Analytical atomic spectroscopy using ion trap devices

Investigations using ion trap devices for analytical atomic spectroscopy purposes have focused on the use of an inductively coupled plasma (ICP) ion source with ion trap mass spectrometric (ITMS) detection. Initial studies were conducted with an instrument assembled by simply appending an ion trap as the detector to a fairly conventional ICP/MS instrument, i.e. leaving an intermediate linear quadrupole between the plasma source and the ion trap. The principal advantages found with this system include the destruction of nearly all problematic and typical ICP/MS polyatomic ions (e.g., ArH(+), ArO(+), ArCl(+), Ar(2)(+), etc) and a dramatic reduction of the primary plasma source ion, Ar(+). These results prompted the development of a second-generation plasma source ion trap instrument in which direct coupling of the ICP and ion trap has been effected (i.e. no intermediate linear quadrupole); the same performance benefits have been largely preserved. Initial operation of this instrument is described, characterized, and compared to the originally described ICP/ITMS and conventional ICP/MS systems. In addition, experiments aimed at improving ICP/ITMS sensitivity and selectivity using broadband resonance excitation techniques are described. Finally, the potential for laser optical detection of trapped ions for analytical purposes is speculated upon.     

Dissociation of polyatomic ions in the inductively coupled plasma

A general method for identifying the origin of a particular polyatomic ion is described. Based on a postulated dissociation reaction, measured ion signal ratios (e.g. Ar₂⁺/Ar⁺) are combined with mass bias corrections and estimates of the density of the neutral product (usually Ar, O or H atoms) to determine a gas kinetic temperature T_gas. The temperature can also be measured by the reduction in pressure when the ICP is sampled (compared to room temperature argon), or by other means. Dissociation energies and spectroscopic constants for the ions are necessary. For the particular instrument used, some of the findings of this study are: (a) ArO⁺ and ArN⁺ can be either dissociated (if the plasma potential is high) or created (if the plasma potential is low) by collisions between the sampler and skimmer; (b) the strongly-bound oxide ions O₂⁺ and MO⁺ for the rare earths are observed at levels consistent with T_gas ∼5300 K in a ‘hot’ plasma, but ClO⁺ is formed in excess; and (c) the abundances of most other polyatomic ions like H₂O⁺ and ArH⁺ correspond to higher densities than would be expected in the ICP itself.     

Electrospray ionization and atmospheric pressure matrix‐assisted laser desorption/ionization mass spectrometry of antioxidants applied in lubricants

The aim of this study was to investigate the utility of ion trap mass spectrometry (ITMS) in combination with the two desorption/ionization methods, electrospray (ESI) and atmospheric pressure matrix‐assisted laser desorption/ionization (AP‐MALDI), for the detection of antioxidants which are applied in lubricants. These experiments should form the base for future investigations of antioxidants in tribologically formed thin layers on the surface of frictional systems. Seventeen different antioxidants were selected out of the group of hindered phenolic and aromatic aminic compounds. Practically all antioxidants could be characterized by positive ion ESI‐ and AP‐MALDI‐ITMS, forming various types/species of molecular ions (e.g. [M]^{+ .} , [M+H]⁺, [M+Na]⁺ or [M–2H+H]⁺). A few compounds could be analyzed by negative ion ESI‐MS, too, but none by negative ion AP‐MALDI‐MS. The influence of target materials in AP‐MALDI‐MS (gold‐ and titanium nitride (TiN)‐covered stainless steel, micro‐diamond‐covered hard metal, hand‐polished and sand‐blasted stainless steel targets) with respect to the molecular ion intensity and type of molecular ion of two selected antioxidants was evaluated. The surface properties are of particular interest because in friction tests different materials with different surface characteristics are used. However, the MS results indicate that optimal target surfaces have to be found for individual antioxidants in AP‐MALDI‐MS but in general smooth surfaces were superior to rough surfaces. Finally the gold‐covered stainless steel MALDI target provided the best mass spectra and was selected for all the antioxidants investigated. Copyright © 2009 John Wiley & Sons, Ltd.     

