Secondary electrospray ionization-ion mobility spectrometry for explosive vapor detection

Carbohydrate structure characterization by tandem ion mobility mass spectrometry (IMMS)2

A high resolution ion mobility spectrometer was interfaced to a Synapt G2 high definition mass spectrometer (HDMS) to produce IMMS-IMMS analysis. The hybrid instrument contained an electrospray ionization source, two ion gates, an ambient pressure linear ion mobility drift tube, a quadrupole mass filter, a traveling wave ion mobility spectrometer (TWIMS), and a time-of-flight mass spectrometer. The dual gate drift tube ion mobility spectrometer (DTIMS) could be used to acquire traditional IMS spectra but also could selectively transfer specific mobility selected precursor ions to the Synapt G2 HDMS for mass filtration (quadrupole). The mobility and mass selected ions could then be introduced into a collision cell for fragmentation followed by mobility separation of the fragment ions with the traveling wave ion mobility spectrometer. These mobility separated fragment ions are finally mass analyzed using a time-of-flight mass spectrometer. This results in an IMMS-IMMS analysis and provides a method to evaluate the isomeric heterogeneity of precursor ions by both DTIMS and TWIMS to acquire a mobility-selected and mass-filtered fragmentation pattern and to additionally obtain traveling wave ion mobility spectra of the corresponding product ions. This new IMMS(2) instrument enables the structural diversity of carbohydrates to be studied in greater detail. The physical separation of isomeric oligosaccharide mixtures was achieved by both DTIMS and TWIMS, with DTIMS demonstrating higher resolving power (70-80) than TWIMS (30-40). Mobility selected MS/MS spectra were obtained, and TWIMS evaluation of product ions showed that isomeric forms of fragment ions existed for identical m/z values.

Super-atmospheric pressure chemical ionization mass spectrometry

Super-atmospheric pressure chemical ionization (APCI) mass spectrometry was performed using a commercial mass spectrometer by pressurizing the ion source with compressed air up to 7 atm. Similar to typical APCI source, reactant ions in the experiment were generated with corona discharge using a needle electrode. Although a higher needle potential was necessary to initiate the corona discharge, discharge current and detected ion signal were stable at all tested pressures. A Roots booster pump with variable pumping speed was installed between the evacuation port of the mass spectrometer and the original rough pumps to maintain a same pressure in the first pumping stage of the mass spectrometer regardless of ion source pressure. Measurement of gaseous methamphetamine and research department explosive showed an increase in ion intensity with the ion source pressure until an optimum pressure at around 4-5 atm. Beyond 5 atm, the ion intensity decreased with further increase of pressure, likely due to greater ion losses inside the ion transport capillary. For benzene, it was found that besides molecular ion and protonated species, ion due to [M + 2H](+) which was not so common in APCI, was also observed with high ion abundance under super-atmospheric pressure condition.

High mass resolution breath analysis using secondary electrospray ionization mass spectrometry assisted by an ion funnel

In this study, we used secondary electrospray ionization mass spectrometry assisted by an ion funnel (IF) operating at ambient pressure to find compounds in the mass range of 100-500 m/z in online breath fingerprinting experiments. In low-resolution experiments conducted on an ion trap instrument, we found that pyridine is present in breath of individuals long after drinking coffee. In high-resolution experiments conducted on a Fourier transform ion cyclotron resonance, we found more than 30 compounds in the mass range of 100-500 m/z in analogous online breath experiments. More than a third of these compounds have molecular weights above 200 Daltons and have not been mentioned in previous studies. In low-resolution experiments as well as experiments without the IF, these compounds could not be detected.

