Geometry Mass Spectrometer Research Articles

Rotation of focal plane of magnetic analyzer in an isotope ratio mass spectrometer using curved shims

The inclination of focal plane from beam axis of the magnetic sector analyzer in an isotopic ratio mass spectrometer is an important design feature. For a stigmatic geometry mass spectrometer, the focal plane is normally inclined by an angle of ∼25° with respect to the principal trajectory which presents various complications in case the number of collectors is more than five. An attempt has been made by us to rotate this plane along a line normal to the principal trajectory (by using shims on exit side of magnetic analyzer) resulting in ease of design for the collector system. This paper presents theoretical explanation, computer simulation and experimental results on this development. Simulated results on optimum shim curvature are in good agreement with experimental data.

Characterization of an argon microwave plasma torch coupled to a Mattauch–Herzog geometry mass spectrometer

An atmospheric pressure argon microwave plasma torch (MPT) ionization source was coupled to a Mattauch–Herzog +mass spectrometer (MHMS) for elemental analysis. The operating conditions of the Ar MPT-MHMS were optimized for signal level, peak shape and resolution. The background spectrum was characterized, with most peaks arising from species that contain argon and nitrogen. Detection limits were in the tens to hundreds of pptr range while sensitivities are in the 106 counts s−1 ppm−1 regime for most elements. The major weakness of the Ar MPT-MHMS was isotope-ratio accuracy and precision.

A novel geometry mass spectrometer, the Q-TOF, for low-femtomole/attomole-range biopolymer sequencing.

Ultra-high-sensitivity, biopolymer sequencing is a goal in many fields of molecular biology, and collisionally activated decomposition electrospray mass spectrometry (CAD ES MS/MS) using a triple quadrupole mass spectrometer has become a method of choice for work in the high- to mid-femtomole range. However, when the detection of ions becomes statistical, as it may in that range, the mass assignment of fragment ions is inaccurate and either sequencing becomes impossible or ambiguities result due, for example, to the closeness in amino acid residue masses (I/L, N or K/Q, E). Some ambiguities may be resolved by synthesizing possible sequences, but this is unsatisfactory. In considering the limitations of triple quadrupole MS/MS with respect to scanning ion detection, resolution, transmission, and mass accuracy, we reasoned that a novel geometry quadrupole orthogonal acceleration time-of-flight (Q-TOF) instrument would have special merit for ultra-high-sensitivity MS/MS sequencing, and suggested its construction for this purpose some three years ago. A prototype Q-TOF has now been built by Micromass [Morris et al. (1996), Rapid Commun. Mass Spectrom. 10, 889-896], and in the first research on the instrument, including MHC antigen and filarial nematode glycoprotein studies, we demonstrate low-femtomole- and attomole-range sequencing with mass accuracy of better than 0.1 Da throughout the daughter-ion spectrum, thus removing sequencing ambiguities in some of the most challenging work demanding the highest sensitivity.

A study of acceleration region processes in a double-focusing mass spectrometer. Part 1

The ion beam issuing from the Nier-type source of a conventional sector mass spectrometer is subjected immediately to an accelerating field of the order of 10 4 V cm −1. A variety of processes may occur to a given ion during its flight through the acceleration region, each giving rise to a continuum of mass spectral signals in momentum-to-charge—energy-to-charge space. Ion signal intensities are 10 −5 to 10 −6 of that of the main beam and are not observed in conventional electron ionization mass spectra; however, such peaks can make significant contributions to ion kinetic energy spectra and are often dismissed as “interference peaks” or “artifact peaks”. A description is presented here of the mathematical considerations governing acceleration region processes, including charge permutation reactions; in addition, acceleration region processes are portrayed on an ion incidence map. Experimental results are presented of the investigation of acceleration region processes which occur in the Argon system in a commercial reverse geometry mass spectrometer. The interpretation of some anomalous ion signals observed during this work has been facilitated by computer simulation.

Structure determination of ceramides and neutral glycosphingolipids by collisional activation of [M + Li] + ions

