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The New Exclusive SRI-MS Reagent Ion: NH4+ and Its Benefits

A novel proprietary method of NH4+ production for IONICON customers

In PTR-TOF-MS traditionally H3O+ reagent ions produced from H2O vapor are used. NH4+ reagent ions have been mentioned in literature as being particularly beneficial because of higher sensitivity (less fragmentation) and higher selectivity (higher proton affinity). However, so far toxic and corrosive ammonia was needed for NH4+ production.

We developed a novel method of producing NH4+ solely from H2O vapor and N2 (gas cylinder or filtered air) in the established hollow cathode ion source, without the need of any ammonia, by adjusting the pressure and electric fields in the source (PCT application submitted). That is, the reagent ions can be rapidly switched.

Disadvantages of “classic” NH4+ ion production in IMR-MS instruments

Until now in order to produce NH4+ reagent ions IMR-MS instruments had to use NH3 gas cylinders or ammonia solutions as a NHsource. The first approach requires a high level of safety precautions and is prohibited in some areas, the latter does not provide a stable NH3 concentration over time. Moreover NH3 can damage important parts of the instrument, such as lines, lenses, vacuum pumps, valves, flow controllers, etc. The instrument’s exhaust is contaminated with toxic NH3 and surfaces inside the IMR-MS instrument get covered with NH3 which desorbs very slowly after switching to a different reagent ion, i.e. switching from NH4+ to H3O+ takes tens of minutes if not hours.

Benefits of the novel IONICON NH4+ production method

All of these downsides of traditional methods have been overcome by IONICON’s invention which is now available exclusively to our customers. Producing NH4+ solely from H2O vapor and N2 in our ULTRA-PURE ion source eliminates all NH3 sources, reduce health and safety risks associated with NH3 as well the instrument corrosion potential. Our new method also enables fast switching between the reagent ions within seconds. As such NH4+ ionization incl. rapid switching is now part of the SRI technology for IONICON analyzers.

Advantages of using NH4+ reagent ions

  • improved selectivity: in PTR-MS often two or more compounds are detected at the same nominal m/z (e.g. isobars or isomers). The higher proton affinity (PA) of NH3 compared to H2O enables the ionization of certain species only, thus in some cases not all but selected isomers are detected which also leads to
  • simplification of mass spectra: only sample molecules with a higher PA than NH3 will be detected, all others will be blanked out, leading to a simpler spectrum than using H3O+ ions.
  • less fragmentation: compounds with high PAs often show high levels of fragmentation upon PTR ionization from H3O+, but less with NH4+, which in turn fosters identification and quantification of e.g. explosives or chemical warfare agents (CWAs).

All IONICON SRI-MS customers can now benefit from our proprietary and novel method of producing NH4+ solely from H2O vapor and N2.

Do you need NH4+ ionization? In our next story we will tell you more about a promising application where fast switching NH4+ ionization is crucial…

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