Double-focusing Sector Field Mass Spectrometer Research Articles

Hugo Bondy (1900–1985): an Austrian scientist of mass spectrometry in the twentieth century

Hugo Bondy (1900–1985), a fairly unknown Austrian scientist working in academia in the twentieth century made important contributions to the development of double-focusing sector field mass spectrometers. This article describes his life and scientific contributions in a fascinating biography meriting his achievements in the development and construction of new mass spectrometers. The article divides into three sections according to Bondy’s biography. In the first section, his education, career in academia, and research in mass spectrometry are displayed. The second section reconstructs his expulsion from the university, his persecution and resistance in the Nazi era, and his failed attempt to be reinstated at his position as a scientist. In the third and last section, Bondy’s career in the Austrian Federal Ministry of Education is depicted by focusing on his youth work, a controversy of his political affiliations, and his accomplishments and character in the civil service. Bondy’s life is taken as an example to uncover the process of exclusion in science with all its consequences. But also, the ethical aspects of science for education and the application of a scientific attitude to society are revealed by his actions.Graphical abstract

Investigation of mass-scale drift effects in the milli-mass range: Influence on high mass resolution mode multicollector-inductively coupled plasma mass spectrometer isotope ratio measurements.

The consequences, possible origins, and prevention of mass-scale drifts in the high mass resolution mode (HR, M/ΔM ≈ 8000) under constant conditions were investigated and simulated in case of a multicollector-inductively coupled plasma mass spectrometer (MC-ICP-MS) using silicon enriched in 28 Si as the main element in this survey. A drifting mass scale strongly impairs the precise and accurate determination of isotope ratios, depending especially on the peak/plateau width. For example, 29 Si+ in Si highly enriched in 28 Si has an extremely small mass plateau width of ΔM ≤ 4 × 10-3 u, compare ΔM(56 Fe+ ) ≈ 18 × 10-3 u, which is to our knowledge one of the smallest plateaus routinely investigated in isotope ratio measurements, thus requiring extreme stability. During warm-up of the double-focusing sector field mass spectrometer, a mass drift up to ΔM/Δt ≥ 0.006 u/hr has been observed. Long-term studies on mass scale stability and simulations concerning fluctuations of the magnetic field B, acceleration voltage Uacc and ESA voltage UESA are reported. A change of one of these quantities of 0.01% induces changes of the mass scale of 6 × 10-3 u, 3 × 10-3 u, and 1 × 10-3 u in the case of B, Uacc , and UESA , respectively. After identifying electrical charging/discharging effects in the mass spectrometer affecting the mass scale stability, the instrument was completely dismantled and carefully reinstalled. Additional stability tests using silicon, strontium, and lead finally yielded a mass drift of ΔM/Δt ≤ 0.001 u/8 h in the case of silicon. This enhanced stability guarantees measurements of isotope ratios on smallest plateaus with lowest uncertainty. The importance of a stable mass scale is pointed out and the relevant quantities of a typical magnetic sector field mass spectrometer are discussed.

A systematic study on the influence of carbon on the behavior of hard-to-ionize elements in inductively coupled plasma–mass spectrometry

A systematic study on the influence of carbon on the signal of a large number of hard-to-ionize elements (i.e. B, Be, P, S, Zn, As, Se, Pd, Cd, Sb, I, Te, Os, Ir, Pt, Au, and Hg) in inductively coupled plasma–mass spectrometry has been carried out. To this end, carbon matrix effects have been evaluated considering different plasma parameters (i.e. nebulizer gas flow rate, r.f. power and sample uptake rate), sample introduction systems, concentration and type of carbon matrix (i.e. glycerol, citric acid, potassium citrate and ammonium carbonate) and type of mass spectrometer (i.e. quadrupole filter vs. double-focusing sector field mass spectrometer). Experimental results show that P, As, Se, Sb, Te, I, Au and Hg sensitivities are always higher for carbon-containing solutions than those obtained without carbon. The other hard-to-ionize elements (Be, B, S, Zn, Pd, Cd, Os, Ir and Pt) show no matrix effect, signal enhancement or signal suppression depending on the experimental conditions selected. The matrix effects caused by the presence of carbon are explained by changes in the plasma characteristics and the corresponding changes in ion distribution in the plasma (as reflected in the signal behavior plot, i.e. the signal intensity as a function of the nebulizer gas flow rate). However, the matrix effects for P, As, Se, Sb, Te, I, Au and Hg are also related to an increase in analyte ion population caused as a result of charge transfer reactions involving carbon-containing charged species in the plasma. The predominant specie is C+, but other species such as CO+, CO2+, C2+ and ArC+ could also play a role. Theoretical data suggest that B, Be, S, Pd, Cd, Os, Ir and Pt could also be involved in carbon based charge transfer reactions, but no experimental evidence substantiating this view has been found.

