spectrometry under ambient conditions. Furthermore, quite a few environmental ionization methods, with ionization mechanisms similar to ESI and APCI, have been developed over the past two decades [3]. In one of the methods developed, the analyzed solution enters directly into the input capillary of the mass spectrometer (solvent assisted inlet ionization, SAII). In the capillary, the liquid breakup into droplets with the formation of gaseous ions [4,5]. The ion formation mechanisms are similar in the SAII and ESI: droplet charging, solvation shell evaporation, and Coulomb fission. Similarly, the droplet assisted inlet ionization method (DAII) was developed for the aerosol ionization, where aerosol enters the inlet capillary of the mass spectrometer [6]. It is believed that in these methods the ionization depends on the pressure gradient at the inlet, which determines the droplet breakup. This is consistent with the charge separation mechanism in the aerodynamic breakup of water droplets [7]. In both SAII and DAII, heating of the inlet capillary of the mass spectrometer increases the number of ions [6,8]. The mechanical breakup of the drops leads to a statistical ionization of the drops by Dodd [9]. The resulting drops are mostly singly charged and of micrometer size [10,11]. According to the Dodd, the smaller droplets the more is the total charge. One of the ways to obtain smaller droplets is the use of infrared radiation [12]. Under the short pulse of an infrared laser, a thin stream of water or micrometer-sized water



droplets breaks up into charged nanometer-sized droplets. In the heated capillary, there is infrared radiation that can be considered as a radiation of a completely black body [13], which leads to the breakup of the droplets. In our works [14,15], the heated capillary is also used to generate ions (however, it is separated from the input capillary of the mass spectrometer, unlike the SAII and DAII methods). In the review “Mass spectrometry for trace analysis of explosives in water” [16] it is reported that increasing temperature during ionization often leads to thermal decomposition of explosive compounds preventing their detection. On the other hand, the solvent can protect the analyte from thermal decomposition in the inlet capillary of the mass spectrometer [17]. Additionally, it was mentioned that the number of negative ions was significantly less than positive ions for SAII [4,5]. It is the negative ionization that is used in the mass spectrometry of most of the explosive compounds from solutions. Because of the foregoing, it was interesting to perform the mass spectrometric analysis of TNT from solutions (water, ethanol, acetonitrile) in the negative ion mode using the Aerodynamic Thermal Breakup Droplet Ionization. The experimental results obtained would be discussed using the model of the ionization process proposed in this work. It should be noted that in all our experiments, even at minimum capillary temperatures, an ion current is observed. This is a consequence of the ionization of the droplets by the aerodynamic breakup occurring in the nebulizer (studied earlier Aerodynamic Breakup Droplet Ionization, ABDI [14]). However, the use of a hot capillary leads to the additional ionization, as discussed below. Therefore, the method must be renamed as Aerodynamic Thermal Breakup Droplet

Corresponding author. E-mail address: [email protected] (D.G. Sheven).

https://doi.org/10.1016/j.talanta.2020.120770 Received 12 November 2019; Received in revised form 20 January 2020; Accepted 21 January 2020 Available online 22 January 2020 0039-9140/ © 2020 Elsevier B.V. All rights reserved.

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Ionization (ATBDI). In this article, we discuss the model for ionization in ATBDI. The efficiency of ATBDI in ionization of TNT from solutions (water, ethanol, acetonitrile) is considered in terms of the total ion current of the mass spectrometer and the intensity of the peaks of the target compound. Also, the limit of detection (LOD) is determined for solutions of TNT in ethanol and water when using the ATBDI method. The limit of detection is defined in a standard way as that concentration at which the signal to noise (S/N) ratio is equal to 3.

