G Model CHROMA-353717; No. of Pages 7 ARTICLE IN PRESS Journal of Chromatography A, xxx (2012) xxx–xxx Contents lists available at SciVerse ScienceDirect Journal of Chromatography A journal homepage: www.elsevier.com/locate/chroma Thermal Solid Sample Introduction–Fast Gas Chromatography–Low Flow Ion Mobility Spectrometry as a field screening detection system Saeed Hajialigol ∗ , Seyed Alireza Ghorashi, Amir Hossein Alinoori, Amir Torabpour, Mehdi Azimi Iranian Space Agency, Engineering Research Institute, Engineering Research Center of Esfahan, Esfahan, Iran a r t i c l e i n f o Article history: Received 7 July 2012 Received in revised form 2 October 2012 Accepted 8 October 2012 Available online xxx Keywords: Gas Chromatography–Ion Mobility Spectrometry (GC–IMS) Thermal Solid Sample Introduction (TSSI) Hyphenated technique Field analysis a b s t r a c t The potential of Thermal Solid Sample Introduction (TSSI)–Fast Gas Chromatography (GC)–Low Flow Ion Mobility Spectrometry (LF-IMS) having been designed and constructed in Engineering Research Center of Esfahan, detector group was investigated for chemical detection capabilities. Customizing the configuration of fast GC–IMS as a high technology, provides unique solutions for rapid detection of a broad range of chemical mixtures in many operational environments. TSSI configuration provides fast and easily applied method for direct detection with no additional sample preparation or extraction. The time required for total analysis, less than 265 s, was determined by the wide range of solid matrixes, including nitrate esters, nitroaromatics, and a nitramine. The fast extraction together with the short separation time limits degradation of the thermally labile compounds and decreases the peak widths, which results in larger peak intensities and a simultaneous improvement in detection limits. For signal-to-noise ratio equals to 5, the detection limits for instrument for TNT, DNT and RDX were attained 15, 10 and 50 ng/␮l respectively. The combination of short analysis time and low detection limits make this instrument a potential candidate for field screening techniques. © 2012 Elsevier B.V. All rights reserved. 1. Introduction Ion Mobility Spectrometry (IMS) is a vanguard analytical technique that provides analytical information in a simple, rapid, and inexpensive manner. For example, an ion mobility spectrometer can be used to extract global information about the volatile profile of a sample [1,2]. Some advantages of IMS include the following: low detection limits for single-component samples (nanogram detection limits), fast analysis time (provides output data in seconds), no required sample pretreatment of solids or analytes in solution, operation at atmospheric pressure eliminating bulky vacuum systems, detection of both positive and negative ions which provides good selectivity, capability to be miniaturized and battery operated, and comparably less expensive to purchase and operate than other trace detection technologies [3–9]. IMS instruments are the-state-of-the art for on-site detection of chemical warfare agents, explosives and drugs [10] as well as air-quality monitoring [11]. More recent areas of application have included process control [12], rocket fuel leak detection [13], identification of wood species [14], detection of bacteria [15], determining the pesticides in agriculture products [16,17] and monitoring ammonia ∗ Corresponding author at: Iranian Space Agency, Engineering Research Institute, Engineering Research Center of Esfahan, 7th Km Imam Khomeini Street, P.O. Box 81955/174, Esfahan, Iran. Tel.: +98 311 3222439; fax: +98 311 3222446. E-mail address: s hajialigol@yahoo.com (S. Hajialigol). levels in water [18]. IMS was used during space planned experiments for the Rosetta Comet Mission [19] and an IMS instrument configured as a gas chromatography detector will be used to monitor cabin atmospheres in the next generation of space stations [20]. Thus, IMS has developed in recent years from few successful niche applications to a general purpose analyzer suitable for a broad range of applications. The theory and operation of Ion Mobility Spectrometry has been extensively described elsewhere [21]. While many IMS instruments are used in the field to locate explosives, as with all detection techniques, there are limitations. IMS does not handle chemical mixtures adequately because of the complex interactions in the IMS source that leads to obscure or less distinct plasmagrams [22–24]. IMS instruments are