Thin Layer Chromatography-Ion Mobility Spectrometry (TLC-IMS

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Thin Layer Chromatography-Ion Mobility Spectrometry (TLC-IMS) Vahideh Ilbeigi, and Mahmoud Tabrizchi Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/ac502685m • Publication Date (Web): 24 Nov 2014 Downloaded from http://pubs.acs.org on December 3, 2014

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Analytical Chemistry

Thin Layer Chromatography-Ion Mobility Spectrometry (TLC-IMS) Vahideh Ilbeigi, Mahmoud Tabrizchi* [email protected] v. [email protected] Department of Chemistry, Isfahan University of Technology, Isfahan, 84156-83111, Iran.

ABSTRACT Ion mobility spectrometry (IMS) is a fast and sensitive analytical method which operates at the atmospheric pressure. To enhance the capability of IMS for the analysis of mixtures, it is often used with pre-separation techniques such as GC or HPLC. Here, we report for the first time the coupling of the thin-layer chromatography and IMS. A variety of coupling schemes were tried that included direct electrospray from the TLC strip tip, indirect electrospray from a needle connected to the TLC strip, introducing the moving solvent into the injection port, and, the simplest way, offline introduction of scratched or cut pieces of strips into the IMS injection port. In this study a special solvent tank was designed and the TLC strip was mounted horizontally where the solvent would flow down. A very small funnel right below the TLC tip collected the solvent and transferred it to a needle via a capillary tubing. Using the TLC-ESI-IMS technique, acceptable separations were achieved for two component mixtures of morphine-papaverine and acridine-papaverine. A special injection port was designed to host the pieces cut off the TLC. The method was successfully used to identify each spot on the TLC by IMS in a few seconds.

Keywords: Ion Mobility Spectrometry, Thin layer chromatography, Electrospray, Direct coupling, Indirect coupling. 1 ACS Paragon Plus Environment


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INTRODUCTION Ion mobility spectrometry (IMS) is an analytical technique in which gas phase ions are separated while travelling in a weak electric field under atmospheric pressure. In this method, the sample is vaporized, ionized, and the ions are separated in a drift tube based on their mobilities in a fashion similar to the time-of flight technique. While the technique is simple, inexpensive, and fast, it also offers very low detection limits (down to ppb or even ppt) for many compounds such as narcotics, explosives, and many drugs.1 Due to its high speed of analysis (only a few seconds), it is vastly used by security forces in airports and embassies to screen luggage and passengers. It is also used as a laboratory technique in the detection and identification of harmful chemicals in air and water as well as toxins in food and fruits.2,3,4,5,6,7 IMS has recently found medical applications, especially in breath analysis and cancer diagnosis. In many cases, IMS can be assumed as an alternative analytical instrument to GC and HPLC when the analyte is pure or a simple matrix is involved.8 However, for complicated matrices, a pre-separation step is required due to the competition among different components for ionization. The ionization techniques commonly used in IMS are atmospheric pressure chemical ionization mainly with a radioactive source, corona discharge, and electrospray. The dominant ionization mechanism in these techniques is the proton transfer reaction. Hence, the ionization efficiency of the compounds depends on their proton affinity. In the simplest case, when the analyte is a two-component mixture, the compound with the higher proton affinity does not allow the other to be ionized. To overcome this problem and to enhance IMS capability, different pre-separation techniques have been coupled to IMS. Some hyphenated techniques reported in the literature are GC-IMS,9,10 multi-capillary columns (MCC)-IMS,11,12,13 solid-phase microextraction (SPME)-IMS,

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LC-IMS-TOF,18 and HPLC-IMS.19 In this work, we describe the use of thin layer chromatography (TLC) as a pre-separation step for IMS. TLC is a simple and inexpensive type of chromatography that is extensively used in organic chemistry to identify synthesized products. In this technique, two phases are involved: the stationary phase which is typically an adsorbent made of silica gel or alumina layered onto a glass, plastic, or aluminum plate; and the mobile phase that is a solvent or a mixture of solvents with different polarities. Separation in TLC is based on differences in analyte affinities for the stationary versus the mobile phases. This technique has been widely used for the identification of many narcotics such as methadone, cocaine, and methapyrilene. The compounds separated on the TLC plate form spots which are usually detected with UV light, iodine vapor, or other visualization reagents. However, some spots may be missed in visualization. In addition, the visualization methods are not capable of identifying the nature of the separated compounds. To overcome this limitation, mass spectrometric methods have been developed for identifying the compounds separated on the TLC plates. The methods include scratching the spots of interest,20 combination of TLC with GC-MS,21 electrospray ionization mass spectrometry (ESI/MS),22 atmospheric pressure chemical ionization mass spectrometry (APCI-MS),23 fast atom bombardment (FAB),24 laser desorption ionization (LDI),25 matrixassisted