Often mass spectra obtained with ATBDI are characterized by a high content of clusters and dimeric ions. To obtain pure mass spectra, we applied the fragmentor voltage (the voltage between the end of the input capillary of the mass spectrometer and the skimmer, Fig. S1). This voltage can be changed during the experiment from 0 to 350 V. 2.3. Mass spectrometric conditions The ESI was implemented with the same Agilent 6130 mass spectrometer. The ESI conditions were as follows: nitrogen as a drying gas, the temperature of 350 °C, the flow rate of 7 L min−1; sprayer pressure (nitrogen) of 60 psig; the voltage on the capillary of 4000 V. The analysis was performed in the range of 100–1500 Da. The SCAN mode was used for charged ions. To obtain and process the mass spectra, the mathematical software (Agilent ChemStation) provided by the manufacturer of the mass spectrometer was used. The experiments with ATBDI and ESI were carried out at least three times for all samples. Standard deviations were calculated using Microsoft Excel. Mean values and error bars of three measurements are shown in the figures shown below.

2. Experimental 2.1. Chemicals and reagents Distilled water used in the experiments was prepared by the services of the Institute for internal needs. Acetonitrile “HPLC grade” and ethanol were purchased from Cryohrom Ltd (St.-Petersburg, Russia). Technical TNT was provided as part of the research work “Investigation of the possibility of registering explosives substances using gas analysis methods”, carried out by the “Special equipment and communications” police department of Russia (Siberian Branch). The TNT was recrystallized once before use. The stock TNT solutions (10−3 g mL−1) for each solvent were prepared by dissolving the sample and stored at T ~4 °C. Solutions of the required concentrations were prepared before the experiment by diluting the initial solution with the definite solvent.

3. Results and discussion 3.1. Model We consider the following mechanism of droplets breakup under the heating. In equilibrium the pressure inside a spherical drop of radius r is greater than the ambient pressure (Young–Laplace equation) [18]:

2.2. Aerodynamic thermal breakup droplet ionization (ATBDI) implementation

Pd = P0 + 2 / r

Aerodynamic Thermal Breakup Droplet Ionization was implemented by us with commercially available mass spectrometer Agilent 6130; the assembly is described in detail in Ref. [14] (Fig. S1, supplementary material (SM)). Briefly, the analyzed compound in a polar solvent is sprayed in the form of large neutral or single-charged droplets with a size of about 3–5 μm. The droplets obtained in the nebulizer enter the heated tube (in our experiments, with an inner diameter of 1 mm, a length of 130 mm, and a temperature, Tsuck, varied in the range of 20–340 °C). The temperature was measured using a copper-constantan thermocouple (wire thickness of 0.1 mm). The thermocouple was inserted directly into the suction capillary to a depth of 7 cm while airflow containing an aqueous aerosol simultaneously passed through the capillary. Depending on the voltage on the heater, the temperature was measured; the calibration curve was later used by us to estimate the capillary temperature during the experiments. In the tube, the drops are rapidly heated by the radiation from the heated walls of the capillary (Fig. 1), the surface tension of the drop decreases, its explosive breakup occurs and heir drops are ionized according to the Dodd [9]. At the same time, intensive evaporation of the solvent and the formation of gaseous ions take place. As a result of the breakup, the droplets are electrically charged (without using high voltage or using high energy particles), thus a bipolar aerosol comes out of the suction tube and enters the inlet capillary of the mass spectrometer.

(1)

where Pd is the pressure inside the drop, P0 is the ambient pressure and σ is the surface tension of the liquid. Suggested, the droplet breakup if the surface of the droplet (or the part of the surface) heats up quickly to a temperature close to the critical [19]. It can be explained by the fact that the surface tension of the liquid decreases when temperature increases [20]; such behavior is described by the Van der Waals equation [21]:

~(Tc

T )3/2

(2)

where Tc – is the critical temperature. Thus, heating of the droplets shifts the equilibrium and increased pressure inside the droplet results in its explosive breakup [22]. The phrase “heats up quickly”, used above, means that cooling and the droplet size reducing during the evaporation are relatively slow and the equilibrium described by the equation (1) is not maintained. Regarding this interpretation, the explosive breakup is more likely to occur at rapid heating (by the laser or by the sharp increase in temperature in the inlet capillary of the mass spectrometer, as shown in Fig. 1). Otherwise, the size of the droplet decreases by evaporation giving the equilibrium described by the equation (1) [23]. Following the Dodd approach [9], there are neutral drops of solution (with volume V) containing positive and negative ions (with equal

