Burnaby Munson

High pressure charge exchange mass spectrometry

Abstract Several compounds (He, Ne, Ar, Kr, Xe, N 2 , CO, CO 2 , O 2 , CF 4 , NO) have been used as reactant gases for charge exchange reactions at high pressures in a modified CEC 21-110B mass spectrometer. Carbon monoxide gave relatively abundant molecular ions and structurally useful fragment ions. Nitric oxide gave mostly molecular ions and addition ions, (M+NO) + , with very little fragmentation. The effects of pressure and repeller field on the distribution of ions in the reactant gases and the charge exchange spectra were investigated. Spectra of several types of compounds were obtained and compared with electron ionization spectra.

Mass spectral characterization of tetracyclines by electrospray ionization, H/D exchange, and multiple stage mass spectrometry

Electrospray ionization (ESI) and collisionally induced dissociation (CID) mass spectra were obtained for five tetracyclines and the corresponding compounds in which the labile hydrogens were replaced by deuterium by either gas phase or liquid phase exchange. The number of labile hydrogens, x, could easily be determined from a comparison of ESI spectra obtained with N2 and with ND3 as the nebulizer gas. CID mass spectra were obtained for [M + H]+ and [M – H]− ions and the exchanged analogs, [M(Dx) + D]+ and [M(Dx) − D]−, and produced by ESI using a Sciex API-IIIplus and a Finnigan LCQ ion trap mass spectrometer. Compositions of product ions and mechanisms of decomposition were determined by comparison of the MSN spectra of the un-deuterated and deuterated species. Protonated tetracyclines dissociate initially by loss of H2O (D2O) and NH3 (ND3) if there is a tertiary OH at C-6. The loss of H2O (D2O) is the lower energy process. Tetracyclines without the tertiary OH at C-6 lose only NH3 (ND3) initially. MSN experiments showed easily understandable losses of HDO, HN(CH3)2, CH3 - N=CH2, and CO from fragment ions. The major fragment ions do not come from cleavage reactions of the species protonated at the most basic site. Deprotonated tetracyclines had similar CID spectra, with less fragmentation than those observed for the protonated tetracyclines. The lowest energy decomposition paths for the deprotonated tetracyclines are the competitive loss of NH3 (ND3) or HNCO (DNCO). Product ions appear to be formed by charge remote decompositions of species de-protonated at the C-10 phenol.

Electrospray ionization mass spectrometry of tetracycline, oxytetracycline, chlorotetracycline, minocycline, and methacycline.

The effects of mobile-phase additives and analyte concentration on electrospray ionization mass spectra of a series of tetracyclines were investigated in both positive and negative ion modes. Only [M + H](+) and [M - H](-) ions were observed. The greatest sensitivity as [M + H](+) ions was obtained with 1% acetic acid and the greatest sensitivity as [M - H](-) ions was obtained using 50 mM ammonium hydroxide. Sensitivities in the positive ion mode were greater than those in the negative ion mode. The sensitivity as [M + H](+) showed no systematic variation with pH; however, the sensitivity as [M - H](-) did increase with increasing pH. A larger linear range was observed for [M - H](-) than for [M + H](+) ions. Both [M + Na](+) and [M + H](+) ions were observed with 0.5 mM sodium acetate and sodium iodide, but no adduct ions were observed with ammonium acetate. Some M(2)H(+) ions were observed at higher concentrations. Cluster ions, Na(NaOAc)(n)(+) or Na(NaI)(n)(+), but no sample ions were observed using 5 mM salts. The data suggest that mechanisms in addition to solution ionization are involved in the formation of the ESI sample ions. The utility of mobile phases containing 1% HOAc or 50 mM NH(4)OH was demonstrated for chromatographic separations.

Collisionally-induced dissociation of purine antiviral agents: mechanisms of ion formation using gas phase hydrogen/deuterium exchange and electrospray ionization tandem mass spectrometry.

Electrospray ionization (ESI) and collision-induced dissociation (CID) mass spectra were obtained for two purine nucleoside antiviral agents (acycloguanosine and vidarabine) and one purine nucleotide (vidarabine monophosphate) and for the corresponding compounds in which the labile hydrogens were replaced by deuterium gas-phase exchange. The number of labile hydrogens, x, was determined from a comparison of ESI spectra obtained with N2 and with ND3 as the nebulizer gas. CID mass spectra were obtained for [M + H]+ and [M – H]− ions and the exchanged analogs, [M(Dx) + D]+ and [M(Dx) – D]−, produced by ESI using a Sciex API-IIIplus mass spectrometer. Compositions of product ions were determined and mechanisms of decomposition elucidated by comparison of the CID mass spectra of the undeuterated and deuterated species. Protonated purine antiviral agents dissociate through rearrangement decompositions of base-protonated [M + H]+ ions by cleavage of the glycosidic bonds to give the protonated bases with a sugar moiety as the neutral fragment. Cleavage of the same bonds with charge retention on the sugar moiety gives low abundance ions, due to the low proton affinity of the sugar moiety compared with that of the purine base. CID of protonated purine bases [B + H]+ occurs through two major pathways: (1) elimination of NH3 (ND3) and (2) loss of NH2CN (ND2CN). Minor pathways include elimination of HNCO (DNCO), loss of CO and loss of HCN (DCN). Deprotonated acycloguanosine and vidarabine exhibit the deprotonated base [B – H]− as a major fragment from glycosidic bond cleavage and charge delocalization on the base. Deprotonated vidarabine monophosphate, however, shows predominantly phosphate-related product ions. CID of deprotonated guanine shows two principal pathways: (1) elimination of NH3 (ND3) and (2) loss of NH2CN (ND2CN). Minor pathways include elimination of HNCO (DNCO), loss of CO and loss of HCN (DCN). The dissociation reactions of deprotonated adenine, however, proceed by elimination of HCN and elimination of NCHNH (NCHND). The mass spectra of the antiviral agents studied in this paper may be useful in predicting reaction pathways in other heteroaromatic ring decompositions of nucleosides and nucleotides.

