Ion/molecule reactions for detecting ammonia using miniature cylindrical ion trap mass spectrometers

Gaseous ammonia, a common toxic industrial compound, is not detected readily in ion trap mass spectrometers because its molecular ion falls below the low-mass cutoff (~m/z 40) normally used when examining organic compounds. Instead, reactions of ammonia with halobenzene radical cations were used with internal electron ionization in two cylindrical ion trap miniature mass spectrometers to create a characteristic product ion by which to identify and quantify ammonia. Ammonia showed a linear response over the concentration range studied (parts per million [ppm] to parts per billion [ppb]) with limits of detection of 17 ppm and 220 ppb for experiments involving direct introduction and thermal desorption after pre-concentration, respectively. These values are comparable to ammonia's permissible exposure limit (50 ppm) and odor threshold (5 ppm). Receiver operating characteristic (ROC) curves were used to describe the method sensitivity, the probability of true positives, and the false positive rate for ammonia. A customized reaction scan function was created to select the species available for the ion/molecule reaction and set the amount of time the product ion could be accumulated in the trap. Product ion identity was verified using tandem mass spectrometry. Similar reactions with methylamine, ethylamine and the two nitriles, acetonitrile and benzonitrile, were explored.     

Enantioselective alkene radical cations reactions

The reaction of enantiomerically enriched 2-methyl-2-nitro-3-(diphenylphosphatoxy)alkyl radicals with tributyltin hydride and AIBN in benzene at reflux results in the formation of alkene radical cation/anion pairs, which are trapped intramolecularly by amine nucleophiles, leading to pyrrolidine and piperidine systems with memory of stereochemistry. The scope and limitations of the system are explored with respect to nucleophile, leaving group, and substituents within the substrate backbone.     

Ion/molecule reactions of the protonated serine octamer

The protonated homochiral octamer of serine exchanges all 33 of its labile hydrogens with CH(3)OD and undergoes ligand switching reactions with amines in a quadrupole ion trap mass spectrometer.     

Kinetics of radical heterolysis reactions forming alkene radical cations

Rate constants for heterolytic fragmentation of beta-(ester)alkyl radicals were determined by a combination of direct laser flash photolysis studies and indirect kinetic studies. The 1,1-dimethyl-2-mesyloxyhexyl radical (4a) fragments in acetonitrile at ambient temperature with a rate constant of k(het) > 5 x 10(9) s(-1) to give the radical cation from 2-methyl-2-heptene (6), which reacts with acetonitrile with a pseudo-first-order rate constant of k = 1 x 10(6) s(-1) and is trapped by methanol in acetonitrile in a reversible reaction. The 1,1-dimethyl-2-(diphenylphosphatoxy)hexyl radical (4b) heterolyzes in acetonitrile to give radical cation 6 in an ion pair with a rate constant of k(het) = 4 x 10(6) s(-1), and the ion pair collapses with a rate constant of k < or = 1 x 10(9) s(-1). Rate constants for heterolysis of the 1,1-dimethyl-2-(2,2-diphenylcyclopropyl)-2-(diphenylphosphatoxy)ethyl radical (5a) and the 1,1-dimethyl-2-(2,2-diphenylcyclopropyl)-2-(trifluoroacetoxy)ethyl radical (5b) were measured in various solvents, and an Arrhenius function for reaction of 5a in THF was determined (log k = 11.16-5.39/2.3RT in kcal/mol). The cyclopropyl reporter group imparts a 35-fold acceleration in the rate of heterolysis of 5a in comparison to 4b. The combined results were used to generate a predictive scale for heterolysis reactions of alkyl radicals containing beta-mesyloxy, beta-diphenylphosphatoxy, and beta-trifluoroacetoxy groups as a function of solvent polarity as determined on the E(T)(30) solvent polarity scale.     

Formation and reactions of aryldiazomethane radical cations

Dicyanoanthracene-sensitized photolysis of aryldiazomethanes affords stilbenes with cis/trans ratios of ～3 via free radical cations; anodic oxidation gives similar results but via radical cations adsorbed on electrode surface.     

Ion/molecule reactions in a miniature RIT mass spectrometer

Ion/molecule reactions were explored in a newly developed miniature mass spectrometer fitted with a rectilinear ion trap (RIT) mass analyzer. The tandem mass spectrometry performance of this instrument is demonstrated using collision induced dissociation (CID) and ion/molecule reactions. The latter includes Eberlin transacetalization reactions and electrophilic additions. Selective detection of the chemical warfare simulant dimethyl methyl phosphonate (DMMP) was achieved through selective Eberlin reactions of its characteristic phosphonium fragment ion CH3OP(+)(O)CH3 (m/z 93), with 1,4-dioxane or 1,3-dioxolane. Efficient adduct formation as a result of electrophilic attack by the phosphonium ion on various nucleophilic reagents, including 1,1,3,3-tetramethyl urea, methanesulfonic acid methyl ester, dimethyl sulfoxide and methyl salicylate, was also observed using the RIT device. The product ions of these reactions were analyzed using CID and the characteristic fragmentation patterns of the ionic addition products were recorded using multiple-stage experiments in the miniature RIT instrument. This study clearly demonstrates that a small, home-built, miniature RIT mass spectrometer can be used to perform analytically useful ion/molecule reactions and also that instruments like this have the potential to provide a portable platform for in situ detection of organophosphorus esters and related compounds with high specificity using tandem mass spectrometry.     

