Ion-shift reagent binding energy and the mass-mobility shift correlation in ion mobility spectrometry.

RATIONALE: Ion mobility spectrometry (IMS) detects illegal substances and explosives in airports, ports, and customs. This is complicated by false positives caused by overlapping peaks. Shift reagents selectively change ion mobilities through adduction with analyte ions. This discriminates false positives because interferents and illegal substances respond differently to shift reagents. METHODS: We introduced five different shift reagents using electrospray ionization-IMS-mass spectrometry to study the effect of interaction energy, intermolecular bonds, and analyte size on ion mobility shifts. Analyte ion-shift reagent interactions were calculated using Gaussian. RESULTS: The mobility shifts showed a decreasing trend as the molecular weight increased for a series of ten compounds. The shift in drift time better reflected the pure effect of shift reagents. Valinol was an exception to this trend because it had a low binding energy interaction with all shift reagents and, consequently, its clusters were short-lived. This produced fewer collisions against the buffer gas and a shorter drift time, compared to ions of similar molecular weight. CONCLUSIONS: The results of this investigation are important for understanding the behavior of shift reagents in resolving overlapping peaks that cause interferences. The suppression of false positives eases the transit of passengers and cargos, increases the confiscation of illicit substances, and saves money and distress due to needless delays in customs and airports.

Effective collisional cross-section of small ions in the gas phase: Application to ion mobility spectrometry.

RATIONALE: The observed drift times of monoatomic ions, including alkali metal ions and halide anions, are not fully consistent with their size. When the effect of mass is included through the Mason-Schamp equation, the deviation gets worse so that the trend of the experimental collisional cross-sections becomes completely opposite to what is expected. This is attributed to the stronger local electric field around smaller ions. The strong electric field in the vicinity of a small ion leads to strong ion-neutral interactions and creates a drag force against ion motion. The smaller the ions, the stronger the interaction, because of the higher charge density. METHODS: In view of this, a modified equation is introduced to describe the relationship between the observed drift times or ion mobilities and the cross-sections of small ions. Here, for small ions with high charge density, the experimental collision cross-section is expressed as the effective collision cross-section, Ωeff = σi (1 + α/ri 3 ), that takes into account both intrinsic ion size, σi , and the ion-molecule interactions through a correction term of α/ri 3 , which is proportional to the charge density. RESULTS: A linear fit of the drift times of alkali metal ions and halide anions to the proposed equation showed relative deviations of <8.2%. The model successfully predicted the drift time of other small diatomic ions with reasonable error. CONCLUSIONS: The proposed model can be used as a simple and efficient relationship in predicting the effective cross-section of small ions.

Humidity Effect on the Drift Times of the Reactant Ions in Ion Mobility Spectrometry.

The effect of moisture content on the drift times of NH4+ and H3O+ reactant ions at different temperatures was experimentally and theoretically studied using an ion mobility spectrometer (IMS). The peak positions of the ions shifted to higher drift times as the humidity of the drift gas increased. The peak displacements were attributed to the consecutive formation of hydrated ion clusters, RI+(H2O)n. Using chemical equilibrium relations and thermodynamic data derived from DFT calculation, a model was proposed to formulate the change in the drift times as 1/td = 1/tdΘ - ßT log[H2O], where ß is a constant and T is temperature. [H2O] is the concentration of water in ppm and, tdΘ is the drift time at the standard condition of [H2O] = 1 ppm. The proposed equation perfectly predicted the change in the drift times of the reactant ions as a function of the moisture in the drift gas. Accordingly, standard mobility, K = KΘ - γT ln[H2O], was defined, which is independent of the moisture level of the drift gas and reflects the chemical reactivity. In this work, it is proposed to correct the reported reduced mobilities for the moisture to a standard condition of 1 ppm water concentration, in a similar manner to the corrections to the standard temperature and pressure of 273 K and 760 mbar, respectively. Finally, the likelihood that different hydrates forms of the reactant ion exist is discussed based on the entropy concept.

Ion Mobility Spectrometry of Heavy Metals.

A simple, fast, and inexpensive method was developed for detecting heavy metals via the ion mobility spectrometry (IMS) in the negative mode. In this method, Cl(-) ion produced by the thermal ionization of NaCl is employed as the dopant or the ionizing reagent to ionize heavy metals. In practice, a solution of mixed heavy metals and NaCl salts was directly deposited on a Nichrome filament and electrically heated to vaporize the salts. This produced the IMS spectra of several heavy-metal salts, including CdCl2, ZnSO4, NiCl2, HgSO4, HgCl2, PbI2, and Pb(Ac)2. For each heavy metal (M), one or two major peaks were observed, which were attributed to M·Cl(-) or [M·NaCl]Cl(-)complexes. The method proved to be useful for the analysis of mixed heavy metals. The absolute detection limits measured for ZnSO4 and HgSO4 were 0.1 and 0.05 µg, respectively.

