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Direct Liquid Sampling for Corona Discharge Ion Mobility Spectrometry Martin Sabo,† Michaela Malásková,† Olga Harmathová,† Jasna Hradski,‡ Marián Masár,‡ Branislav Radjenovic,§ and Štefan Matejčík*,† †

Department of Experimental Physics, Faculty of Mathematics, Physics and Informatics, Comenius University in Bratislava, Mlynská dolina F2, 842 48, Bratislava, Slovakia ‡ Department of Analytical Chemistry, Faculty of Natural Sciences, Comenius University in Bratislava, Mlynská dolina, Ilkovičova 6, 842 15, Bratislava, Slovakia § Institute of Physics, University of Belgrade, P.O. Box 57, 11080 Belgrade, Serbia ABSTRACT: We present a new technique suitable for direct liquid sampling and analysis by ion mobility spectrometry (IMS). The technique is based on introduction of a droplet stream to the IMS reaction region. The technique was successfully used to detect explosives dissolved in methanol and oil as well as to analyze amino acids and dipeptides. One of the main advantages of this technique is its ability to analyze liquid samples without the requirement of any special solution.

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water resources,9 head space multicapillary column gas chromatography (HS-MCCGS) was successfully applied for the determination of three different categories of virgin olive oils,10 membrane extraction (ME) was used for the analysis of chlorinated hydrocarbons in water,11 and stir bar sorptive extraction (SBSE) was applied for the detection of explosives in liquid samples.12 More applications, advantages, and drawbacks of these extraction techniques can be found in review articles.13,14 The most important development in the field of liquid sample analysis by IMS was achieved by the introduction of an electrospray ionization source (ESI).15 The ESI interface has been applied for the analysis of proteins,15,16 peptides,17 amino acids,18 explosives,19 and food contaminants,20 and it has been incorporated into human blood profiling.21 Corona discharge for liquid sampling in IMS was reported for the first time by Shumate and Hill.22 They applied this coronaspray technique for the simultaneous nebulization and ionization of several organic acids. An appropriate technique for liquid sampling in IMS should be selected on the basis of the required parameters, such as geometric dimension, time consumption, sample preparation, and sensitivity. In this work, we present, for the first time, a new liquid sampling technique for IMS. This technique is based on

ost modern analytical techniques are based on solutionphase analysis. Therefore, it is apparent that there is an increasing demand for analysis of liquids in different fields of science and applications. Forensic and security applications require liquids to be monitored in order to ensure public health and safety. The analysis of liquid samples in medical and biochemical applications plays an important role in understanding physiological mechanisms and protecting human health. In addition, monitoring wastewater is important for industrial applications and environmental protection. Ion mobility spectrometry (IMS) is, due to its compact design, high sensitivity, and fast response, a valuable analytical tool in many of the above-mentioned applications.1,2 IMS is a separation technique based on the drift of ions in the buffer gas in a weak homogeneous electric field. Among other factors, the separation depends on the mass of the ions and their geometry. The most relevant applications of this technique include detection of explosives,3 warfare agents,4 environmental monitoring,5 human breath analysis,6 pharmaceutical applications,7 and air quality monitoring (e.g., International Space Station).8 Although IMS is readily applicable for the detection of trace gases, analyzing liquid samples is a more challenging task for this technique. Several solutions for the analysis of volatile and semivolatile compounds in the liquid phase have been proposed. Solid-phase microextraction (SPME) was successfully used for the determination of parabens in pharmaceutical formulations, 7 dispersive liquid−liquid microextraction (DLLME) was used for the detection of malathion in various © XXXX American Chemical Society

Received: April 27, 2015 Accepted: June 25, 2015

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DOI: 10.1021/acs.analchem.5b01585 Anal. Chem. XXXX, XXX, XXX−XXX