prone to inaccurate detection and false positive responses when chemical interferents are present in samples. This technique is also relatively easy to saturate and the result is a linear response range that is often limited to one to two orders of magnitude. Thus, sample amount must be carefully controlled at the instrument’s entrance to prevent saturation or nonlinear response [23,25–27]. If these limitations can be mitigated, IMS can be a more powerful tool in detecting explosives under a variety of confounding sampling situations. Coupling a gas chromatograph (GC) to the IMS detector is an effective way to overcome IMS limitations [28–32]. While two chemicals can have identical IMS ion mobilities, the chemicals typically have different GC retention times, which will be separated prior to entering an IMS [33,34]. The addition of GC to IMS has 0021-9673/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.chroma.2012.10.010 Please cite this article in press as: S. Hajialigol, et al., J. Chromatogr. A (2012), http://dx.doi.org/10.1016/j.chroma.2012.10.010 G Model CHROMA-353717; No. of Pages 7 2 ARTICLE IN PRESS S. Hajialigol et al. / J. Chromatogr. A xxx (2012) xxx–xxx Fig. 1. Two designs for connecting capillary column to IMS (a) side connection and (b) axial connection. demonstrated improved trace organic chemical detection through better resolution, reduced detector saturation, and increased IMS linear range [35–38]. The modern age for mobility measurement after capillary gas chromatography can be tracked back to the work of Baim and Hill [28]. IMS was further demonstrated as a selective and nonselective detector for GC in another report by Baim et al. when terpenes from orange extracts were monitored for all product ions having drift times between specific intervals [39]. The IMS constructed for interfacing to a GC was also investigated in negative ion mode [3]. The detector was tuned to monitor chloride ion, and five pesticides at 100 pg cm−3 were investigated. Other studies that were carried out with the constructed instrument included the positive ion mode quantification of 2,4-dichlorophenoxyacetic acid residues in soil extracts [40]. DeBono et al. developed GC–IMS technique for separation of different groups of mixtures such as narcotics, explosives, lutidine isomers and pesticides [41,42]. Also in another study by them, rapid analyses of pesticides on imported fruits were performed by GC–IMS [43]. An important aspect of the interface created for GC–IMS is the transfer line. A number of prominent factors need to be considered when developing a transfer line for an interface between a GC and an IMS. The most important factors are maintaining the integrity of the resolution, achieving quantitative transfers, and matching flows [31]. There are two possible methods to connect a capillary column to a mobility spectrometer including the attachment of the column to the drift tube on axis to the ion source with a concentric inner tube (Fig. 1b) or through the side of the drift tube (Fig. 1a). The side design is effective, although complicated by the requirements of a gas-tight connection between the column and the drift tube. In the second design, the capillary column is extended on axis into the ion source, from the end of the drift tube. In the axial design, response is sensitive to the position of the capillary column in the ion region. When a make up flow is used, the interface of the GC to the IMS is uncomplicated and less sensitive to the location of the capillary outlet, although the response is affected by the flow of gas in the make up. The flow of the make up gas can be employed to extend the linear range of response for a GC–IMS [21]. Achieving the minimal time of operation in gas chromatography (GC) has been a research topic ever since the introduction of GC. Today, revived interest in fast GC is seen to be driven by applications, such as process control and high-throughput analysis, or by the desire to reduce the costs of operation and ownership in routine analysis. Having a newly designed oven with rapid temperature programming capability (typically greater than 1 ◦ C/s) and rapid cool down times on the order of 2 min, the fast GC instruments provides an effective injection system, advanced pneumatic controls and fast data acquisition speeds. The net effect of these instrumental designs is a GC analyses with analytical cycle times in the range of 5 min or less. The goal of faster GC is to obtain the information required from a certain sample in a