Engineering synthetic breath biomarkers for respiratory disease.

Human breath contains many volatile metabolites. However, few breath tests are currently used in the clinic to monitor disease due to bottlenecks in biomarker identification. Here we engineered breath biomarkers for respiratory disease by local delivery of protease-sensing nanoparticles to the lungs. The nanosensors shed volatile reporters upon cleavage by neutrophil elastase, an inflammation-associated protease with elevated activity in lung diseases such as bacterial infection and alpha-1 antitrypsin deficiency. After intrapulmonary delivery into mouse models with acute lung inflammation, the volatile reporters are released and expelled in breath at levels detectable by mass spectrometry. These breath signals can identify diseased mice with high sensitivity as early as 10 min after nanosensor administration. Using these nanosensors, we performed serial breath tests to monitor dynamic changes in neutrophil elastase activity during lung infection and to assess the efficacy of a protease inhibitor therapy targeting neutrophil elastase for the treatment of alpha-1 antitrypsin deficiency.

Optimization of ultraviolet Raman spectroscopy for trace explosive checkpoint screening.

Raman spectroscopy has long been considered a gold standard for optically based chemical identification, but has not been adopted in non-laboratory operational settings due to limited sensitivity and slow acquisition times. Ultraviolet (UV) Raman spectroscopy has the potential to address these challenges through the reduction of fluorescence from background materials and increased Raman scattering due to the shorter wavelength (relative to visible or near-infrared excitation) and resonant enhancement effects. However, the benefits of UV Raman must be evaluated against specific operational situations: the actual realized fluorescence reduction and Raman enhancement depend on the specific target materials, target morphology, and operational constraints. In this paper, the wavelength trade-space in UV Raman spectroscopy is evaluated for one specific application: checkpoint screening for trace explosive residues. The optimal UV wavelength is evaluated at 244, 266, and 355 nm for realistic trace explosive and explosive-related compound (ERC) residues on common checkpoint materials: we perform semi-empirical analysis that includes the UV penetration depth of common explosive and ERCs, realistic explosive and ERC residue particle sizes, and the fluorescence signal of common checkpoint materials. We find that while generally lower UV wavelength provides superior performance, the benefits may be significantly reduced depending on the specific explosive and substrate. Further, logistical requirements (size, weight, power, and cost) likely limit the adoption of optimal wavelengths. Graphical abstract.

Theoretical Bounds on Electron Energy Filtering in Disordered Nanomaterials.

Electron energy filters have recently been proposed as a method of reducing the effects of thermal broadening in device and sensing applications, enabling substantial improvements in their room temperature performance. Nanostructured materials can act as electron energy filters by funneling thermally broadened electrons through discrete energy levels. In this study, we develop a theoretical model of the electron filtering properties of nanostructured materials that explicitly includes the effects of thermal broadening and size heterogeneity on the heterogeneity of nanostructure energy levels. We find that under certain conditions quantum dot solids can perform as effective electronic energy filters. We identify a material-specific length scale parameter, Lcrit, that specifies the maximum mean quantum dot size that can yield effective energy filtering. Moreover, we show that energy filtering materials composed of quantum dots with size near Lcrit are maximally robust to heterogeneity in quantum dot size, tolerating variations ∼10% of the mean size. The length scale Lcrit can be estimated directly from the widely tabulated density of states effective mass and shows that semiconductors with light conduction band electrons, such as III-V type materials InSb and GaAs, are the most forgiving for energy filtering applications. Taken together, these results provide a practical set of quantitative design principles for semiconductor electron filters.

Rapid Quantitative Analysis of Multiple Explosive Compound Classes on a Single Instrument via Flow-Injection Analysis Tandem Mass Spectrometry.