Laser ablation-inductively coupled plasma-mass spectrometry using a double-focusing sector field mass spectrometer of Mattauch–Herzog geometry and an array detector for the determination of platinum group metals and gold in NiS buttons obtained by fire assay of platiniferous ores

This paper explores the potential of laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) for the determination of Au, Ir, Pd, Pt, Rh and Ru in NiS buttons obtained by fire assay of platiniferous ores using a new type of ICP-MS instrument equipped with an array detector Mattauch–Herzog spectrometer. The method evaluated comprises the NiS fire assay of a representative amount of the ore sample (40–75 g), grinding of the NiS buttons subsequently obtained, pelleting of the resulting powders using polyethylene wax as a binder, and LA-ICP-MS analysis of the sample using in-house matrix-matched standards for calibration. The use of this new ICP-MS device proved very beneficial in this context, offering a remarkable level of precision (2–3% RSD for the most abundant analytes, Pt, Pd, Rh and Ru, and 6–11% RSD for Au and Ir) owing to its simultaneous capabilities and its extended linear range, which enabled an improved performance of the internal standard (61Ni). Moreover, the low level of argide-based interferences and the detection power of the instrument provided low limits of detection (10 ng g−1 level) even for those elements that could be potentially affected by ArNi+ and ArCu+ overlap in this particular matrix (Pd, Rh, Ru). The accuracy finally obtained (90% of results within 10% of the reference value, and 70% of results within 5%) is fit-for-purpose for this application. The resulting method can be thus considered very attractive as a faster and greener strategy for the control of these types of samples in the platinum group metal industry, circumventing the need for the cumbersome and environmentally unfriendly digestion procedures currently employed.

IonCCD™ for Direct Position-Sensitive Charged-Particle Detection: from Electrons and keV Ions to Hyperthermal Biomolecular Ions

A novel, low-cost, pixel-based detector array (described elsewhere Sinha and Wadsworth (76(2), 1) is examined using different charged particles, from electrons to hyperthermal (<100 eV) large biomolecular positive and negative ions, including keV small atomic and molecular ions. With this in mind, it is used in instrumentation design (beam profiling), mass spectrometry, and electron spectroscopy. The array detector is a modified light-sensitive charge-coupled device (CCD) that was engineered for direct charged-particle detection by replacing the semiconductor part of the CCD pixel with a conductor Sinha and Wadsworth (76(2), 1). The device is referred to as the IonCCD. For the first time, we show the direct detection of 250-eV electrons, providing linearity response of the IonCCD to the electron beam current. We demonstrate that the IonCCD detection efficiency is virtually independent from the particle energy (250 eV, 1250 eV), impact angle (45(o), 90(o)) and flux. By combining the IonCCD with a double-focusing sector field mass spectrometer (MS) of Mattauch-Herzog geometry (MH-MS), we demonstrate fast data acquisition. Detection of hyperthermal biomolecular ions produced using an electrospray ionization source (ESI) is also presented. In addition, the IonCCD was used as a beam profiler to characterize the beam shape and intensity of 15 eV protonated and deprotonated biomolecular ions at the exit of an rf-only collisional quadrupole. This demonstrates an ion-beam profiling application for instrument design. Finally, we present simultaneous detection of 140 eV doubly protonated biomolecular ions when the IonCCD is combined with the MH-MS. This demonstrates the possibility of simultaneous separation and micro-array deposition of biological material using a miniature MH-MS.