N eutral glycosphingolipids are a general class of biological molecules that are important components of cell membranes. Their basic structure (1) comprises one or more neutral sugar residues attached to a ceramide residue. The ceramide group (R = H) is a long-chain sphingoid base, such as (4E)-sphingenine [R’ = CH=CH(CH,),CH,] or sphinganine [R’ = (CH,),CHJ, that is substituted at the amino group by a fatty acyl group (R” = alkyl chain). If the R group were a simple sugar, the glycosphingolipid would be called a cerebroside or a 1-@-glycosylceramide [ 11. The structure of the glycosyl group can vary, as can the structures of both the sphingoid base and N-a@ chain. Thus, easy but complete structural elucidation of neutral glycosphingolipids has been a challenge in biochemistry, and mass spectrometry has played an important role. Derivatization and gas chromatography/mass spectrometry have been used to separately characterize the sugar, ceramide, sphingoid base, and fatty acyl components [2, 31. Egge and Peter-KataliniC [4] reviewed the many uses of fast atom bombardment (FAB) of [M + HI+, [M + Na]+, and [M HIions for structure elucidation of different types of glycosphingolipids. Of more recent application is the combination of collision-induced dissociation (CID) of FAB-desorbed [M + HI+, [M HI-, and their fragment ions 15-71. Costello and Vath [6] have contributed significantly in this latter area and have recently reviewed their work. As an alternative to using CID of [M + H]+ and [M HIions, we have been exploring the use of alkali [&lo], and alkaline earth [ll], metal ions as adducts to enhance structural determination upon CID. Here, we report our initial results of applying CID of [M + Li]+ ions to the structural determination of six ceramides and 10 neutral glycosphingolipids. All spectra were acquired by using a VG Instruments (Manchester, UK) 70-S forward geometry mass spectrometer and B/E linked scans. Precursor ions were

Interannular proton exchange and fragmentation of carbonyl‐protonated benzophenones

AbstractBenzopbenones a initially protonated at the carbonyl group were prepared by electron‐impact induced dissocation of 1,1‐diphenylpropanols (compounds 1‐5). These protonated ketones decompose in the ion source and the second field‐free region of a reversed geometry mass spectrometer by proton migration to one of the phenyl groups and subsequent elimination of benzene. In the case of derivatives substituted by methoxy groups and trifluoromethyl groups respectively, the proton migrates predominantly to the more bask benzene ring, resulting in the elimination of anisole in the former case and of benzene in die latter case. A study of protonated benzopbenones labelled at the phenyl ring and at the carbonyl group shows that only a few interannular H/D exchange steps precede the fragmentation. This is observed not only for metastable ions in the magnetic sector instilment but also for ions of long lifetimes investigated by Fourier‐transform‐ion cyclotron resonance (FT‐ICR) Spectrometry. This is in contrast to the arene elimination from protonated 1,ω‐dipbenylalkanes and related polyphenylalkanes which fragment by complete positional exchange of all hydrogen atoms at the aromatic rings. The special behaviour of protonated benzophenones is attributed to a low barrier for the decomposition of a chemically activated arenium ion b, which arises from the initial proton transfer. Once b is formed, it decomposes quickly without much interannular proton exchange.

Collision-induced dissociations and B/E linked scans for structural determination of modified fatty acid esters

Structures of modified fatty acid esters are determined by collisionally activating [M + Li] + ions desorptively ionized by fast atom bombardment (FAB). Collision-induced dissociations (CIDs) that occur in the first field-free region of a normal geometry mass spectrometer are observed by using B/E linked scans. The CID spectra provide immediate information about chain length, location and type of substituent in saturated, unsaturated, methyl-branched and hydroxy esters. New spectra of more unusual ethyl esters that contain longer alkyl branches and keto substituents also indicate the location and type of substituent. Novel charge-remote reactions that are directly analogous to thermolytic hydro-acyloxy eliminations and heteroatom analogs of the retro-ene reaction are suggested to explain the formation of some fragment ions from the alkyl and keto esters. Deuterium labeling provides evidence for mechanisms for structurally informative charge-mediated fragmentations of the hydroxy esters. Individual components of complex mixtures can be determined, and the detection limit for the structural determination of methyl palmitate is 25 ng. Methods also are described for optimizing the practical use of FAB in conjunction with B/E linked scans, which can be hampered by poor precursor ion resolution.

On the structure, reactivity and relative stability of the large carbon cluster ions C+62, C+60 and C+58

The exclusive metastable loss of C2 from C+62, C+60 and C+58 generated by laser ablation from graphite is studied using a high resolution reverse geometry mass spectrometer. Kinetic energy release distributions are peaked at low energies (≈0.2 eV) and fall off quasi-exponentially at higher energies. Successful modeling by statistical phase space theory indicates no reverse activation energy for C2-loss. A molten droplet of spheroidal geometry for the structure of these ions best fits the data. The modeling also indicates C2 is bound to C+62 by 3.0 eV and to C+60 and C+58 by ≈4.6 eV. Electronic rather than structural effects are suggested as the cause of the instability of C+62 relative to C+60 and C+58. Kinetic effects that reflect the instability of C+62 and to a lesser extent the stability of C+60 are suggested as the principal reasons why C+60 can be a dominant feature in the mass spectrum under favorable experimental conditions.