Fig. 1. The schematic diagram of the ATBDI. 2

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concentrations, ci, cm−3), which break up into N identical drops of volume V/N. The probability P(k, k + ) of finding k + positive ions in the droplet obtained after the breakup is determined by the Poisson distribution:

P (k, k+) = (k k +/k + !) e

(3)

k

where k = ci × V/N is an average number of positive ions in the obtained droplets (k ~60 for a water droplet with a volume of 1 μm3, with pH = 7). For negative ions, the distribution is the same (with k-). The droplet is charged if there are different amounts of positive and negative ions in the droplet after the breakup (k + ≠ k-). The probability of such an event is P(k + , k-) = P(k, k + ) × P(k, k-). The full charge distribution P(q) (q = 1, 2, 3, …) can be obtained by summarizing P (k + , k-) over all k + and k-. Using the Gaussian formula, P(q) is described rather simply [24]:

P(q)

(4)

exp( q2/4k) 1/2

with the width of the charge distribution P(q) equal to (2k) . The behavior of the ion current obtained as a result of droplet breakup is quite complex. However, from equation (4) it is obvious that for k » 1 the distribution is wide and almost all droplets are charged (except the droplets with zero value of q). In this case, the number of charge carriers is proportional to N, i.e. the finer the fragmentation of the initial droplet, the higher the ion current. On the other hand, for k « 1, the distribution is narrow and most of the droplets are not charged. Physically, this means that there are too many drops and not enough charges available in the initial drop. In this case, further fragmentation of the droplet is pointless if one wants to increase the ion current. As mentioned above, the evaluated value of k is ~60 for a droplet of pure water with a volume V = 1 μm3. The nebulizer used in this work produces initial droplets with a linear size of about 3 μm [10,11], i.e. V = 10 μm3 and k ~600. The width of the charge distribution in such a droplet is (2 × 600)1/2 ≈ 24.5. Taking into account this consideration, it can be suggested that the further splitting of the drops in the ATBDI results in the increase in ion current proportionally to the N. It should be noted that the decrease in the droplet volume to 0.01 μm3 leads to the distribution width of 1, and this is the threshold value giving the increase in the ion current. Thus, an increase in the ion current in our experiments is supposed to be associated with the number of droplets obtained during the thermal breakup of the initial droplet.

Fig. 2. The total ion current of the mass spectrometer (m/z: 150–1500) versus the temperature of the suction capillary, Tsuck. The data are shown for 10−4 g mL−1 solutions of TNT in: 1 – water (black circles), 2 – acetonitrile (red squares), 3 – ethanol (blue triangles). The ATBDI method, an applied fragmentation voltage – 0 V. For clarity, the current values are normalized to the value measured at a temperature of 50 °C.. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

“Model”. The critical temperature, surface tension, and boiling point are shown in Table 1. Also, in the table, there are data on the change in the total ion current with an increase in the temperature of the capillary (I235°C/I50°C). It is noteworthy that for water the critical temperature is more than 100 °C higher than for other solvents. Therefore, according to equation (2), the surface tension drops, droplets breakup, and the charges appear at a higher temperature; this is exactly observed in the experiment (Fig. 2). It should be noted that the temperature indicated in the figure is the temperature of the gas flowing in the capillary (see “Experimental”). The temperature of the droplet may be higher since it absorbs infrared radiation from the walls of the capillary stronger. Furthermore, the surface tension of water is the highest. This gives the highest pressure inside the drop according to the equation (1) and consequently more energetic explosive breakup of a droplet. Therefore, it is more likely that droplets resulting from an explosive breakup are charged. It is seen from Fig. 1, but more clearly seen from the data on the target compound (see below). Considering the decrease in the total ion current during thermal spraying of ethanol solution, it should be noted that its boiling point is the lowest of all the solvents studied. Consequently, the evaporation of ethanol presumably occurs before the explosive decomposition could occur under the conditions of our experiment (that is, the “slow heating” is realized, Fig. 1). This evaporation reduces the size of the droplets and they diffuse more readily to the walls of the capillary. The charged droplet loss to the wall leads to a decrease in the total ion current, even if they were initially in the flow. Nevertheless, the mass spectra of the TNT can be obtained from alcohol solutions with the ATBDI method. The temperature dependence of the peak of the deprotonated TNT (m/z 226, (С6H2CH3(NO2)3–H)-) was investigated for different solvents (concentration of TNT equal to 10−4 g mL−1) and the results are shown in Fig. 3. Since the solvent makes it difficult to detect the target peak, the fragmentor voltage of 100 V was applied (the raw spectra obtained by ATBDI for water, ethanol, and acetonitrile at fragmentation voltages of 0 and 50 V can be