shorter time. In fast GC, the peaks elute in a shorter time frame and the registration system should be able to record and process the information generated in real time, irrespective of which method of faster GC has been used. In faster chromatography, more peaks are generated per unit of time. This means that the peak broadening caused by the detector must be small enough to preserve the column efficiency. In addition, the sampling frequency of the detector must be high enough to provide some 15–20 data points across the peak for an accurate representation of the peak. Complete characterization of a hazardous waste site can involve a large amount of money, manpower and time. Sometimes, multimillion dollar analysis plans are carried out, only to find that whether contamination levels are within standard guidelines. On other occasions, a catastrophic environmental release requires fast sample turnaround times to deal effectively with the release. In these instances, classical methods of shipping samples to an analytical laboratory are either a waste of resources or too time consuming to provide timely data. Idealistically, having the ability to identify and quantify analytes, a handheld, rugged field screening instrument should provide instantaneous sample information with high signal to noise ratio and resolution. Realistically, a functional field instrument needs to meet some criteria including deploying by one person without extraneous equipment, being sufficiently sensitive to detect compounds of interest, being capable of discriminating among the various analytes (even with possible interferences present), providing a full analysis in less than 10 min and being rugged enough to be moved about a sampling site without adverse effects. The need for such an analytical instrument is considerable and its development would make the characterization of chemical contamination extremely easier [44]. For increasing the capabilities of the GC–IMS as a field screening instrument, a Thermal Solid Sample Introduction (TSSI) module is needed for injection the real solid samples to gas chromatograph without preparation of samples in laboratory. TSSI is suitable for the fast and uncomplicated analysis of various analytes. In this work a home-made fast GC was designed and constructed for rapid separation of complex mixtures and minimum degradation of thermally labile compounds. Also a modified design of IMS was made for interfacing it to capillary GC columns and rapid clear down times. The performance of instrument was investigated by injecting nitrate esters, nitroaromatics, and a nitramine individually and in complex mixtures through the constructed TSSI. Results of experiments revealed the validity of TSSI–GC–IMS as an effective instrument for field screening of explosives. 2. Instrumentation TSSI–GC–IMS has the potential to satisfy the requisites for a realistic and useful field screening instrument. TSSI–GC–IMS can be divided in four main parts: Thermal Solid Sample Introduction, Gas Chromatograph, Transfer line of GC to IMS and Ion Mobility Spectrometer. 2.1. Thermal Solid Sample Introduction TSSI as a simple, fast, precious and versatile sample preparation technique works by vaporization for introducing a solid sample Please cite this article in press as: S. Hajialigol, et al., J. Chromatogr. A (2012), http://dx.doi.org/10.1016/j.chroma.2012.10.010 G Model CHROMA-353717; No. of Pages 7 ARTICLE IN PRESS S. Hajialigol et al. / J. Chromatogr. A xxx (2012) xxx–xxx Fig. 2. Side view of TSSI assembly. into gas chromatograph or gas detectors. It can be as a modern rapid alternative to complicated and laborious solid sample preparation and injection technology with no solvent extraction requirement for gas analyzers. TSSI is able to introduce separately volatile, semi-volatile, and non-volatile organic compounds, from the real solid sample matrices to fast response gas detectors. It can introduce solid samples into analytical equipment in a manner that optimizes the concentration of the analyte of interest and minimizes the concentration of interferences. This increases the detector sensitivity for the analytes and lowers the detection limits. Advantages of TSSI versus solvent extraction include: no manual sample preparation, no analytical interference from the solvent, selective focusing/extraction of compounds of interest, cost effective reusable samplers and readily automated. 