David A. Atkinson

Construction and characterization of a high-flow, high-resolution ion mobility spectrometer for detection of explosives after personnel portal sampling

A novel analysis of explosives via the coupling of an airline passenger personnel portal with a high-flow (HF), high-resolution (HR) ion mobility spectrometry (IMS) was shown for the first time. The HF-HR-IMS utilized a novel ion aperture grid design with a (63)Ni ionization source while operating at ambient pressure in the positive ion mode at 200 degrees C. The HF-HR-IMS response characteristics of 2,4,6-trinitrotoluene (TNT), 4,6-dinitro-o-cresol (4,6DNOC), and cyclo-1,3,5-trimethylene-2,4,6-trinitramine (RDX) were investigated. Modifications made to the HF-HR-IMS exhaust and ionization source created an 800% increase in the total ion current (TIC), from 0.85 to 6.8 nA. This translated into a 65% ion response increase for TNT when compared with a traditional IMS. A mixture of TNT and (4,6DNOC) was used to successfully demonstrate the resolving power of the species with similar reduced mobility constants (K(o)), 1.54 and 1.59, respectively. The reactant ion (H(2)O)(n)H(+), peak was also used to measure the resolving power of the spectrometer while varying the internal diameter of three different aperture openings from 1.00 to 3.54cm. This provided a resolving power range of 50-60, double that typically achievable by commercial IMS instruments. Most important, these changes made in this new instrumental design can be implemented to all existing and future IMSs to greatly enhance the achievable IMS resolving power.

The role of oxygen in the formation of TNT product ions in ion mobility spectrometry

The atmospheric pressure ionization of 2,4,6-trinitrotoluene (TNT) in air yields the (TNT-H)− product ion. It is generally accepted that this product ion is formed by the direct proton abstraction of neutral TNT by O2− reactant ions. Data presented here demonstrate the reaction involves the formation of an intermediate (TNT·O2)−, from the association of either TNT+O2− or TNT−+O2. This intermediate has two subsequent reaction branches. One of these branches involves simple dissociation of the intermediate to TNT−; the other branch is a terminal reaction that forms the typically observed (TNT-H)− ion via proton abstraction. The dissociation reaction involving electron transfer to TNT− appeared to be kinetically favored and prevailed at low concentrations of oxygen (less than 2%). The presence of significant amounts of oxygen, however, resulted in the predominant formation of the (TNT-H)− ion by the terminal reaction branch. With TNT− in the system, either from direct electron attachment or by simple dissociation of the intermediate, increasing levels of oxygen in the system will continue to reform the intermediate, allowing the cycle to continue until proton abstraction occurs. Key to understanding this complex reaction pathway is that O2− was observed to transfer an electron directly to neutral TNT to form the TNT−. At oxygen levels of less than 2%, the TNT− ion intensity increased with increasing levels of oxygen (and O2−) and was larger than the (TNT-H)− ion intensity. As the oxygen level increased from 2 to 10%, the (TNT-H)− product ion became predominant. The potential reaction mechanisms were investigated with an ion mobility spectrometer, which was configured to independently evaluate the ionization pathways.

Resolving interferences in negative mode ion mobility spectrometry using selective reactant ion chemistry

During the investigation of the degradation products of 2,4,6-trinitrotoluene (TNT) using ion mobility spectrometry (IMS), 2,4-dichlorophenoxyacetic acid (2,4-D) and 2,4-dichlorophenol (DCP) were found to have IMS responses which overlapped those of the TNT degradation products. It was observed that the Cl(-) reactant ion chemistry, often used for explosives analysis, was not always successful in resolving peak overlap of analytes and interferents. It is shown here that resolution of the analytes and interferences can sometimes be achieved using only air for the formation of reactant ions, at other times through the use of Br(-) as an alternative to Cl(-) for producing reactant ions, and also through the promotion of adduct stability by lowering the IMS temperature.