3.2. TNT To study the ionization of TNT during the thermal breakup of droplets, we have chosen three solvents – water, acetonitrile, and ethanol, thus, the effect of solvent will be investigated. We wanted to establish the limits of ionization for TNT in solutions when passing through a hot capillary. In Fig. 1, there are dependences of the total ion current of the mass spectrometer on the temperature of the suction capillary Tsuck in the range of 150–1500 m/z. In these experiments, the TNT concentration in water, acetonitrile, and ethanol was 10−4 g mL−1, however, peaks corresponding to TNT were not distinguished, and only the total current of the mass spectrometer was measured. For clarity, the current values are normalized to the value measured at a temperature of 50 °C. From Fig. 2, it can be seen that the ATBDI gives significantly different results for different solvents. For ethanol, for example, there is a slight increase in the ion current when starting the heating. A further increase in the temperature of the capillary leads to a decrease in the current. When acetonitrile is used as a solvent, the ion current increases monotonously with increasing capillary temperature. For water, the effect appears at a higher temperature (as compared with the other solvents), but the ion current increases sharply giving the ionization efficiency higher than acetonitrile. To understand this behavior, we considered the properties of the studied solvents, important in view of the theory described in the 3

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Table 1 Properties of the solvents. a

water acetonitrile ethanol a b

Critical temperature (TC), °C

374 272 241

a

Surface tension (σ), mN/m

72.75 29.3 22.75

a

Boiling temperature (TBoil), °C

100 82 78

b

Total current change (I235°C/I50°C)

3.2 2.8 0.8

The data are taken from the reference book [25]. the data are obtained in this paper.

unwanted peaks (Fig. 4c). In Fig. 4, there are mass spectra for water, ethanol, and acetonitrile solutions of TNT (10−4 g mL−1) obtained using the ATBDI with an applied fragmentor voltage of 100 V. The most simple and obvious spectrum was obtained for an aqueous solution (Fig. 4a). Besides the deprotonated monomer and dimer (m/z 226, (С6H2CH3(NO2)3–H)- and m/z 453, ((С6H2CH3(NO2)3)2–H)-, respectively) the oxidized form of TNT (m/z 242, (С6H2CH3(NO2)3+O–H)-) is shown. Water clusters with these ions can also be seen in the spectrum. When ethanol is used as a solvent (Fig. 4b), the total intensity of the spectrum decreases, and the m/z 406 peak starts to dominate (we previously assigned this peak to the deprotonated TNT dimer with the loss of the HNO2 fragment, ((С6H2CH3(NO2)3)2–HNO2–H)-. The m/z 406 peak is visible in the ATBDI mass spectra no matter what solvent is used. This peak can be the product of decomposition of the TNT dimer in the heated capillary. Nevertheless, the deprotonated TNT monomer and corresponding clusters with ethanol are visible in the spectrum allowing the use of ethanol in experiments with TNT. The most weak, complex, and difficult for interpretation mass spectrum was obtained for the acetonitrile solution (Fig. 4c). It can be seen that single peaks are decomposed into several peaks and the most intense peak in the spectrum is associated with the fragment involving solvent molecules. This additionally supports the assumption that the ketenimine form of acetonitrile contributes to the neutral clustering with TNT and prevents the appearance of simple TNT ions. This information is important for qualitative and quantitative mass spectrometric analysis. Considering the intensity and the overall view of the spectra recorded, we investigated the concentration dependences only for aqueous and ethanol solutions of TNT (Fig. 5). It should be noted that the peak of the deprotonated TNT monomer is absent in the ESI mass spectra obtained from the acetonitrile solution of TNT (Fig. S5a, SM), which confirms the hypothesis about strong interaction of TNT and acetonitrile molecules. On the contrary, in the ESI mass spectra obtained from aqueous and aqueous-ethanol solutions of TNT, the ion at m/z 226, (С6H2CH3(NO2)3–H)-, was detected (Fig. S5 b and c, SM). However, the quality of these mass spectra is worse than the quality of the ATBDI mass spectra obtained from an aqueous solution of TNT (the intensity is lower and the background peaks are present, Fig. 4a). Thus, in the case of an aqueous TNT solution with a concentration of 10−4 g mL−1 the ATBDI gives the peak of the target compound at m/z 226 almost an order of magnitude higher than the ESI (Figs. 4a and S5b). It should be noted that water is not the best solvent for ESI ionization. Moreover, the ATBDI ionization efficiency only decreases when ethanol or acetonitrile is added to water. On the contrary, water/acetonitrile 70/30 v/v and water/methanol 50/50 v/v mixtures acidified to ~0.1% are considered standard mobile phases in ESI-MS. Considering the observed phenomenon, it can be proposed that the addition of acetonitrile or ethanol to water results in a decrease in the surface tension. Thus, instead of the explosive breakup, the solvent evaporation takes place in charged droplets formed from the solution. In Fig. 5, there are concentration dependences of the most intense TNT peaks for aqueous and ethanol solutions of TNT obtained with ATBDI (the m/z 226 in case of aqueous solution and m/z 406 in case of ethanol solution, Fig. 4 a, b). The concentration dependences were also obtained for other peaks at different capillary temperatures and