2.1.1. Principle of TSSI operation The TSSI consists of five major parts: heated sample sweeper, entrance slit, inlet of TSSI, six port valve and sample collecting loop (Fig. 2). Samples are collected by suction with portable rechargeable sampler onto porous mesh filter mounted in sample collector. To begin an analysis the sample collector is inserted into the entrance slit on the front of the TSSI–GC–IMS and start button is pressed. When the analysis cycle starts, the heated sample sweeper moves up and seals the sample collector to the inlet of TSSI. As the sample is heated, the sample carrier gas sweeps the desorbed vapors into a heated six port valve and cooled sample collecting loop where it is trapped. For the sample analysis, the six port valve switches position and the loop is rapidly heated. GC carrier gas purges the loop and transfers the collected sample into the GC column. At the end of analysis, the valve switches back into previous position and the loop is then cooled for the next analysis. 2.1.2. Optimization of TSSI conditions 2.1.2.1. Decreasing the dead volume of desorption region and transfer lines. Eliminating or reducing dead volume in TSSI is one of the prominent aspects in designing and constructing this region. The end of the TSSI is connected to gas chromatography capillary column. Internal diameter of capillary columns varies between 0.25 and 0.53 mm. To decline the dead volume in this area, the internal diameter of transfer lines and fittings need to be the same or near the diameter of GC column. This critical issue can be exacerbated in the inlet of TSSI which has 10 mm diameter according to diameter of porous mesh filter. The quantity of particles swept on the target collecting surface is usually very small, often in the range of only nanograms or even picograms per square centimeter. Inasmuch as the porous mesh filter should collect maximum amounts of sample particles from environment and transfer them to TSSI for analyzing, it needs to be provided with diameter at least 10 mm. On the other side, mesh filters having bigger diameters would absorb the temperature of the desorption region, the space between heated sample sweeper and 3 inlet of TSSI, and cause a salient drop in its temperature. Thus, the particles collected on the filter cannot evaporate considerably and a few amounts of them will be delivered to the instrument. With regard to the aforementioned remarks, the inlet diameter of TSSI should be equal to 10 mm similar to the diameter of the porous mesh filter. Moreover, to fit this diameter with the required one (about 0.5 mm), the multi-stage decrease should be implemented. It goes without saying that if we perform sudden, unorganized reduction, we will produce some steps in the gas transfer lines causing noticeable dead volumes. On the contrary, by decreasing the diameter in various stages as well as arranging cone-shaped structures, we overcome this problem effectively. The end of the TSSI was connected to gas chromatography capillary column. Internal diameter of capillary columns varies between 0.25 and 0.53 mm. A key factor in the performance of coupling with capillary GC is the absence of any dead volume. The term ‘dead-volume’ refers to volumes within the chromatographic system which are not swept by the mobile phase. By dead volume we mean the empty spaces in the TSSI and connecting tubing before the column and the connecting tubing and ion mobility spectrometer after the column. In these sections, band broadening occurs without realizing separation of components. Also dead volume causes to loss some of analytes during transferring to GC capillary column and IMS. It results loss of efficiency to the chromatographic system and increasing detection limit [45]. Eliminating or reducing dead volume in TSSI is one of the important aspects of design and construction of TSSI. For reducing dead volume, internal diameter of transfer lines and fittings should be equal or near to internal diameter of GC capillary column. On the other side, this critical problem can be found in the inlet of TSSI. TSSI instrument works by releasing analytes from a solid sample collected on porous mesh filter by thermally desorbing in desorption region and then transferring into inlet of TSSI which dead volume is reduced in it. 2.1.2.2. Decreasing the sample loss in desorption region. The instrument should provide a signal that is proportional to the concentration of target molecule vapor. This concentration is further dependent on the equilibrium vapor pressure of target molecule, the vaporization temperature, the total flow rate of non-target gas