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Analytical Chemistry

managed by the control unit. During the experiment, IMS was operated in both polarities and the drift field intensity used was 583 V cm−1. The injection time of the Bradbury−Nielsen-type shutter grid was 10 μs with a duty cycle of 50 Hz. Sixty four IMS spectra were averaged and recorded by the control unit. The drift gas flow was 1 L min−1, and the sample flow was 0.6 L min−1. The IMS operated at a pressure of 500 mbar; the pressure inside the drift tube was permanently controlled and recorded. Under these conditions, ambient air was constantly sucked into the IMS reaction region via the sample inlet capillary. The IMS instrument was calibrated on the basis of reduced mobility of 2,6-di-tert-butylpyridine.23 The operating parameters of the instrument are summarized in Table 1.

the thermal spray of the liquid, where the mixture of sprayed vapors and droplets is directly injected into the IMS instrument. Subsequently chemical ionization of the sample occurs in the IMS reaction region. In contrast to the work of Shumate and Hill,22 in the present work, the processes of nebulization and ionization are strictly separated. The technique was successfully tested to detect explosives in different solvents as well as to analyze amino acids and dipeptides.

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EXPERIMENTAL SECTION IMS Instrument. The IMS instrument used in this work was developed at the Department of Experimental Physics, Comenius University in Bratislava. A 3D model of the instrument is shown in Figure 1a. The instrument is equipped with a corona discharge (CD) ionization source; the length of the reaction region is 1.5 cm, and the length of the drift tube is 11.05 cm. The drift field intensity, drift tube temperature, gas flows, and pressure inside the drift tube are optional and fully

Table 1. Operating Parameters of the IMS Instrument IMS drift tube length drift field intensity IMS operating temperature IMS operating pressure shutter grid pulse width shutter grid frequency drift gas flow sample gas flow

11.05 cm 583 V cm−1 345 K (negative polarity) 318 K (positive polarity) 500 mbar 10 μs 50 Hz 1 L min−1 0.6 L min−1

Direct Liquid Sampling (DLS) Unit. A detailed view of the DLS unit is shown in Figure 1b, which demonstrates that the DLS is a small, compact unit with three capillary inlets. The first capillary (o.d. 1.5 mm, i.d. 0.3 mm) was used for liquid sample introduction to the DLS. The liquid flow rate of the DLS unit can be set as optional. The liquid flow used in the experiment ranged from 20 to 200 μL min−1. The second capillary (o.d. 1.5 mm, i.d. 1.2 mm), the so-called cooling capillary, was used for atmospheric air (temperature 296 K) injection. The air flowing out of the second capillary cooled and protected the first capillary against the heated environment because the evaporation of the liquid sample inside the first capillary can result in capillary clogging. The third capillary (o.d. 1.5 mm, i.d. 1.2 mm) was used for heating the atmospheric air to 680 K. The heated air was mixed together with the protection gas and liquid sample in order to promote the evaporation of the liquid. The result is that the DLS unit is able to generate a droplet stream, as seen in Figure 1c. The temperature of the droplet stream used in the experiment was 480 K. The droplet stream was located directly opposite the IMS sample capillary inlet. The droplet stream was sucked into the heated capillary inlet and transported into the IMS reaction region, where chemical ionization of the analyte occurred. Limit of Detection (LOD) Calculation. The ability to adjust the liquid flow of the DLS unit allows the flow rate of the liquid injected into the IMS instrument to be changed. The amount of sample injected into the IMS is directly proportional to the rate of the liquid flow, and various liquid flows result in the analytes responding differently. Figure 2 demonstrates different IMS responses for TNT dissolved in methanol at a 10 μg mL−1 concentration under liquid flow rates from 20 to 180 μL min−1. Thus, the response represents not only the concentration of the analyte in the liquid sample but also the amount of analyte injected into the IMS in 1 s (ng s−1). For this reason, the LOD was also given in nanograms per second and was measured

Figure 1. (a) Three-dimensional model of the IMS instrument with a DLS unit; (b) DLS unit details: (1) capillary for liquid sample, (2) cooling gas capillary, and (3) heating capillary; (c) photograph of a droplet stream from the DLS unit. B


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Figure 2. IMS response for TNT at a 10 μg mL−1 concentration dissolved in methanol under different liquid flow rates.