laser

desorption/ionization

(MALDI),26,27

and

surface-assisted

laser

desorption/ionization (SALDI).28,29 A detailed description of these techniques may be found in a review article by Shiea and coworkers.30 A drawback in using the mass spectrometer for TLC is that the mass spectrometer operates under vacuum while TLC is prepared at ambient pressure. In this situation, the TLC strip has to be either brought into the vacuum chamber or washed and electrosprayed. Alternatively, it can be desorbed either by heating the spot or under the laser



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light. Recently, TLC-MS techniques have been developed which operate under ambient pressure .31,32,33,34 To the best of our knowledge, there is no report on using TLC with IMS. The aim of this work is to report the coupling of TLC to IMS. Like the mass spectrometer, the IMS adds another dimension to the TLC data. However, unlike mass spectrometry which needs vacuum pumps, IMS operates at atmospheric pressure. This advantage makes the TLCIMS much easier than TLC-MS coupling. In TLC-IMS, IMS serves as a powerful detector for TLC while TLC separates components prior to IMS. In fact, the two techniques compensate for each other’s limitations and integrate the advantages of both techniques. To evaluate the capability of the new technique for the analysis of mixtures, we used narcotics as test compounds.

EXPRIMENTAL SECTION Ion Mobility Spectrometry. The IMS instrument used in the present study was manufactured by TOF Tech. Pars Company at Isfahan University of Technology, Iran (Fig. 1). It consisted of ionization and drift regions housed in a small thermostat oven (15×20×32 cm) with temperatures ranging from room temperature to 500K within ±2 K. The ionization region consisted of four aluminum rings 0.95 cm thick, 20 mm ID, and 55 mm OD. The drift tube consisted of 11 similar aluminum rings of 36 mm ID. An additional similar ring hosting a shutter grid was placed between the two regions in order to separate the ionization region from the drift one. Thin Teflon insulators were inserted between the rings. Each ring was connected to the adjacent one via a 5 ΜΩ resistor to create a potential gradient. A voltage of 7 kV was applied across the entire cell to create a drift field of 437 V.cm-1. The shutter grid was operated by applying a 100 µs pulse of


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110 V amplitude. This allowed ions to pass through during the short period of removed voltage. A faraday cup detector was located at the end of the drift tube encapsulated in a stainless steel chamber. The collected signal was amplified with a gain of 109 V/A and fed to a computer via a digital oscilloscope (Picotech, UK). Nitrogen was used as both the drift and the carrier gas with flow rates of 700 and 350 ml/min, respectively. The instrument was operated at 473 K. These conditons were found to be the optimum conditons for detection of narcotics. The ionization region was especially designed to have an open end. This made the instrument capable of coupling different ionization sources such as filament, UV lamp, laser light, corona discharge, and electrospray. In addition, the heated IMS cell housing, separated from the electronics, could be positioned with different horizontal, vertical, and inclined orientations. This advantage facilitated the coupling of any source in any direction. A detail description of the instrument may be found in our previous works.35,36,37,38 A home made isolated high voltage power supply was used for electrospray. The high voltage were operated by only trained personnel.

Figure 1. Schematic diagram of the ion mobility spectrometer.



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Materials. Nicotine and acridine (97%) were purchased from Sigma (Sigma-Aldrich). Methanol (99.5% v/v), was obtained from Merck and used as the solvent and the mobile phase. Morphine and papaverine were obtained from Anti-Narcotics Police (Tehran, Iran). Stock solutions (100 mg/mL) of chemicals were prepared in methanol. TLC plates (silica gel matrix coated on alumina plates obtained from Fluka) were cut to strips 3 mm wide and 5 cm long. In the electrospray experiments, the TLC strips were cut on one end in sharp triangles as shown in Fig. 2. Vertical development of the plates was carried out in an especially designed chamber to be described below. Sample solutions were manually applied to the TLC plates using a capillary tube.

Interfacing TLC to IMS. A variety of schemes were tried for coupling TLC to IMS. The schemes included direct electrospray from the tip of the TLC strip, indirect electrospray from a needle connected to the TLC strip, introducing the carrier solvent into the injection port, and offline introduction of scratched or cut pieces of strips into the injection port.

