Dilshadbek T. Usmanov

Atmospheric pressure chemical ionization of explosives using alternating current corona discharge ion source: APCI of explosives using ac corona discharge ion

The high-sensitive detection of explosives is of great importance for social security and safety. In this work, the ion source for atmospheric pressure chemical ionization/mass spectrometry using alternating current corona discharge was newly designed for the analysis of explosives. An electromolded fine capillary with 115 µm inner diameter and 12 mm long was used for the inlet of the mass spectrometer. The flow rate of air through this capillary was 41 ml/min. Stable corona discharge could be maintained with the position of the discharge needle tip as close as 1 mm to the inlet capillary without causing the arc discharge. Explosives dissolved in 0.5 µl methanol were injected to the ion source. The limits of detection for five explosives with 50 pg or lower were achieved. In the ion/molecule reactions of trinitrotoluene (TNT), the discharge products of NOx (-) (x = 2,3), O3 and HNO3 originating from plasma-excited air were suggested to contribute to the formation of [TNT - H](-) (m/z 226), [TNT - NO](-) (m/z 197) and [TNT - NO + HNO3 ](-) (m/z 260), respectively. Formation processes of these ions were traced by density functional theory calculations. Copyright

Detection of explosives using a hollow cathode discharge ion source.

RATIONALE For public security and safety, it is highly desirable to develop an ion source for the detection of explosives that is highly sensitive, compact in size, robust, and does not use any special carrier gases such as helium. In this work, a hollow cathode discharge (HCD) ion source was developed for the detection of explosives using ambient air as a carrier gas. METHODS To detect nonvolatile and thermally unstable explosives with high sensitivities, a new HCD ion source was designed and coupled with an ion trap mass spectrometer. RESULTS Five explosives--hexamethylene triperoxide diamine (HMTD), 1,3,5-trinitroperhydro-1,3,5-triazine (RDX), pentaerythritol tetranitrate (PETN), nitroglycerin (NG) and trinitrotoluene (TNT)--were detected with limits of detection of lower than ng. The intensities of the NO3(-) adduct ions with RDX, PETN, and NG showed a marked increase with increase in ion source pressure in the range of 1-28 Torr. CONCLUSIONS Because the major NOx(-) ions (x = 2, 3) produced in the plasma act as reagent ions in ion-molecule reactions of explosives, air is best suited as a carrier gas for the detection of explosives. It is proposed that the NOx(-) (x = 2, 3) and O3 contributed to the formation of [TNT-H](-) and [TNT-NO](-) ions, via the reactions NOx(-) + TNT → [TNT-H](-) + HNOx and [TNT](-) + O3 → [TNT-NO](-) + NO2 + O2.

Probe electrospray ionization (PESI) mass spectrometry with discontinuous atmospheric pressure interface (DAPI).

Probe electrospray ionization (PESI) using a 0.2 mm outside diameter titanium wire was performed and the generated ions were introduced into the mass spectrometer via a discontinuous atmospheric pressure interface using a pinch valve. Time-lapse PESI mass spectra were acquired by gradually increasing delay time for the pinch valve opening with respect to the start of each electrospray event when a high voltage was applied. The opening time of the pinch valve was 20 ms. Time-resolved PESI mass spectra showed marked differences for 10 mM NaCl, 10−5 M gramicidin S and insulin in H2O/CH3OH/CH3COOH (65/35/1) with and without the addition of 10 mM CH3COONH4. This was ascribed to the pH change of the liquid attached to the needle caused by electrochemical reactions taking place at the interface between the metal probe and the solution. NaCl cluster ions appeared only after the depletion of analytes. For the mixed solution of 10−5 M cytochrome c, insulin, and gramicidin S in H2O/CH3OH/CH3COOH (65/35/1), a sequential appearance of analyte ions in the order of cytochrome c → insulin → gramicidin S was observed. The present technique was applied to three narcotic samples, methamphetamine, morphine, and codeine. Limits of detection for these compounds were 10 ppb in H2O/CH3OH (1/1) for the single sampling with a pinch valve opening time of 200 ms.