Strategies for generating peptide radical cations via ion/ion reactions

Several approaches for the generation of peptide radical cations using ion/ion reactions coupled with either collision induced dissociation (CID) or ultraviolet photo dissociation (UVPD) are described here. Ion/ion reactions are used to generate electrostatic or covalent complexes comprised of a peptide and a radical reagent. The radical site of the reagent can be generated multiple ways. Reagents containing a carbon–iodine (C―I) bond are subjected to UVPD with 266‐nm photons, which selectively cleaves the C―I bond homolytically. Alternatively, reagents containing azo functionalities are collisionally activated to yield radical sites on either side of the azo group. Both of these methods generate an initial radical site on the reagent, which then abstracts a hydrogen from the peptide while the peptide and reagent are held together by either electrostatic interactions or a covalent linkage. These methods are demonstrated via ion/ion reactions between the model peptide RARARAA (doubly protonated) and various distonic anionic radical reagents. The radical site abstracts a hydrogen atom from the peptide, while the charge site abstracts a proton. The net result is the conversion of a doubly protonated peptide to a peptide radical cation. The peptide radical cations have been fragmented via CID and the resulting product ion mass spectra are compared to the control CID spectrum of the singly protonated, even‐electron species. This work is then extended to bradykinin, a more broadly studied peptide, for comparison with other radical peptide generation methods. The work presented here provides novel methods for generating peptide radical cations in the gas phase through ion/ion reaction complexes that do not require modification of the peptide in solution or generation of non‐covalent complexes in the electrospray process. Copyright © 2015 John Wiley & Sons, Ltd.     

Ion/molecule reactions of ammonia and methylamine with chloro- and nitrobenzene. A comparison of chemical ionization and ion cyclotron resonance experiments

It is shown by ion cyclotron resonance measurements that ion/molecule reactions, leading to substitution or reduction product ions from chloro- and nitrobenzene with the title amines, are those between the molecular ions [RNH₂]⁺ or [C₆H₅X]⁺˙ and their respective counterparts C₆H₅X or RNH₂. The protonated reagent gas ions [RNH₃]⁺ are not involved in these reactions. In the case of nitrobenzene, adduct ions [C₆H₅NO₂·RNH₃]⁺ do not decompose within the time scale of the measurements. The results obtained are compared with those found under chemical ionization conditions.     

Substituent effects in pericyclic reactions of radical cations: the ring opening of 3-substituted cyclobutene radical cations

The substituent effects on the ring-opening reaction of cyclobutene radical cations have been studied at the Becke3LYP/6-31G* level of theory. The effect on the reaction energies and activation energies of the concerted and stepwise pathways of electron-donating substituents such as methyl and methoxy as well as electron-withdrawing substituents such as nitrile and carboxaldehyde in the 3-position of the cyclobutene is discussed. The exothermicity of the reaction correlates well with the ability of the substituent to stabilize the 1,3-butadiene radical cation by electron donation or conjugation. The relative stability of the (E) and (Z) isomers of the resulting 1,3-butadiene radical cations depends largely on steric effects. Similarly, steric effects are responsible for the relative energies of the different diastereomeric transition structures. The cyclopropyl carbinyl intermediate of the stepwise pathway resembles the nonclassical carbocation and is stabilized by electron-donating substituents. In the case of electron-donating substituents, this species becomes a minimum on the potential energy hypersurface, whereas unstabilized or destabilized cyclopropyl carbinyl radical cations are not minima on the hypersurface. The stabilization of the cyclopropyl carbinyl radical cation by substituents correlates qualitatively with the Brown-Okamoto substituent parameter sigma+. However, in all cases studied here, the concerted mechanism is the lowest energy pathway.     

Gas-phase basicities of the isomeric dihydroxybenzoic acids and gas-phase acidities of their radical cations

The thermochemical acid/base properties of the six dihydroxybenzoic acids (x,y-DHB) as prototypical matrices used in matrix-assisted laser desorption/ionization (MALDI) have been investigated. The ground-state gas-phase basicities (GB) of the six DHB isomers and the gas-phase acidities (deltaG acid) of the corresponding radical cations ([x,y-DHB]*+) have been determined by Fourier-transform ion cyclotron resonance mass spectrometry employing the thermokinetic method. The gas-phase basicities vary from 814 kJ mol-1 for the least basic isomer, 3,5-DHB, to 831 kJ mol-1 for the most basic isomer, 2,4-DHB. The obtained gas-phase acidities of the corresponding radical cations vary from 815 kJ mol-1 for the most acidic species, 3,4-DHB, to 858 kJ mol-1 for the least acidic one, 2,5-DHB. The results indicate that ground-state proton transfer from the matrix radical cations to the analyte may play a role in the ionization process of MALDI, whereas proton transfer from protonated matrix molecules can be excluded.     

