Fourier transform ion cyclotron resonance mass spectrometry: a primer

This review offers an introduction to the principles and generic applications of FT-ICR mass spectrometry, directed to readers with no prior experience with the technique. We are able to explain the fundamental FT-ICR phenomena from a simplified theoretical treatment of ion behavior in idealized magnetic and electric fields. The effects of trapping voltage, trap size and shape, and other nonidealities are manifested mainly as perturbations that preserve the idealized ion behavior modified by appropriate numerical correction factors. Topics include: effect of ion mass, charge, magnetic field, and trapping voltage on ion cyclotron frequency; excitation and detection of ICR signals; mass calibration; mass resolving power and mass accuracy; upper mass limit(s); dynamic range; detection limit, strategies for mass and energy selection for MSn; ion axialization, cooling, and remeasurement; and means for guiding externally formed ions into the ion trap. The relation of FT-ICR MS to other types of Fourier transform spectroscopy and to the Paul (quadrupole) ion trap is described. The article concludes with selected applications, an appendix listing accurate fundamental constants needed for ultrahigh-precision analysis, and an annotated list of selected reviews and primary source publications that describe in further detail various FT-ICR MS techniques and applications.     

An external source 7 T Fourier transform ion cyclotron resonance mass spectrometer with electrostatic ion guide

An external source 7 T Fourier transform ion cyclotron resonance (FTICR) mass spectrometer offers three main novel features. First, a 9-way ion-source cross allows for mounting of up to three ionization sources simultaneously, thereby minimizing 'downtime' for changing ion sources. Second, an electrostatic (wire-in-cylinder) ion guide transports the ions approximately 1.5 m from the ion source to the ion trap for mass analysis, through a large magnetic field gradient. Third, the system operates from a modular data system described elsewhere in this issue. Luteinizing hormone-releasing hormone (LH-RH) matrix-assisted laser desorption/ionization (MALDI) FTICR positive-ion mass spectra exhibit signal-to-noise ratio greater than 1000:1 and mass resolving power, m/delta m 50% > 100,000. Laser-induced fragmentation of bradykinin demonstrates the ability of the ion guide to transmit both molecular and fragment ions simultaneously. Ultra-high resolution (average resolving power approximately 400,000) was achieved for poly(ethylene glycol) of specified number-average molecular weight, Mn approximately 3400. Future installation of an electrospray source to the ion-source cross should allow for better characterization of the performance of the ion guide.     

Enhanced mass resolving power, sensitivity, and selectivity in laser desorption Fourier transform ion cyclotron resonance mass spectrometry by ion axialization and cooling

Ion cooling and axialization produced by azimuthal quadrupolar excitation in the presence of ion-neutral collisions are applied to laser desorption Fourier transform ion cyclotron resonance mass spectrometry (LD/FT/ICR-MS). With this technique, the large kinetic and internal energies of ions generated by laser desorption processes can be cooled effectively by collisions of ions with neutral argon atoms (at > 5 x 10(-7) Torr). After sufficient cooling in the source compartment of a dual ion trap, the axialized ions may be transferred to the analyzer compartment for detection at much lower pressure (and thus much higher mass resolving power). Enhancements in both FT/ICR mass resolving power and sensitivity are observed; moreover, ion isolation with high selectivity at high pressure is also demonstrated.     

Counting individual sulfur atoms in a protein by ultrahigh-resolution Fourier transform ion cyclotron resonance mass spectrometry: experimental resolution of isotopic fine structure in proteins

A typical molecular ion mass spectrum consists of a sum of signals from species of various possible isotopic compositions. Only the monoisotopic peak (e.g., all carbons are 12C; all nitrogens are 14N, etc.) has a unique elemental composition. Every other isotope peak at approximately integer multiples of approximately 1 Da higher in nominal mass represents a sum of contributions from isotope combinations differing by a few mDa (e.g., two 13C vs. two 15N vs. one 13C and one 15N vs. 34S, vs. 18O, etc., at approximately 2 Da higher in mass than the monoisotopic mass). At sufficiently high mass resolving power, each of these nominal-mass peaks resolves into its isotopic fine structure. Here, we report resolution of the isotopic fine structure of proteins up to 15.8 kDa (isotopic 13C,15N doubly depleted tumor suppressor protein, p16), made possible by electrospray ionization followed by ultrahigh-resolution Fourier transform ion cyclotron resonance mass analysis at 9.4 tesla. Further, a resolving power of m/Deltam50% approximately 8,000,000 has been achieved on bovine ubiquitin (8.6 kDa). These results represent a 10-fold increase in the highest mass at which isotopic fine structure previously had been observed. Finally, because isotopic fine structure reveals elemental composition directly, it can be used to confirm or determine molecular formula. For p16, for example, we were able to determine (5.1 +/- 0.3) the correct number (five) of sulfur atoms solely from the abundance ratio of the resolved 34S peak to the monoisotopic peak.     

