Generating Reagent Ions Other Than

The use of reagents such as NO +, O2 and Kr + moves us into a realm beyond the strict definition of proton transfer reaction mass spectrometry, but this additional flexibility in the means of analyte ionization does have the potential to broaden the scope of the technique and, in the case of Kr +, rather dramatically so. With the availability of several alternative ionizing reagents it would be more appropriate to refer to the technique as chemical ionization reaction mass spectrometry (CIR-MS) than PTR-MS. Indeed the former term has already been used in the research literature when referring to the availability of H3O+, NO+ and O2 reagent ions from a single source in a PTR-MS instrument [16]. However, the term PTR-MS is now well established in the literature and at present there seems to be no widespread desire to change to CIR-MS. 

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This new, active, and extremely attractive field of MS has room for new ideas and improvements. In this sense, atmospheric pressure glow discharges (APGD) can generate a much larger number and wider variety of reagent ions than other chemical ionization techniques [29], and hence is tunable for a larger variety of applications. 

The radioactive source shown in Figure 3.9 has also been used as a clean source of chemical ionization (Cl) reagents other than H3O+ [16,17]. In particular, both NO+ and O2 + ion streams have been generated by this means and offer useful alternatives to H3O + for certain applications, as already discussed in Section 2.3. 

The less polar nature of disulfides and sulfenamides makes the addition of these reagents to alkenes inefficient. Although thiolates can be generated under reducing conditions,the activation of disulfides and sulfenamides is routinely performed by Lewis acids to enable the introduction of new u bonds other than C—S bonds. As a consequence, the stereospecificity and reactivity observed in the Lewis-acid-promoted addition of disulfides and sulfenamides to alkenes is similar to that of sulfenylation by sulfenyl halides and thi-osulfonium salts. Likewise, the accepted reaction mechanism includes the coordination of Lewis acid, resulting in a thio-sulfonium-like species that can further react with alkene either directly or indirectly by way of dithiosuhonium ions. 

There is direct evidence, from ir and nmr spectra, that the f-butyl cation is quantitatively formed when f-butyl chloride reacts with A1CI3 in anhydrous liquid HCI.246 In the case of olefins, Markovnikov s rule (p. 750) is followed. Carbocation formation is particularly easy from some reagents, because of the stability of the cations. Triphenylmethyl chloride247 and 1-chloroadamantane248 alkylate activated aromatic rings (e.g., phenols, amines) with no catalyst or solvent. Ions as stable as this are less reactive than other carbocations and often attack only active substrates. The tropylium ion, for example, alkylates anisole but not benzene.249 It was noted on p. 337 that relatively stable vinylic cations can be generated from certain vinylic compounds. These have been used to introduce vinylic groups into aryl substrates.250... 

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