Low-sample flow secondary electrospray ionization: improving vapor ionization efficiency

In secondary electrospray ionization (SESI) systems, gaseous analytes exposed to an elecrospray plume become ionized after charge is transferred from the charging electrosprayed particles to the sample species. Current SESI systems have shown a certain potential. However, their ionization efficiency is limited by space charge repulsion and by the high sample flows required to prevent vapor dilution. As a result, they have a poor conversion ratio of vapor into ions. We have developed and tested a new SESI configuration, termed low-flow SESI, that permits the reduction of the required sample flows. Although the ion to vapor concentration ratio is limited, the ionic flow to sample vapor flow ratio theoretically is not. The new ionizer is coupled to a planar differential mobility analyzer (DMA) and requires only 0.2 lpm of vapor sample flow to produce 3.5 lpm of ionic flow. The achieved ionization efficiency is 1/700 (one ion for every 700 molecules) for TNT and, thus, compared with previous SESI ionizers coupled with atmospheric pressure ionization-mass spectrometry (API-MS) (Mesonero, E.; Sillero, J. A.; Hernández, M.; Fernandez de la Mora, J. Philadelphia PA, 2009) has been improved by a large factor of at least 50-100 (our measurements indicate 70). The new ionizer coupled with the planar DMA and a triple quadrupole mass spectrometer (ABSciex API5000) requires only 20 fg (50 million molecules) to produce a discernible signal after mobility and MS(2) analysis.

Sensitive detection of drug vapors using an ion funnel interface for secondary electrospray ionization mass spectrometry

In this study, we use an ion funnel (IF) at ambient pressure to enhance the sensitivity of secondary electrospray ionization (SESI). Atenolol, salbutamol and cocaine as test compounds are delivered to the SESI interface in the gas phase and are charged with three nano electrosprays. In our experiments, we show that the compounds can be detected at concentrations in the low pptv range, which is an increase of two orders of magnitude compared with the results without the IF. With a standard SESI interface, the compounds could not be detected at all. With the use of the SESI IF interface for the headspace analysis of bananas and limes, we can detect many more compounds and at higher intensities than with a standard SESI interface.

Comparison of rectangular and bisinusoidal waveforms in a miniature planar high-field asymmetric waveform ion mobility spectrometer

High-field asymmetric waveform ion mobility spectrometry (FAIMS) separates ions by utilizing the mobility differences of ions at high and low fields. The shape of the waveform is one of the essential features affecting the resolution, transmission, and separation of FAIMS. Due to practical circuitry advantages, sinusoidal asymmetric waveforms are typically used in FAIMS, whereas theoretical studies indicate that square asymmetric waveforms improve ion separation, resolution, and sensitivity. Results from FAIMS using square and sinusoidal waveforms are presented, and effects of the waveforms on ion separation are discussed. A FAIMS system interfaced with a quadrupole ion trap mass spectrometer was used in this study. FAIMS spectra were generated by scanning the compensation voltage (CV) while operating the mass spectrometer in total ion mode. The identification of ions was accomplished through mass spectra acquired at fixed values of ions' CVs. Square waveform evaluation was done by acquiring data at three frequencies and six duty cycles of the square waveform generator. The performance of FAIMS using square and sinusoidal waveforms at 250, 333, and 500 kHz frequencies was compared, and trends were identified. For all frequencies, the best response of FAIMS was achieved at the lower amplitudes and under the lower duty cycles of the square waveform generator. The separation of FAIMS was better at the higher frequencies. These results demonstrate the potential to incorporate square-wave FAIMS into the design of a miniature device for detection of explosives in the field. SIMION version 8.0, the ion trajectory modeling program, was utilized to optimize the performance of the miniature FAIMS cell and to validate experimental results.

Ion spectrometric detection technologies for ultra-traces of explosives: a review

In recent years, explosive materials have been widely employed for various military applications and civilian conflicts; their use for hostile purposes has increased considerably. The detection of different kind of explosive agents has become crucially important for protection of human lives, infrastructures, and properties. Moreover, both the environmental aspects such as the risk of soil and water contamination and health risks related to the release of explosive particles need to be taken into account. For these reasons, there is a growing need to develop analyzing methods which are faster and more sensitive for detecting explosives. The detection techniques of the explosive materials should ideally serve fast real-time analysis in high accuracy and resolution from a minimal quantity of explosive without involving complicated sample preparation. The performance of the in-field analysis of extremely hazardous material has to be user-friendly and safe for operators. The two closely related ion spectrometric methods used in explosive analyses include mass spectrometry (MS) and ion mobility spectrometry (IMS). The four requirements-speed, selectivity, sensitivity, and sampling-are fulfilled with both of these methods.