Deconvolution of kinetic‐energy release signals from singly, doubly, and triply charged metastable ions

AbstractA new geometric model has been developed and applied to the deconvolution of ion kinetic‐energy release signals observed from fragmentation of metastable ions in the second field‐free region of a reversed geometry mass spectrometer. The model takes account of: (i), fragmentations occurring throughout the entire length of the field‐free region; (ii), the difference in ion beam intensity before and after the intermediate slit; (iii), the slight difference in angular acceptance between backward‐ and forward‐scattered product ions; (iv), fragmentations of singly and multiply charged ions; and (v), ion kinetic‐energy releases of any magnitude. Several examples of metastable peaks are given together with the associated kinetic‐energy release distribution and the theoretical metastable peak generated from the kinetic‐energy release distribution.

Collision‐energy ramp. A modification to an RF‐only quadrupole collision cell

AbstractA modification to the electronics controlling the collision energy of an RF‐only quadrupole collision cell in a BEqQ geometry mass spectrometer is described. The modification allows the collision energy to be ramped from 0–500 eV while scanning the quadrupole mass analyser. The collision‐induced dissociation spectra generated using a ramped collision energy exhibit a 10%–100% increase in product‐ion abundance.

The determination of collisionally induced consecutive dissociations of ions by collision cell potential dependence

Consecutive two-step dissociations of parent ions inside and after a collision cell in the second field-free region of a reversed geometry mass spectrometer to give grand-daughter ions can be detected if a potential is applied to the collision cell. A quantitative method to assign the masses of the ions involved in the consecutive dissociations, based on the unique collision cell potential dependences of the kinetic energies of the grand-daughter ions formed, is presented. The practical utility of the method is examined for a parent anion and a parent cation. It is proven that in both cases consecutive dissociations occur which provide important information about the dissociation patterns of these parent ions. A brief comparison of the capability of the method to detect, distinguish and determine consecutive dissociations with that applied in three-sector instruments is also given. It is suggested that the method can yield additional information if only molecular or pseudo-molecular ions are generated in the ion source.

A new method for studying carbon clusters in the gas phase: Observation of size specific neutral fragment loss from metastable reactions of mass selected C+n, n≤60

A new method for studying the generation and structure of carbon cluster ions is described. A rotating, translating carbon rod is subjected to the focused output of an excimer laser operating at 308 nm. The resulting plume is directly sampled by a high resolution reverse geometry mass spectrometer operating between 2 and 8 kV. In this paper we report initial results for the positive cluster ions up to n=60. The mass spectra are bimodal with a minimum occurring between C+26 and C+31 in agreement with findings of other workers using related techniques. Metastable neutral loss is observed for all cluster ions with n≥5. Below C+30 the most intense neutral loss is C3 but intense C1 and C5 loss is observed for a significant number of systems and C10 and C14 for a few systems. Above C+30 the metastable loss spectra dramatically change and only C2, and a minor amount of C4 loss, is observed. The C+60 ion is slightly enhanced in the mass spectrum but its metastable reactivity is no different than any of the other C+n ions above n=30. The detailed results are interpreted in terms of stable structures of both the parent ions and neutral fragments that have been predicted theoretically.

A data acquisition system for mass‐analyzed ion kinetic energy spectra using a personal computer

AbstractAn inexpensive data acquisition system has been developed for mass‐analyzed kinetic energy experiments which involve scanning the electrostatic analyzer of a reversed geometry mass spectrometer. The various hardware and software design features are described. Results for data obtained with a commercially available VG ZAB‐2F mass spectrometer are presented and discussed.

The Measurement of Leucine Enkephalinat the Femtomole Level by Fast Atom Bombardment Mass Spectrometry/Mass Spectrometry Methods

Abstract FAB-MS/MS methods are used to quantify the neuropeptide leucine enkephalin (LE = YGGFL) in synthetic solutions. Maximum molecular specificity is provided by monitoring two metastable transitions from the LE (M + H)+, 556 → 425 and 556 → 336, in a forward geometry (E, B) mass spectrometer using a B/E linked-field selected reaction monitoring technique. Obtained sensitivity is 40 pg LE, which equals 72 fmol. The statistics of the best-fit straight lines are, for m/z 425: y = 34 + 166 (r = 0.999), and for m/z 336: y = 2.5x + 17.2 (r = 0.996).

Gas-phase thermochemical information from triple quadrupole mass spectrometers: Relative proton affinities of amines

Proton-bound dimers of amines fragment to give the individual protonated bases in proportions which depend upon the proton affinities of the amines, the collision energy, and the target gas thickness in a triple quadrupole mass spectrometer. The behavior of these dimers observed with the triple quadrupole is similar to that observed with a reversed geometry mass spectrometer suggesting that the kinetic method of assigning amine proton affinities is applicable to the triple quadrupole. This is shown explicitly. Furthermore, the effect of precursor-ion internal energy on this measurement is delineated through MS/MS experiments covering excitation through collisions at various energies and target pressures. The effect of varying target thickness and collision energy are shown to be small when structurally similar amines comprise the dimer, but in the cases of dimers of phenylethylamine and pyridine with primary alkylamine partners, differences between literature values of proton affinities and the measured gas-phase basicities are observed. These entropic effects are interpreted on the basis of the relative steric hindrance to protonation of phenylethylamine (PEA), pyridine (PYR) and the primary alkylamines (ALK), that is, the order of steric hindrance appears to be PEA > ALK > PYR. For the primary alkylamine dimers, entropic effects appear to cancel within experimental error. Using data taken under various collision conditions, the proton affinity of 3-aminopentane is assigned as 221.7±0.3 kcal mol −1.