Formation of halide reactant ions and effects of excess reagent chemical on the ionization of TNT in ion mobility spectrometry

The efficiency of chloride reactant ion formation, when chlorinated hydrocarbon reagent chemicals were added to the ionization region of an ion mobility spectrometer, corresponded to the electron attachment rate constant of the chemical. The chemicals investigated here included chloromethane, dichlormethane, trichloromethane, tetrachloromethane and chlorobenzene, with tetrachloromethane producing the greatest amount of chloride reactant ions for the amount of chemical added. Reagent chemicals with smaller electron attachment rate constants required the addition of more chemical to reach functional reactant ion levels. The excess neutral reagent molecules clustered to the chloride reactant ions and reduced the effectiveness of abstracting a proton from 2,4,6-trinitrotoluene (TNT). The effect of clustering was different for each chemical. Tetrachloromethane, which had the least exothermic clustering reaction, had the most effective production of the (TNT-H)(-) product ion per mole of reagent chemical. Bromide and iodide ions were also investigated as potential reactant ions. Bromide was found to effectively produce the proton abstracted (TNT-H)(-) ion. Iodide, however, was not a strong enough base to form (TNT-H)(-) from TNT. There was no apparent transfer of an electron to TNT by chloride, bromide or iodide.

Electrospray ionization-ion mobility spectrometry: a rapid analytical method for aqueous nitrate and nitrite analysis

This paper reports the first example of electrospray ionization (ESI) for the separation and detection of anions in aqueous solutions by ion mobility spectrometry (IMS). Standard solutions of arsenate, phosphate, sulfate, nitrate, nitrite, chloride, formate, and acetate were analyzed using ESI-IMS and distinct peak patterns and reduced mobility constants (K(0)) were observed for respective anions. Real world water samples were analyzed for nitrate and nitrite to determine the feasibility of using ESI-IMS as a rapid analytical method for monitoring nitrate and nitrite in water systems. The data showed satisfactory correlation between the measured value ([similar]0.16 ppm) and the reported maximum nitrate-nitrogen concentration (0.2 ppm) found in a local drinking water system. For on-site measurement applications, direct sample introduction and air as an alternate drift gas to nitrogen were evaluated. The identities of the nitrite and nitrate mobility peaks were verified by comparison of reduced mobility constants with mass identified nitrate and nitrite ions reported in literature. In the mixing ratio, a linear dynamic range of 3 orders of magnitude and instrument detection limits of 10 ppb for nitrate and 40 ppb for nitrite were obtained. The calibration curves showed r(2) value of 0.98 and slope of 0.06 for nitrate and r(2) value of 0.99 and slope of 0.11 for nitrite.

Quantification of mammalian lignans in biological fluids using gas chromatography with ion mobility detection

A method is presented to quantify selected mammalian lignans in human physiological fluids by gas chromatography (GC) coupled with ion mobility spectrometry (IMS). The use of IMS following GC permitted the selective and sensitive measurement of 2,3-bis(3-hydroxybenzyl)butane-1,4-diol (i.e., enterodiol) and trans-2,3-bis(3-hydroxybenzyl)-gamma-butyrolactone (i.e., enterolactone) concentrations in urine and plasma following dietary supplementation with whole wheat/flaxseed bread high in mammalian lignan precursors. Following six weeks of flaxseed feeding, urinary and plasma levels of enterodiol and enterolactone were elevated, exceeding the amounts found at baseline by a factor of 3-5. The approach to mammalian lignan methodology presented herein provides novel analytical phytochemical procedures for assessing the impact of lignan consumption in human health and disease.

Direct Real-Time Detection of Vapors from Explosive Compounds

The real-time detection of vapors from low volatility explosives including PETN, tetryl, RDX, and nitroglycerine along with various compositions containing these substances was demonstrated. This was accomplished with an atmospheric flow tube (AFT) using a nonradioactive ionization source coupled to a mass spectrometer. Direct vapor detection was accomplished in less than 5 s at ambient temperature without sample preconcentration. The several seconds of residence time of analytes in the AFT provided a significant opportunity for reactant ions to interact with analyte vapors to achieve ionization. This extended reaction time, combined with the selective ionization using the nitrate reactant ions (NO3(-) and NO3(-)·HNO3), enabled highly sensitive explosives detection from explosive vapors present in ambient laboratory air. Observed signals from diluted explosive vapors indicated detection limits below 10 ppqv using selected ion monitoring (SIM) of the explosive-nitrate adduct at m/z 349, 378, 284, and 289 for tetryl, PETN, RDX, and NG, respectively. Also provided is a demonstration of the vapor detection from 10 different energetic formulations sampled in ambient laboratory air, including double base propellants, plastic explosives, and commercial blasting explosives using SIM for the NG, PETN, and RDX product ions.