Environmental analysis by inductively coupled plasma mass spectrometry

This article reviews the numerous ways in which inductively coupled plasma mass spectrometry has been used for the analysis of environmental samples since it was commercially introduced in 1983. Its multielemental isotopic capability, high sensitivity and wide linear dynamic range makes it ideally suited for environmental analysis. Provided that some care is taken during sample preparation and that appropriate calibration strategies are used to circumvent non-spectroscopic interferences, the technique is readily applicable to the analysis of a wide variety of environmental samples (natural waters, soils, rocks, sediments, vegetation, etc.), using quadrupole, time-of-flight or double-focusing sector-field mass spectrometers. In cases where spectroscopic interferences arising from the sample matrix cannot be resolved, then separation methods can be implemented either on- or off-line, which can simultaneously allow analyte preconcentration, thus further decreasing the already low detection limits that are achievable. In most cases, the blank, prepared by following the same steps as for the sample but without the sample, limits the ultimate detection limits that can be reached.

Evidence of near-field laser ablation inductively coupled plasma mass spectrometry (NF-LA-ICP-MS) at nanometre scale for elemental and isotopic analysis on gels and biological samples

Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) has been established as a powerful surface analytical method for local elemental analysis on metallic, ceramic, geological or biological sample surfaces. Here we show a new way of nanometre scale analysis of elements on sample surfaces by near-field LA-ICP-MS (NF-LA-ICP-MS). This technique uses the near-field enhancement effect on the tip of a thin silver needle in a laser beam (Nd:YAG laser, wavelength 532 nm) on the sample surface. The thin silver needle was etched electrolytically in an electrochemical cell using a droplet of citric acid as electrolyte. For nanolocal analysis by NF-LA-ICP-MS on soft matter (e.g., on 2-D gels and biological samples) a small volume transparent laser ablation chamber was constructed and coupled to a double-focusing sector field inductively coupled plasma mass spectrometer (ICP-MS). A small amount of soft sample material is ablated at atmospheric pressure by a single laser shot in the near-field of the silver tip in the defocused Nd:YAG laser beam. The ablated material is transported with argon as carrier gas into the inductively coupled plasma (ICP) ion source of the sensitive double-focusing sector field mass spectrometer with reverse Nier–Johnson geometry. By single-shot analysis on 2-D gels and biological surfaces doped with uranium in the μg g−1 range using NF-LA-ICP-MS an enhancement of ion intensities of transient signals in comparison with the background signal of up to factor 60 was observed. In gels doped with isotopically enriched 65Cu and 67Zn spikes by NF-LA-ICP-MS (single shot analysis) ion intensities up to the n × 105 cps range and isotope ratios (235U/238U, 65Cu/63Cu and 67Zn/64Zn) were measured at a lateral resolution in the nanometre scale. Using the near-field effect in LA-ICP-MS, it was demonstrated that nanolocal analysis is possible in single-shot measurements of elements on biological samples and on a gel surface with spatial resolution at the hundreds of nanometres range. This first experiment on near-field LA-ICP-MS opens up a new, challenging path for future applications in nanoimaging of elements in life science, biology and medicine, e.g., for analyses of single cells, cell organelles or biological structures at nanometre range in order to detect neurodegenerative diseases, but also in material science, nanotechnologies and nanoelectronics.

A LASER PROBE COUPLED WITH ICP DOUBLE-FOCUSING SECTOR-FIELD MASS SPECTROMETER FOR IN SITU ANALYSIS OF GEOLOGICAL SAMPLES AND U Pb DATING OF ZIRCON

Quadrupole ICP–MS is currently the method of choice for laser-ablation applications, since sector-field instruments are generally considered too slow for the acquisition of rapidly varying signals. However, the new generation of double-focusing sector-field mass spectrometers is capable of rapidly scanning the mass spectrum both magnetically and electrostatically, and the combination of these two scan modes approaches the performance of quadrupoles for most practical applications. We present the instrumental configuration and operating conditions of a LA–ICP–MS system, assembled by coupling a 266 nm laser source to a double-focusing magnetic-sector mass spectrometer (“Element”, Finnigan MAT). The capability of the adopted configuration to furnish high-quality in situ trace-element analysis of minerals and U–Pb geochronological data is described. Concentrations of thirty geochemically relevant elements ( LILE , HFSE , REE , actinides) were determined on geological samples with good precision and accuracy (<10%), evaluated using two USGS basaltic glass reference materials, BCR–2 and BIR–1. The high sensitivity and low background allowed extremely low limits of detection (down to the ppb level) for the heaviest elements, and between 10 and 100 ppb for the lighter masses. The first results on U:Pb ratio determinations are also very positive. By adopting an external matrix-matched standard for corrections of mass bias and laser-induced fractionation of elements, precision and accuracy in age determinations to better than ~2% are attained on the zircon standard 91500.