directly while taking into consideration a signal-to-noise ratio of 3 (S/N = 3). Gases and Chemicals. Atmospheric air at a flow rate of 1 L min−1, additionally purified by molecular sieve traps (Agilent), was used as the drift gas. Explosives. The explosives, 2,4,6-trinitrotoluene (TNT), cyclotrimethyl-enetrinitramine (RDX), pentaerythritol tetranitrate (PETN), SEMTEX, and C4, were obtained from the Ministry of Defense of Slovak Republic with purities up to 99%. The liquid samples were prepared in the following manner. A specific amount of TNT was dissolved in methanol (analytical grade purity), and the TNT solution was directly analyzed by DLS-IMS. RDX, PETN, SEMTEX, and C4 were dissolved in acetone (analytical grade purity). These solutions were further diluted with methanol until the required concentration of explosive in the solution was achieved. Samples of the explosives in oil (diesel fuel obtained from a local gas station) were prepared in the following manner. Two milligrams of explosive was added to 5 mL of oil. The explosives were left to dissolve in the oil for 2 days under laboratory temperature. During this period, only TNT was fully dissolved; the other explosives dissolved only partially. To determine the LOD, the oil was diluted with methanol and the solution was introduced into the DLS unit for further investigation. Amino Acids and Dipeptides. Amino acids glycine (Gly), serine (Ser), leucine (Leu), isoleucine (Ile), and phenylalanine (Phe), as well as dipeptides glycine−alanine (Gly−Ala) and aspartame, all of analytical grade purity, were obtained from Sigma-Aldrich. The standard solutions were prepared at 1 mg mL−1 concentrations by dissolution in deionized water. The solutions were then diluted with deionized water in order to reach the desired concentration.

Figure 3. DLS-IMS response for (a) TNT, RDX, and PETN, (b) C4, and (c) SEMTEX.

speculate that molecules of the methanol solvent, injected into the IMS together with the explosives, form strong clusters of Cl− ions. Thus, we assigned the RIP to Cl−·(CH4O)n clusters. The response of the IMS instrument for the explosive samples results in the formation of three additional peaks with drift times of 4.84, 5.07, and 6.09 ms and with corresponding K0 values of 1.54, 1.48, and 1.24 cm2 V−1 s−1, respectively. The IMS peak with a K0 value of 1.54 cm2 V−1 s−1 was assigned to a proton abstracted anion of TNT, (TNT − H)−, the peak with a K0 value of 1.48 cm2 V−1 s−1 was assigned to a Cl−·RDX cluster, and the peak with a K0 value of 1.24 cm2 V−1 s−1 was assigned to a Cl−·PETN cluster. The reduced mobilities of these species are in very good agreement with values

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RESULTS AND DISCUSSION Explosives Detection. The IMS spectrum of the mixture of investigated explosives (4 μg mL−1 TNT, 10 μg mL−1 RDX, 10 μg mL−1 PETN) diluted in methanol (liquid flow, 40 μL s−1) is shown in Figure 3a. The admixture of CCl4 dopant to CD24 results in the formation of a reactant ion peak (RIP) with a drift time of 3.26 ms and reduced ion mobility (K0) of 2.27 cm2 V−1 s−1. The RIP is composed of Cl− ions. The reduced mobility of Cl− ions in IMS is usually significantly greater.25 We C


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Analytical Chemistry reported in the literature.3,26 The response of the instrument to the plastic explosives C4 and SEMTEX results in the detection of the explosives of which they are composed. Figure 3b shows that a Cl−·RDX peak with a K0 value of 1.48 cm2 V−1 s−1 was found for C4 as an RDX-based plastic explosive. Similarly, Figure 3c shows the formation of a Cl−·PETN peak (K0 of 1.24 cm2 V−1 s−1) in the case of SEMTEX (a PETN-based explosive material). The results obtained for plastic explosives by DLSIMS are similar to those obtained by ESI-IMS.27 The LODs obtained for the tested explosives were as follows: 840 pg s−1 for TNT, 2 ng s−1 for RDX, 1.7 ng s−1 for PETN, 1.4 ng s−1 for C4, and 2.2 ng s−1 for SEMTEX, as listed in Table 2. Table 2. LOD of Explosives LOD (ng s−1) methanol oil

TNT

RDX

PETN

C4 (RDX)

SEMTEX (PETN)

