Hyperfine structure induced isotopic effects in krypton resonance ionization mass spectrometry

A time-of-flight mass spectrometer combined with a pulsed laser system has been used for determination of krypton isotope ratios of air samples containing 10⁶-10⁷ krypton atoms. A three-color, doubly resonant ionization scheme employing a 116.48 nm transition from the ground into the first 4s²4p⁵(²P°_1/2)5s J = 1 excitation level has been applied. The magnitude of a hyperfine splitting of this level has been estimated for ⁸¹Kr, ⁸³Kr and ⁸⁵Kr isotopes. Hyperfine structure induced isotopic effects have been investigated under different saturation conditions and found to be negligible when the first and the second transitions are strongly saturated.     

Resonance ionisation mass spectrometry of krypton and its applications in planetary science

A new resonance ionisation time-of-flight mass spectrometer for determining krypton isotope ratios in extraterrestrial samples is presented. Laser heating is used to extract gas from mg-size samples. A cryogenic sample concentrator is employed. Atoms continuously condense on a 75 K stainless steel substrate at the back plate of a Wiley-McLaren laser ion source from where they are desorbed by a pulsed 1064 nm laser and resonantly ionized in the plume. A three-colour (116.5 nm, 558.1 nm and 1064 nm) excitation scheme is used. Tuneable coherent Vacuum Ultraviolet (vuv) radiation near 116.5 nm is generated by four-wave sum frequency mixing of 252.5 nm and 1507 nm pulsed dye laser beams in a binary mixture of negatively and positively dispersive gases (Xe and Ar). Isotope effects have been observed that reduce the reproducibility of isotope ratio measurements between odd-mass, non-zero nuclear spin isotopes and even-mass, zero nuclear spin isotopes. This can be minimised and stabilised by controlling the laser fluences, experimental geometry, and the population of the magnetic sub-levels of the excited atomic states used in the ionisation process. Once stability is achieved, sample-standard bracketing (during which the known isotope ratios of a standard are determined before and after the measurements of the sample under the same conditions) allows precision and reproducibility of $\sim $ 1 % for the major isotope ratios to be achieved in samples $\sim 10^{6}$ krypton atoms. Detection limits of $<1000$ atoms/isotope have been demonstrated, ratios of $^{81}$ Kr in meteorites have been made with $\sim $ 5–10 % precision. Applications of the instrument in various areas of planetary science are also discussed.     

Ion cyclotron resonance mass spectrometry of acetylene

Ion-molecule reactions of acetylene have been studied by ion cyclotron resonance mass spectrometry in the pressure range 1–10 × 10⁻⁶ torr. The results are in good agreement with those obtained using tandem and high-pressure source instruments. The major products from the molecular ion are C₄H₂⁺ and C₄H₃⁺ and at 12.5 eV, I(C₄H₂⁺)/I(C₄H₃⁺) was found to be 0.455. This ratio was found to vary slightly with electron energy. Double-resonance experiments showed that the ions C⁺, CH⁺, C₂⁺, C₂H⁺, and C₂H₂⁺ all react with acetylene in reactions of the type

and that these reactions are major sources of C₃ and C₄ ions. The variation of double-resonance line-shapes with the amplitude of the irradiating oscillator is discussed.     

Porphyrinogen fragmentation profiles by ultra-high-performance liquid chromatography/electrospray ionisation tandem mass spectrometry

An ultra-high-performance liquid chromatography/electrospray ionisation tandem mass spectrometry system is described for the separation and characterisation of uroporphyrinogen, heptacarboxylic acid porphyrinogen, hexacarboxylic acid porphyrinogen, pentacarboxylic acid porphyrinogen and coproporphyrinogen. The separation was carried out on a 100 mm × 2.1 mm Thermo-Hypersil BDS column (2.4 μm average particle size) by gradient elution with a mixture of acetonitrile, methanol and 1 mol/L aqueous ammonium acetate buffer, pH 5.16, as eluent. The fragmentation pattern of each compound was established by collision-induced dissociation tandem mass spectrometry. The most characteristic fragmentation was ring opening at one of the four methylene bridges of the protonated porphyrinogen molecule followed by further cleavages of methylene bridges linking the four pyrrole rings at various points to give product ions with methylenepyrrolenine, methylene-dipyrrolenine and methylene-tripyrrolenine structures.     