Proton-decoupled 31P chemical shift imaging of the human brain in normal volunteers

Proton-decoupled, 31P three-dimensional (3-D) chemical shift imaging (CSI) spectra have been acquired from the entire human brain using a new dual tuned resonator. The resonator operates in quadrature mode to provide improved sensitivity, excellent B1 homogeneity and reduced power deposition at both frequencies. Proton-decoupled and fully NOE enhanced, 31P spectra were acquired from normal volunteers using Waltz-4 proton decoupling with continuous wave bi-level excitation applied through a second radio frequency channel. Well resolved peaks in the phosphomonoester (PME) and phosphodiester regions were obtained from nonlocalized FIDs and spectra localized with 3-D CSI without processing for resolution enhancement. pH measurements made over large regions of the brain using the P(i) resonance show no significant variations (6.9 +/- 0.02) for a single individual. The improved spectral resolution and sensitivity of the PME resonances results in more well defined metabolite images of the PME peak region.     

Identification of pollutants in a municipal well using high resolution mass spectrometry

An elevated incidence of childhood cancer was observed near a contaminated site. Trace amounts of several isomeric compounds were detected by gas chromatography/mass spectrometry (GC/MS) in a concentrated extract of municipal well water. No matching library mass spectra were found and Fourier transform IR and NMR analyses were not feasible due to the low concentration of the compounds. Mass peak profiling from selected-ion-recording data (MPPSIRD) provided the sensitivity and scan speed necessary to acquire mass peak profiles at mass resolutions of 10,000 to 20,000 for the molecular ion (M+) and 10 fragment ions as capillary GC peaks eluted. Using a profile generation model (PGM), the elemental composition of the molecular ion was determined from the exact masses and abundances of the M, M + 1 and M + 2 profiles. Fragment ion compositions were determined from their exact masses based on the elements in the molecular ion. Exact mass differences between the molecular and fragment ions corresponded to unique combinations of atoms for the neutral losses. Consequent reduction of the number of possible structures for the fragment ions simplified mass spectral interpretation. After inspecting library mass spectra for smaller molecules, isomeric structures were hypothesized with cyano and alkylcyano groups attached to tetralin. A literature search found such isomers produced by an industrial polymer synthesis. Three isomers in a standard form polymerization of styrene and acrylonitrile provided the same mass spectra and GC retention times as isomers in the extract.     

Study of mechanisms involved in the collision-induced dissociation of carboxylate anions from glycerophospholipids using negative ion electrospray tandem quadrupole mass spectrometry

The collision-induced dissociation of the carboxylate anions from human blood phosphatdilycholine (PC), phosphatidylethanolamine (PE), phosphatidylserine (PS), phosphatidylinositol (PI) and phosphatidic acid (PA) containing the C18:0 (sn-1) and C20:4 (sn-2) fatty acyl residues was studied using normal phase liquid chromatography coupled with negative ion electrospray tandem mass spectrometry. The product ion peak area ratio of C18:0 to C20:4 was calculated for each phospholipid species and was found to increase with increasing collision energy for all classes. For the phospholipids with a net neutral charge (PE, PC) there was a preferential loss of the sn-2 carboxylate anion (C20:4) at low collision energy, while at higher energy there was a preferential loss of the sn-1 carboxylate anion (C18:0). For the phospholipids with a net negative charge (PI, PA, PS) the intensity of the sn-1 carboxylate anion peak was equal to or higher than the sn-2 carboxylate anion peak at the energies measured. At a given collision energy the product ion peak area ratio decreased in the order PA > or = PS > PI. Studying PS and PE species at different collision energies, it was found that for both classes the increase in the abundance ratio with increasing collision energy was largely dependent on the chain length and degree of unsaturation of the sn-2 acyl chain.     

Application of capillary zone electrophoresis in cephalosporin analysis

Cephalosporins have structures and antibiotic activity similar to those of penicillins which represent a class of compounds with closely related structures. Most of the cephalosporins contain aromatic groups and show distinctive UV spectra. Separating the different types of cephalosporins is a challenging task for HPLC. but the resolving power of capillary zone electrophoresis (CZE) makes this separation fast and simple. The present study reports the application of CZE for cephalosporin analysis and the separation of cephalosporins from plasma. Both field strength and temperature were shown to influence the plate number. The influence of injection time on the peak height was studied. Furthermore, the influence of pH value on the separation of cephalosporins by CZE was investigated. The low sample amount required and the relatively short analysis time are the main advantages of this method.     