A novel method for the determination of three volatile organic compounds in exhaled breath by solid-phase microextraction-ion mobility spectrometry.

A method was carried out for the quantitative determination of the concentrations of volatile organic compounds (VOCs) using solid-phase microextraction and ion mobility spectrometry (SPME-IMS). This method was optimized and evaluated. The best results were obtained at sorption temperature 70 °C, desorption temperature 200 °C, and extraction time 15 min. Under the optimized conditions, the linear dynamic range was found to be 0.01-4.0 ppb (R(2) > 0.995), 2.3-400 ppm (R(2) > 0.994), and 2.5-76 ppb (R(2) > 0.998) for acetone, acetaldehyde, and acetonitrile, respectively. The detection limits for acetone, acetaldehyde, and acetonitrile were 0.001 ppb, 0.18 ppm, and 0.22 ppb, respectively. As a practical application, the method was applied for the determination of acetone, acetaldehyde, and acetonitrile in human breath matrix. Therefore, the proposed method was found to be effective and simple enough to be strongly recommended for real sample analysis.

Determination of Xylene and Toluene by Solid-Phase Microextraction Using Au Nanoparticles-Thiol Silane Film Coupled to Ion Mobility Spectrometry.

A simple, rapid and sensitive method for analysis of xylene and toluene was developed using the solid-phase microextraction (SPME) combined with ion mobilityspectrometry (IMS). Gold nanoparticles-thiol silane film (Au NPs-TSF) was applied as a newsorbent for SPME. Thiol silane film was formed on the surface of a stainless steel wire through incorporating Au nanoparticles during synthesis using sol-gel technique. The extraction properties of the fiber to xylene and toluene were examined, using a SPME device and thermal desorption ininjection port of IMS. The calibration graphs were linear in the range of 1.10-32.0 and 16.0-112.0 ppb for xylene and toluene, respectively. The detection limits for xylene and toluene were 0.7 and 8.0 ppb, respectively. The proposed method was applied without any considerable sample pretreatment and it was found to be simple enough to be strongly recommended for real sample analysis.

Thin layer chromatography-ion mobility spectrometry (TLC-IMS).

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 preseparation 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.

Field Switching Combined with Bradbury-Nielsen Gate for Ion Mobility Spectrometry.

Bradbury-Nielsen gate (BNG) is commonly used in ion mobility spectrometers. It, however, transmits only a small fraction of the ions into the drift region, typically 1%. In contrast, all ions in the ionization chamber could be efficiently compressed into the drift region by the field switching gate (FSG). We report in this paper on the simultaneous use of BNG and field switching (FS) to enhance ion utilization of the BNG. In this technique, the FS collects the ions existing in the region between the FS electrode and the BNG and drives them quickly, going through the BNG in the period of gate opening. The BNG acts as the retarding field in the reported FSG to stop ions from diffusing into the drift region in the period of gate closing. Using this technique, an increase of at least 10-fold in the ion peak height without any loss of resolution is achieved for acetone compared with the BNG-only approach at a gate pulse width of 150 µs, and an even larger improvement factor of 21 is achieved for heavier DMMP dimer ions. This technique can be adapted to the current BNG-based ion mobility instruments to significantly enhance their sensitivity without any modification of the drift tube hardware.

Effect of hydration on the kinetics of proton-bound dimer formation: experimental and theoretical study.

A kinetic study was performed on the proton-bound dimer formation of cyclopentanone, cyclohexanone, and cycloheptanone at atmospheric pressure with ion mobility spectrometry (IMS) at the temperature range of 30 to 70 °C. Measured rate constants were in the range of 9.5 × 10(-11) to 4.5 × 10(-10) cm(3) s(-1). Rate constants were also calculated using average dipole orientation (ADO) theory employing density functional theory (DFT). Calculated rate constants were in the range of 1.0-5.5 × 10(-9) cm(3) s(-1). The difference between experimental and calculated rate constants was interpreted based on the hydration of the protonated monomers so that water molecules were replaced with a neutral monomer molecule in the process of dimer formation. This process requires activation energy for the formation of dimer and consequently reduces the rate constants. To verify our hypothesis, an effective rate constant (keff) was introduced, which accounted for the energetically activated water-monomer replacement in the dimer formation reactions. A good agreement was observed between the experimental rate constants and calculated keff, confirming the validity of the proposed model in explaining the kinetics of dimer formation in atmospheric pressure.