Quantification routines for adsorption studies in static secondary ion mass spectrometry and the effect of ionisation probability

It is shown that the usual method of quantification of surface composition in static secondary ion mass spectrometry (SSIMS), which is purely comparative in nature, is unsuitable for adsorption studies by SSIMS. This is because of the effect of the ionisation efficiency and ion stability of a particular ion produced from a molecule adsorbed on the surface of a substrate. The established routine results in a non-linear relationship between calculated relative surface coverage and the ion selected to characterise the adsorbate. The application of a new normalisation routine to time-of-flight secondary ion mass spectrometry (ToF-SIMS) data has been used to account for this discrepancy, and also takes into account the effect of a possible contribution from the clean substrate to the ion selected to characterise the adsorbate molecule. This routine is suggested only for use with organic secondary ions, where the ionisation potential of such ions is of a comparable magnitude, and should prove particularly useful in the application of surface analysis techniques to adsorption studies.     

Dependence of the energy of the resonance states of an acceptor in silicon on the host isotopic mass

It has been found that the energy of the transitions of acceptors to resonance states under a spin-split valence subband depends on the isotope content of the silicon matrix. An estimate of the isotope effect for the spin-orbit splitting of the valence band of silicon has been obtained.     

Measuring technique for thermal ionisation mass spectrometry of human tracer kinetic study with stable cerium isotopes

Thermal ionisation mass spectrometry (TIMS) method has been developed for the simultaneous detection of different cerium isotopes in biological samples (i.e., blood and urine) at very low concentrations. The work has been done in the frame of a biokinetic study, where different stable cerium isotopes have been administered orally and intravenously as tracers to the human body. In order to develop an appropriate detection method for the tracers in the biological samples, an optimum sample preparation technique has been set and adapted to the specific requirements of the analysis technique used, i.e., TIMS. For sample evaporation and ionisation, the double tantalum filament technique showed the best results. The ions produced were simultaneously collected on a secondary electron multiplier so that the isotopic ratios of the cerium isotopes in the biological samples could be measured. The technique has been optimised for the determination of cerium down to 1?ng loaded on the evaporation filament corresponding to cerium concentrations of down to 1?ng?ml(-1) in the blood or urine samples. It has been shown that the technique is reliable in application and enables studies on cerium metabolism and biokinetics in humans without employing radioactive tracers.     

Tunable VUV light generation for resonance ionization mass spectrometry of Krypton

Tunable coherent VUV radiation from 115.8 to 116.9 nm has been produced by non-linear four-wave sum frequency mixing in a xenon-argon mixture. 116.5 nm light generated by this means has been used as the first step in a three color, doubly resonant ionization scheme for Kr. In the process of validating the system the xenon refractive index per atom (STP) at 116.5 nm has been determined to be (n_(Xe) − 1)/N_Xe = −6.8(±0.8) × 10⁻²³ cm³.     

Identification of wheat varieties using matrix-assisted laser desorption/ionisation time-of-flight mass spectrometry and an artificial neural network

A novel tool for variety identification of wheat (Triticum aestivum L.) has been developed: an artificial neural network (ANN) is used to classify the gliadin fraction analysed by matrix-assisted laser desorption/ionisation time-of-flight mass spectrometry (MALDI-TOFMS). The robustness of this novel method with respect to various experimental parameters has been tested. The results can be summarised: (i) With this approach 97% of the wheat varieties can be classified correctly with a corresponding correlation coefficient of 1.0, (ii) The method is fast since the time of extracting gliadins from flour can be reduced to 20 min without significant decrease in overall performance, (iii) The storage of flour or extracts under standard conditions does not influence the classification ability (i. e. the generalisation ability) of the method, and (iv) The classification obtained is not influenced by the identity of the operator making the analysis. This study demonstrates that a combination of an ANN and MALDI-TOFMS analysis of the gliadin fraction provides a fast and reliable tool for the variety identification of wheat. Copyright 1999 John Wiley & Sons, Ltd.     

Reliability evaluation for failed fuel identification using resonance ionization mass spectrometry

In the fast reactors, rapid and accurate identification of a fuel failure event is essential for ensuring safety operation. Isotopic analysis of krypton (Kr) and xenon (Xe) using resonance ionization mass spectrometry (RIMS) is an effective identification tool, in which Kr and Xe atoms are resonantly ionized by a pulsed laser at 216.7 nm and 249.6 nm, respectively, and then three isotopic ratios: ⁷⁸Kr/⁸⁰Kr, ⁸²Kr/⁸⁰Kr and ¹²⁶Xe/¹²⁹Xe are measured to detect the location of the failed fuel assembly. In this paper, we report on the required analytical precision of RIMS estimated from simulation studies as well as the analytical performance of our spectrometer to evaluate the availability of RIMS to the failed fuel identification technique in the fast reactors.     

