Keyong Hou

Dopant-assisted negative photoionization ion mobility spectrometry for sensitive detection of explosives.

Ion mobility spectrometry (IMS) is a key trace detection technique for explosives and the development of a simple, stable, and efficient nonradioactive ionization source is highly demanded. A dopant-assisted negative photoionization (DANP) source has been developed for IMS, which uses a commercial VUV krypton lamp to ionize acetone as the source of electrons to produce negative reactant ions in air. With 20 ppm of acetone as the dopant, a stable current of reactant ions of 1.35 nA was achieved. The reactant ions were identified to be CO(3)(-)(H(2)O)(n) (K(0) = 2.44 cm(2) V(-1) s(-1)) by atmospheric pressure time-of-flight mass spectrometry, while the reactant ions in (63)Ni source were O(2)(-)(H(2)O)(n) (K(0) = 2.30 cm(2) V(-1) s(-1)). Finally, its capabilities for detection of common explosives including ammonium nitrate fuel oil (ANFO), 2,4,6-trinitrotoluene (TNT), N-nitrobis(2-hydroxyethyl)amine dinitrate (DINA), and pentaerythritol tetranitrate (PETN) were evaluated, and the limits of detection of 10 pg (ANFO), 80 pg (TNT), and 100 pg (DINA) with a linear range of 2 orders of magnitude were achieved. The time-of-flight mass spectra obtained with use of DANP source clearly indicated that PETN and DINA can be directly ionized by the ion-association reaction of CO(3)(-) to form PETN·CO(3)(-) and DINA·CO(3)(-) adduct ions, which result in good sensitivity for the DANP source. The excellent stability, good sensitivity, and especially the better separation between the reactant and product ion peaks make the DANP a potential nonradioactive ionization source for IMS.

Single photon ionization and chemical ionization combined ion source based on a vacuum ultraviolet lamp for orthogonal acceleration time-of-flight mass spectrometry.

A novel combined ion source based on a vacuum ultraviolet (VUV) lamp with both single photon ionization (SPI) and chemical ionization (CI) capabilities has been developed for an orthogonal acceleration time-of-flight mass spectrometer (oaTOFMS). The SPI was accomplished using a commercial 10.6 eV krypton discharge lamp with a photon flux of about 10(11) photons s(-1), while the CI was achieved through ion-molecule reactions with O(2)(+) reactant ions generated by photoelectron ionization at medium vacuum pressure (MVP). To achieve high ionization efficiency, the ion source pressure was elevated to 0.3 mbar and the photoionization length was extended to 36 mm. As a result, limits of detection (LODs) down to 3, 4, and 6 ppbv were obtained for benzene, toluene, and p-xylene in MVP-SPI mode, and values of 8 and 10 ppbv were obtained for toluene and chloroform, respectively, in SPI-CI mode. As it is feasible to switch between MVP-SPI mode and SPI-CI mode rapidly, this system is capable of monitoring complex organic mixtures with a wide range of ionization energies (IEs). The analytical capacity of this system was demonstrated by measuring dehydrogenation products of long-chain paraffins to olefins through direct capillary sampling and drinking water disinfection byproducts from chlorine through a membrane interface.

Sensitive Detection of Black Powder by a Stand-Alone Ion Mobility Spectrometer with an Embedded Titration Region

Sensitive detection of black powder (BP) by stand-alone ion mobility spectrometry (IMS) is full of challenges. In conventional air-based IMS, overlap between the reactant ion O2(-)(H2O)n peak and the sulfur ion peak occurs severely; and common doping methods, providing alternative reactant ion Cl(-)(H2O)n, would hinder the formation of ionic sulfur allotropes. In this work, an ion mobility spectrometer embedded with a titration region (TR-IMS) downstream from the ionization region was developed for selective and sensitive detection of sulfur in BP with CH2Cl2 as the titration reagent. Sulfur ions were produced via reactions between sulfur molecules and O2(-)(H2O)n ions in the ionization region, and the remaining O2(-)(H2O)n ions that entered the titration region were converted to Cl(-)(H2O)n ions, which avoided the peak overlap as well as the negative effect of CH2Cl2 on sulfur ions. The limit of detection for sulfur was measured to be 5 pg. Furthermore, it was demonstrated that this TR-IMS was qualified for detecting less than 5 ng of BP and other nitro-organic explosives.