Improved performance in angle-resolved mass spectrometry

A new experimental arrangement for angle-resolved mass spectrometry is described featuring a needle which delivers collision gas to the source slit of a reversed geometry mass spectrometer. In this experiment the scattering angle is defined by pre-collision electrical deflection of the parent ion beam, as contrasted with post-collision selection of the direction of the daughter ion beam. Advantages are rapid access to any desired angle, coupled with an improvement in ion intensity and in angular resolution evident from measured breakdown curves. Collision-induced dissociation spectra of the methanol molecular ion, recorded over a range of scattering angles, agree well with previous results. Experiments which record ions that survive collisions, i.e., scatter without fragmentation, indicate that scattering of polyatomic ions through substantial angles (0–6°) makes a significant contribution to the phenomena underlying angle-resolved mass spectrometry.

Studies of consecutive reactions of quinones in a reversed geometry mass spectrometer

AbstractConsecutive reactions involving the ejection of molecules of carbon monoxide from quinone systems have been studied. Using a reversed geometry double focusing mass spectrometer, the individual steps of a consecutive reaction may be separated into different field free regions. The structure of ions formed in the ion source may be compared with those formed from the fragmentation of metastable ions in a field free region by studying differences in the pattern of translational energies released when they break. In the case of anthraquinone it is found that two structures exist for the [MCO]+˙ ion. Critical energy measurements strengthen this argument. Furthermore, 18O labelling has been used to distinguish between the loss of carbon monoxide from different positions on the molecular ion of 1‐hydroxyanthraquinone.

Studies of consecutive reactions of organic ions in a reversed geometry mass spectrometer

AbstractOver thirty consecutive reactions of a type m1+ → m2+ → m3+ have been studied in a variety of organic compounds using a reversed geometry mass spectrometer. Two field free regions allow the separation of the two steps that make up the consecutive reaction sequence. Translational energy release measurements are used in the comparison of m2+ ions formed as a result of a high energy process in the ion source with m2+ ions formed as a product of a metastable decomposition. In some cases the structures of such ions have been found to be different. Examples have also been found where consecutive fragmentations of metastable ions do not occur although, when higher energy ions within the source are studied, the two‐step reaction does take place. Furthermore, it has been found that a control over ion internal energy may be achieved by selecting portions of a peak due to the fragmentation of a metastable ion. Unimolecular reactions may then be used to study the reactivity of such ‘energy‐selected’ ions; collision induced reactions can be used to study the structure of both the reactive and unreactive energy selected ions.

New methods of identifying organic compounds

A 'reversed' geometry mass spectrometer, in which the ion beam passes through the magnetic sector before the electric sector, offers several advantages for the study of large organic molecules. The method used is to select individual ionic species formed in the ion source in turn by using the magnet and to study the fragmentation of these species in the field-free region in front of the electric sector. Either unimolecular or collision-induced fragmentations can be investigated, the masses of the daughter species being determined by scanning the electric sector. By selecting a variety of individual ions, a comprehensive fragmentation ‘ map' of the molecular species can be constructed. Because it is a voltage that is scanned, the instrument can readily be computer controlled which gives improved reproducibility of scanning, together with other advantages. The several pathways that often link a particular fragment ion with the molecular ion provide complementary information concerning ion structure. The fragmentation pattern of any ion is often sufficiently characteristic of the ion structure to allow direct identification of structural features present to be made by comparing the pattern from the relevant ion with that of an ion formed from a known reference compound. By using these methods the molecular structure of large organic molecules can often be deduced. Large isomeric molecules such as steroids, differing only in the structure of a side chain, can be distinguished by selecting only ions containing the side chain for study. The new methods also offer advantages for the detection and identification of individual components in mixtures.

Observation De Fragmentations De Seconde Région Libre De Champ D'Un Spectromètre De Masse à Géometrie Inversée Lors Du Balayage De La Tension D'Accélération

AbstractIn a reversed geometry mass spectrometer first field free region fragmentations are usually detected by scanning the accelerating voltage. In some favourable instances, artifacts corresponding to second field free region processes are observed; the influence of a collision gas on their relative intensities is discussed.