A flow-injection analysis tandem mass spectrometry (FIA MSMS) method was developed for rapid quantitative analysis of 10 different inorganic and organic explosives. Performance is optimized by tailoring the ionization method (APCI/ESI), de-clustering potentials, and collision energies for each specific analyte. In doing so, a single instrument can be used to detect urea nitrate, potassium chlorate, 2,4,6-trinitrotoluene, 2,4,6-trinitrophenylmethylnitramine, triacetone triperoxide, hexamethylene triperoxide diamine, pentaerythritol tetranitrate, 1,3,5-trinitroperhydro-1,3,5-triazine, nitroglycerin, and octohy-dro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine with sensitivities all in the picogram per milliliter range. In conclusion, FIA APCI/ESI MSMS is a fast (<1 min/sample), sensitive (~pg/mL LOQ), and precise (intraday RSD < 10%) method for trace explosive detection that can play an important role in criminal and attributional forensics, counterterrorism, and environmental protection areas, and has the potential to augment or replace several of the existing explosive detection methods.

Characterization of nitrated sugar alcohols by atmospheric-pressure chemical-ionization mass spectrometry.

RATIONALE: The nitrated sugar alcohols mannitol hexanitrate (MHN), sorbitol hexanitrate (SHN) and xylitol pentanitrate (XPN) are in the same class of compounds as the powerful military-grade explosive pentaerythritol tetranitrate (PETN) and the homemade explosive erythritol tetranitrate (ETN) but, unlike for PETN and ETN, ways to detect MHN, SHN and XPN by mass spectrometry (MS) have not been fully investigated. METHODS: Atmospheric-pressure chemical-ionization mass spectrometry (APCI-MS) was used to detect ions characteristic of nitrated sugar alcohols. APCI time-of-flight mass spectrometry (APCI-TOF MS) and collision-induced dissociation tandem mass spectrometry (CID MS/MS) were used for confirmation of each ion assignment. In addition, the use of the chemical ionization reagent dichloromethane was investigated to improve sensitivity and selectivity for detection of MHN, SHN and XPN. RESULTS: All the nitrated sugar alcohols studied followed similar fragmentation pathways in the APCI source. MHN, SHN and XPN were detectable as fragment ions formed by the loss of NO2 , HNO2 , NO3 , and CH2 NO2 groups, and in the presence of dichloromethane chlorinated adduct ions were observed. It was determined that in MS/MS mode, chlorinated adducts of MHN and SHN had the lowest limits of detection (LODs), while for XPN the lowest LOD was for the [XPN-NO2 ]- fragment ion. Partially nitrated analogs of each of the three compounds were also present in the starting materials, and ions attributable to these compounds versus those formed from in-source fragmentation of MHN, SHN, and XPN were distinguished and assigned using liquid chromatography APCI-MS and ESI-MS. CONCLUSIONS: The APCI-MS technique provides a selective and sensitive method for the detection of nitrated sugar alcohols. The methods disclosed here will benefit the area of explosives trace detection for counterterrorism and forensics. Copyright © 2016 John Wiley & Sons, Ltd.

Biomimetic Sniffing Improves the Detection Performance of a 3D Printed Nose of a Dog and a Commercial Trace Vapor Detector.

Unlike current chemical trace detection technology, dogs actively sniff to acquire an odor sample. Flow visualization experiments with an anatomically-similar 3D printed dog's nose revealed the external aerodynamics during canine sniffing, where ventral-laterally expired air jets entrain odorant-laden air toward the nose, thereby extending the "aerodynamic reach" for inspiration of otherwise inaccessible odors. Chemical sampling and detection experiments quantified two modes of operation with the artificial nose-active sniffing and continuous inspiration-and demonstrated an increase in odorant detection by a factor of up to 18 for active sniffing. A 16-fold improvement in detection was demonstrated with a commercially-available explosives detector by applying this bio-inspired design principle and making the device "sniff" like a dog. These lessons learned from the dog may benefit the next-generation of vapor samplers for explosives, narcotics, pathogens, or even cancer, and could inform future bio-inspired designs for optimized sampling of odor plumes.

Reagent approaches for improved detection of chlorate and perchlorate salts via thermal desorption and ionization.