Substrate-Assisted Laser Desorption/Ionization Time-of-Flight Mass Spectrometry of Some Small Biomolecules Using TiO2-Nanotubes: The Effect of Nanotube Diameter and Salt Addition.

Substrate-assisted laser desorption/ionization (SALDI) is a kind of soft ionization method that is most suitable for the analysis of low molecular weight analytes when it is coupled with a time-of-flight mass spectrometer. Unlike the conventional matrix-assisted laser desorption/ionization, there is no interference in the SALDI with matrices for the low mass analyte peaks (m/z < 700). The focus of this work is to develop substrates based on nanomaterials to obtain higher sensitivity, better reproducibility, and easier preparation. The mass spectra of some small molecules (capecitabine, hemin, methadone, noscapine, oxycodone, thebaine, malathion, chlorpyrifos, ethion, permethrin, and phosalone) deposited on the TiO2-nanotube (TiO2-NTs) plate by the SALDI-TOF-MS technique are reported. The nanotubes are synthesized in different diameter sizes of nanotubes via the anodizing method. The intensity of the analyte peaks and the softness of ionization are optimized by varying the diameter of nanotubes and adding relevant alkali salts to the analytes. In addition, the reproducibility of the signal intensity of analytes is optimized by changing the surface hydrophilicity of the TiO2-NT plate.

Peak-peak repulsion in ion mobility spectrometry.

The space charge effect has an important role in instruments dealing with ion packets and charged particles in gas phase such as the mass spectrometer and ion mobility spectrometer (IMS). It has been shown that the space charge is partially responsible for peak broadening in IMS depending on the ion density. Here, we explore the effect of space charge on peak shifting in IMS. We show that the field created by a large peak influences the drift time of a neighboring small peak. An experimental method was introduced to accurately measure the effect of space charge between two peaks. In this method, a double pulse was applied to the shutter grid to create two closed ion packets with a given initial spacing. The final spacing was then measured at the collector through the separation of the two peaks. This study shows that space charge repulsion must be considered for accurate measurements of ion mobilities. The experiments were performed in both normal and inverse modes. A theoretical model was also proposed to describe the repulsion between two ion packets in IMS.

Dynamic Response of Ion Mobility Spectrometry.

Ion mobility spectrometers (IMS) are widely used in the security industry as well as in analytical measurements. Nevertheless, the IMS input is dynamic in nature because the sampling input can vary over time. Similar to most instruments, IMS is unable to respond immediately to variations in the input sample. Therefore, the measurements are made under transient conditions, which may affect the results. This work investigates the dynamic response of an IMS to different inputs, including steps, pulsed, exponential, and Gaussian functions. A theoretical model was developed based on two phenomena: the accumulation or dilution of a sample in the ionization region and the adsorption or desorption inside the injection port. Both processes have a charging/discharging nature. Thus, a mathematical expression was derived that takes into account two RC circuits in series. Fitting the output signal of the experimental data to the expression obtained from the model gave reasonable time constants of 2-4 s and 15-20 s for the dilution and the desorption processes, respectively. The model performance was evaluated by comparing the output with the experimental results, which were in excellent correlation. IMS was also found to behave in a manner similar to a second-order instrument, in which the output is related to the input via a second-order differential equation. These results are applicable to GC-IMS and IMS-based detectors.

Improved design for the atmospheric pressure photoionization source.

A different design for the atmospheric pressure photoionization (APPI) source, other than commercially available sources, such as PhotoSpray and PhotoMate, has been proposed. Unlike PhotoSpray, this design applies an electric field to separate photoions and electrons. In addition, the UV radiation is parallel to the gas stream toward the mass spectrometer sampling aperture. The total ion current obtained using this geometry, for dopant only, could be an order of magnitude larger than that obtained using the PhotoSpray design. Additionally, to prevent the negative effect of solvent on the photoionization yield, a curtain electrode was mounted in front of the UV lamp to divide the ionization zone into two distinct regions: the dopant and the solvent regions. Dopant was introduced in the vicinity of the lamp, and vaporized solvent was introduced into the solvent region. The curtain electrode prevented the solvent from entering the dopant region where dopant was directly photoionized. This design consumes much less dopant (approximately 1/10 less) than the conventional source, which minimizes the presence of photofragmented radicals and dopant trace contaminants in the ionization region. As a result, unlike PhotoSpray, the mass spectra contained mainly the analyte and solvent peaks. Additionally, the source was tested using an ion mobility spectrometer (IMS). The effect of the curtain electrode on signal intensity and performance of the source using IMS was also proved to be positive.