that dilutes the target vapor and sample losses on desorption region of the TSSI. Sample losses after evaporation of samples in desorption region is one of the major problems in TSSI. During the vaporization process of a liquid or solid sample, the pressure drastically increases in desorption region and causes to lose vapor sample from this part. To overcome this problem, the pressure of this region was reduced by utilizing a sampling pump which sucks the sample gas into the loop from the inlet of TSSI. Also for decreasing sample loss in desorption region during the vaporization process, this region should be sealed. Because of this fact, the material of sample collector was optimized. A sample collector consists of two parts, a porous mesh filter and PTFE card. Due to the flexibility of PTFE in high pressure and temperature, it can be used as a suitable seal material for sample collector. 2.2. Fast Gas Chromatography A home-made fast gas chromatographic system was designed and built to have minimum temperature gradients. The air within the oven is heated with an electric heating element placed within the oven and circulated with a fan to uniformly heat or cool the interior of the oven and the column. Generally, ovens have a rectangular or square interior. The square or rectangular shape of the oven interior makes it difficult to uniformly heat the oven. Also heating time is increased as is the cooling time, because of large thermal mass of Please cite this article in press as: S. Hajialigol, et al., J. Chromatogr. A (2012), http://dx.doi.org/10.1016/j.chroma.2012.10.010 G Model CHROMA-353717; No. of Pages 7 4 ARTICLE IN PRESS S. Hajialigol et al. / J. Chromatogr. A xxx (2012) xxx–xxx responses, a cylindrical make up gas was flown into the interface around the analyte input tube. In brief, with implementing the mentioned method, the advantages of this configuration would be gained including preventing from detector saturation of the instrument, rapid clear down times, extend the linear dynamic range of the instrument, control ion chemistry by maintaining the concentrations of analyte in the source region and high efficiency of dispersion of analyte in reaction region. 2.4. Low flow ion mobility spectrometer Fig. 3. Cross section of cylindrical oven of fast GC. the oven interior. In the TSSI–GC–IMS, the oven cavity is cylindrical (Fig. 3), thereby reducing the surface area and volume of the cavity as compared to square or rectangular oven cavities with removing corner tips, further decreasing the heat load and therefore the heating and cooling time. Fast GC of this instrument provides fast and linear heating rate up to 70 ◦ C/min. Temperature gradient is less than 1 ◦ C for 40 ◦ C/min and around 2 ◦ C for 70 ◦ C/min heating slopes. This allows determining a large number of compounds with wide range of boiling points. Also applying a shorter column length, an optimum carrier gas velocity and faster temperature programming cause to determine temperature labile compounds too. On the other hand, detection system should be sufficiently fast and sensitive. The constructed IMS along with its sophisticated data acquisition system indicated its potential as a detector for fast GC. Baseline width of our peaks in IMS is about 1.2 ms, this requires a sample frequency of 12.5–16.7 kHz to provide 15–20 data points across the peak for an accurate representation. Typical spectral acquisition rate of our IMS is 25 spectra/s, each spectrum consists of 4096 samples collected in 40 ms. These figures implied that the sampling frequency of the system is about 100 kHz which provides our IMS as a well qualified detector system to couple with a GC to form a capable fast GC–IMS. 2.3. Transfer lines of GC to IMS The most important factors are maintaining the integrity of the resolution, achieving quantitative transfers, and matching flows [32]. There are two possible methods to connect a capillary column to a mobility spectrometer including the attachment of the column to the drift tube on axis to the ion source with a concentric inner tube (Fig. 1b) or through the side of the drift tube (Fig. 1a). The flow of the make up gas can be employed to extend the linear range of response for a GC/IMS [46]. Make up gas allows vapors eluting from the fused-silica column to be dispersed into the reaction region effectively because sample and reactant ions are mixed well. When the make up flow is reduced, effluent from the fused-silica column swept along the walls of tubing declines, and the mixing of