David L. Smith

A new tandem mass spectrometer for the study of ion-molecule reactions

Abstract Essential design and construction details and some preliminary results are given for a new tandem mass spectrometer which was designed for the study of low (approximately thermal) energy ion-molecule reactions. The first stage of the instrument consists of a 180 degree, 5.7 cm radius magnetic mass spectrometer which provides a beam of mass analyzed reactant ions. The second stage of the instrument consists of a deceleration lens and an ion cyclotron resonance mass spectrometer. A moderately high pressure source in the first stage of the instrument permits studies of reactant ions which are themselves products of ion-molecule reactions. Differential pumping isolates the ion source from the collision region and therefore permits detailed studies of internal energy effects on ion-molecule reactions.

Stable isotopes of calcium as tracers: methodology

While radioisotopic tracers have been used extensively, stable isotopes may, in principle, give the same information and offer several advantages for some investigations. Since there is no radiation involved, experiments using such high risk groups as children and pregnant women become possible. In addition, some studies may require using several different tracers. In the case of calcium, there are five stable isotopes ( 42Ca, 43Ca, 44Ca, &Ca and 48Ca) which may be used as tracers. If stable isotopic tracers are used, experiments may be done independent of isotope production schedules and the samples may be stored indefinitely. In spite of these advantages, stable isotopes have rarely been used as tracers because there has not been a satisfactory method of detection. Although isotopic abundances of calcium have been determined by both neutron activation analysis [1,2] and thermal ionization mass spectrometry [3,4], neither method has been used extensively for clinical investigation. This is presumably due to requirements for extensive sample preparation, and the necessary instrumentation. It has recently been shown [5] that fast atom bombardment mass spectrometry can be used to measure tracer levels of stable isotopes of calcium in plasma and urine. This technique offers good precision, high sensitivity, rapid analysis, and requires little or no sample preparation. This paper describes the first application of fast atom bombardment mass spectrometry and the double isotope technique to measure the fractional true absorption of calcium.

Low energy study of symmetric and asymmetric charge‐transfer reactions

A modified Varian ion cyclotron resonance (ICR) mass spectrometer was used to measure absolute rate constants for charge transfer from Kr+ to Kr and CO as well as from CO+ to Kr and CO. The technique of double resonance permitted measurement of the rate constants over the translational energy range from 0.1 to 10 eV. Ion motion in an ICR mass spectrometer was analyzed to give an average ion velocity which was used to convert the reaction rate constants to cross sections. These results are compared to the theoretical results of Rapp and Francis, with Langevin and with existing experimental results. One of the most striking results is that at a very low energy the cross sections for the crossed or asymmetric reactions are substantially greater than the cross sections for the symmetric charge‐transfer reactions.

Ion-molecule reactions in H2/rare-gas systems by ion cyclotron resonance II. Reactions in systems of H2 with Ar, Kr and Xe

Abstract Experimental ion cyclotron resonance techniques described in the companion paper have been used to measure rate constants for reaction of hydrogen ions (H 2 + or D 2 + ) with argon, krypton and xenon as well as for reaction of Ar + , Kr + and Xe + with hydrogen or deuterium. For the conditions of lowest energy, the average translational energy is less than 0.1 eV; thus these rate constants are representative of reactions of ions whose translational energy is approximately thermal. The energy dependence of the total rate of reaction was measured using a new technique which is based on the ability to energize an ion in the source and observe ions of the same mass in the analyzer. Rate constants for each reaction channel were measured using standard double resonance methods. Isotope effects, as reflected in the ArH + /ArD + ratio are given as a function of ion energy. The results are compared with previous experimental and theoretical results.

Ion—molecule reactions in the CO2/H2 system by ion cyclotron resonance

Abstract Thermal energy rate constants have been measured for proton and hydrogen atom transfer to CO2 and CO2+ from H2+ and H2, respectively, using an ion cyclotron resonance mass spectrometer. At thermal energies, proton transfer is about 2.5 times more likely than electron transfer. Double resonance experiments show that the two reaction channels are of equal magnitude for an H2+ translational energy of 10 eV. At higher energies the electron transfer reaction dominates. Similar double resonance studies show that the rate constant for H transfer from H2 to CO2+ is independent of CO2+ translation energy up to at least 5 eV. The most striking feature of this work is that the rate constant for proton transfer to CO2 is substantially greater than the collision rate constant calculated from the usual Langevin polarization theory of ion—molecule reactions. The plausibility of this large rate constant is discussed in terms of the ion—quadrupole interaction potential.