0.8 1.7

2 100

1.7 100

1.4 33

2.2 56

The detection of explosives present in different samples is required to ensure safety and security against terrorism and other hazards. Therefore, the ability of the DLS-CD-IMS technique to directly analyze explosives present in oil was also demonstrated. In the model experiment, 2 mg of explosive was added to 5 mL of oil. After 2 days, the small volume of oil was mixed with methanol (typically, 0.1 mL of oil was mixed with 0.9 mL of methanol), and the sample was then analyzed by DLS-CD-IMS. The present method is suitable for the direct analysis of oil samples as well as for analysis of the sample after dilution with methanol. The results for both cases will be shown. An increase in the oil/methanol liquid flow rate from 0 to 100 μL min−1 resulted in a shift of the Cl− RIP from a drift time of 2.82 ms (K0 of 2.63 cm2 V−1 s−1) to a drift time of 3.15 ms (K0 of 2.35 cm2 V−1 s−1), as can be seen in Figure 4a. We assign the shift in the RIP to the clustering of Cl− ions25 as the liquid flow rate increases. The presence of oil also resulted in the formation of two additional peaks with drift times of 6.81 and 7.52 ms (K0 of 1.09 and 0.99 cm2 V−1 s−1, respectively). On the other hand, increasing the liquid flow rate does not affect the drift time of the explosives. Figure 4b demonstrates the stability of (TNT − H)− ions as a function of the liquid flow rate. RDX, C4, PETN, and SEMTEX were also analyzed in oil samples, as seen in Figure 4c. The LODs of explosives dissolved in oil were in the range 1.7−100 ng s−1. The LOD values obtained for explosives dissolved in oil and methanol are listed in Table 2. As seen from this table, the LOD for TNT dissolved in oil is comparable to that of TNT dissolved in methanol. However, the LOD values of other explosives dissolved in oil are nearly 2 orders of magnitude higher than those when these compounds are dissolved in methanol. The main reason for these differences we ascribe to the high solubility of TNT in oil; the other explosives were not completely dissolved in oil. From Table 2, it is also evident that the LOD values of RDX and PETN in oil are higher than those of plastic explosives based on these compounds. We associate this with the additives present in plastic explosives, which can be the reason that these compounds dissolve better in oil samples. Amino Acids. Amino acids are the building blocks of proteins, and they are essential for many life processes. The possibility of using DLS-IMS to detect and analyze these organic compounds was also examined in this work. IMS

Figure 4. (a) Shift in the RIP as a function of the liquid flow rate; (b) stability of the (TNT − H)− peak as a function of the liquid flow rate; (c) response of RDX, C4, PETN, and SEMTEX in oil samples.

spectra of the investigated amino acids are shown in Figure 5a. Contrary to the polarity used when analyzing the explosives, amino acids were analyzed in positive polarity mode. The amino acids were diluted in deionized water without any other solvents. In positive mode, an RIP with a drift time of 3.8 ms (K0 of 2.12 cm2 V−1 s−1) is composed of H3O+·(H2O)n ions. With IMS, the amino acids examined result in the formation of peaks with drift times of 4.35, 4.53, 5.04, 4.97, and 5.35 ms (K0 of 1.86 (Gly), 1.78 (Ser), 1.61 (Leu), 1.62 (Ile), and 1.51 cm2 V−1 s−1 (Phe), respectively). Leu and Phe were also D


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we suggest that the difference in the drift gases used is responsible for this disagreement. The LOD values obtained were in the range 6−60 ng s−1 for the studied amino acids. The LOD values of amino acids are also listed in Table 3. Table 3. LOD of Amino Acids and Dipeptides LOD (ng s−1) Gly