reactant ions and sample is reduced. Consequently, response is variable, depending upon make up flows. In the constructed TSSI–GC–IMS, the axial design with unidirectional flow was used for connection of GC column and IMS. A gas chromatography–ion mobility spectrometer inlet based on this configuration causes to rapid clear down times, an important feature for gas chromatography detectors. In order to improve The low flow IMS was designed and constructed at Engineering Research Center of Esfahan (Esfahan, Iran). The developed configuration constitutes an open-system easy to manufacture at low cost. The device comprises the parts of a regular ion mobility spectrometer, namely, an ionization chamber, shutter-grids, a separation chamber, an ion collector and voltage generators. The device is equipped with a radioactive 63 Ni ionization source (St. Petersburg, Russia), with an activity of 12 mCi which consists of a cylindrical cavity of 1 cm in length and 1 cm in diameter houses the Ni-foil. A sample inlet lets a continuous stream of the gas carrying the analytes passing through the ionization chamber, where the ions are effectively focused before entering the separation chamber. To transport ions from the ionization chamber into the drift tube, a shutter-grid built from parallel steel wires is used. The configuration type of the shutter follows a Bradbury–Nielsen gate design. Electrical components allow adjusting the opening time of the grid to let the ions pass at short pulses towards the separation chamber. The drift tube, with an inner diameter of 15 mm and 75 mm in length, was designed on the basis of avoiding ion repulsion and charge effects and allowing homogeneity of the drift field. In this separation chamber, kept at 120 ◦ C, ions with different mobility are separated within an electrical field of 280 V/cm at a fixed voltage of 3 kV hold by means of 10 parallel drift rings constructed from steel 316 which are coaxial cylinders. A high voltage supply is connected to the drift rings placed in equal distance to create the electric field in the drift tube. Isolators made from Teflon and resistors are placed between the drift rings. The influence of the voltage in the signal and separation of the ions was characterized using acetone as exemplary substance for IMS separation. The optimized value is 3 kV as a compromise between sensitivity and the better resolution. The drift gas (nitrogen) flows in the opposite direction of the ions in order to prevent non-ionized impurities caused by water molecules or volatile components of the column stationary phase, from entering the separation chamber. A gas inlet at the front of the drift tube allows the entrance of the carrier gas and an outlet line makes the gaseous streams exit the instrument. Finally, the separated ions reach a Faraday plate of 35 mm in diameter and the signal is delivered to the computer. Home-made data acquisition and analysis software was used to acquire the signal of ions collected by the Faraday plate. The modification on conventional IMS was done to improve the sensitivity and use as a GC detector. In this manner, the ionization cell volume was reduced. The reduction of ionization cell volume decreases the residence time of a compound in the ionization region to a few tenths of a second, maintaining the integrity of the gas chromatographic separation and improve sensitivity. Also this modification causes a decrease in gas consumption of IMS. Similar to uni-directional flow design in transfer line, reduction of ionization cell volume causes rapid clear down time of IMS. Consequently the consumed drift gas would be reduced twenty times compared to the volume of drift gas in conventional home-made IMS. According to Table 1 sum of drift gas and make up gas was reduced from 550 ml/min to 25 ml/min in low flow IMS. Please cite this article in press as: S. Hajialigol, et al., J. Chromatogr. A (2012), http://dx.doi.org/10.1016/j.chroma.2012.10.010 ARTICLE IN PRESS G Model CHROMA-353717; No. of Pages 7 S. Hajialigol et al. / J. Chromatogr. A xxx (2012) xxx–xxx Table 1 Comparison of the characteristics of the low flow ion mobility spectrometer and conventional IMS. Parameter Low flow IMS Traditional IMS Ionization source Length of the drift region Full length of the spectrometer Electrical field strength Make up flow Drift flow 63 63 Ni (12 mC) 75 mm 110 mm 280 V/cm 10 ml/min 15 ml/min Ni (16 mC) 120 mm 230 mm 240 V/cm 250 ml/min 300 ml/min Comparison of the characteristics of both detectors (conventional and low flow IMS) is summarized in Table 1. 