Statistical mechanics of Ar2+ in an inductively coupled plasma

In the mass spectrum of an argon inductively coupled plasma (ICP), there is a peak due to the presence of the argon dimer ion, Ar₂⁺. Using elementary statistical mechanics, an attempt is made to elucidate the mechanism responsible for this ion's presence in the ICP, The assumption of local thermodynamic equilibrium (LTE) in the ICP leads to three possible mechanisms that could be responsible for the presence of the argon dimer ion, however, the results of the calculations show that only one of the mechanisms agrees with experiment. The experimental measurements of the number density ratio of Ar₂⁺ to Ar⁺, against which the theoretical values are compared, were taken using inductively coupled plasma mass spectrometry (ICP-MS),     

Identification of organic hydroperoxides and hydroperoxy acids in secondary organic aerosol formed during the ozonolysis of different monoterpenes and sesquiterpenes by on‐line analysis using atmospheric pressure chemical ionization ion trap mass spectrometry

On‐line ion trap mass spectrometry (ITMS) enables the real‐time characterization of reaction products of secondary organic aerosol (SOA). The analysis was conducted by directly introducing the aerosol particles into the ion source. Positive‐ion chemical ionization at atmospheric pressure (APCI(+)) ITMS was used for the characterization of constituents of biogenic SOA produced in reaction‐chamber experiments. APCI in the positive‐ion mode usually enables the detection of [M+H]⁺ ions of the individual SOA components. In this paper the identification of organic peroxides from biogenic volatile organic compounds (VOCs) by on‐line APCI‐ITMS is presented. Organic peroxides containing a hydroperoxy group, generated by gas‐phase ozonolysis of monoterpenes (α‐pinene and β‐pinene) and sesquiterpenes (α‐cedrene and α‐copaene), could be detected via on‐line APCI(+)‐MS/MS experiments. A characteristic neutral loss of 34 Da (hydrogen peroxide, H₂O₂) in the on‐line MS/MS spectra is a clear indication for the existence of an organic peroxide, containing a hydroperoxy functional group. Copyright © 2009 John Wiley & Sons, Ltd.     

Probing trapped ion energies via ion-molecule reaction kinetics: Quadrupole ion trap mass spectrometry

We present a detailed study of the energies of the ions stored in a quadrupole ion trap mass spectrometer (QITMS). Previous studies have shown that the rate constant, k, for the charge exchange reaction Ar⁺ N⁺ ₂ →, N⁺ ₂+Ar increases with increasing ion-molecule center-of-mass kinetic energy (K.E._cm). Thus, we have determined k for this chemical “thermometer” reaction at a variety of Ar and N₂ pressures and have assigned K.E._cm values as a function of the q₂ of the Ar⁺ ion both with and without He buffer gas present in the trap. The K.E._cm energies are found to lie within the range 0.11–0.34 eV over the variety of experimental conditions investigated. Quantitative “cooling” effects due to the presence of He buffer gas are reported, as are increases in K.E._cm due to an increase in the q₂ of the Ar⁺ ion. “Effective” temperatures of the Ar⁺ ions in He buffer are determined based on a Maxwell-Boltzmann distribution of ion energies. The resulting temperatures are found to lie within the range ≈ 1700–3300 K. We have also examined the K.E._cm, values arising from the chemical thermometer reaction of O⁺ ₂ with CH₄, as previous assignments of effective ion temperatures based on this reaction have been called into question.     

Methylene substitution reactions in a quadrupole ion trap selectivity of ethylene,ethylene oxide,and dimethyl ether reactive ions

To elucidate the selectivity of methylene substitution reactions of monosubstituted and disubstituted oxyaromatic compounds in a low pressure quadrupole ion trap environment, the relative abundances of covalently bound and loosely bound adducts formed by ion/molecule reactions with ethylene (ET), ethylene oxide (ETOX), and dimethyl ether (DME) were compared. Adduct ions of all three reagent gases were formed in both a conventional ion source and a quadrupole ion trap and characterized by collisionally activated dissociation. For DME and ET, the covalently bound adducts formed at (M + 45)⁺ and (M + 41)⁺, respectively, are direct precursors to the methylene substitution product ions at (M + 13)⁺. ETOX and ET do not demonstrate the same functional group selectivity for methylene substitution as previously observed for DME. This is attributed to differences in reaction exothermicities and competing reactions.     