Ion-molecule reactions in the vinyl methyl ether system studied by ion cyclotron resonance mass spectrometry

The main reactions of the molecular ion of vinyl methyl ether with its parent molecule are the formation of the ions C₄H₈O₂⁺, C₅H₈O⁺ and C₃H₇O₂⁺ with relative rate constants of 0.20, 0.10 and 0.70 respectively. These reactions and those in the presence of acetone-d₆ and hexan-2-one clearly distinguish the ion from other C₃H₆O⁺ isomers, and this is supported by observation of the metastable transitions of the ion. Deuterium labelling studies indicate that H/D scrambling occurs only in the formation of the C₄H₈O₂⁺ ion and possible reaction mechanisms are discussed in the light of this.     

Stable isotopic analysis of pyrogenic organic matter in soils by liquid chromatography-isotope-ratio mass spectrometry of benzene polycarboxylic acids

Pyrogenic organic matter (PyOM), the incomplete combustion product of organic materials, is considered stable in soils and represents a potentially important terrestrial sink for atmospheric carbon dioxide. One well-established method of measuring PyOM in the environment is as benzene polycarboxylic acids (BPCAs), a compound-specific method, which allows both qualitative and quantitative estimation of PyOM. Until now, stable isotope measurement of PyOM carbon involved measurement of the trimethylsilyl (TMS) or methyl (Me) polycarboxylic acid derivatives by gas chromatography-combustion-isotope ratio mass spectrometry (GC-C-IRMS). However, BPCA derivatives can contain as much as 150% derivative carbon, necessitating post-analysis correction for the accurate measurement of δ13C values, leading to increased measurement error. Here, we describe a method for δ13C isotope ratio measurement and quantification of BPCAs from soil-derived PyOM, based on ion-exchange chromatography (IEC-IRMS). The reproducibility of the δ13C measurement of individual BPCAs by IEC-IRMS was better than 0.35‰ (1σ). The δ13C-BPCA analysis of PyOM in soils, including at natural and artificially enriched 13C-abundance, produced accurate and precise δ13C measurements. Analysis of samples that differed in δ13C by as much as 900‰ revealed carryover of <1‰ between samples. The weighted sum of individual δ13C-BPCA measurements was correlated with previous isotopic measurements of whole PyOM, providing complementary information for bulk isotopic measurements. We discuss potential applications of δ13C-BPCA measurements, including the study of turnover rates of PyOM in soils and the partitioning of PyOM sources based on photosynthetic pathways.     

Measurements of235U/238U isotopic ratio in the photoproduct UF5 by multiphoton ionization and time-of-flight mass spectrometry

A MultiPhoton-Ionization Time-Of-Flight Mass Spectrometry (MPI/TOFMS) apparatus was developed for real-time measurement of the uranium isotopic ratio in nascent UF₅ formed by the 266 nm photolysis of effusive UF₆ ( < 300 K, 1.3 × 10^–4 Pa). The UF₅ was selectively and efficiently multiphoton ionized by 532 nm radiation at appreciably low fluences ( < 10 J/cm²). The main ions observed, U⁺ and U²⁺, were subsequently analyzed with a TOFMS with mass resolution of 1190 to separate²³⁵U ⁿ⁺ and²³⁸U ⁿ⁺ completely. The isotopic ratio measurements showed good precision resulting from the excellent agreement which was observed between the isotopic ratios in UF₅ products and those in a parent UF₆ sample. These results suggested that the MPI/TOFMS method can be applied to the real-time analysis of separation factors in the molecular laser isotope separation of uranium by ionization of UF₅ following the infrared photodissociation of UF₆.     

Development of high resolution resonance ionization mass spectrometry for trace analysis of 93mNb

⁹³Nb(n, n′)^93mNb reaction allows retrospective estimation of integrated fast neutron dose in nuclear reactor. We proposed isomer-selective trace analysis of ^93mNb by Resonance Ionization Mass Spectrometry (RIMS) combined with a gas-jet atomic source and an injection locked Ti:Sapphire laser system operated at several kHz. Resonant ionization spectroscopy of Nb in gas-jet using Ti:Sapphire laser was demonstrated.