Ser

Leu

Ile

Phe

Gly−Ala

aspartame

22

60

9

6

17

35

160

The response of DLS-IMS to amino acids depends strongly on the solvent in which they are dissolved. Figure 5b,c demonstrates this for Leu and Phe dissolved in deionized water and methanol. Close inspection of the spectra shows that changing the solvent from deionized water to methanol results in a shift of the peaks from a drift time of 5.04 ms (K0 of 1.61 cm2 V−1 s−1) to 5.17 ms (K0 of 1.56 cm2 V−1 s−1) for Leu (Figure 5b) and from a drift time of 5.35 ms (K0 of 1.51 cm2 V−1 s−1) to 5.4 ms (K0 of 1.49 cm2 V−1 s−1) for Phe (Figure 5c). The shift in the positions of the peaks is a result of the strong clustering of amino acids. The MS investigations of this behavior show that only water clusters were present in the spectrum when deionized water was used as a solvent, whereas in the case of methanol, methanol clusters of the analytes were observed. Contrary to MS analysis, the ions and their clusters are usually not separated by IMS due to their drift in thermodynamic equilibrium.28 The changes in RIP can also be observed in Figure 5b,c. In the case of water as the solvent, the RIP was composed only of protonated water clusters H+·(H2O)n. However, changing the solvent also resulted in a change in the RIP to methanol trimmers H+·M3 due to the higher proton affinity of methanol molecules. Dipeptides. Dipeptides Gly−Ala and aspartame were also investigated by the DLS-IMS technique. Figure 6a shows the IMS response of Gly−Ala dissolved in deionized water. The IMS response of this dipeptide resulted in the formation of two peaks with drift times of 4.92 and 5.18 ms (K0 of 1.65 and 1.56 cm2 V−1 s−1, respectively). We suggest that the peak with a K0 value of 1.65 cm2 V−1 s−1 is composed of protonated dipeptide Gly−Ala ions H+·M, whereas the second peak with a K0 value of 1.56 cm2 V−1 s−1 can be assigned to ammonia clusters NH4+· M. The IMS response of aspartame molecules also resulted in the formation of two peaks, as seen in Figure 6b. The peak with a drift time of 5.35 ms (K0 of 1.51 cm2 V−1 s−1) was assigned to Phe. Since Phe is one of the amino acids present in aspartame, the presence of these species can be explained by the thermal decomposition of aspartame molecules during their transport in the DLS unit. The second peak, with a drift time of 6.68 ms (K0 of 1.21 cm2 V−1 s−1), was assigned to protonated molecules of aspartame H+·M.29 Considering the differences in the drift gas composition in our work (purified air) and in the work of Fernandez-Maestre and Hill29 (N2), the values of the reduced mobilities are in a fairly good agreement. The LODs for the studied dipeptides were as follows: 35 ng s−1 for Gly−Ala and 160 ng s−1 for aspartame (Table 3).

Figure 5. (a) DLS-IMS response of amino acids Gly, Ser, Leu, Ile, and Phe; (b) DLS-IMS response of Leu in deionized water and methanol; (c) DLS-IMS response of Phe in deionized water and methanol.

investigated by mass spectrometry (MS), where the DLS unit was interfaced to a hybrid IMS-orthogonal acceleration time-offlight MS (oaTOFMS) instrument.28 We detected exclusively the protonated adduct ions, M·H2O·H+. Accordingly, we assume that the protonated adduct ions, M·H2O·H+, are also formed by the reaction of RIP with other amino acids. The reduced mobility values reported in this work slightly differ from those reported by Beegle et al.18 The main reason for this is the use of a different drift gas in this study compared to that by Beegle et al. While Beegle et al. used high-purity N2, in our case, purified atmospheric air was used. Since the drift of ions in IMS is strongly dependent on the composition of the drift gas,1

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CONCLUSIONS A new liquid sampling technique for direct liquid analysis by IMS was presented. The DLS unit was constructed and E