Thermal energy reactions of H3+ with methanol

Abstract A tandem mass spectrometer has been used to measure the ion-product distribution for reaction of H 3 + with CH 3 OH as an explicit function of the average number of H 3 + -H 2 + collisions occurring before reaction. As the number of H 3 + -H 2 + collisions is changed from zero to several, large changes in the product distribution are observed and demonstrate the effectiveness of such collisions in deactivating the vibrationally excited H 3 + ions. The present results suggest that 5–10H 3 + -H 2 collisions are required to deactivate the H 3 + . Isotope studies suggest that reaction occurs initially by proton transfer to give a CH 3 OH 2 + ion which, if energetically possible, either eliminates H 2 or cleaves the CO bond. Because the present experiments are performed under conditions of low pressure and long reaction time, observation of the CH 3 OH 2 + product is used to show that the H 2 product of the initial protonation reaction carries away a disproportionately large share of the enthalpy of reaction as either vibrational or translational energy.

Reactions of the carcinogen N-acetoxy-4-acetamidostilbene with nucleosides

The carcinogen N-acetoxy-4-acetamidostilbene (N-AcO-AAS) yields multiple products in reactions with guanosine, adenosine or cytidine in aqueous acetone. The major product from the reaction with cytidine is a deamination product, 1-(4-acetamidophenyl)-1-(3-uridyl)-2-hydrosy-2-phenylethane. Three minor products were unstable and were characterized only by their UV spectra and pK values. Adenosine yielded two major products, one of them 1-(4-acetamidophenyl)-1-(N6-adenoxyl)-2-hydroxy-2-phenylethane, and the second 3-(beta-D-ribosyl)-7-phenyl-8-(4-acetamidophenyl)-7,8 dihydroimidazo [2,1-i] purine. The major adduct with guanosine is 1-(4-acetamidophenyl)-1-(1-guanosyl)-2-hydroxy-2-phenylethane. One minor adduct also appears to be a guanosine-N-1 derivative, while two other minor adducts yield 1-(4-acetamidophenyl)-2-phenyl-1, 2-ethanediol on acid hydrolysis, and thus appear to be O6-derivatives. None of the guanine adducts isolated had the properties of N-7, C-8 or N2 adducts. In this respect, N-Aco-AAS appears to behave more like a classical alkylating agent than like previously studied N-acetoxy-N-arylacetamides, although the target organs of 4-acetamidostilbene are the same as those of other N-arylacetamides.

Ion-molecule reactions in H2/rare-gas systems by ion cyclotron resonance I. Reactions with He and Ne

Abstract Ion cyclotron resonance (ICR) techniques have been applied in a detailed study of ion-molecule reactions in hydrogen/rare-gas systems. Absolute rate constants are reported for both the total rate of reaction as well as for each of the possible reaction channels for reaction of H2+ with X and X+ with H2, where X is either helium or neon. The average translational energy of the reactant ions is less than 0.1 eV, and thus these rate constants are indicative of reaction of ions whose translational energy may be considered as approximately thermal. The energy dependence of the total rate of reaction was measured using a new technique which is based on the ability to energize an ion in the source and observe ions of the same mass in the analyzer. Additionally, the rate constants for each reaction channel were measured using standard double-resonance methods. Isotope effects, as reflected in the product-ion ratio XH+/XD+ of reaction products from the reaction of HD+ with He and Ne, were also examined as a function of energy. The results are compared with theoretical predictions and with previous experimental studies.

Capillary gas chromatography of pyrimidines and purines: N,O-peralkyl and trifluoroacetyl-N,O-alkyl derivatives

Abstract Preparation and capillary gas chromatographic properties of volatile derivatives of eighteen pyrimidine and purine nucleic acid bases are described. N,O-peralkylation using methylsulfinyl carbanion, methyl or ethyl iodide reagent, and alkylation preceded by N-trifluoroacetylation produced derivatives having minimal adsorption and tailing compared with trimethylsilyl derivatives. Relative retention times and linearity of flame ionization or nitrogen—phosphorus detector response were measured using polar (Superox-FA) and apolar (SE-30) liquid phases. Application of gas chromatography—mass spectrometry to derivatives of DNA hydrolysates using mass chromatography is demonstrated.

Ion-molecule reaction of CH3+, C2H5+, C3H5+ and C3H7+ with unsaturated C2, C3 and C4 hydrocarbons

Abstract The reaction rate constants and product distributions for reaction of CH3+ and C2H5+ with ethylene, C2H5+ with propene and cyclopropane, and C3H5+ and C3H7+ with the four butene isomers are reported here. The results were obtained using a Dempster-ion cyclotron resonance tandem mass spectrometer which mass analyzes and decelerates the reactant ions to less than 0.1 eV. The internal excitation energy of all but the methyl reactant ions may be varied through unreactive collisions in the first stage of the instrument. Deuterated reactant ions have been used in an attempt to classify reaction channels according to models such as hydride abstraction, proton transfer or complex formation. Most of the major reaction channels studied here are best described as proceeding through a short-lived intermediate complex. The reactions of C2H5+ with propene and cyclopropane are particularly interesting since the major product, C4H9+, is produced through both direct proton transfer and complex formation.