Characterizing bacterial volatiles using secondary electrospray ionization mass spectrometry (SESI-MS)

Secondary electrospray ionization mass spectrometry (SESI-MS) is a method developed for the rapid detection of volatile compounds, without the need for sample pretreatment. The method was first described by Fenn and colleagues¹ and has been applied to the detection of illicit drugs² and explosives^3-4, the characterization of skin volatiles⁵, and the analysis of breath^6-7.

SESI ionization occurs by proton transfer reactions between the electrospray solution and the volatile analyte, and is therefore suitable for the analysis of hetero-organic molecules, just as in traditional electrospray ionization (ESI). However, unlike standard ESI, the proton transfer process of SESI occurs in the vapor phase rather than in solution (Fig. 1), and therefore SESI is best suited for detecting organic volatiles and aerosols.

We are expanding the use of SESI-MS to the detection of bacterial volatiles as a method for bacterial identification and characterization⁸. We have demonstrated that SESI-MS volatile fingerprinting, combined with a statistical analysis method, can be used to differentiate bacterial genera, species, and mixed cultures in a variety of growth media.⁸ Here we provide the steps for obtaining bacterial volatile fingerprints using SESI-MS, including the instrumental parameters that should be optimized to ensure robust bacterial identification and characterization.

Contribution of liquid-phase and gas-phase ionization in extractive electrospray ionization mass spectrometry of primary amines

In this study, we investigated how binary mixtures of compounds influence each other's signal intensity in electrospray ionization (ESI), extractive electrospray ionization (EESI) and secondary electrospray ionization (SESI) experiments. The experiments were conducted using a series of homologous primary amines (from 1-butyl to 1- decylamine). In every experiment, two of the amines were present, and all 21 possible combinations were measured with EESI, ESI and SESI as ionization sources. Except for the volatility, which decreases with increasing molecular weight, the physico-chemical properties of the amines are very similar, so that the intensity ratio obtained in each experiment provides information about discrimination effects occurring during the ionization process. The results show that for the relatively volatile compounds investigated, the EESI ionization mechanism resembles the SESI-like gas-phase charge transfer more than ESI-like analyte ionization in solution. In addition, almost no discrimination effects were observed in the spectra obtained in EESI experiments. Quantitative EESI experiments with nonylamine as internal standard showed that EESI is capable of providing both more accurate and more precise results than SESI and ESI.

Identification of explosives and explosive formulations using laser electrospray mass spectrometry

Mass analysis is demonstrated for the detection of sub-microgram quantities of explosive samples on a metallic surface at atmospheric pressure using laser electrospray mass spectrometry (LEMS). A non-resonant femtosecond duration laser pulse vaporizes native samples for subsequent electrospray ionization and transfer into a time-of-flight mass spectrometer. LEMS was used to detect 2,3-dimethyl-2,3-dinitrobutane (DMNB), 1,3,5-trinitroperhydro-1,3,5-triazine (RDX), 3,4,8,9,12,13-hexaoxa-1,6-diazabicyclo[4.4.4]tetradecane (HMTD), and 3,3,6,6,9,9-hexamethyl-1,2,4,5,7,8-hexaoxacyclononane (TATP) deposited on a steel surface. LEMS was also used to directly analyze composite propellant materials containing an explosive to determine the molecular composition of the explosive pellets at atmospheric pressure.

Ambient mass spectrometry: bringing MS into the "real world"

Mass spectrometry has recently undergone a second contemporary revolution with the introduction of a new group of desorption/ionization (DI) techniques known collectively as ambient mass spectrometry. Performed in an open atmosphere directly on samples in their natural environments or matrices, or by using auxiliary surfaces, ambient mass spectrometry (MS) has greatly simplified and increased the speed of MS analysis. Since its debut in 2004 there has been explosive growth in the applications and variants of ambient MS, and a very comprehensive set of techniques based on different desorption and ionization mechanisms is now available. Most types of molecules with a large range of masses and polarities can be ionized with great ease and simplicity with the outstanding combination of the speed, selectivity, and sensitivity of MS detection. This review describes and compares the basis of ionization and the concepts of the most promising ambient MS techniques known to date and illustrates, via typical analytical and bioanalytical applications, how ambient MS is helping to bring MS analysis deeper than ever into the "real world" open atmosphere environment--to wherever MS is needed.