Characterization of triacetone triperoxide by ion mobility spectrometry and mass spectrometry following atmospheric pressure chemical ionization.

The atmospheric pressure chemical ionization of triacetone triperoxide (TATP) with subsequent separation and detection by ion mobility spectrometry has been studied. Positive ionization with hydronium reactant ions produced only fragments of the TATP molecule, with m/z 91 ion being the most predominant species. Ionization with ammonium reactant ions produced a molecular adduct at m/z 240. The reduced mobility value of this ion was constant at 1.36 cm(2)V(-1)s(-1) across the temperature range from 60 to 140 °C. The stability of this ion was temperature dependent and did not exist at temperatures above 140 °C, where only fragment ions were observed. The introduction of ammonia vapors with TATP resulted in the formation of m/z 58 ion. As the concentration of ammonia increased, this smaller ion appeared to dominate the spectra and the TATP-ammonium adduct decreased in intensity. The ion at m/z 58 has been noted by several research groups upon using ammonia reagents in chemical ionization, but the identity was unknown. Evidence presented here supports the formation of protonated 2-propanimine. A proposed mechanism involves the addition of ammonia to the TATP-ammonium adduct followed by an elimination reaction. A similar mechanism involving the chemical ionization of acetone with excess ammonia also showed the formation of m/z 58 ion. TATP vapors from a solid sample were detected with a hand-held ion mobility spectrometer operated at room temperature. The TATP-ammonium molecular adduct was observed in the presence of ammonia and TATP vapors with this spectrometer.

Bayesian Integration and Classification of Composition C-4 Plastic Explosives Based on Time-of-Flight-Secondary Ion Mass Spectrometry and Laser Ablation-Inductively Coupled Plasma Mass Spectrometry

Time-of-flight-secondary ion mass spectrometry (TOF-SIMS) and laser ablation-inductively coupled plasma mass spectrometry (LA-ICPMS) were used for characterization and identification of unique signatures from a series of 18 Composition C-4 plastic explosives. The samples were obtained from various commercial and military sources around the country. Positive and negative ion TOF-SIMS data were acquired directly from the C-4 residue on Si surfaces, where the positive ion mass spectra obtained were consistent with the major composition of organic additives, and the negative ion mass spectra were more consistent with explosive content in the C-4 samples. Each series of mass spectra was subjected to partial least squares-discriminant analysis (PLS-DA), a multivariate statistical analysis approach which serves to first find the areas of maximum variance within different classes of C-4 and subsequently to classify unknown samples based on correlations between the unknown data set and the original data set (often referred to as a training data set). This method was able to successfully classify test samples of C-4, though with a limited degree of certainty. The classification accuracy of the method was further improved by integrating the positive and negative ion data using a Bayesian approach. The TOF-SIMS data was combined with a second analytical method, LA-ICPMS, which was used to analyze elemental signatures in the C-4. The integrated data were able to classify test samples with a high degree of certainty. Results indicate that this Bayesian integrated approach constitutes a robust classification method that should be employable even in dirty samples collected in the field.

Chapter 1 General Detection Problems in SFC

Publisher Summary This chapter presents the general detection problems faced in supercritical fluid chromatography (SFC). When the separation stage of a hyphenated system is SFC, special interfacing problems exist between the SFC and the spectrometer due to the unique properties of super critical fluids. Phase changes, varying sample introduction rates, mobile phase compatibility, mobile phase elimination, integrity of the SFC separation, and ambiguous detector terminology are all problems with SFC hyphenated analytical methods. Perhaps the most difficult problem associated with SFC detection is compatibility with the mobile phase. Carbon dioxide has emerged as one of the principal mobile phases used in SFC not only because of its convenient critical parameters and non-toxicity but also because of its compatibility with both flame ionization and UV absorption detection. Nevertheless, the use of modifiers with CO 2 to increase polarity is severely limited by the detection method employed. Comparative evaluation of hyphenated detection methods for SFC can also present a problem since such a wide variety of analytical methods can be interfaced with SFC.