Low-pressure barrier discharge ion source using air as a carrier gas and its application to the analysis of drugs and explosives

In this work, a low-pressure air dielectric-barrier discharge (DBD) ion source using a capillary with the inner diameter of 0.115 and 12 mm long applicable to miniaturized mass spectrometers was developed. The analytes, trinitrotoluene (TNT), 1,3,5-trinitroperhydro-1,3,5-triazine (RDX), 1,3,5,7-tetranitroperhydro-1,3,5,7-tetrazocine (HMX), pentaerythritol tetranitrate (PETN), nitroglycerine (NG), hexamethylene triperoxide diamine (HMTD), caffeine, cocaine and morphine, introduced through the capillary, were ionized by a low-pressure air DBD. The ion source pressures were changed by using various sizes of the ion sampling orifice. The signal intensities of those analytes showed marked pressure dependence. TNT was detected with higher sensitivity at lower pressure but vice versa for other analytes. For all analytes, a marked signal enhancement was observed when a grounded cylindrical mesh electrode was installed in the DBD ion source. Among nine analytes, RDX, HMX, NG and PETN could be detected as cluster ions [analyte + NO3 ](-) even at low pressure and high temperature up to 180 °C. The detection indicates that these cluster ions are stable enough to survive under present experimental conditions. The unexpectedly high stabilities of these cluster ions were verified by density functional theory calculation.

Desorption in Mass Spectrometry

In mass spectrometry, analytes must be released in the gas phase. There are two representative methods for the gasification of the condensed samples, i.e., ablation and desorption. While ablation is based on the explosion induced by the energy accumulated in the condensed matrix, desorption is a single molecular process taking place on the surface. In this paper, desorption methods for mass spectrometry developed in our laboratory: flash heating/rapid cooling, Leidenfrost phenomenon-assisted thermal desorption (LPTD), solid/solid friction, liquid/solid friction, electrospray droplet impact (EDI) ionization/desorption, and probe electrospray ionization (PESI), will be described. All the methods are concerned with the surface and interface phenomena. The concept of how to desorb less-volatility compounds from the surface will be discussed.

Mass spectrometric monitoring of oxidation of aliphatic C6 − C8 hydrocarbons and ethanol in low pressure oxygen and air plasmas

Experimental and theoretical studies on the oxidation of saturated hydrocarbons (n-hexane, cyclohexane, n-heptane, n-octane and isooctane) and ethanol in 28 Torr O2 or air plasma generated by a hollow cathode discharge ion source were made. Ions corresponding to [M + 15]+ and [M + 13]+ in addition to [M - H]+ and [M - 3H]+ were detected as major ions where M is the sample molecule. The ions [M + 15]+ and [M + 13]+ were assigned as oxidation products, [M - H + O]+ and [M - 3H + O]+ , respectively. By the tandem mass spectrometry analysis of [M - H + O]+ and [M - 3H + O]+ , H2 O, olefins (and/or cycloalkanes) and oxygen-containing compounds were eliminated from these ions. Ozone as one of the terminal products in the O2 plasma was postulated as the oxidizing reagent. As an example, the reactions of C6 H14+• with O2 and of C6 H13+ (CH3 CH2 CH+ CH2 CH2 CH3 ) with ozone were examined by density functional theory calculations. Nucleophilic interaction of ozone with C6 H13+ leads to the formation of protonated ketone, CH3 CH2 C(=OH+ )CH2 CH2 CH3 . In air plasma, [M - H + O]+ became predominant over carbocations, [M - H]+ and [M - 3H]+ . For ethanol, the protonated acetic acid CH3 C(OH)2+ (m/z 61.03) was formed as the oxidation product. The peaks at m/z 75.04 and 75.08 are assigned as protonated ethyl formate and protonated diethyl ether, respectively, and that at m/z 89.06 as protonated ethyl acetate. Copyright

Dipping probe electrospray ionization/mass spectrometry for direct on-site and low-invasive food analysis

Rapid, direct, on-site and noninvasive food analysis is strongly needed for quality control of food. To satisfy this demand, the technique of dipping probe electrospray ionization/mass spectrometry (dPESI/MS) was developed. The sample surface was pricked with a fine acupuncture needle and a sample of ∼200 pL was captured at the needle tip. After drying the sample, the needle tip was dipped into the solvent for ∼50 ms and was moved upward. A high-voltage was applied to the needle to generate electrospray when the needle reached the highest position, and mass spectra were measured with a time-of-flight mass spectrometer. For evaluation of the method, the technique was used to analyze foods such as vegetables, salmon flesh, cows milk, yogurt, and soy-bean milk. The detected major ions for cows milk and yogurt were [(Lac)n + Ca]2+ with n = 1-6 (where (Lac) is lactose), indicating that Ca2+ is tightly bound by Lac molecules.