Direct electrospray from the TLC tip. As the simplest method for electrospray, the IMS housing

was mounted vertically above the TLC tank and a high voltage was applied to the solution in the tank. Similar design in which electrosprayed ions from an overrun TLC strip or paper sprayed ions entering mass spectrometer can be found in references. 39,40,41,42 The tip of the TLC strip was placed at a distance of ∼2 cm from the entrance to the IMS ionization region. A peak was observed for acridine when its spot reached the tip. However, in this geometry, the TLC plate was not in contact with the saturated vapor of the solvent and it sometimes got dry, resulting in 6 ACS Paragon Plus Environment

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an unstable signal. To avoid this, the strip was housed in a small cylindrical chamber shown in Fig. 2a. An electrode was then mounted at the bottom of the tank to hold the strip and to connect the electrospray power supply. A disk-shaped aperture electrode with a 3-mm hole covered the top end of the tank. The cover was meant to prevent the drying up of the strip while the electrosprayed ions escaped through the aperture. A separate power supply was employed to establish an electric field between the aperture electrode and the entrance to the IMS ionization region which allowed the ions to be extracted. A voltage of 200 Volts was also applied to create an electric field between the aperture electrode and the first IMS guard ring and about 2 kV was applied between the TLC tip and the aperture electrode to establish the electrospray or the corona spray. Acridine used as the test compound was placed on the TLC plate. Due to low surface affinity, it was moving with the same velocity as the solvent. A stable signal was observed for acridine when the spot reached the tip. However, the acridine signal lasted long, which indicated that the sample accumulated on the TLC tip and that it was not removed fast enough to make a fresh surface for the next spot. In addition, in this case the solvent in the TLC plate was moved upward by only capillary action and the solvent flow rate was not high enough to generate an electrospray plume at the tip of the TLC plate. It was also not possible to use a makeup solvent flow to induce ESI. We think, in this case, that corona discharge APCI instead of ESI was responsible for the ionization of the analyte. In the next try, we changed the geometry to revert the whole system vertically (upside down) as shown in Fig. 2 b. In this geometry, gravity helped the solvent to flow faster. To have a reservoir of the solvent above the TLC strip, a silicon disk was placed near the top in the cylinder, holding the TLC plate and the solvent. The problem with this geometry was the accumulation of the solvent on the tip and the formation of droplets. 7 ACS Paragon Plus Environment


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Figure 2. Two different geometries for direct coupling of TLC to IMS by electrospray, a) normal, b) upside down

Indirect electrospray. The problem with the upside down geometry was solvent dripping from the

tip. In order to prevent droplet formation, a very small funnel was mounted just below the TLC tip. The solvent was collected in the funnel and transferred to a needle (ID 0.1 mm and OD 0.35 mm ) via capillary tubing (L-geometry). The IMS housing was positioned horizontally and the capillary was bent to hold the needle in front of the IMS entrance. The setup is shown in Fig. 3. An auxiliary solvent moving at a flow rate of 7 mL/min was used to enhance the solvent flow on the surface, help washing the surface, and create a more stable electrospray. The L-geometry stopped the dripping of the liquid solvent into the IMS ionization region. The needle voltage was set to about 2 kV to observe corona discharge ions. When the solvent reached the needle, the corona spray 43 started and the sample peak appeared. 8 ACS Paragon Plus Environment

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Figure 3. Schematic diagram of the indirect coupling of TLC to IMS by electrospray

Scratching or cutting the TLC spots (Offline coupling). A very easy way to transport the TLC-

separated analytes to the IMS involves the mere scratching of the surface where a spot is observed and transferring the spot into the injection port. This, however, may in the long run cause such problems for the instrument as solid accumulation in the injection port. An alternative way is to cut the strip and put the cutting inside the injection port. The newly designed injection port (Fig. 4) has the capability to host small pieces of the TLC strip. A mesh was mounted inside the injection port to hold the strip cuttings. The injection port temperature was set to 473 K for all the substances. The important point was that the carrier gas was shut during the transfer of the TLC pieces. Immediately after closing the injection port tap, the carrier gas was opened to carry the evaporated material into the IMS cell, where they are ionized via the corona discharge and separated in the drift region. Subsequent analyses were performed by changing the pieces. It took only a few seconds for the analysis of each cutting and recording of its spectra. As the carrier gas



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was closed and reopened during the operation, the evaporated materials were accumulated to yield an enhanced signal intensity. Fortunately, IMS does not respond to the material used in the TLC plates (SiO2 and Al metal). However, the TLC plates were preconditioned prior to analysis by baking in an oven at 423 K for 15 minutes. This was meant to remove any desorbed organics on the surface and to clear off any possible adhesive or additives.