Investigation of fragmentation behaviors of steroidal drugs with Li+, Na+, K+ adducts by tandem mass spectrometry aided with computational analysis

In this study, we have investigated the fragmentation of the widely used steroidal pharmaceutical drugs (n = 14), complexed by a singly charged proton or alkali metal ion (Li⁺, Na⁺, K⁺) using Ion trap and quadrupole time-of-flight mass spectrometers. Spectra were collected by LC-MS/MS analysis using system automated collision energy i.e., of 25–60 eV. Theoretical calculations were also calculated using DFT software. The metal complexes showed different fragmentation pathways not commonly observed for protonated compounds. There was a distinct difference observed in the relative intensities of some common fragments for free vs. metallated drugs. Some major fragments from protonated and lithium adducts showed close resemblance, while sodium and potassium adducts showed different fragments. Theoretical calculations showed a distinct difference in the position of attachment of proton and metals. This adducts ion fragmentation information will be helpful for the identification of these compounds in complex samples.     

Study on thermal decomposition of polymers by simultaneous measurement of TG–DTA and miniature ion trap mass spectrometry equipped with skimmer-type interface

Simultaneous thermogravimetry–differential thermal analysis and miniature ion trap mass spectrometry (TG–DTA–ITMS) instrument equipped with a skimmer-type interface has been successfully developed. The system allows precise real-time monitoring analysis of activated organic compounds such as pyrolysates because gaseous transformation of the evolved gases from the TG–DTA is suppressed by the skimmer-type interface. Also, excessive fragmentation during ionization of molecules can be avoided with the soft ionization method by photoionization. In addition, the miniaturized ITMS is equipped with unique tandem MS capability. These features permit a better understanding of the complicated thermal behavior and the precise pyrolysates of materials. The pyrolysis of various standard reagent polymers such as polymethyl methacrylate, polystyrene, and polyphenylene sulfide has been examined by the TG–DTA–ITMS in inert atmosphere. The synergy effect of the skimmer-type interface and the ITMS was evaluated comparatively with conventional quadrupole mass spectrometry results. It was confirmed that the real-time monitoring ITMS/MS worked satisfactorily. Here, we demonstrate a valuable application of the TG–DTA–ITMS in the detailed analysis of commercial polymers.     

The use of static pressures of heavy gases within a quadrupole ion trap

The performance of quadrupole ion traps using argon or air as the buffer gas was evaluated and compared to the standard helium only operation. In all cases a pure buffer gas, not mixtures of gases, was investigated. Experiments were performed on a Bruker Esquire ion trap, a Finnigan LCQ, and a Finnigan ITMS for comparison. The heavier gases were found to have some advantages, particularly in the areas of sensitivity and collision-induced dissociation efficiency; however, there is a significant resolution loss due to dissociation and/or scattering of ions. Additionally, the heavier gases were found to affect ion activation and deactivation during MS/MS, influencing the product ion intensities observed. Finally, the specific quadrupole ion trap design and the ion ejection parameters were found to be crucial in the quality of the spectra obtained in the presence of heavy gases. Operation with static pressures of heavy gases can be beneficial under certain design and operating conditions of the quadrupole ion trap.     

Determination of Methylmercury in Waters Using Sodium Tetraphenylborate Derivatization/Solvent Extraction and Gas Chromatography-Ion Trap Mass Spectrometry

Abstract

An analytical approach involving aqueous-phase derivatization followed by gas chromatography/ion trap mass spectrometry (GC/ITMS) for the determination of methylmercury in waters is described. The aqueous-phase derivatization is based on the formation of a more hydrophobic compound between MeHg⁺ and BPh^? ₄, i.e. MeHgPh, which is extractable into dichloromethane. The detection limit of the method for MeHg is 1 ng/mL for Hg when 100 ml water was analyzed. Recoveries from standard addition to tap water, lake water are 96 ± 8% and 92 ± 8%, respectively.     