Probing isomeric differences of phosphorylated carbohydrates through the use of ion/molecule reactions and FT-ICR MS

Through the use of ion/molecule reactions and tandem mass spectrometry, phosphate position is assigned in both phosphorylated monosaccharides and oligosaccharides. In previous work phosphate moieties of monosaccharides were stabilized under collisional activation, by first derivatizing the deprotonated monosaccharide with trimethyl borate through an ion/molecule reaction, and the phosphate position determined through marker ions generated in tandem mass spectra. In this work, the methodology is extended to larger phosphorylated oligomers employing chlorotrimethylsilane (TMSCl) as the ion/molecule reagent. Phosphorylated monosaccharides were first investigated to determine diagnostic ions for phosphate linkage in monomeric standards. It was observed that the diagnostic ions showed both linkage and some monosaccharide stereochemical information. Furthermore, it was observed that TMS addition stabilized the phosphate moiety under collisionally activated conditions. Upon identification of the diagnostic ions, the methodology was applied to lactose-1-phosphate. It was found that TMSCl, stabilized the phosphate moiety upon collisional activation, and furthermore, the phosphate linkage could be determined through tandem mass spectrometric analysis. As a further extrapolation to biologically relevant problems, the methodology was applied to a lipophosphoglycan analog from the protozoan parasite Leishmania. This sample contains bridging phosphates which were converted to terminal phosphates through collision induced dissociation. The sample was then analyzed in the same manner as lactose-1-phosphate, yielding phosphate linkage information and stereochemical information. This study showed that, using the developed methodology, phosphate linkage can be determined from both monosaccharides and larger oligosaccharides; furthermore it is applicable to samples in which the phosphates are either terminating or bridging.     

Side-chain radical losses from radical cations allows distinction of leucine and isoleucine residues in the isomeric peptides Gly-XXX-Arg

Sequencing of peptides via low-energy collision-induced dissociation of protonated peptides typically yields b(n) and y(n) sequence ions. The isomeric residues leucine and isoleucine rarely can be distinguished in these experiments since they give b(n) and y(n) sequence ions of the same m/z. Siu's pioneering work on electrospray ionization of copper complexes of peptides (Chu IK, Rodriquez CF, Lau TC, Hopkinson AC, Siu KWM. J. Phys. Chem. B 2000; 104: 3393) provides a way of forming radical cations of peptides in the gas phase. This method was used to generate M(+ small middle dot) ions of the two isomeric peptides Gly-Leu-Arg and Gly-Ile-Arg in order to compare their fragmentation reactions. Both radical cations fragment to give even electron y(2) and y(1) sequence ions as well as side-chain radical losses of CH(3) and CH(3)CH(2) for isoleucine and (CH(3))(2)CH for leucine. In contrast the [M + H](+) and [M + 2H](2+) ions do not allow distinction between the isomeric leucine and isoleucine peptides.     

Ion/molecule reactions performed in a miniature cylindrical ion trap mass spectrometer

A recently constructed miniature mass spectrometer, based on a cylindrical ion trap (CIT) mass analyzer, is used to perform ion/molecule reactions in order to improve selectivity for in situ analysis of explosives and chemical warfare agent simulants. Six different reactions are explored, including several of the Eberlin reaction type (M. N. Eberlin and R. G. Cooks, Org. Mass Spectrom., 1993, 28, 679-687) as well as novel gas-phase Meerwein reactions. The reactions include (1) Eberlin transacetalization of the benzoyl, 2,2-dimethyloximinium, and 2,2-dimethylthiooximinium cations with 2,2-dimethyl-1,3-dioxolane to form 2-phenyl-1,3-dioxolanylium cations, 2,2-dimethylamine-1,3-dioxolanylium cations and the 2,2-dimethylamin-1,3-oxathiolanylium cations, respectively; (2) Eberlin reaction of the phosphonium ion CH3P(O)OCH3+, formed from the chemical warfare agent simulant dimethyl methylphosphonate (DMMP), with 1,4-dioxane to yield the 1,3,2-dioxaphospholanium ion, a new characteristic reaction for phosphate ester detection; (3) the novel Meerwein reaction of the ion CH3P(O)OCH3+ with propylene sulfide forming 1,3,2-oxathionylphospholanium ion; (4) the Meerwein reaction of the benzoyl cation with propylene oxide and propylene sulfide to form 4-methyl-2-phenyl-1,3-dioxolane and its thio analog, respectively; (5) ketalization of the benzoyl cation with ethylene glycol to form the 2-phenyl-1,3-dioxolanylium cation; (6) addition/NO2 elimination involving benzonitrile radical cation in reaction with nitrobenzene to form an arylated nitrile, a diagnostic reaction for explosives detection and (7) simple methanol addition to the C7H7+ ion, formed by NO2 loss from the molecular ion of p-nitrotoluene to form an intact adduct. Evidence is provided that these reactions occur to give the products described and their potential analytical utility is discussed.     

Electron transfer free radical mechanism in the reactions of arenediazonium cations with grignard reagents

The major pathway in the reactions of arenediazonium cations with certain Grignard reagents is found to involve an electron transfer from the latter to the π-system of the former reactant and radicals are the immediate precursors of the final products.