5 Table 2 TSSI–GC–IMS experimental conditions. Conditions IMS operating mode Sampling time (s) TSSI temperature (◦ C) Drift tube temperature (◦ C) Loop (preconcentrator) temperature (◦ C) Valve temperature (◦ C) Oven initial temperature (◦ C) Oven initial hold (s) Oven ramp rate (◦ C/min) Oven final temperature (◦ C) Final hold (s) Transfer line temperature (◦ C) Total analysis time (sampling + analyzing) Negative 60 235 110 240 220 140 30 25 200 30 220 264 4. Results and discussion 3. Materials and methods 4.1. TSSI–GC–IMS analysis of individual explosives All reagents were of analytical grade. Acetone was purchased from Merck. Trinitrotoluene, dinitrotoluene and 1,3,5-trinitroperhydro-1,3,5-triazine were purchased from Sigma–Aldrich Chemical Company (St. Louis, MO, USA). The standards were weighed into 2 ml amber vials and were dissolved with 1 ml of acetone, and then the vials were placed into an ultrasonic bath for effective dissolution for 10 min then diluted to 10 ml for making stock solutions. Other solutions were made by successive dilution of the stock solutions. In the TSSI–GC–IMS, analytes are thermally desorbed in a Thermal Solid Sample Introduction (TSSI). Then desorbed analytes enter a sample loop which acts as a preconcentrator for GC separation. The sample loop (preconcentrator) is subsequently heated and sample vapors are carried into the GC column (15 m DB-1; 0.53 mm internal diameter; 1.0 ␮m film thickness). Experimental conditions for each of the three explosives analyzed are listed in Table 2. To accurately transfer a known quantity of explosive to each swab, 1 ␮l of a dissolved explosives solution was directly deposited onto sample collector. The solvent was allowed to evaporate (5 s), leaving the desired mass of explosive on the swab before it was placed into the TSSI. The TSSI–GC–IMS chromatogram of 40 ng TNT is shown in Fig. 4a. The spectrum shows an intense peak at a retention time of 162 s. The response of instrument to DNT is more interesting due to the fact that the TSSI–GC–IMS indicates its potential to separate two isomers of DNT. The results of introducing 80 ng DNT has been shown in Fig. 4b in which two distinct peaks can be observed, a sharp peak with retention time of 140 s and a small one with shorter retention time of 135 s related to 2,4-DNT and 2,6-DNT respectively. Eventually, the chromatogram of 100 ng RDX has been revealed in Fig. 4c. Although this figure demonstrates a keen peak with retention time of 80 s which is assigned to byproducts of RDX, the intrinsic peak of this compound can be found in 200 s. 4.2. Identification of interested compounds in mixture sample In this study, 1 ␮l standard mixed solutions of 40 ppm 2,4,6-trinitrotoluene, 50 ppm 2,4-dinitrotoluene and 100 ppm 1,3,5-trinitroperhydro-1,3,5-triazine were directly deposited onto sample collector. The solvent was allowed to evaporate leaving the desired mass load of explosives on the sample collector before it Fig. 4. Chromatogram of (a) TNT, (b) DNT, (c) RDX and (d) mixture of explosives. Please cite this article in press as: S. Hajialigol, et al., J. Chromatogr. A (2012), http://dx.doi.org/10.1016/j.chroma.2012.10.010 G Model CHROMA-353717; No. of Pages 7 6 ARTICLE IN PRESS S. Hajialigol et al. / J. Chromatogr. A xxx (2012) xxx–xxx was placed into the TSSI. The chromatogram of these compounds has been shown in Fig. 4d. Recent findings reveal that the designed TSSI–GC–IMS has been well qualified to separate and identify each of the compounds in the complicated matrix. 4.3. Detection limits In TSSI–GC–IMS, the instrument is designed to pass sample and carrier gas for a preset period of time through a preconcentrator described earlier which essentially acts as a trapping medium. Subsequently, a six-port valve reverses flow and the preconcentrator is simultaneously heated to desorb and transfer the analytes into the GC column for chemical separation. All preconcentrators have a limited trapping capacity, which can differ among compounds. Once a preconcentrator reaches its maximum trapping capacity for an analyte, any subsequent analyte will simply vent and may never be transferred into the GC column or, in this case, the IMS. The TSSI–GC–IMS response in comparison with the stand alone IMS response has lower peak amplitude at nearly all concentrations. The decreased signal response for TSSI–GC–IMS at the same concentrations may be primarily attributed to the limiting effect of the sample loop, since analyte is lost