Accurate mass spectrometry of trapped ions

The Penning trap Ion Cyclotron Resonance (ICR) method we use to weigh atomic masses is reviewed, and our plans for future measurements, new methods, and apparatus improvements are discussed. Our ultimate goal is to develop a new technique for measuring atomic masses with an accuracy of a few parts in 10¹². We will do this by comparing the cyclotron frequencies of two simultaneously trapped ions. In order to successfully implement this new method we are developing a quieter, more sensitive DC SQUID-based detector and a new more harmonic trap, and we plan to use our classical squeezing techniques to reduce the effects of thermal noise. With our improved apparatus we will weigh Cs and Rb to help determine the fine structure constant α, weigh ²⁹Si and ³⁰Si as part of the current effort to replace the artifact kilogram standard with a Si crystal containing a known number of atoms, and measure the ³H-³He mass difference to help set a limit on the mass of the electron neutrino. Our higher accuracy will also enable us to ``weigh' the neutron capture gamma rays of <sup>28</sup>Si, <sup>32</sup>S, and <sup>48</sup>Ti to help determine the molar Planck constant N<sub>A</sub>h and the fine structure constant α. Finally, with a mass measurement accuracy \sim 10<sup>-12</sup> we will be able to ``weigh' chemical bonds. This revised version was published online in August 2006 with corrections to the Cover Date.     

Charge state of ions in liquid metal field ion sources

The post-ionization model of field evaporation is shown to be consistent with observations of singly and doubly charged ions in liquid metal field ion sources. The model can be used to estimate the field strength at the apex of the Taylor cone which is found to be 1.9–2.0 V/Å for a Ga source. Experiments to test the post-ionization model and to determine the apex field strength more accurately are suggested. A possible method of obtaining ～μA currents of highly charged ions, e.g. Zr⁴⁺, Ta⁴⁺, Ga³⁺, As³⁺, is proposed.     

High-accuracy mass spectrometry with stored ions

Like few other parameters, the mass of an atom, and its inherent connection with the atomic and nuclear binding energy is a fundamental property, a unique fingerprint of the atomic nucleus. Each nuclide comes with its own mass value different from all others. For short-lived exotic atomic nuclei the importance of its mass ranges from the verification of nuclear models to a test of the Standard Model, in particular with regard to the weak interaction and the unitarity of the Cabibbo–Kobayashi–Maskawa quark mixing matrix. In addition, accurate mass values are important for a variety of applications that extend beyond nuclear physics. Mass measurements on stable atoms now reach a relative uncertainty of about 10^-11${10}^{-11}$. This extreme accuracy contributes, among other things, to metrology, for example the determination of fundamental constants and a new definition of the kilogram, and to tests of quantum electrodynamics and fundamental charge, parity, and time reversal symmetry. The introduction of Penning traps and storage rings into the field of mass spectrometry has made this method a prime choice for high-accuracy measurements on short-lived and stable nuclides. This is reflected in the large number of traps in operation, under construction, or planned world-wide. With the development and application of proper cooling and detection methods the trapping technique has the potential to provide the highest sensitivity and accuracy, even for very short-lived nuclides far from stability. This review describes the basics and recent progress made in ion trapping, cooling, and detection for high-accuracy mass measurements with emphasis on Penning traps. Special attention is devoted to the applications of accurate mass values in different fields of physics.     

Multiple peak formation in laser ionization mass spectrometry

Molecules in the source region of a time of flight mass spectrometer are ionized by ultraviolet laser radiation. Under certain conditions mass spectra consisting of a remarkably periodic set of ion peaks are generated. It is demonstrated that these result from spatially periodic ionization within the mass spectrometer which is a consequence of the development of standing electromagnetic waves.     

Ionization of atoms and ions by intense laser radiation

The ionization of negative ions by ultrashort electromagnetic pulses, the multiphoton ionization of atoms beyond the perturbation theory taking into account the Coulomb interaction, and the relativistic theory of tunneling in application to the ionization problems have been analyzed. The main results have been obtained using the imaginary time method.     