RATIONALE: Techniques for improving the detectability of chlorate and perchlorate salts with thermal desorption based ionizers (i.e. radioactive, corona discharge and photoionization-based) are desired. This work employs acidic reagents to chemically transform chlorate and perchlorate anions into traces of chloric and perchloric acid. These high vapor pressure acids are easier to detect than the originating salts. METHODS: The efficacy of the reagent chemistry was quantified with a triple-quadrupole mass spectrometer interfaced with a custom-built thermal-desorption atmospheric-pressure chemical ionization (TD-APCI) source. Additional experiments were conducted using tandem IMS/MS instrumentation. Reagent pKa and pH values were varied in order to gain a better understanding of how those parameters affect the degree of observed signal enhancement. RESULTS: Samples of chlorates and perchlorates treated with liquid acidic reagents exhibit signal enhancement of up to six orders of magnitude compared with signals from untreated analytes. Three orders of magnitude of signal enhancement are demonstrated using solid-state reagents, such as weakly acidic salts and polymeric acids. Data is presented that demonstrates the compatibility of the solid-state approach with both MS and IMS/MS platforms. CONCLUSIONS: Several methods of acidification were demonstrated for enhanced vaporization and detection of chlorates and perchlorates. For applications where rapid surface collection and analysis for chlorates and perchlorates are desired, the solid-state approaches offer the simplest means to integrate the reagent chemistry into MS or IMS detection.

Vapor Pressure of Hexamethylene Triperoxide Diamine (HMTD) Estimated Using Secondary Electrospray Ionization Mass Spectrometry.

A rapid method for vapor pressure measurement was developed and used to derive the vapor pressure curve of the thermally labile peroxide-based explosive hexamethylene triperoxide diamine (HMTD) over the temperature range from 28 to 80 °C. This method uses a controlled flow of vapor from a solid-phase HMTD source that is presented to an ambient-pressure-ionization mass spectrometer equipped with a secondary-electrospray-ionization (SESI) source. The subpart-per-trillion sensitivity of this system enables direct detection of HMTD vapor through an intact [M + H](+) ion in real time at temperatures near 20 °C. By calibrating this method using vapor sources of cocaine and heroin, which have known pressure-temperature (P-T) curves, the temperature dependence of HMTD vapor was determined, and a Clausius-Clapeyron plot of ln[P (Pa)] vs 1/[T (K)] yielded a straight line with the expression ln[P (Pa)] = {(-11091 ± 356) × 1/[T (K)]} + 25 ± 1 (error limits are the standard error of the regression analysis). From this equation, the sublimation enthalpy of HMTD was estimated to be 92 ± 3 kJ/mol, which compares well with the theoretical estimate of 95 kJ/mol, and the vapor pressure at 20 °C was estimated to be ∼60 parts per trillion by volume, which is within a factor of 2 of previous theoretical estimates. Thus, this method provides not only the first direct experimental determination of HMTD vapor pressure but also a rapid, near-real-time capability to quantitatively measure low-vapor-pressure compounds, which will be useful for aiding in the development of training aids for bomb-sniffing canines.

Reagent assessment for detection of ammonium ion-molecule complexes.

RATIONALE: Ammonia (NH3) is an important chemical target in sensor applications such as trace explosives detection of ammonium nitrate (NH4NO3) and environmental monitoring. Ion-molecule reagent chemistries show potential to increase sensitivity in detection systems relying on atmospheric pressure ionization (API) of reagent-ammonium (M + NH4(+)) complexes. Gas-phase reagent selection assessment is based on mass spectrometric (MS) determination of binding constants relative to competitive ions and critical energies for ion-molecule complex dissociation. METHODS: Eight ammonium complexation reagents were identified and gas-phase ion-molecule interactions were studied using electrospray ionization. Binding constants were determined, in Log(K), using the competition method for one host molecule with three guests (NH4(+), Na(+), and K(+)) in single quadrupole MS. Critical energy determination was based on calibration of threshold activation voltage using collision-induced dissociation (CID) tandem mass spectrometry (MS/MS). RESULTS: This assessment informs selective binding affinity and intrinsic ion-molecule critical energy for dissociation. Relative NH4(+) binding affinity was highest for sucrose and 4-tert-butylcalix[6]arene, while 4-tert-butylcalix[6]arene and methyl acetoacetate showed the highest preferential binding of NH4(+) versus Na(+) and K(+). The intrinsic critical energy for NH4(+) binding was highest for crown ethers, tetraglyme and methyl acetoacetate. CONCLUSIONS: An MS-based framework was developed to quantitatively assess API ion-molecule reagent chemistries based on ammonium selectivity versus competing ions, and intrinsic ammonium binding strength and complex survivability for detection. Methyl acetoacetate is an attractive ammonium reagent for vapor-phase API techniques given its high vapor pressure, preferential selectivity, and high critical energy for dissociation.