Selective ion-molecule reactions of lactams with dimethyl ether ions

The ion-molecule reactions of dimethyl ether ions CH₃OCH₃ ⁺ and (CH₃OCH₃)H⁺, and four- to seven-membered ring lactams with methyl substituents in various positions were characterized by using a quadrupole ion trap mass spectrometer and a triple-quadrupole mass spectrometer. In both instruments, the lactams were protonated by dimethyl ether ions and formed various combinations of [M + 13] ⁺, [M + 15] ⁺, and [M + 45] ⁺ adduct ions, as well as unusual [M + 3] ⁺ and [M + 16] ⁺ adduct ions. An additional [M + 47] ⁺ adduct ion was formed in the conventional chemical ionization source of the triple-quadrupole mass spectrometer. The product ions were isolated and collisionally activated in the quadrupole ion trap to understand formation pathways, structures, and characteristic dissociation pathways. Sequential activation experiments were performed to elucidate fragment ion structures and stepwise dissociation sequences. Protonated lactams dissociate by loss of water, ammonia, or methylamine; ammonia and carbon monoxide; and water and ammonia or methylamine. The [M + 16] ⁺ products, which are identified as protonated lactone structures, are only formed by those lactams that do not have an N-methyl substituent. The ion-molecule reactions of dimethyl ether ions with lactams were compared with those of analogous amides and lactones.     

A comparison of electrospray tandem mass spectra of some sialic acid derivatives: Ion trap and high resolution QqToF mass spectrometers

Using O-acetyl-N-acyl derivatives of O-methyl sialoside methyl esters, it was shown that an ion trap and a hybrid analyzer (linear quadrupole–time-of-flight analyzer, reflectron) give comparable, though not identical secondary mass spectra for the [M + Na]⁺ and [M + K]⁺ ions. A parallel use of an ion trap and a hybrid QqToF instrument gives information about the fragmentation pathways of ions of sialic acid derivatives under collisional activation. In this case, the sequence of fragmentation may be established using an ion trap, whereas a QqToF instrument offers a possibility of revealing the elemental composition of fragment ions quickly and unequivocally.     

Fragmentation pathways of 2,3‐dimethyl‐2,3‐dinitrobutane cations in the gas phase

2,3‐Dimethyl‐2,3‐dinitrobutane (DMNB) is an explosive taggant added to plastic explosives during manufacture making them more susceptible to vapour‐phase detection systems. In this study, the formation and detection of gas‐phase [M+H]⁺, [M+Li]⁺, [M+NH₄]⁺ and [M+Na]⁺ adducts of DMNB was achieved using electrospray ionisation on a triple quadrupole mass spectrometer. The [M+H]⁺ ion abundance was found to have a strong dependence on ion source temperature, decreasing markedly at source temperatures above 50°C. In contrast, the [M+Na]⁺ ion demonstrated increasing ion abundance at source temperatures up to 105°C. The relative susceptibility of DMNB adduct ions toward dissociation was investigated by collision‐induced dissociation. Probable structures of product ions and mechanisms for unimolecular dissociation have been inferred based on fragmentation patterns from tandem mass (MS/MS) spectra of source‐formed ions of normal and isotopically labelled DMNB, and quantum chemical calculations. Both thermal and collisional activation studies suggest that the [M+Na]⁺ adduct ions are significantly more stable toward dissociation than their protonated analogues and, as a consequence, the former provide attractive targets for detection by contemporary rapid screening methods such as desorption electrospray ionisation mass spectrometry. Copyright © 2009 Commonwealth of Australia. Published by John Wiley & Sons, Ltd.     

Photodissociation of the dimer ions Ar+2, Ne+2, and (CO2)+2

The tandem quadrupole photodissociation mass spectrometer has been used to study photodissociation reactions of Ar⁺₂, Ne⁺₂, and (CO₂)⁺₂. The cross sections for photodissociation of Ar⁺₂ exhibited a strong dependence on ion source pressure, varying from 2 × 10 ^?18cm² at 0.1 torr to 6 × 10^?19cm² at 0.5 torr. A large photodissociation cross section (2 × 10^?17cm² for the reaction (CO₂)⁺₂ → CO⁺₂+ CO₂ was observed at the red end of the visible spectrum (580–620 nm) suggesting that this may be an important reaction in CO₂ rich planetary ionspheres such as that of Mars.     