Selective detection of diethylene glycol in toothpaste products using neutral desorption reactive extractive electrospray ionization tandem mass spectrometry

A rapid, sensitive method based on neutral desorption (ND) reactive extractive electrospray ionization mass spectrometry (EESI-MS) has been established for the selective quantitative detection of diethylene glycol (DEG) in toothpaste products without any sample pretreatment. The sensitivity and specificity of DEG detection were enhanced by implementing selective ion/molecule reactions in the EESI process, featuring the EESI mass spectra with the characteristic signals of DEG. The method provided a low limit of detection (LOD) (approximately 0.00002%, weight percent of DEG in toothpaste), reasonable recovery (97.6-102.4%), and acceptable relative standard deviations (RSD < 8%, n = 8) for direct measuring of DEG in the spiked toothpaste samples. Trace amounts of DEG in commercial toothpaste products have been quantitatively detected without any sample manipulation. The results demonstrate that nonvolatile compounds such as DEG can be sensitively liberated using the neutral gas beam for quantitative detection from the extremely viscous toothpaste containing solid nanoparticles, showing that ND-EESI-MS is a useful tool for the rapid characterization of highly complex and/or viscous samples at molecular levels.

What can we learn from ambient ionization techniques?

Ambient mass spectrometry-mass spectrometric analysis with no or minimal effort for sample preparation-has experienced a very rapid development during the last 5 years, with many different methods now available for ionization. Here, we review its range of applications, the hurdles encountered for its quantitative use, and the proposed mechanisms for ion formation. Clearly, more effort needs to be put into investigation of matrix effects, into defining representative sampling of heterogeneous materials, and into understanding and controlling the underlying ionization mechanisms. Finally, we propose a concept to reduce the number of different acronyms describing very similar embodiments of ambient mass spectrometry.

On-line detection of human skin vapors

Vapors released by the skin in the hand of one human subject are detected in real time by sampling them directly from the ambient gas surrounding the hand, ionizing them by secondary electrospray ionization (SESI, via contact with the charged cloud from an electrospray source), and analyzing them in a mass spectrometer with an atmospheric pressure source (API-MS). This gas-phase approach is complementary to alternative on-line surface ionization methods such as DESI and DART. A dominating peak of lactic acid and a complete series of saturated and singly unsaturated fatty acids (C(12) to C(18)) are observed, in accordance with previous off-line studies by gas chromatography-mass spectrometry. Several other metabolites have been identified, including ketomonocarboxylic and hydroxymonocarboxylic acids.

Ion mobility spectrometry coupled with multi-capillary columns for metabolic profiling of human breath

Recently, ion mobility spectrometry (IMS) started to be used for direct breath analysis with respect to metabolic profiling, biomarker finding and gas trace analysis. The present review describes the basic operation of an ion mobility spectrometer including the ionization process, humidity effects and sampling procedures. To enhance the resolution, pre-separation by multi-capillary columns (MCCs) is discussed and examples for IMS chromatograms are presented. The focus is to review the analytical method IMS with respect to potential use for direct investigations of humid air in direct breath analysis but not on detailed discussion of results of specific medical application of MCC/IMS or on specific analytes found in exhaled air.

Generation of multiply charged peptides and proteins by radio frequency acoustic desorption and ionization for mass spectrometric detection

The design and implementation of a radio frequency acoustic desorption ionization (RADIO) source has been demonstrated for the analysis of multiply charged peptides and proteins. One muL aliquots of melittin, BNP-32, and ubiquitin ( approximately 1 mug of analyte) were deposited onto a quartz crystal microbalance (QCM) electrode before radio frequency actuation for desorption. Continuous electrospray parallel to/above the sampling surface enabled the ionization of desorbed species. Detection by a hybrid linear ion trap Fourier transform ion cyclotron resonance mass spectrometer confirmed the intact and dissociated species observed during MS and MS/MS experiments, respectively.

