adduct ions. EGDN also produces an intense NO2+ fragment, and. DMNB generates a [MâNO2]+ fragment that is strongly affected by temperature and humidity.
ASPECTS OF EXPLOSIVE TAGGANTS ION FORMATION IN ATMOSPHERIC PRESSURE CHEMICAL IONIZATION ION SOURCE (WITH LOW GAS LOAD) Alexey
1 Makas ,
Andrey
1,2 Kudryavtsev ,
Mikhail
1 Troshkov ,
Ochir Ochirov
1,2
(1) Laboratory of Spectrometry, Trofimuk Institute of Petroleum Geology and Geophysics, Russian Academy of Sciences, Siberian Branch, Novosibirsk, Russia (2) Novosibirsk State University, Novosibirsk, Russia
INTRODUCTION In an effort to deter terrorism, The International Civil Aviation Organization Montreal Convention of 1991 mandated that complying nations ensure that all plastic explosives manufactured within their borders include volatile chemical markers, or “taggants”, to help improve explosives detection: Ethylene glycol dinitrate (EGDN), 2,3-Dimethyl 2,3-Dinitrobutane (DMNB) and para-Mononitrotoluene (p-MNT). There has been a growing interest in atmospheric pressure mass spectrometry for wider implementation in the detection of explosives and other controlled substances. However, to the present moment no deliberate research has been carried out regarding the aspects of taggants’ ion formation under APCI. While pursuing the miniaturization of the mass spectrometric detector, the authors have been developing a concept of a positive ion atmospheric pressure chemical ionization (APCI) source with reduced gas load [1]. In the process of coupling of the detector with the fast enrichment/separation system based on direct flash thermal desorption [2], it was found that the ionization properties of the above-named taggants vary depending on the system settings: temperature, humidity of carrier gas, etc. The objective was to study the ionization aspects of DMNB, EGDN, and p-MNT taggants to optimize the analytical performance under APCI with low gas load and specifically in case of coupling with the fast enrichment/separation system.
ANALYTE ION COMPOSITION AND COLLISION-INDUCED DISSOCIATION PATHWAYS MASS SPECTRA AT LOW DECLUSTERING VOLTAGE
HUMIDITY DEPENDENCIES OF ANALYTE IONS T=150°C Humidity dependencies were determined by exponential dilution of wet carrier gas by dried air in buffer.
FRAGMENTATION CURVES
EXPERIMENTAL SECTION LOW SIZE MASS-SPECTROMETRIC DETECTOR WITH APCI [1,2] • mass analyzer 5 cm long monopole mass filter which retains resolution and transmission at a working pressure of up to 10-3 Torr (0.13 Pa); • 0.1 L/s rotary pump; • 100 L/s turbomolecular hybrid pump with reduced rotation frequency
LOW GAS LOAD ION SOURCE
There are two reasons to reduce the inlet flow into APCI ion source: 1) lower power requirements for the vacuum system, which determines the size and the weight of the whole system; 2) reduction of sample dilution, if the pulse inlet system with repacking is used (GC peak or thermal desorption). It is common practice to use a fairly high gas flow of about 1 L/min in ion sources operating at atmospheric pressure, to ensure a greater ion yield from the ionization region by means of their gas-dynamic transportation. On the other hand, a low input flow rate is preferable when the sample is transferred into carrier gas entering the ionization region as a pulse, specifically as result of flash desorption or as a chromatographic peak. In this case, lower carrier gas flow rate results in a higher concentration of analyte in the sample. Furthermore, narrowing the inlet aperture of the Ion source provides for better transmission through the declustering region of the apparatus because of lower ion beam divergence. Specifications of the ion source: • Kambara’s two stage ion interface • Inlet flow: 10 ml/min • Positive corona discharge: 1 μA • Pressure in the declustering region: about 2 Torr (267 Pa) • Up to 40% ion transmission in the second stage • Output current: 0.4 nA • Diameter of output ion beam (without additional focusing): 0.25 mm The mass spectrometer’s vacuum system has about fivefold excess of power now with the inlet flow restricted to 10 ml/min. The structure of supersonic jet in Kambara’s two stage ion interface. Conventional ion source, 1L/min
Low gas load ion source, 10 mL/min
HUMIDITY DEPENDENCY OF REACTANT ION COMPOSITION RH 20%
RH 0.5%
CONCLUSION It is established that, in a positive corona discharge in air, taggants EGDN, DMNB, and p-MNT produce [M+H]+, [M+NO]+ and [M+NO2]+ adduct ions. EGDN also produces an intense NO2+ fragment, and DMNB generates a [M–NO2]+ fragment that is strongly affected by temperature and humidity. [M+H]+ protonated molecules of the three taggants show highly different humidity dependences due to dissimilarity in their gas phase ion energetics. The increase in [M+NO]+ and [M+NO2]+ adduct ion formation correlates with the humidity dependence of the NO+ and NO2+ reactant ions. For EGDN, the [M+NO]+ adduct ion dominates over [M+H]+. Besides, EGDN is notable for a dramatic, more than a 100-fold rise in ionization efficiency as the air dries. At normal humidity there are not specific ions of EGDN at all. A probable reason for this is very low stability of water cluster that is formed by the [M+H]+ protonated molecule, as P.Kebarle supposed [3]. To optimize the simultaneous determination of the three taggants it is necessary to provide dehumidified air as carrier gas. A moderate compromise temperature of the ion source is required to reduce the fragmentation effect on DMNB. For EGDN, the [M+NO]+ adduct ion is preferable for specific detection.
SIMION modeling of ion transport
REFERENCES 1.
2.
3.
A.L. Makas, M.L. Troshkov, A.S. Kudryavtsev, V.M. Lunin, Miniaturized mass-selective detector with atmospheric pressure chemical ionization , Journal of Chromatography B. 800 (2004) 63-67. A.S. Kudryavtsev, A.L. Makas, M.L. Troshkov, M.А. Grachev, S.P. Pod’yachev. The method for on-site determination of trace concentrations of methyl mercaptan and dimethyl sulfide in air using a mobile mass spectrometer with atmospheric pressure chemical ionization, combined with a fast enrichment/separation system, Talanta, Volume 123 (2014) 140–145. Sunner J., Nicol G., Kebarle P. Factors Determining Relative Sensitivity of Analytes in Positive Mode Atmospheric Pressure Ionization Mass Spectrometry. Anal. Chem., 1988, v.80, p.1300-1307