Fast Switching of CO3–(H2O)n and O2–(H2O)n Reactant Ions in Dopant-Assisted Negative Photoionization Ion Mobility Spectrometry for Explosives Detection

Ion mobility spectrometry (IMS) has become the most deployed technique for on-site detection of trace explosives, and the reactant ions generated in the ionization source are tightly related to the performances of IMS. Combination of multiform reactant ions would provide more information and is in favor of correct identification of explosives. Fast switchable CO3(-)(H2O)n and O2(-)(H2O)n reactant ions were realized in a dopant-assisted negative photoionization ion mobility spectrometer (DANP-IMS). The switching could be achieved in less than 2 s by simply changing the gas flow direction. Up to 88% of the total reactant ions were CO3(-)(H2O)n in the bidirectional mode, and 89% of that were O2(-)(H2O)n in the unidirectional mode. The characteristics of combination of CO3(-)(H2O)n and O2(-)(H2O)n were demonstrated by the detection of explosives, including 2,4,6-trinitrotoluene (TNT), cyclo-1,3,5-trimethylene-2,4,6-trinitramine (RDX), ammonium nitrate fuel oil (ANFO), and black powder (BP). For TNT, RDX, and BP, product ions with different reduced mobility values (K0) were observed with CO3(-)(H2O)n and O2(-)(H2O)n, respectively, which is a benefit for the accurate identification. For ANFO, the same product ions with K0 of 2.07 cm(2) V(-1) s(-1) were generated, but improved peak-to-peak resolution as well as sensitivity were achieved with CO3(-)(H2O)n. Moreover, an improved peak-to-peak resolution was also obtained for BP with CO3(-)(H2O)n, while the better sensitivity was obtained with O2(-)(H2O)n.

Sampling wand for an ion trap mass spectrometer.

A new sampling wand concept for ion trap mass spectrometers equipped with discontinuous atmospheric pressure interfaces (DAPI) has been implemented. The ion trap/DAPI combination facilitates the operation of miniature mass spectrometers equipped with ambient ionization sources. However, in the new implementation, instead of transferring ions pneumatically from a distant source, the mass analyzer and DAPI are separated from the main body of the mass spectrometer and installed at the end of a 1.2 m long wand. During ion introduction, ions are captured in the ion trap while the gas in which they are contained passes through the probe and is pumped away. The larger vacuum volume due to the extended wand improves the mass analysis sensitivity. The wand was tested using a modified hand-held ion trap mass spectrometer without additional power or pumping being required. Improved sensitivity was obtained as demonstrated with nano-electrospray ionization (ESI), atmospheric pressure chemical ionization (APCI), and low temperature plasma (LTP) probe analysis of liquid, gaseous, and solid samples, respectively.

Bipolar ionization source for ion mobility spectrometry based on vacuum ultraviolet radiation induced photoemission and photoionization.

A novel bipolar ionization source based on a commercial vacuum-UV Kr lamp has been developed for ion mobility spectrometry (IMS), which can work in both negative and positive ion mode. Its reactant ions formed in negative ion mode were predominantly assigned to be O(3)(-)(H(2)O)(n), which is different from that of the (63)Ni source with purified air as carrier and drift gases. The formation of O(3)(-)(H(2)O)(n) was due to the production of ozone caused by ultraviolet radiation, and the ozone concentration was measured to be about 1700 ppmv by iodometric titration method. Inorganic molecules such as SO(2), CO(2), and H(2)S can be easily detected in negative ion mode, and a linear dynamic range of 3 orders of magnitude and a limit of detection (S/N = 3) of 150 pptv were obtained for SO(2). Its performance as a negative ion source was investigated by the detection of ammonium nitrate fuel oil explosive, N-nitrobis(2-hydroxyethyl)amine dinitrate, cyclo-1,3,5-trimethylene-2,4,6-trinitramine, and pentaerythritol tetranitrate (PETN) at 150 degrees C. The limit of detection was reached at 45 pg for PETN, which was much lower than the 190 pg using (63)Ni ion mobility spectrometry under the same experimental condition. Also, its performance as an ordinary photoionization source was investigated in detecting benzene, toluene, and m-xylene.