Capabilities of inductively coupled plasma mass spectrometry for the measurement of Fe isotope ratios

Reliable isotopic analysis of Fe by means of inductively coupled plasma mass spectrometry (ICP-MS) is traditionally hindered by spectral overlap of the analyte signals with those of Ar- and Ca-based molecular ions. The merits of several approaches for overcoming spectral interferences—operation of the ICP under cool plasma conditions, membrane desolvation of the sample aerosol, use of a double-focusing sector field mass spectrometer operated at a higher mass resolution and ion-molecule chemistry in a dynamic reaction cell (DRC)—for solving this particular problem were critically compared and the most successful approaches were subsequently used for the determination of Fe isotope ratios in a human serum reference material. Although beneficial to some extent and/or for some of the Fe nuclides, cool plasma conditions and membrane desolvation were shown to offer no sufficient reduction in the intensity of the interfering ions. With DRC-ICP-MS, all reaction gases tested, NH3, CO and N2, showed a similar behaviour. An excellent performance in terms of both freedom from spectral overlap and isotope ratio precision, not exceeding the theoretical precision calculated on the basis of counting statistics and ≤ 0.2% RSD for 5 successive measurements, was established for aqueous standard solutions. In the presence of Ca, however, the detrimental influence of spectral interferences precluded accurate determination of 57Fe/56Fe at the standard RPq setting. When using a matrix-matched blank in addition to CO as a reaction gas to correct for the remaining overlap, 54Fe/56Fe could, however, be accurately and precisely measured in human serum. It was established that, with DRC-ICP-MS, the matrix composition affected the mass discrimination to a significant extent, such that the isotopic standard used for mass discrimination also required matrix-matching. Further optimisation of the RPq value resulted in an improved signal-to-background ratio at a mass-to-charge ratio of 57. Despite a marked increase in mass discrimination, 54Fe/56Fe could still be accurately determined under these conditions (deviation from true value <0.1%) while the result for 57Fe/56Fe was substantially improved. The total uncertainty on a single determination (5 replicate measurements of 60 s each) of the 54Fe/56Fe ratio typically amounted to approximately 0.5%. When using sector field ICP-MS operated at a mass resolution of 3000, the analyte signals could be resolved from those of the above-mentioned molecular ions. For aqueous standard solutions, the isotope ratio precision attainable with sector field ICP-MS, typically 0.2–0.4% for 54Fe/56Fe and 57Fe/56Fe and ≥1% for 58Fe/56Fe, was significantly worse than that obtained with DRC-ICP-MS. The deviation between the experimental values for 54Fe/56Fe and 57Fe/56Fe in human serum and the corresponding true values, however, was <0.05%, while the total uncertainty on the ICP-MS results was approximately 0.5% (5 replicate measurements of 90 s each). Due to the low isotopic abundance of 58Fe, the uncertainty for 58Fe/56Fe deteriorated to ~2.5%. Conclusions concerning the applicability of ICP-MS for isotopic analysis of Fe are presented and some ideas for further research are discussed.

Sub-miniature ExB sector-field mass spectrometer

A novel sub-miniature double-focusing sector-field mass spectrometer has been fabricated at the University of Minnesota using a combination of conventional machining methods and thin film patterning techniques typically used in the sensor technology industry. Its design is based on the mass separation capabilities of a 90° cylindrical crossed electric and magnetic sector-field analyzer with a 2-cm radius, which under proper conditions is able to effectively cancel the angular and chromatic dispersion of the ion beam, thus improving the resolving power of the instrument. Simulations using finite element analysis and computer modeling were employed to verify and optimize the performance of the proposed instrument before and during its fabrication. The prototype was able to attain a resolving power of 106 full-width at half-maximum (FWHM), a detection limit close to 10 parts per million, a dynamic range of 5 orders of magnitude and a mass range up to 103 Da. Its overall size, including the magnet assembly, is 3.5 cm wide, 6 cm long and 7.5 cm tall, it weighs 0.8 kg, and its power consumption was measured to be 2.5 W. The performance of the instrument was found to be comparable to that of commercial residual gas analyzers, at a fraction of the cost. All these characteristics make this miniature mass spectrometer suitable for portable and low-cost analytical instrumentation.