Luminescence Properties of Green-Emitting Phosphor (Ba1-x,Srx)2SiO4:Eu2+ for White LEDS

The (Ba1-xSrx)2SiO4∶Eu2 green-emitting phosphors were synthesized by conventional solid-state reaction in a CO-reductive atmosphere, and their luminescent properties were investigated. The XRD data show that the Ba/Sr ratio not only affects the lattice parameters, but also influences the emission peak. The excitation spectra indicate that this phosphor can be effectively excited by UV light from 370 to 470 nm. The emission band is due to the 4f65d1→4f7 transition of the Eu2 ion. With an increase in x, the emission band shifts to longer wavelength and the reason was discussed. The emission spectra exhibit a satisfactory green performance under different excitation wavelength(380, 398, 412, 420, 460 nm). (Ba1-xSrx)2SiO4∶Eu2 is a promising phosphor for green white-lighting-emission diode by ultraviolet chip.     

Red-Emitting Ga2O3:Eu3+ Powder Derived by Sol-Gel Processing

Ga₂O₃: Eu³⁺ Powder was synthesized by sol-gel method with inorganic salt raw materials, and the crystal structure and luminescent properties were investigated. The structures studied by X-ray diffractionmeter showed that single monoclinism could be obtained using this simple processing. Along with the rise of the calcining temperature, the crystallinity of Ga₂O₃:Eu³⁺ powder increased and the particle size grew larger. The excitation spectra of the powder showed a wide CTS band and typical excitation from Eu³⁺ ion, while the emission spectra had a strongest red emission peak at 613 nm. The excitation and emission intensities changed along with the calcining temperature from 650 to 1050 °C and had an optimal calcining temperature. The excitation and emission intensities also changed along with the doping concentration of Eu³⁺ ion from 0.25% to 4% (atom fraction) and achieve a maximum at a certain concentration which differed for direct Eu³⁺ excitation or CTS excitation. The electroluminescent spectra indicated the typical red emission from Eu³⁺ ion.     

A spectrometric method for determining the capability of intermolecular interaction: mono- and dideoxynucleotides

A method for detecting the dimer of mono- and oligodeoxynucleotides is presented by using fast atom bombardment mass spectrometry (FAB-MS). The mass spectrum of a mononucleotide dpC showed an intense peak of the dimeric ion dpC:::dpC, compared with those of dpA, dpG and dpT. The mass spectrum of an equimolar mixture of dpC and dpG showed the highest peak of the dimeric ion dpC:::dpG, compared with all other combinations. The dimeric ion peaks in the mass spectra of dinucleotides d(ApT) and d(CpG) were relatively high and it may reflect the base-pairing energy due to hydrogen bonding. The degradation patterns of each dimers for d(ApT) and d(CpG) obtained by using the collision-induced dissociation (CID) technique which reflected the strength of each base-pairing.     

ESI/ion trap/ion mobility/time-of-flight mass spectrometry for rapid and sensitive analysis of biomolecular mixtures

An ion trap/ion mobility/time-of-flight mass spectrometry technique is shown to be a rapid and sensitive means of analyzing peptide/protein mixtures. In this approach, an ion trap is used to accumulate ions that have been electrosprayed from a mixture into concentrated packets. The ion packets are injected into a drift tube where components of the mixture are separated based on differences in mobility through a buffer gas. Ions that exit the drift tube are dispersed in a time-of-flight mass spectrometer for mass-to-charge (m/z) determination. The gas-phase separation strategy reduces congestion in the mass spectrum, and experimental mobilities complement m/z measurements in assigning peaks. Examples of the application of the approach to identification of peptides (from tryptic digests) and to separation of charge-state distributions from electrospray of a mixture containing ubiquitin and myoglobin are presented. Most peptides that are observed from tryptic digests of proteins such as cytochrome c and myoglobin can be identified from data that are acquired in under 1 min; studies of mixtures with known compositions indicate that detection limits are approximately 0.5-3 pmol for individual components. Factors that may influence the distributions that are observed, such as storage time in the trap, injection voltages used for the mobility experiment, and variations in ion cross section with charge state, are discussed.     