Heating coil

Figure 4. Schematic diagram of the injection port hosting the TLC cuttings.

RESULTS TLC/ESI-IMS. As described in Section 3, different geometries of the TLC/ESI-IMS were tried. The most stable results were obtained from the L-geometry, shown in Fig. 3, which we present here. To evaluate the proposed technique, narcotics were used as test compounds. Initially, 2 µL of a 100 ppm papaverine and morphine (1:1) solution in methanol was applied to the surface of 10 ACS Paragon Plus Environment

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the TLC strip and housed in the normal TLC tank to ensure separation of the two components. As shown in Fig. 5a, the two analytes were well separated. Morphine, with two hydroxyl groups and oxygen exhibits a stronger reaction with the polar surface (SiO2) than papaverine does; papaverine moves faster than morphine on the surface by the methanol solvent. In one run, the same mixture was applied to the TLC strip and placed in the special tank shown in Fig. 3. The results obtained from the electrospray of the solvent and the components falling off the needle are shown in Fig. 5b. In the next run, the mixture of papaverine and acridine was used. Acridine with groups of lower affinity to attach to the surface (oxygen, nitrogen or hydroxyl) moves faster than papaverine on the TLC plates. The results of the electrospray process using the setup in Fig. 3 are presented in Fig. 5.c and d. It is clear that the acridine peak appeared earlier than that of papaverine.



Time (ms)

(a)

(b)

Intensity (mV)

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Time (ms)

(d)

(c)

Figure 5. a) Separation of morphine-papaverine mixture by the conventional TLC method, b) ion mobility spectra of the mixture of morphine-papaverine and c) the mixture of acridine and papaverine by the TLC/ESI- IMS using the setup in Fig. 3, d) Top view of the 3-D ion mobility spectra of separation of acridine-papaverine


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In continuation, a mixture containing nicotine and papaverine was analyzed using the indirect electrospray probe simply to observe a similar separation pattern.

Cutting TLC Strips. To evaluate the cuttings of the TLC strip, mixtures of different analytes were applied to the TLC strips and put in a conventional tank. After separation and drying, the spots were visualized under UV light. The corresponding spots were then cut and consecutively introduced into the injection port of the IMS. The results for two sets of experiments are shown in Fig. 6. Each spot produced its own ion mobility spectrum without contamination from other spots. This is the simplest way to identify the compound in a spot.

Figure 6. Ion mobility spectra obtained from the cut TLC strips: a) nicotine and papaverine, b) nicotine and noscapine.



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CONCLSION In this article, it was shown that IMS is a useful technique for the identification of components separated on the TLC plate. In fact, it not only serves as an excellent detector for TLC but also identifies the separated components to some extent. Compared to other techniques coupled to TLC, such as mass spectrometry, IMS is simple, fast, and inexpensive while it does not depend on complicated sampling procedures. Compared to GC, TLC does not enjoy a very high separation power. Similarly, compared to mass spectrometry, IMS does not provide a high resolution. However, its resolution and speed are high enough to identify the compounds already separated by TLC. Indeed, the use of TLC prior to IMS adds an extra dimention to the analysis. Therefore, the combination of the two techniques can not only compensate for each technique’s inadequacies but also restrengthen their advantages in complex analysis. In fact, TLC-IMS, compared to TLC-MS, is a cost effective and time-saving method for most applications in which high separation power and mass resolution are not needed. The advantage of TLC-IMS is that both techniques work at atmospheric pressure, which makes the coupling simpler than that of TLC-MS. In the TLC-MS, the scratched material cannot be directly introduced into the mass spectrometer because the vacuum pumps may be damaged. Normally, the scratched material or spots are solvent extracted, filtered, and concentrated prior to introduction into the mass spectrometer. However, in TLC-IMS, the scratched TLC or its pieces can be directly introduced into the IMS without any need for the time-consuming sample preparation procedures. TLC-IMS can also be an alternative to GC-IMS, since it is simpler and faster with adequate resolution for many applications.


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All the different combinations for TLC-MS coupling reported in the literature can also be used for the TLC-IMS. Research is being currently conducted on the coupling of TLC with IMS based on laser desorption and other heating techniques using automatic scanning.

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Thin Layer Chromatography-Ion Mobility Spectrometry (TLC-IMS

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