应用激光质谱法选择探测一氧化碳

此文解决了激光质谱法中从含同质量数的N2分子的混合气体中选择CO分子这一问题,由于用紫外266nm激光不能从含N2分子的混合气体中选择CO分子,因此采用了可见激光来探测,发现用可见激光430～435nm的激光能成功地将CO分子选择探测出来。此外还着重分析了在可见波段和266nm激光下产生CO+离子的机理：在可见光波段443.55nm所对应的共振峰是CO分子吸收3光子的激光能量与A1Π态的振动量子数为2的振动态共振产生的;434.3nm所对应的共振峰是CO分子吸收3光子的激光能量与A1Π态的振动量子数为3的振动态共振产生的;CO分子吸收两光子的266nm激光能量与A1Π态第7振动态的某一高转动态相共振,处于该振动态的CO分子再继续吸收一个光子的266nm激光的能量至其电离态电离产生CO+离子。     

Investigation of trace p-dichlorobenzene using laser mass spectrometry

The time-of-flight mass spectrum (TOFMS) relative to the resonant two-photon ionization of gas phase pdichlorobcnzene was obtained in the wavelength range of 240 - 250 nm by a home-made system. A special design was made to reduce the effect of memory on the inner wall of the sample inlet system suitable for the investigation of semi-volatile organic compound. In this wavelength range, p-dichlorobenzene molecules firstly absorbed one photon to be excited from the ground state 1A9(So) to the first excited state 1B2u (S1), then absorbcd another photon to be ionized. The relationship between the signal intensity of p-dichlorobenzene molccular ion C6H435Cl at 248-nm wavelength and the laser power was given. The 1.52 power index of C6H435Cl2 was a typical identification of the 3/2 power law. The relationship between the ion signal intensity of C6H435Cl and the sample concentration was close to a linear one in the ppm(V/V) range, which led to a detection limit of 125 ppb(V/V) for p-dichlorobenzene.     

Investigation of trace laser mass p-dichlorobenzene using spectrometry

The time-of-flight mass spectrum （TOFMS） relative to the resonant two-photon ionization of gas phase p- dichlorobenzene was obtained in the wavelength range of 240 - 250 nm by a home-made system. A special design was made to reduce the effect of memory on the inner wall of the sample inlet system suitable for the investigation of semi-volatile organic compound. In this wavelength range, p-dichlorobenzene molecules firstly absorbed one photon to be excited from the ground state ^1Ag（So） to the first excited state ^1B2u （S1）, then absorbed another photon to be ionized. The relationship between the signal intensity of p-dichlorobenzene molecular ion C6H4^35Cl2^＋ at 248-nm wavelength and the laser power was given. The 1.52 power index of C6H4^35Cl2^＋ was a typical identification of the 3/2 power law. The relationship between the ion signal intensity of C6H4^35Cl2^＋ and the sample concentration was close to a linear one in the ppm（V/V） range, which led to a detection limit of 125 ppb（V/V） for p-dichlorobenzene.     

On the abundance of molecular ions in secondary ion mass spectrometry

Changes in molecular secondary ion intensities brought about by working in an environment of oxygen can be rationalised in simple statistical terms.     

Uniform molecular analysis using femtosecond laser mass spectrometry

The potential of femtosecond laser time-of-flight mass spectrometry (FLMS) for uniform quantitative analysis of molecules has been investigated. Various samples of molecular gases and vapours have been studied, using ultra-fast ( approximately 50 fs) laser pulses with very high intensity (up to 1.6 x 10(16) Wcm(-2)) for non-resonant multiphoton ionisation/tunnel ionisation. Some of these molecules have high ionisation potentials, requiring up to ten photons for non-resonant ionisation. The relative sensitivity factors (RSF) have been determined as a function of the laser intensity and it has been demonstrated that for molecules with very different masses and ionisation potentials, uniform ionisation has been achieved at the highest laser intensities. Quantitative laser mass spectrometry of molecules is therefore a distinct possibility. Copyright 1999 John Wiley & Sons, Ltd.     