Using surfactants as matrix for the matrix-assisted laser desorption/ionization time of flight mass spectrometry (MALDI-TOF-MS) of amino acids: Sodium dodecyl sulfate (SDS) and sodium octyl sulfate (SOS).

In the present study, the applicability of two surfactants including sodium dodecyl sulfate (SDS) and sodium octyl sulfate (SOS) as the matrix for the Matrix-Assisted Laser Desorption/Ionization (MALDI) of several amino acids (phenylalanine (Phe), valine (Val), proline (Pro), alanine (Ala), and tyrosine (Tyr)) is investigated. Also, the effect of the material of the repeller plate of the ionization part of the used time of flight (TOF) mass spectrometer on the spectral patterns of the amino acids is studied. Furthermore, the recorded MALDI spectra of amino acids are compared with their corresponding direct laser desorption/ionization (direct-LDI) TOF mass spectra. It is observed that the SDS is an appropriate matrix for the Na+ transfer to the Phe and Val amino acids, especially, when the Ag metal is selected as the material of the repeller plate. In this case, the peaks of the [M + Na]+ and [M-H + 2Na]+ species are considerably more intense compared to when the NaF salt is used as a Na+ source in the LDI of these amino acids. Unlike Phe and Val, the SDS is not a good matrix for the other selected amino acids. The decrease of the carbonic chain length of the surfactant on the MALDI spectrum of Phe is investigated and it is seen that the mentioned important peaks disappeared in the presence of SOS as the matrix. The density functional theory (DFT) calculation is employed to characterize the structure of [M + Na]+ and [M-H + 2Na]+ species and determine the interaction sites of amino acids for the Na+ attachment. Also, the change in standard Gibbs free energy (∆G°) of the M + Na+ â†’ [M + Na]+ and [M + Na]+ + Na+ â†’ [M-H + 2Na]+ + H+ reactions are calculated. Based on the values of ∆G°, the attachment of the first Na+ to the amino acid takes place in the gas phase while the attachment of the second one to [M + Na]+ is not a favorable process in the gas phase.

Inverse ion mobility spectrometry.

A novel method is proposed for enhancing the separation power of ion mobility spectrometry (IMS) and other similar pulsed techniques, such as time-of-flight mass spectrometry. In this technique, rather than generating an ion packet, a dip is created in the ion beam. This is achieved by an inverse pulse applied to the shutter grid. The dip moves with the same velocity as the ion packet, and the detector reads an inverse peak at the same drift time as that of the normal operation. Using this technique, we achieved 30-60% higher resolution compared to the normal method. In addition, two close peaks that were not resolved via normal IMS were well resolved to the baseline using this technique. The main reason for the increased resolution is likely the absence of space charge in the dip.

Online detection and measurement of elemental mercury vapor by ion mobility spectrometry with chloroform dopant.

A rapid and simple method is proposed for detection of elemental mercury (Hg) vapor by ion mobility spectrometry (IMS). Negative corona discharge (CD) as the ionization source and chloroform as the dopant gas were used to produce Cl- reactant ion. A mass spectrum of the product ions confirmed that the mechanism of ionization is based on Cl- anion attachment to Hg and formation of HgCl- ion. It was found that the optimum drift gas temperature for Hg detection was about 160 °C and the drift gas flow rate should be minimized and just sufficient to clear contaminants and carry-over from the drift cell. The drift time of the HgCl- peak relative to that of the Cl- peak at 160 °C is 1.52 ms corresponding to the reduced mobility of 1.90 cm2/Vs. Because many volatile organic compounds (VOCs) such as alcohols, amines, aldehydes, ketones, and alkanes are not ionized in the negative mode of CD-IMS, these compounds do not interfere with the detection of Hg. Mercaptans peaks also did not show any interference with the Hg signal. Hence, the method is highly selective for detection of Hg in natural gas containing sulfur compounds. The detection limit of Hg obtained by the proposed method was 0.07 mg/m3. The method was successfully verified in determination of the mercury vapor content of a fluorescent lamp, as a real sample.

Rapid screening of methamphetamines in human serum by headspace solid-phase microextraction using a dodecylsulfate-doped polypyrrole film coupled to ion mobility spectrometry.