Analysis of GaAs using a combined r.f. glow discharge and inductively coupled plasma source mass spectrometer

A radiofrequency (r.f.) glow discharge ion source was coupled to a double-focusing sector field mass spectrometer with reverse Nier–Johnson geometry. The glow discharge cell powered by a 13.56 MHz generator was connected directly to the interface of the mass spectrometer. The r.f. glow discharge ion source operates optimally at an argon pressure of 2.5 hPa and radiofrequency powers of 30 W. With increasing argon pressure more complex mass spectra were observed due to the higher molecular ion formation rate. The analytical performance of r.f. glow discharge mass spectrometry was investigated for the trace elemental analysis of semi-insulating gallium arsenide crystals. Using ICP–MS after matrix separation for a better quantification of multielement determination of trace impurities, detection limits comparable to r.f. GDMS in the low ng/g concentration range are obtained.

Electron impact ionization of tetramethylsilane (TMS)

We studied the electron impact ionization of tetramethylsilane (TMS), Si(CH 3) 4, which is utilized in plasma polymerization applications, using various mass spectrometric techniques. Absolute partial cross-sections for the formation of the parent ion and 15 fragment ions were measured in a high resolution double-focusing sector field mass spectrometer from threshold to 90 eV. Complementary measurements of mass spectral cracking patterns of TMS were also carried out using two quadrupole mass spectrometers. Dissociative ionization of TMS was found to be the dominant process. The Si(CH 3) 3 + ion is by far the most abundant fragment ion with a maximum cross-section of about 1 × 10 −15cm 2. In contrast, the maximum parent ionization cross-section is less than 2% of that value. On the basis of the measured partial ionization cross-sections, mass spectral cracking patterns and appearance energies of the various ionic products, we propose a collision-induced decomposition scheme for the TMS molecule. Three major decomposition channels were identified which all involve the total or partial removal of a methyl group. The agreement of our measured absolute partial ionization cross-sections with earlier data obtained by a different technique is generally poor.

Interaction of electrons with C 60+ and C 60−: post-ionization, dissociation and kinetic energy release

We report on the interaction of energetic electrons with size- and charge-selected fullerene ions in the second field-free region of a double-focusing sector-field mass spectrometer. C 60 2+ and C 60 + are formed from C 60 + and C 60 −, respectively. The kinetic energy released in the electron-induced formation of the fragment ions, C 58 2+ and C 58 +, is measured and compared to the kinetic energy released in metastable dissociations.

Dynamics of Formation and Evaporation of Mixed Alkali Halide Nanocrystals: A Case of Comparable Lattice Energies

Abstract : The relative intensity distribution of the (Rb(n)K(14-n)I(13)(+) mixed nanocrystals containing a 'magic' number of 14 metal cations and 13 iodide anions is examined. These nanocrystals were generated through sputtering of mixed RbI/KI solids using fast atom bombardment, and analyzed by use of a double-focusing sector-field mass spectrometer. The relative intensities of the observed peaks is found to have a 'bimodal' distribution with a maximum near n=7. This observation is explained by assuming: (a) a rate of formation that is proportional to the number of isomers that can be formed for each mixed nanocrystal, and (b) a rate of dissociation that is insensitive to the composition of the mixed nanocrystal. The later assumption leads to the conclusion that the internal energies of these mixed nanocrystals are similar and independent of their composition. The conclusion is supported by the results of Monte Carlo simulation annealing as well as by the similarity of the known lattice energies of KI and RbI bulk crystals.

Dissociative ionization of an O 2 molecule in Ar nO 2 clusters: a new method for monitoring excited states and for studying the dynamics of energy transfer processes in clusters?

The dynamics of energy- and charge-transfer from cluster atoms to an embedded impurity molecule is studied in Ar n O 2 clusters excited and ionized by electron impact ionization. It is shown that the first excited state of teh Ar +* n cluster ion corresponding to the trapped n = 1 exciton (i.e., the 3Σ u Ar * 2 excimer) is quenched by the presence of an oxygen molecule. A simultaneous excitation- and charge-transfer from this state leads to a dissociative ionization of the oxygen molecule within the cluster. The energy released in this process is sufficient for the formation of an O + atomic ion in the first excited state O + ( 2D). The Ar n O + cluster ions so produced decay in the metastable time regime (i.e., several microseconds after the ionization) by the evaporation of up to five Ar monomers. The resulting fragmentation pattern, measured in a double-focusing sector-field mass spectrometer, is compared with results from molecular dynamics simulations. Two fragmentation mechanisms are discussed, i.e.: (i) a mechanism where the charge-transfer reaction O + ( 2D) + Ar → O + Ar + is occurring within the cluster, which is followed by a sequence of curve-crossing transitions leading to the release of kinetic energy and to subsequent fragmentation of the cluster; (ii) a mechanism which involves the radiative decay of the O +( 2D) metastable excited state embedded within the cluster, followed by the contraction of ArO + bonds and by the vibrational excitation and subsequent fragmentation of the cluster.