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REFERENCES

(1) Eiceman, G. A.; Karpas, Z.; Hill, H. H. Ion Mobility Spectrometry, 3rd ed.; CRC Press: Boca Raton, FL, 2013. (2) Borsdorf, H.; Mayer, T.; Zarejousheghani, M.; Eiceman, G. A. Appl. Spectrosc. Rev. 2011, 46, 472. (3) Ewing, R. G.; Atkinson, D. A.; Eiceman, G. A.; Ewing, G. J. Talanta 2001, 54, 515. (4) Makinen, M. A.; Anttalainen, O. A.; Sillanpaa, M. E. T. Anal. Chem. 2010, 82, 9594. (5) Koester, C. J.; Moulik, A. Anal. Chem. 2005, 77, 3737. (6) Baumbach, J. I. J. Breath Res. 2009, 3, 034001. (7) Lokhnauth, J. K.; Snow, N. H. Anal. Chem. 2005, 77, 5938. (8) Johnson, P. V.; Beegle, L. W.; Kim, H. I.; Eiceman, G. A.; Kanik, I. Int. J. Mass Spectrom. 2007, 262, 1. (9) Jafari, M. T.; Riahi, F. J. Chromatogr. A 2014, 1343, 63. (10) Garrido-Delgado, R.; Arce, L.; Valcarcel, M. Anal. Bioanal. Chem. 2012, 402, 489. (11) Du, Y.; Zhang, W.; Whitten, W.; Li, H.; Watson, D. B.; Xu, J. Anal. Chem. 2010, 82, 4089. (12) Lokhnauth, J. K.; Snow, N. H. J. Chromatogr. A 2006, 1105, 33. (13) Holopainen, S.; Nousiainen, M.; Anttalainen, O.; Sillanpaa, M. E. T. TrAC, Trends Anal. Chem. 2012, 37, 124. (14) Arce, L.; Menendez, M.; Garrido-Delgado, R. G.; Valcarcel, M. TrAC, Trends Anal. Chem. 2008, 27, 139. (15) Wittmer, D.; Chen, Y. H.; Luckenbill, B. K.; Hill, H. H. Anal. Chem. 1994, 66, 2348. (16) Wu, C.; Siems, W. F.; Asbury, G. R.; Hill, H. H. Anal. Chem. 1998, 70, 4929. (17) Wu, C.; Siems, W. F.; Klasmeier, J.; Hill, H. H. Anal. Chem. 2000, 72, 391. (18) Beegle, L. W.; Kanik, I.; Matz, L.; Hill, H. H. Anal. Chem. 2001, 73, 3028. (19) Asbury, G. R.; Klasmeier, J.; Hill, H. H. Talanta 2000, 50, 1291. (20) Midey, A. J.; Camacho, A.; Sampathkumaran, J.; Krueger, C. A.; Osgood, M. A.; Wu, C. Anal. Chim. Acta 2013, 804, 197. (21) Dwivedi, P.; Schultz, A. J.; Hill, H. H. Int. J. Mass Spectrom. 2010, 298, 78. (22) Shumate, C. B.; Hill, H. H. Anal. Chem. 1989, 61, 601. (23) Eiceman, G. A.; Nazarov, E. G.; Stone, J. A. Anal. Chim. Acta 2003, 493, 185. (24) Puton, J.; Nousiainen, M.; Sillapaa, M. Talanta 2008, 76, 978. (25) Mayer, T.; Borsdorf, H. Anal. Chem. 2014, 86, 5069. (26) Crawford, C. L.; Hill, H. H. Talanta 2013, 107, 225. (27) Hilton, C. K.; Krueger, C. A.; Midey, A. J.; Osgood, M.; Wu, J.; Wu, C. Int. J. Mass Spectrom. 2010, 298, 64. (28) Sabo, M.; Matejčík, Š. Anal. Chem. 2012, 84, 5327. (29) Fernandez-Maestre, R.; Hill, H. H. Int. J. Ion Mobility Spectrom. 2009, 12, 91.

Figure 6. IMS spectra of (a) Gly−Ala and (b) aspatrame.

successfully tested for the analysis and detection of explosives, amino acids, and dipeptides. One of the main advantages of the developed DLS-IMS technique in comparison to ESI-IMS is its ability to analyze any liquid sample without the requirement of any special solution. The DLS unit provides many opportunities for further applications in the environmental monitoring, security, forensic, and pharmaceutical areas. We believe that the DLS-IMS technique reported in this work is a step toward the development of new combined techniques with analytical separation methods performed on the microscale suitable for the analysis of liquid samples. Interfacing these techniques together will increase their high-throughput and analytical power for solving difficult (bio)analytical tasks and challenges.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; Fax: +421-2-6542-9980. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by Slovak Research and Development Agency project nos. APVV-0733-11 and APVV-0259-12. We acknowledge instrumental support by MaSa Tech company (Slovakia). F


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