Vacuum Ultraviolet Lamp Based Magnetic Field Enhanced Photoelectron Ionization and Single Photon Ionization Source for Online Time-of-Flight Mass Spectrometry

A magnetic field enhanced photoelectron ionization (MEPEI) source combined with single photon ionization (SPI) was developed for an orthogonal acceleration time-of-flight mass spectrometer (oaTOFMS). A commercial radio frequency (rf) powered vacuum ultraviolet (VUV) lamp was used as SPI light source, and the photoelectrons generated by photoelectric effect were accelerated to induce electron ionization (EI). The MEPEI was obtained by applying a magnetic field of about 800 G with a permanent annular magnet. Compared to a nonmagnetic field photoelectron ionization source, the signal intensities for SO(2), SF(6), O(2), and N(2) in MEPEI were improved more than 2 orders with the photoelectron energy around 20 eV, while most of the characteristics of soft ionization still remained. Simulation with SIMION showed that the sensitivity enhancement in MEPEI was ascribed to the increase of the electron moving path and the improvement of the electrons transmission. The limits of detection for SO(2) and benzene were 750 and 80 ppbv within a detection time of 4 s, respectively. The advantages of the source, including broad range of ionizable compounds, reduced fragments, and good sensitivity with low energy MEPEI, were demonstrated by monitoring pyrolysis products of polyvinyl chloride (PVC) and the intermediate products in discharging of the SF(6) gas inpurity.

Long-Term Real-Time Monitoring Catalytic Synthesis of Ammonia in a Microreactor by VUV-Lamp-Based Charge-Transfer Ionization Time-of-Flight Mass Spectrometry

With respect to massive consumption of ammonia and rigorous industrial synthesis conditions, many studies have been devoted to investigating more environmentally benign catalysts for ammonia synthesis under moderate conditions. However, traditional methods for analysis of synthesized ammonia (e.g., off-line ion chromatography (IC) and chemical titration) suffer from poor sensitivity, low time resolution, and sample manipulations. In this work, charge-transfer ionization (CTI) with O2(+) as the reagent ion based on a vacuum ultraviolet (VUV) lamp in a time-of-flight mass spectrometer (CTI-TOFMS) has been applied for real-time monitoring of the ammonia synthesis in a microreactor. For the necessity of long-term stable monitoring, a self-adjustment algorithm for stabilizing O2(+) ion intensity was developed to automatically compensate the attenuation of the O2(+) ion yield in the ion source as a result of the oxidation of the photoelectric electrode and contamination on the MgF2 window of the VUV lamp. A wide linear calibration curve in the concentration range of 0.2-1000 ppmv with a correlation coefficient (R(2)) of 0.9986 was achieved, and the limit of quantification (LOQ) for NH3 was in ppbv. Microcatalytic synthesis of ammonia with three catalysts prepared by transition-metal/carbon nanotubes was tested, and the rapid changes of NH3 conversion rates with the reaction temperatures were quantitatively measured with a time resolution of 30 s. The high-time-resolution CTI-TOFMS could not only achieve the equilibrium conversion rates of NH3 rapidly but also monitor the activity variations with respect to investigated catalysts during ammonia synthesis reactions.

Quasi-trapping chemical ionization source based on a commercial VUV lamp for time-of-flight mass spectrometry.

The application of VUV lamp-based single photon ionization (SPI) was limited due to low photon energy and poor photon flux density. In this work, we designed a quasi-trapping chemical ionization (QT-CI) source with a commercial VUV 10.6 eV krypton lamp for time-of-flight mass spectrometry. The three electrode configuration ion source with RF voltage on the second electrode constitutes a quasi-trapping region, which has two features: accelerating the photoelectrons originated from the photoelectric effect with VUV light to trigger the chemical ionization through ion-molecule reaction and increasing the collisions between reactant ion O2(+) and analyte molecules to enhance the efficiency of chemical ionization. Compared to single SPI based on VUV krypton lamp, the QT-CI ion source not only apparently improved the sensitivity (e.g., 12-118 fold enhancement were achieved for 13 molecules, including aromatic hydrocarbon, chlorinated hydrocarbon, hydrogen sulfide, etc.) but also extended the range of ionizable molecules with ionization potential (IP) higher than 10.6 eV, such as propane, dichloroethane, and trichloromethane.

Note: Design and construction of a simple and reliable printed circuit board-substrate Bradbury-Nielsen gate for ion mobility spectrometry

A less laborious, structure-simple, and performance-reliable printed circuit board (PCB) based Bradbury-Nielsen gate for high-resolution ion mobility spectrometry was introduced and investigated. The gate substrate was manufactured using a PCB etching process with small holes (Φ 0.1 mm) drilled along the gold-plated copper lines. Two interdigitated sets of rigid stainless steel spring wire (Φ 0.1 mm) that stands high temperature and guarantees performance stability were threaded through the holes. Our homebuilt ion mobility spectrometer mounted with the gate gave results of about 40 for resolution while keeping a signal intensity of over 0.5 nano-amperes.