Secondary electrospray ionization (SESI) of ambient vapors for explosive detection at concentrations below parts per trillion

We determine the sensitivity of several commercial atmospheric pressure ionization mass spectrometers towards ambient vapors, ionized by contact with an electrospray of acidified or ammoniated solvent, a technique often referred to as secondary electrospray ionization (SESI). Although a record limit of detection of 0.2 x 10(-12) atmospheres (0.2 ppt) is found for explosives such as PETN and 0.4 ppt for TNT (without preconcentration), this still implies the need for some 10(8)-10(9) vapor molecules/s for positive identification of explosives. This extremely inefficient use of sample is partly due to low charging probability ( approximately 10(-4)), finite ion transmission, and counting probability in the mass spectrometer (1/10 in quadrupoles), and a variable combination of duty cycle and background noise responsible typically for a 10(3) factor loss of useful signal.

Selective detection of gas-phase TNT by integrated optical waveguide spectrometry using molecularly imprinted sol–gel sensing films

A chemical sensor was developed to detect the explosive 2,4,6-trinitrotoluene (TNT) utilizing planar integrated optical waveguide (IOW) attenuated total reflection spectrometry. Submicron thick films of organically modified sol-gel polymers were deposited on the waveguide surface as the sensing layer. Sol-gels were molecularly imprinted for TNT using covalently bound template molecules linked to the matrix through 1 or 2 carbamate linkages. Upon chemical cleavage of the template and displacement of the TNT-like pendant groups from the matrix, shape-selective binding sites were created that possess a primary amine group. The amine was used to deprotonate bound TNT yielding an anionic form that absorbs visible light. Binding of TNT and subsequent conversion to the anion results in the attenuation of light propagating through the waveguide, thus creating a spectrophotometric device. Sensitivity can be achieved by taking advantage of the substantial pathlength provided by the use of single mode IOWs. The limit-of-detection to gas-phase TNT was found to be five parts-per-billion (ppbV) in ambient air at a flow rate of 40 mL min(-1) given a 60 s sampling time. The sensor is highly selective for TNT due to the selectivity of binding site recognition of TNT and the subsequent generation of the TNT anion. Response to TNT is not reversible which results in an integrating sensor device which, in theory, can improve the ability to detect small amounts of the explosive if the exposure time is sufficient in length.

Mechanical ion gate for electrospray-ionization ion-mobility spectrometry

A novel ion gate for electrospray-ionization atmospheric-pressure ion-mobility spectrometry (ESI-IMS) has been constructed and evaluated. The ion gate consisted of a chopper wheel with two windows--one for periodic ion passage from the ESI source into the drift region and the other for timing and synchronization purposes. The instrument contained a 45.0 cm long drift tube comprising 78 stainless steel rings (0.12 cm thick, 4.90 cm o.d., 2.55 cm i.d.). The rings were connected together in series with 3.34-MOmega resistors. The interface plate and the back plate were also connected with the first and the last rings, respectively, of the drift tube with 3.34-MOmega resistors. A potential of -20.0 kV was applied to the back plate and the interface plate was grounded. The drift tube was maintained at an electric field strength of approximately 400 V cm-1. An aperture grid was attached to the last ring in front of a Faraday plate detector, center-to-center. Several sample solutions were electrosprayed at +5.0 kV with +500 V applied to the ion gate. Baseline separations of selected benzodiazepines, antidepressants, and antibiotics were observed with moderate experimental resolution of approximately 70.

Detection of explosives on solid surfaces by thermal desorption and ambient ion/molecule reactions

A simple, fast and direct method is presented for detecting traces of solid explosives on cotton swabs or in particulate samples: ions are transferred into a mass spectrometer after thermal desorption and corona discharge chemical ionization in ambient air; specificity is enhanced using ambient ion/molecule reactions or by conventional tandem mass spectrometry.