Regulated In Situ Generation of Molecular Ions or Protonated Molecules under Atmospheric-Pressure Helium-Plasma-Ionization Mass Spectrometric Conditions

Abstract

Abstract. In an enclosed atmospheric-pressure helium-plasma ionization (HePI) source engulfed with dehumidified ambient gases, molecular cations are generated from compounds such as toluene, bromobenzene, and iodobenzene. Evidently, the ionization is effected by a direct Penning mechanism attributable to interactions of the gas-phase analyte with metastable helium atoms. It is widely known that secondary ions generated from ambient gases also play an important role in the overall ionization process. For example, when the ambient gases bear even traces of moisture, the analytes are ionized by proton transfer reactions with gaseous H 3 O + . In this study, we demonstrate how a controlled variation of experimental conditions can manipulate the abundance of molecular ions and protonated molecules in a HePI source.

Helium-Plasma-Ionization Mass Spectrometry of Metallocenes and Their Derivatives

“…During this investigation, the logical question arose about the origin of the proton that generates the (M + H) + analyte ions, because all compounds tested were sampled as solids directly taken from their containing bottles, placed on glass capillary tubes, and inserted into the enclosed ion source. Protonated molecules of ambient water vapor (H 3 O + ) are typically considered as the proton donors; however, our attempts at saturating the humidity of the ion source by inserting a wetted cotton swab along with a small dish with water were not very effective; the peak intensity ratio M +• : (M + 1) + did not change significantly even after 15 min of repeated insertions of moist cotton swabs (Supporting Information Figure S2). It must be pointed out that during this experiment, the heating gas supply was switched off; only the heat of the source block (120 °C) and the desolvation heater (200 °C) were used to indirectly warm up the added water sources.…”

“…In fact, protonation is so effectual in the HePI-QDa source by means of the desolvation gas that even the trace amounts of water vapor present in the latter are sufficient for the process to take place almost instantaneously. We have previously described a similar procedure for selectively generating and switching between molecular ions and protonated species from halobenzenes in the enclosed source of a Quattro Ultima instrument engulfed with various gases . However, it must be pointed out that the relatively smaller size of the HePI-QDa ion source provides for a far greater efficiency and speed for the switching process.…”

“…We have previously described a similar procedure for selectively generating and switching between molecular ions and protonated species from halobenzenes in the enclosed source of a Quattro Ultima instrument engulfed with various gases. 37 However, it must be pointed out that the relatively smaller size of the HePI-QDa ion source provides for a far greater efficiency and speed for the switching process. We utilized this finding while we were subsequently investigating in-source hydrogen−deuterium exchanges of metallocenes (vide infra).…”

“…Although plasma-ionization approaches, such as direct analysis in real time (DART) and flowing atmospheric-pressure afterglow (FAPA), appear to be appealing because samples need not be dissolved in a solvent prior to analysis, these ambient methods have been applied thus far only infrequently for the analysis of organometallics. ,,− The recent review by Vikse and McIndoe does not cite any plasma-based ambient ionization methods for MS analysis of organometallic compounds . Among the ambient methods available, helium-plasma ionization (HePI) , is a simple ionization method for the MS analysis of organic − and even rather involatile chemicals such as inorganic nitrates and mercury salts. , Compared to common APCI techniques, which require high-voltage corona discharges, the glow discharge of HePI is generated at low voltages. Moreover, HePI does not employ a corona needle; it uses a discharge capillary to generate the microplasma.…”

Screening freshness of seafood by measuring trimethylamine (TMA) levels using helium-plasma ionization mass spectrometry (HePI-MS)

“…HePI is a versatile ambient-ionization MS technique, applicable to the analysis of a wide variety of samples, both organic and inorganic. It has been applied, for example, to the analysis of energetic materials (Yang et al 2012), pharmaceutical preparations (Attygalle et al 2014a, 2014b, Xia et al 2016, halobenzenes (Attygalle et al 2014a(Attygalle et al , 2014bGangam et al 2015), phenolics and quinones (Hassan et al 2017), inorganic nitrates (Pavlov and Attygalle 2013), and inorganic mercury compounds (Weerasinghe et al 2014). A major advantage of HePI is that it is highly portable and adaptable: any mass spectrometer with an electrospray ion source can be transformed into an ambient HePI instrument with ease, and no extensive hardware modifications are necessary (see Experimental Section).…”