A simple, rapid and highly sensitive method for simultaneous analysis of methamphetamine (MA) and 3,4-methylenedioxy methamphetamine (MDMA) in human serum was developed using the solid-phase microextraction (SPME) combined with ion mobility spectrometry (IMS). A dodecylsulfate-doped polypyrrole (PPy-DS) was applied as a new fiber for SPME. Electrochemically polymerized PPy is formed on the surface of a platinum wire and will contain charge-compensating anion (dodecylsulfate) incorporated during synthesis using cyclic voltammetry (CV) technique. The extraction properties of the fiber to MA and MDMA were examined, using a headspace-SPME (HS-SPME) device and thermal desorption in injection port of IMS. The results show that PPy-DS as a SPME fiber coating is suitable for the successful extraction of these compounds. This method is suitable for the identification and determination of MAs, is not time-consuming, requires small quantities of sample and does not require any derivatization. Parameters like pH, extraction time, ionic strength, and temperature of the sample were studied and optimized to obtain the best extraction results. The HS-SPME-IMS method provided good repeatability (RSDs<7.8 %) for spiked serum samples. The calibration graphs were linear in the range of 20-4000 ng ml(-1) (R(2)>0.99) and detection limits for MDMA and MA were 5 and 8 ng ml(-1), respectively. HS-SPME-IMS of non-spiked serum sample provided a spectrum without any peak from the matrix, supporting an effective sample clean-up. Finally, the proposed method was applied for analysis one of the ecstasy tablet.

Detection of explosives using negative ion mobility spectrometry in air based on dopant-assisted thermal ionization.

The ionization source is an essential component of most explosive detectors based on negative ion mobility spectrometry. Conventional ion sources suffer from such inherent limitations as special safety regulations on radioactive sources or generating interfering ions (for non-radioactive sources) such as corona discharge operating in the air. In this study, a new negative ion source is introduced for ion mobility spectrometry that is based on thermal ionization and operates in the air, applicable to explosives detection. Our system consists of a heating filament powered by an isolated power supply connected to negative high voltage. The ionization is assisted by doping chlorinated compounds in the gas phase using chlorinated hydrocarbons in contact with the heating element to yield Cl- reactant ions. Several chlorinated hydrocarbons are evaluated as the reagent chemicals for providing Cl- reactant ions, of which CCl4 is identified as the best ionizing reagent. The ion source is evaluated by recording the ion mobility spectra of common explosives, including TNT, RDX, and PETN in the air. A detection limit of 150 pg is obtained for TNT. Compared to other ionization sources, the new source is found to be low-cost, simple, and long-lived, making it suited to portable explosives detection devices.

Laser desorption-ion mobility spectrometry as a useful tool for imaging of thin layer chromatography surface.

We present a novel method for coupling thin layer chromatography (TLC) with ion mobility spectrometry (IMS) using laser desorption technique (LD). After separation of the compounds by TLC, the TLC surface was sampled by the LD-IMS without any further manipulation or preparation. The position of the laser was fixed and the TLC plate was moved in desired directions by the motorized micro-positioning stage. The method was successfully applied to analyze the TLC plates containing explosives (tri nitro toluene, 1,3,5-trinitro- 1,3,5-triazacyclohexane, pentaerythritol tetranitrate, 2,4-dinitro toluene and 3,4-dinitro toluene), amino acids (alanine, proline and isoleucine), nicotine and diphenylamine mixtures and detection limits for these compounds were determined. Combination of TLC with LD-IMS technique offers additional separation dimension, allowing separation of overlapping TLC analytes. The time for TLC sampling by LD-IMS was less than 80s. The scan rate for LD is adjustable so that fast and effective analysis of the mixtures is possible with the proposed method.

Using corona discharge-ion mobility spectrometry for detection of 2,4,6-Trichloroanisole.

In this work possible application of the corona discharge-ion mobility spectrometer (CD-IMS) for detection of 2,4,6-Trichloroanisole (TCA) has been investigated. We applied CD-IMS interfaced with orthogonal acceleration time of flight mass spectrometer (CD-IMS-oaTOF) to study the ion processes within the CD-IMS technique. The CD-IMS instrument was operated in two modes, (i) standard and (ii) reverse flow modes resulting in different chemical ionisation schemes by NO3(-)(HNO3)n (n=0,1,2) and O2(-)(H2O)n (n=0,1,2), respectively. The O2(-)(H2O)n ionisation was associated with formation of Cl(-) and (TCA-CH3)(-) ions from TCA. The NO3(-)(HNO3)n ionisation, resulted in formation of NO3(-)(HNO3)(TCA-Cl) adduct ions. Limit of detection (LOD) for TCA was determined in gas (100 ppb) and solid phases (150 ng).