Metastable fragmentation of Ne +4 cluster ion initiated by excimer decay

A new metastable decay reaction of the Ne + 4 cluster ion produced by electron-impact ionization has been studied with a double-focusing sector field mass spectrometer. Appearance energy measurements of this decay reaction Ne + 4 → Ne + 2 + 2Ne provide quantitative support in favour of the suggested decay model, i.e. the radiative decay of an excimer dimer embedded in the rare-gas cluster ion is the origin of this new, unusual metastable fragmentation channel leading to delayed ejection of more than one monomer. Moreover, the dependence of the metastable fraction on the time interval between ionization and fragmentation obtained supports this model qualitatively.

Unimolecular metastable decay of molecular clusters ions via monomer-fragment ejection: [(O 2) n] −*→[(O 2) n−1 O] −+O

The stability of (O 2) n − ions produced by electron attachment to a neutral cluster beam has been studied with a double-focusing sector field mass spectrometer. Besides monomer evaporation via metastable dissociation reactions (O 2) n −*→(O 2) n−1 −*+O 2 it was possible to observe for the first time metastable cluster ion dissociations via ejection of a monomer-fragment, i.e. [(O 2) n ] −*→[(O 2) n−1 O] −+O. Mechanism, dynamics and energetics of this reaction are presented and discussed.

Transfer of vibrational energy in nitrogen clusters doped with O 2: New evidence for the origin of quantum effects in the metastable decay series of (N 2) n+*

The decomposition of metastable (N 2) n +*, ((N 2) nO 2) +* and (0 2) n +* ions ( n up to 10) has been studied quantit a double-focusing sector field mass spectrometer as a function of cluster size n, and the number of evaporated neutral monomers p (with p up to n-1 for each cluster ion series). Whereas (N 2) n +* cluster ions show the known oscillatory pattern in the evapor probability versus the number of evaporated monomers (attributed to radiationless relaxation of vibrationally excited N 2 molecules in clusters), nitrogen cluster ions doped with one O 2 molecule do not show this behavior. In accordance with observations in liquids, this finding is interpreted as being due to an increased relaxation rate in the presence of the O 2 dopant.

Appearance size and appearance energy of doubly and triply charged ammonia clusters

Multiply charged ammonia clusters are produced by electron impact ionization of neutral NH 3 clusters formed by nozzle expansion. The properties of these ions are studied with a high-resolution double-focusing sector field mass spectrometer. The observed critical appearance sizes are n 2=51 and n 3=121 for doubly and triply charged clusters, respectively. The appearance energy of doubly and triply charged ammonia clusters is found to be shifted way below the respective appearance energy of the monomer. This is in accordance with a recently suggested sequential ionization mechanism for the production of stable multiply charged van der Waals cluster ions.

Quantized sequential decay series of metastable N 2 cluster ions (N 2) n+* → (N 2) n-1 +* → … → N 2+

The first experimental observation of sequential decay of metastable molecular cluster ions is reported. N 2 cluster ions are produced by electron impact ionization of a supersonic N 2 cluster beam and analyzed by a double-focusing sector field mass spectrometer (reversed geometry). It is found that metastable N 2 cluster ions lose more than one N 2 molecule in the microsecond time regime and decay predominantly via sequential series (N 2) n +* → (N 2) n-1 +* → … → N 2 +, evaporating a single monomer in each of these successive decay steps. The metastable decay rates determined in detail for cluster sizes 2 ⩽ n ⩽ 6 and n = 20 lie between 1 and 10 6 s −1 and depend strongly on the time elapsed after ion formation. The decay rates exhibit a quasi-periodic pattern, consistent with the assumption that up to four vibrational quanta (stored in neutral N 2 molecules within the cluster) relax into the phonon bath of the cluster during the sampling time.