Fig. 3. The TNT peak intensity (m/z 226, (C6H2CH3(NO2)3–H)-) versus the temperature of the suction capillary, Tsuck. The data are shown for 10−4 g mL−1 solutions of TNT in: 1 – water (black circles), 2 – acetonitrile (red squares), 3 – ethanol (blue triangles). The ATBDI method with an applied fragmentation voltage of 100 V was used.. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

seen in Supplementary material (SM), Figs. S2–S4). Water is the most effective solvent again, and the ionization efficiency of the target compound increases by an order of magnitude when heating the suction capillary. Assuming the ion current proportional to the number of drops in our experiments (see the “Model”) the initial drop of water divides into about 10 parts during the explosive breakup. Although for ethanol solution of TNT the total ion current decreases with increasing the temperature of the suction capillary (Fig. 2), the target peak of TNT, m/z 226, increases up to a temperature of the suction capillary equal to 265 °C (Fig. 3). This is indicative that explosive droplet breakup still occurs for ethanol. The survival of charged droplets with TNT ions in a heated capillary is more probable since the diffusion of lighter particles without TNT ions occurs faster to the capillary walls. It can be proposed that multiply charged drops also survive poorly in the suction capillary: such particles are attracted more strongly to the capillary walls by inducing the “mirror” charge. The weakest signals of deprotonated TNT was obtained when spraying the acetonitrile solution (Fig. 3), although the total ion current in acetonitrile is rather high. It is known that acetonitrile inhibits the ionization of compounds containing carbonyl or peroxide groups in the ESI and APCI [26]. This is likely due to the neutral clustering of molecules. A similar effect may occur in the case of ATBDI of acetonitrile solutions. Additionally, since nitrile groups are highly polar possessing large dipole moments [27], a ketenimine form exists in acetonitrile (i.e., N^C–CH3↔HN]C–CH3) [28]. Although the ketenimine form of acetonitrile is present in an insignificant amount, an excess of acetonitrile leads to a neutral clustering of TNT molecules with the ketenimine form of acetonitrile, which prevents the appearance of deprotonated TNT ions and contributes to the appearance of numerous 4