Gaseous ionic reactions in tetramethylsilane

Abstract Ion—molecule reactions were studied in tetramethylsilane at pressures up to 0.3 torr at temperatures of 70–170 °C. The major reactions are the collisionally stabilished addition of the trimethylsilyl ion to tetramethylsilane and the subsequent loss of CH4 from this ion. The formation of the addition ion, m/e = 161, shows a pronounced negative temperature coefficient, with an apparent activation energy of −12 kcal/mole. The negative activation energy is attributed to an equilibrium between the trimethylsilyl ion, tetramethylsilane and an excited addition ion.

Proton affinities of saturated aliphatic methyl esters

The kinetic method was used to determine the proton affinities of methyl esters of several saturated fatty acids. Decompositions of the proton-bound dimers of the methyl esters, AHB+, were observed under different conditions with two instruments. The proton affinities (PAs) of the methyl esters increase continually with increasing carbon number in the acid. Equilibrium and initial rate experiments were performed with a Fourier transform ion cyclotron resonance mass spectrometer on the methyl ester of the C22 saturated acid (methyl behenate). These experiments give values for PA (methyl behenate) that are perhaps slightly lower than those obtained with the kinetic method. The PAs of the methyl esters of the fatty acids could be correlated with the equation: PA (ester) = (40.0 ± 2.5)*log(n) + (784.7 ± 3.9) kJ/mol or PA (ester) = (864 ± 2) − (479 ± 41)/n, wheren = number of atoms in the molecule. Proton affinities of smaller sets of 1-alkylamines and 1-alkanols can be fit to similar equations.

On the Proton Affinity of Water

Ion–molecule reactions of proton transfer from H3O+ and to H2O were studied with several mixtures in a Bendix TOF mass spectrometer at pressures of 0.05–0.10 torr and in a CEC 21‐110B mass spectrometer at pressures as high as 0.25 torr. Proton transfer was observed from H3O+ to C3H6, cis‐2‐C4H8, and CH2O; proton transfer was observed to H2O only from C2H5+, and not s‐C3H7+ or s‐C4H9+. These observations support the value of P.A. (H2O) = 164 ± 4 kcal/mole.

The proton affinities of the halogen acids

Abstract The proton affinities of HCl, HBr, and HI have been redetermined. The revised values, PA(HCl) = 135 ± 2 kcal mol−1, PA(HBr) = 140 ± 1 kcal mol−1 and PA(HI) = 147 ± 2 kcal mol−1, exhibit the same trend that is shown for the halogen atoms and alkyl halides. Rate constants for some of the reactions of these species are also reported. The rate constants are not in quantitative agreement with values obtained from theories of interactions of ions with polar molecules.

Correlations of Relative Sensitivities in Gas Chromatography Electron Ionization Mass Spectrometry with Molecular Parameters

The relative molar sensitivities for a number of compounds having a variety of functional groups were obtained in gas chromatography electron ionization mass spectrometry. Comparable results were obtained with a quadrupole and with a magnetic mass spectrometer. The present relative molar sensitivities are in good agreement with relative ionization cross sections obtained by different techniques and different instruments for a variety of compounds with molecular weights below about 200 u. For compounds of higher molecular weight, the present experimental sensitivities are significantly larger than estimates extrapolated from earlier data. The relatively molar sensitivities correlate well with molecular polarizability.

The ion chemistry and thermochemistry of several trimethylsilyl compounds

Abstract The proton affinities of trimethylsilyl alcohol and several trimethylsilyl alkyl ethers have been determined by the bracketing technique in gas chromatography/chemical ionization mass spectrometry. Improvements have been made in this technique which increase the accuracy of the proton affinity measurements. Values of proton affinities obtained by the bracketing technique are in very good agreement with literature values. The protonated species of interest may be synthesized in the mass spectrometer; hence, proton affinities can be measured without having the compounds in question. The proton affinities of the Si-containing compounds are 0.5 kcal mol−1 larger than their carbon analogues. The proton affinity of (CH3)3SiOH is 194.2±0.4 kcal mol−1, in agreement with a recent estimate from an equilibrium measurement on triethylsilanol. From the proton affinities of the alkyl trimethylsilyl ethers and the previously reported dissociation energies for the protonated ethers, one can obtain an estimate of 141 kcal mol−1 for ΔH⦵f[(CH3)3Si+]. One can also estimate the heats of formation of the gaseous neutral trimethylsilyl alkyl ethers.