An isomer-specific high-energy collision-induced dissociation MS/MS database for forensic applications: a proof-of-concept on chemical warfare agent markers

Abstract

Spectra database search has become the most popular technique for the identification of unknown chemicals, minimizing the need for authentic reference chemicals. In the present study, an isomer-specific high-energy collision-induced dissociation (CID) MS/MS spectra database of 12 isomeric O-hexyl methylphosphonic acids (degradation markers of nerve agents) was created. Phosphonate anions were produced by the electrospray ionization of phosphonic acids or negative-ion chemical ionization of their fluorinated derivatives and were analysed in a hybrid magnetic-sector-time-of-flight tandem mass spectrometer. A centre-of-mass energy (E(com)) of 65 eV led to an optimal sequential carbon-carbon bond breakage, which was interpreted in terms of charge remote fragmentation. The proposed mechanism is discussed in comparison with the routinely used low-energy CID MS/MS. Even-mass (odd-electron) charge remote fragmentation ion series were diagnostic of the O-alkyl chain structure and can be used to interpret unknown spectra. Together with the odd-mass ion series, they formed highly reproducible, isomer-specific spectra that gave significantly higher database matches and probability factors (by 1.5 times) than did the EI MS spectra of the trimethylsilyl derivatives of the same isomers. In addition, ionization by negative-ion chemical ionization and electrospray ionization resulted in similar spectra, which further highlights the general potential of the high-energy CID MS/MS technique.

Identification of small molecules using accurate mass MS/MS search

“…Most of the CID MS/MS libraries covered in this review were created using low‐energy CID conditions. Very few examples of high‐energy CID libraries exist, despite the advantage of creating fragment‐rich and reproducible spectra . For low energy CID, one can measure energy resolved breakdown curves for specific ions, by ramping or increasing the collision energy .…”

“…Very few examples of highenergy CID libraries exist, despite the advantage of creating fragment-rich and reproducible spectra. 27,28 For low energy CID, one can measure energy resolved breakdown curves for specific ions, by ramping or increasing the collision energy. 29,30 Energy ramps will allow for finding the optimum fragmentation energies for analysis.…”

Structural determination of nerve agent markers using gas chromatography mass spectrometry after derivatization with 3‐pyridyldiazomethane

“…The mass spectrometric fragmentation of hexyl methylphosphonic acids isomers has also been studied using HE collision‐induced fragmentation in a sector‐TOF tandem mass spectrometer . The tandem mass spectra of the phosphonic anion in the regions for O ‐alkyl fragmentation were analyzed and found to contain two series of ions derived from alkyl chain fragmentation (odd‐ and even‐electron fragments), whereas the picolinyl driven fragmentation reported herein produces a single series of even ion fragments.…”

“…The EI spectra of alkylphosphonic acid derivatives are often very similar and/or their molecular peaks are often very weak or absent. Mass spectrometric separation of O ‐alkyl methylphosphonic acid isomers has been achieved by analyzing the fragments of the phosphonic acid anion in negative chemical ionization using high energy collision‐induced dissociation (HE‐CID) tandem mass spectrometry . However, this technique is not widely available, and so there is a need to develop improved methods that are suitable for general use and can be performed using standard equipment.…”

Distinguishing isomeric aldohexose-ketohexose disaccharides by electrospray ionization mass spectrometry in positive mode

“…Tandem mass spectrometry (MS/MS) is particularly useful, because it is simple, fast and sensitive. Collision‐induced dissociation (CID) is the most common fragmentation technique; however, it provides limited structural information at low collision energy . Recently, other alternative techniques, such as post‐source decay, electron capture dissociation, electron transfer dissociation, infrared multiple‐photon dissociation and higher collision energy dissociation (HCD) have been proposed to distinguish oligosaccharide isomers.…”