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Fig. 4. The mass spectra obtained using the ATBDI with an applied fragmentor voltage of 100 V of 10−4 g mL−1 solutions of TNT in: a – water (black), b – ethanol (blue), c – acetonitrile (red). The capillary temperature was set at 155 °C. The detailed information about the peaks assignments can be found in the text.. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

fragmentor voltages (some of these dependences are shown in Figs. S6 and S7, SM). Unfortunately, the intensity of the studied peaks depends linearly on the concentration in a considerably narrow range for all the solvents. For the ATBDI method, a very limited linear range of concentration dependences was noticed earlier and is most likely associated with the coexistence of several analyte molecules in one drop. For a relatively simple ionization process (for example, by protonation), the way out of this situation is the simultaneous detection of monomeric and dimeric peaks and analysis of their intensity ratio [15]. However, as shown in Fig. 4, the ionization of TNT in the ATBDI is rather complex giving a quasi-dimeric peak (m/z 406) and other peaks; this prevents the use of the simple method. Therefore, the internal standard methods should be used in quantitative ATBDI experiments with TNT. Nevertheless, for the ATBDI the LOD of TNT in aqueous solution was evaluated to be about 10−7 g mL−1 (signal to noise ratio, S/N = 3). Since the solution of TNT in ethanol gives less intense signals, the LOD is approximately an order of magnitude lower in this case. The ATBDI mass spectra of water and ethanol TNT solutions (10−6 g mL−1) around the m/z 226 peak can be seen in Fig. S8. It should be mentioned that the LODs were obtained for a single quadrupole mass spectrometer in scan mode. More advanced

instruments (MS/MS, high-resolution mass spectrometry) give significantly lower limits of detection, for example, for TNT water-methanol solution with the addition of ammonium acetate the LOD of 10−10 g mL−1 was obtained with ESI [29,30]. However, the undoubted advantage of ATBDI over ESI is the surprisingly weak influence of impurities in solution. For example, we have shown that the presence of plant residues in the opium solution does not affect significantly the ATBDI mass spectra [31]. This phenomenon deserves a detailed study and will be investigated in the future. As with any other ionization source, the ATBDI has some disadvantages. They are described in detail in Ref. [31]. Firstly, there is a memory effect (especially unpleasant when working with low concentrated solutions). Secondly, it is necessary to prepare a large volume of the solution (3–5 mL), which is associated with the “dead” volume in the nebulizer. Thirdly, the ATBDI ionization is complicated in the presence of salts with a concentration above 10−4 M. Thus, the proposed device can be improved in terms of both ionization efficiency and the required sample volume. Also, at the moment the ATBDI is not coupled with other separation techniques, for example, liquid chromatography. An improvement of the ATBDI device is necessary and will be performed in our subsequent works.

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Fig. 5. The TNT peaks intensity versus the concentration of TNT in the solution when using the ATBDI with the suction capillary temperature of 155 °C: a – aqueous solution, m/z 226 (circles), b – ethanol solution, m/z 406 (triangles). The curves 1 and 3 correspond to the fragmentor voltage of 50 V, the curves 2 and 4 – to the fragmentor voltage of 100 V.

4. Conclusions

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In this work, the ATBDI ionization was investigated by analysis of TNT in various solvents: acetonitrile, water, and ethanol. The mechanisms of ion formation during the passage of small droplets through a heated suction capillary of a mass spectrometer were discussed. Understanding of the physical principles of the formation of the ions in a heated capillary can help us to improve the ionization efficiency of the target compound. In particular, an increase in the surface tension of the solvent leads to an increase in the yield of ions. This leads to the use of water as the most desirable solvent in the ATBDI in contrast to ESI. Perhaps, the additives changing the structure of the liquid surface can be also efficient. Additionally, the capillary used in the ATBDI can be considered as an ideal-displacement reactor. Therefore, it is possible to investigate chemical reactions using the ATBDI (such as reactions of thermal decomposition). Since microdroplet chemistry is currently intensively studied (the reaction rates can increase up to several orders of magnitude [32]), we are planning to apply the ATBDI in this area. Declaration of competing interest The authors declare no conflict of interest. Acknowledgements This research was supported by Russian Science Foundation under Grant No. 18-79-00136. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.talanta.2020.120770. References [1] J.B. Fenn, M. Mann, C.K. Meng, S.F. Wong, C.M. Whitehouse, Electrospray ionization for analysis of large molecules, Science 246 (1989) 64–67, https://doi.org/

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