Fate dynamics of environmentally exposed explosive traces.

The chemical and physical fates of trace amounts (<50 µg) of explosives containing 2,4,6-trinitrotoluene (TNT), hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX), and pentaerythritol tetranitrate (PETN) were determined for the purpose of informing the capabilities of tactical trace explosive detection systems. From these measurements, it was found that the mass decreases and the chemical composition changes on a time scale of hours, with the loss mechanism due to a combination of sublimation and photodegradation. The rates for these processes were dependent on the explosive composition, as well as on both the ambient temperature and the size distribution of the explosive particulates. From these results, a persistence model was developed and applied to model the time dependence of both the mass and areal coverage of the fingerprints, resulting in a predictive capability for determining fingerprint fate. Chemical analysis confirmed that sublimation rates for TNT were depressed by UV (330-400 nm) exposure due to photochemically driven increases in the molecular weight, whereas the opposite was observed for RDX. No changes were observed for PETN upon exposure to UV radiation, and this was attributed to its low UV absorbance.

The role of stationary phase selection on performance for explosives analysis using GC-ECD.

Gas chromatography with electron capture detection (GC-ECD) analysis of explosive-related nitro organic compounds was performed using four different column stationary phases with the focus being on their impact on analyte stability and transfer efficiency during analysis. All four columns used were 6 m x 0.53 mm, and the four stationary phases were a 1.0-microm thick 5% phenyl siloxane/95% methyl siloxane non-polar phase, a 1.5-microm thick 5% phenyl siloxane/95% methyl siloxane non-polar phase optimized for explosives analysis, an intermediate polarity 0.5-microm thick trifluoropropylmethyl siloxane phase, and a proprietary intermediate polarity 0.5-microm thick phase. Although all exhibited similar recovery (as defined as the detector signal per injected mass) when new, the intermediate polarity phases maintained higher sample recovery over the course of analyzing hundreds of samples than the non-polar phases, particularly for the nitramines hexahydro-1,3,5-trinitro-1,3,5-triazine and octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine, for which a 7x and 3x decrease in recovery were observed, and the nitrate esters nitroglycerin and pentaerythritol tetranitrate, for which a 7x and 11x decrease in recovery were observed. For most other explosive-related compounds, the differences in recovery were between 1.5x and 3x over the same course. Although the detailed chemical formulation of the stationary phases have not been disclosed by their manufacturers, we attribute the observed differences in performance to the stability of their passivation chemistries with regard to other mobile-phase compounds contained in complex field samples. Although these effects have been qualitatively noted in the past and in response, maintenance procedures have been developed to help account for this behavior, the analyst's preference is to use an explosives analysis method that does not require these time-consuming measures. Our desire to prolong this maintenance interval provided the motivation for the assessment reported in this paper. From our assessment, we conclude that manufacturers of GC columns should focus more attention on the stationary phase and passivation chemistries that can lead to the development of a column that is better able to maintain passivation against explosive compound degradation; and users intending to perform large numbers of analyses using GC-ECD should make this a consideration when selecting a column.

Measurement of trace explosive residues in a surrogate operational environment: implications for tactical use of chemical sensing in C-IED operations.

A campaign to measure the amount of trace explosive residues in an operational military environment was conducted on May 27-31, 2007, at the National Training Center at Fort Irwin, CA, USA. The objectives of this campaign were to develop the methods needed to collect and analyze samples from tactical military settings, to use the data obtained to determine what the trace explosive signatures suggest about the potential capabilities of chemical-based means to detect IEDs, and, finally, to present a framework whereby a sound understanding of the signature science can be used to guide development of new sensing technologies and sensor concepts of operation. Through our use of combined background and threat signature data, we have performed statistical analyses to estimate upper limits of notional sensor performance that is limited only by the spatial correlation of the signature chemicals to the threats of interest.