Identification of Fe‐polycarboxylic complexes by electrospray ionization mass spectrometry and reduction of interferences by ion chromatography/inductively coupled plasma mass spectrometry with an octopole reaction system

Stable complexes are required during the ion chromatographic (IC) separation of Fe‐polycarboxylic acid complexes. Electrospray ionization mass spectrometry (ESI‐MS) was used to identify 1:1 stoichiometric complexes of Fe[HEDTA], Fe[EDTA]^1? and Fe[DTPA]^2?, and the spectra showed that these Fe complexes were stable in solution. Furthermore, inductively coupled plasma mass spectrometry (ICP‐MS) using an octopole reaction system (ORS) reduced polyatomic ion ⁴⁰Ar¹⁶O⁺ interference in the detection of ⁵⁶Fe via the addition of either H₂ or He to the ORS, with He at a flow rate 3.5 mL min^?1 being the optimum collision gas. Finally, IC/ICP‐MS was used for the separation and detection of Fe complexes with an eluent containing 30 mM (NH₄)₂HPO₄ at pH 8.0, but only Fe[HEDTA], Fe[EDTA]^1? and Fe[DTPA]^2? were observed within 10 min with reasonable resolution. Detection limits in the range of 10–13 µg L^?1 were achieved using He as the collision gas. The proposed method was used for the determination of Fe species in soil solutions. Copyright © 2010 John Wiley & Sons, Ltd.     

Trapped holes in magic number ion clusters

Observations on the relative intensities of Ar_n·I⁺ and Ar_n·I₂⁺ clusters suggest that stable ion cluster configurations arise from the presence of low ionization potential sites which trap the positive charge.     

Application of a hexapole collision and reaction cell in ICP-MS Part I: Instrumental aspects and operational optimization

The application of an ion-guiding buffer gas-filled hexapole collision and reaction cell in ICP-MS has been studied in order to give a preliminary performance characterization of a new instrument providing this feature for increasing the ion yield and decreasing contributions from Ar induced interfering molecular ions. As buffer gas He was used while H₂ served as reaction gas. Addition of the latter can be an effective means for reduction of typical argon induced polyatomic ions (Ar⁺, ArO⁺, Ar₂ ⁺) by orders of magnitude owing to gas phase reactions. Molecular interferences generated in the cell can be suppressed by a retarding electric field established by a dc hexapole bias potential of –2 V. Received: 10 May 1999 / Revised: 4 June 1999 / Accepted: 12 June 1999     

Quantitative images of metals in plant tissues measured by laser ablation inductively coupled plasma mass spectrometry

Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) was used for quantitative imaging of toxic and essential elements in thin sections (thickness of 30 or 40 μm) of tobacco plant tissues. Two-dimensional images of Mg, Fe, Mn, Zn, Cu, Cd, Rh, Pt and Pb in leaves, shoots and roots of tobacco were produced. Sections of the plant tissues (fixed onto glass slides) were scanned by a focused beam of a Nd:YAG laser in a laser ablation chamber. The ablated material was transported with argon as carrier gas to the ICP ion source at a quadrupole ICP-MS instrument. Ion intensities of the investigated elements were measured together with ¹³C⁺, ³³S⁺ and ³⁴S⁺ within the entire plant tissue section. Matrix matching standards (prepared using powder of dried tobacco leaves) were used to constitute calibration curves, whereas the regression coefficient of the attained calibration curves was typically 0.99. The variability of LA-ICP-MS process, sample heterogeneity and water content in the sample were corrected by using ¹³C⁺ as internal standard. Quantitative imaging of the selected elements revealed their inhomogeneous distribution in leaves, shoots and roots.     

Selective chemical ionization of nitrogen and sulfur heterocycles in petroleum fractions by ion trap mass spectrometry

A procedure is reported for the selective ammonia chemical ionization of some nitrogen and sulfur heterocycles in petroleum fractions using ion trap mass spectrometry (ITMS). The ion trap scan routine is designed to optimize the population of ammonium reagent ions and eject from the trap (by radio frequency/direct current isolation) electron ionization products formed during reagent ion formation prior to ionization of the sample. The ITMS procedure is compared with standard ion trap detector and conventional quadrupole ammonia chemical ionization for the determination of nitrogen and sulfur heterocycles in gas oil and kerosine samples. Greatly enhanced selectivity is shown for the ITMS procedure by suppression of competing charge-exchange processes.