Molecular imaging by Mid-IR laser ablation mass spectrometry

Mid-IR laser ablation at atmospheric pressure (AP) produces a mixture of ions, neutrals, clusters, and particles with a size distribution extending into the nanoparticle range. Using external electric fields the ions can be extracted and sampled by a mass spectrometer. In AP infrared (IR) matrix-assisted laser desorption ionization (MALDI) experiments, the plume was shown to contain an appreciable proportion of ionic components that reflected the composition of the ablated target and enabled mass spectrometric analysis. The detected ion intensities rapidly declined with increasing distance of sampling from the ablated surface to ∼4 mm. This was rationalized in terms of ion recombination and the stopping of the plume expansion by the background gas. In laser ablation electrospray ionization (LAESI) experiments, the ablation plume was intercepted by an electrospray. The neutral particles in the plume were ionized by the charged droplets in the spray and enabled the detection of large molecules (up to 66 kDa). Maximum ion production in LAESI was observed at large (∼15 mm) spray axis to ablated surface distance indicating a radically different ion formation mechanism compared to AP IR-MALDI. The feasibility of molecular imaging by both AP IR-MALDI and LAESI was demonstrated on targets with mock patterns. Presented at the 9-th International Conference on Laser Ablation, 2007 Tenerife, Canary Islands, Spain     

Pulsed laser deposition: metal versus oxide ablation

We present experimental results of pulsed laser interaction with metal (Ni, Fe, Nb) and oxide (TiO₂, SrTiO₃, BaTiO₃) targets. The influence of the laser fluence and the number of laser pulses on the resulting target morphology are discussed. Although different responses for metal and oxide targets to repetitive laser irradiation could be expected due to the different band structures of metals and oxides, the optical response is quite similar for 248-nm laser irradiation. Therefore, the difference in response is largely caused by differences in thermal properties. Metal targets show periodic structures of the order of micrometers after consecutive pulses of laser radiation, while the SrTiO₃ and BaTiO₃ targets show a flat surface after ablation for relatively low fluences (1.0 Jcm^-2). The observed TiO₂ target ablation characteristics fall in between those of the ablated metals and perovskites, because ablation results in the presence of Ti-rich material, which shields the underlying stoichiometric target material from ablation. The final target morphology is dependent on fluence, number of pulses, and the movement of the target itself (rotating, scanning, or stationary). It can take between 15 and 75 pulses to reach a steady-state target morphology on a stationary target. PACS 79.20.Ds; 52.38.Mf; 81.15.Fg     

Charge state and residence time of metal ions generated from amicrosecond vacuum arc

Metal ions generated from a microsecond vacuum arc were measured using a time-of-flight (TOF) method. A point-plane vacuum gap was fired by an impulse voltage to generate metal ions. The risetime and time constant for the decay of the arc current were 0.1 and 4.5 μs, respectively. TOF ion currents were measured for variable ion extraction times after the arc ignition. At a lead cathode, Pb⁺ and Pb ⁺⁺ ions were detected for ion extraction times less than 45 μs. The average charge-state fractions of the Pb⁺ and Pb ⁺⁺ ions were 91 and 9%, respectively. At a copper cathode, Cu ⁺, Cu⁺⁺, and Cu⁺⁺⁺ ions were detected for ion-extraction times less than 12.5 μs, and the average charge-state fractions were 42, 41, and 17%, respectively. The residence times of the generated lead and copper ions were also discussed     

Investigation of carbon graphite-like structures by laser mass spectrometry

Sorbents prepared from industrial waste and some carbon graphite-like materials are studied by laser ablation and time-of-flight mass spectrometry. The mass spectra of all materials show a peak at a mass of 465 u. This peak is found to correspond to a molecule of mass 426 u to which a K⁺ ion is attached. An exponential dependence of the M = 465 u peak intensity on the sorbent specific surface area in the range from 20 to 1200 m²/g is obtained.     

Efficiency of raman conversion of XeCl laser radiation in metal vapors

Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 55, No. 1, pp. 80–83, July, 1991.     

Chemical reactivity in matrix-assisted laser desorption/ionization mass spectrometry

During the control of a multistep organic synthesis on a soluble polymer (PEG) by matrix-assisted laser desorption/ionization (MALDI) mass spectrometry, a chemical reactivity was encountered when the matrix was acidic, for the samples where the amino moiety of the anchored compounds was protected as a Schiff base. Such imine hydrolysis was proven to be solely mediated by the acidic matrix during analyses since the expected protected structures were detected when the experiments were duplicated with a non-acidic matrix. Even if MALDI mass spectrometry was found to be more convenient than electrospray ionization mass spectrometry for the monitoring of liquid phase organic syntheses, the chemical reactivity imparted by the use of a matrix must be taken into account to avoid erroneous spectra interpretations. Copyright 1999 John Wiley & Sons, Ltd.