Atmospheric pressure chemical ionization of explosives induced by soft X-radiation in ion mobility spectrometry: mass spectrometric investigation of the ionization reactions of drift gasses, dopants and alkyl nitrates

Abstract

A promising replacement for the radioactive sources commonly encountered in ion mobility spectrometers is a miniaturized, energy-efficient photoionization source that produce the reactant ions via soft X-radiation (2.8 keV). In order to successfully apply the photoionization source, it is imperative to know the spectrum of reactant ions and the subsequent ionization reactions leading to the detection of analytes. To that end, an ionization chamber based on the photoionization source that reproduces the ionization processes in the ion mobility spectrometer and facilitates efficient transfer of the product ions into a mass spectrometer was developed. Photoionization of pure gasses and gas mixtures containing air, N , CO and N O and the dopant CH Cl is discussed. The main product ions of photoionization are identified and compared with the spectrum of reactant ions formed by radioactive and corona discharge sources on the basis of literature data. The results suggest that photoionization by soft X-radiation in the negative mode is more selective than the other sources. In air, adduct ions of O with H O and CO were exclusively detected. Traces of CO impact the formation of adduct ions of O and Cl (upon addition of dopant) and are capable of suppressing them almost completely at high CO concentrations. Additionally, the ionization products of four alkyl nitrates (ethylene glycol dinitrate, nitroglycerin, erythritol tetranitrate and pentaerythritol tetranitrate) formed by atmospheric pressure chemical ionization induced by X-ray photoionization in different gasses (air, N and N O) and dopants (CH Cl , C H Br and CH I) are investigated. The experimental studies are complemented by density functional theory calculations of the most important adduct ions of the alkyl nitrates (M) used for their spectrometric identification. In addition to the adduct ions [M + NO ] and [M + Cl] , adduct ions such as [M + N O ] , [M + Br] and [M + I] were detected, and their gas-phase structures and energetics are investigated by density functional theory calculations. Copyright © 2016 John Wiley & Sons, Ltd.

Soft X-ray Atmospheric Pressure Photoionization in Liquid Chromatography–Mass Spectrometry

“…examined the formation of reactant ions at atmospheric pressure from different gases and gas mixtures and ionization of alkyl nitrates in the negative ion mode, using 2.8 keV photons produced by a miniaturized soft X-ray APPI source. 12 The same soft X-ray source was applied in the positive ion atmospheric pressure chemical ionization (APCI) of volatile organic compounds produced by various fungi, using gas chromatography-MS. 13 Soft X-ray ion source has also been applied in different APCI methods aiming for selective ionization of atmospheric vapors, such as sulfuric acid, low-volatility organic compounds, and amines, in the negative ion mode. 14 − 16 Thus far, soft X-ray ionization has not been presented as an ionization method in LC-MS.…”

Characterization of volatile metabolites formed by molds on barley by mass and ion mobility spectrometry

“…Our group already demonstrated the application of an alternative APCI source based on soft X‐radiation in MS. In two publications detailing the detection of explosives in the negative mode and the detection of volatile metabolites of fungi in the positive mode, we could show that the underlying ionization mechanism is more selective. These miniature X‐ray sources, which are not subject to any legal regulations in Germany (photon energy E X‐ray < 5 keV), have already been introduced as alternatives for radioactive 63 Ni sources in ion mobility (IM) spectrometers, eg, by the company Bruker Daltonics (Leipzig, Germany).…”

“…The handheld IM spectrometer (Roadrunner, Bruker Daltonics, Leipzig) has an APCI source based on soft X‐radiation (miniature X‐ray tube, 40 mm length, rhodium target on a beryllium window), with an energy of 2.7/2.8 keV ( L α / L β ‐transitions of Rh) and is therefore exempt from registration in Germany. The same type of source was used in the APCI‐MS system (LTQ XL, Thermo Fisher Scientific), where it was integrated into a home‐built ionization chamber . The original application scenario of the Roadrunner instrument is the detection of explosives and drugs on surfaces.…”

Detection of volatile organic compounds in the headspace above mold fungi by GC‐soft X‐radiation–based APCI‐MS

“…The source emits soft X‐rays with an energy of 2.7/2.8 keV (L α /L β ‐transitions of Rh) and is exempt from registration in Germany because of the low energy. The source is located in a self‐developed ionization chamber (Figure B) and has been described previously . The chamber has several functions: (1) sample introduction, (2) temperature control in order to reduce memory effects, (3) shielding of the reaction area (ionization reactions) against environmental influences, (4) shielding of the surroundings against X‐rays, and (5) efficient ion transfer through electrostatic fields.…”

“…In these instruments, the product spectra obtained by the 2 ionization methods are not distinguishable. Our group already demonstrated the use of this X‐ray source in APCI‐MS for the investigation of the ionization mechanisms of explosives . In that paper, a detailed description of the underlying ionization mechanisms can also be found.…”