when the preconcentrator reaches its maximum trapping capacity. The decreased response may also happen due to inefficient transfer of sample vapors into the GC or the IMS. When samples are provided as discrete items, such as filter or swipes or as solid-phase microextraction fibers, MDLs are expressed as picograms per sample or nanograms per sample. This is commonly practiced with semivolatile compounds and when IMS drift tubes are used as chromatographic detectors. Detection limits, defined as the fivefold signal-to-noise ratio, were attained 15, 10 and 50 ng respectively. They were higher than IMS alone. It is worth mentioning that TSSI–GC–IMS, unlike Mass Spectrometry, operates at ambient pressure and for this reason the detection limit of IMS is higher than MS. On the other hand, in IMS only a limited amount of materials are introduced into the drift tube that depends on the time in which the shutter grid is open. Additionally, MS spectrometry has a more powerful ionization source compared to IMS. These reasons exacerbate the detection limit of IMS in contrast with MS. The limits of detection for the GC–MS analyses of TNT and RDX using the Rtx5MS column (15 m length, 0.53 mm inner diameter, 1.5 ␮m film thickness) have been acquired 6 and 3 ng/␮l respectively [45]. 4.4. Linear dynamic range The linear dynamic range (LDR) of the instrument was evaluated. Response curves are shown in Fig. 5 for the three selected explosive compounds. Inasmuch as TSSI–GC–IMS limits the amount of sample entering the IMS, no saturation will happen for the IMS detector and thereby the instrument shows its reliability to have a wide linear dynamic range. Fig. 5a shows a linear range for DNT from 10 to 200 ng which is followed by a second linear region from 200 to 1500 ng, where the ion source is nearly saturated. Thus, the dynamic or working range for DNT is from 10 to 1500 ng. The practical tests for TNT revealed a greater dynamic range, 15–2500 ng; a linear range from 15 to 150 ng and another range from 150 to 2500 ng (Fig. 5b). Albeit TNT, in comparison with DNT, has a narrower dynamic range for low concentrations, it shows a large range for higher ones which is well enough to provide a wider dynamic range in general. The achieved range for RDX is 50–5000 ng including a linear range from 50 to 500 and a second one from 500 to 5000 ng (Fig. 5c). Fig. 5. Response curves for (a) DNT, (b) TNT, and (c) RDX. Table 3 Reproducibility of TSSI–GC–IMS for various explosive. TNT DNT RDX Precision% Reproducibility% 5.34 10.43 17.32 12.04 13.41 15.23 The calibration curves of the compounds showed excellent linearity over a concentration range of at least two orders of magnitude having r2 -values greater than 0.97. These results are especially promising with respect to the need of establishing a quantitative IMS determination method for explosives. With analysis of DNT, TNT and RDX, it appears that the instrument has a greater LDR over that of IMS alone [46]. Commonly, linear ranges of 10–100 have been reported for IMS [21]. 4.5. Precision and reproducibility study of TSSI–GC–IMS Six replicate of 100 ng of the standard solution were analyzed to determine the precision of the instrument. The results revealed that RSDs were less than 18% for all explosives (Table 3). Also in order to assess the reproducibility of the Instrument 100 ng of each solution was analyzed in triplicate on four days during in one month and the RSD for each analyte was determined. It is distinct that TSSI–GC–IMS produces inexhaustible and reliable results for each compound during analysis in one month. 5. Conclusion A TSSI–GC–IMS, suitable for use in explosive field screening detection applications, was designed and constructed. This Please cite this article in press as: S. Hajialigol, et al., J. Chromatogr. A (2012), http://dx.doi.org/10.1016/j.chroma.2012.10.010 G Model CHROMA-353717; No. of Pages 7 ARTICLE IN PRESS S. Hajialigol et al. / J. Chromatogr. A xxx (2012) xxx–xxx instrument has the advantages of a simple, rugged system along with quick response and fast clear down time. It was shown to be selective and sensitive which can be applied to identification of interested analyte in complex mixtures. Results indicate excellent linearity over concentration ranges and reproducibility that make it reasonable for quantitative analysis as well. 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Please cite this article in press as: S. Hajialigol, et al., J. Chromatogr. A (2012), http://dx.doi.org/10.1016/j.chroma.2012.10.010