Remarkable fluorescence enhancement of upconversion composite film and its application on mercury sensing

To achieve a stable, sensitive, and high-efficiency biological probe, a novel NaYF_4:Yb,Er nanocrystals/TiO_2 inverse opal composite film was designed by self-assembly and solvent evaporation methods. 32-fold enhanced upconversion(UC) emission was investigated under 980 nm excitation. According to size-dependency, excitation power density-dependency as well as photonic stop band(PSB)-dependency upconversion spectra, the enhancement mechanism of the composite film was put down to the stochastical diffraction of IOPCs multi-layered structure to the excitation laser. On the basis of the enhancement effect of the composite film, energy transfer between upconversion nanoparticles(UCNPs) and quantum dots(QDs), and the sensitive sensing of CdTe QDs on mercury, the UC composite film was used for sensing of Hg~(2+) in serum. The solid sensor as a mercury detector owns lots of superiorities such as feasible operation, good linear relationship(R=0.997), low limit of detection(70.5 nmol/L) and thus may have broad prospects in the biosensing field.     

Calicheamicin derivatives conjugated to monoclonal antibodies: determination of loading values and distributions by infrared and UV matrix-assisted laser desorption/ionization mass spectrometry and electrospray ionization mass spectrometry

Calicheamicin derivatives (MW approximately 1500) and monoclonal antibodies (MoAbs) conjugated to calicheamicin derivatives (MW approximately 150,000) were analyzed by UV-MALDI/MS, IR-MALDI/MS, and ESI/MS. These materials are potent anticancer agents. Calicheamicin derivatives and conjugates rapidly degrade upon UV irradiation but are relatively stable during IR irradiation and under ESI conditions. A unique feature of IR-MALDI/MS is a 2 times enhancement in resolution relative to UV-MALDI/MS for masses above approximately 50,000 Da resulting in a molecular ion envelope containing a series of partially resolved peaks of the calicheamicin-MoAb conjugates. The mass shift difference between the peak maxima corresponded to the mass change due to the covalent addition of calicheamicin derivatives to the monoclonal antibody. The distribution of the calicheamicin derivatives in the monoclonal antibodies was computed by deconvoluting the partially resolved peak envelope. A unique feature of the ESI mass spectra, under unit resolution conditions, is that the distribution of the carbohydrates can be well resolved for pure MoAbs and can be only partially resolved for conjugated MoAbs. Average loading values for calicheamicia derivatives when conjugated to MoAbs were computed from UV-MALDI/MS, IR-MALDI/MS, and ESI/MS data and the results compared with the average loading values obtained by UV absorption spectrometry. Very low average loading values were computed from UV-MALDI/MS data due to the degradation of the conjugated calicheamicin derivatives during the UV irradiation process. The IR-MALDI/MS average loading values, obtained with glycerol as the matrix, were consistent with the UV absorption spectrometry values for conjugates having hydrolytically stable linkers, but not when the linker contained a hydrolytically labile hydrazone. ESI/MS average loading values were generally lower than the corresponding values obtained by IR-MALDI/MS. The average loading values and distributions obtained using IR-MALDI/MS were more reliable than the corresponding ESI/MS values because the partially resolved, singly and doubly charged peaks in the IR-MALDI spectra can be mathematically deconvoluted, while the overlapping, highly multiply charged peaks of the electrospray spectra can only be partially deconvoluted.     

CaMoO_4∶Eu~(3+),M~+(M=Li,Na,K)粉体的制备与发光性能研究

采用溶胶-凝胶法制备了Ca_(1-1.5x)MoO_4∶xEu~(3+)和Ca_(0.5)MoO_4∶0.25Eu~(3+),M~+(M=Li,Na,K)荧光粉,并对样品的物相结构、颗粒形貌及发光性能进行了分析。结果表明,样品属于四方晶系,颗粒接近八面体形状,大小为2~3μm。激发光谱显示,样品的激发中心分别位于364 nm、386 nm、396 nm、419 nm和466 nm,最大激发峰值位于396 nm。在396 nm近紫外光激发下,样品的发射中心分别位于596 nm、616 nm、656 nm、704 nm,特征发射峰为616 nm,Eu3+离子掺杂浓度为25%(体积分数)时发光强度最强,引入的3种电荷补偿剂M~+(M=Li,Na,K)中,Li+对发光强度的提高最为显著。     

Optical and magnetic properties of GaN epilayers implanted with ytterbium

We have studied the optical and magnetic properties of ytterbium implanted GaN epilayer grown on(0001)sapphire by metalorganic chemical vapor by deposition(MOCVD).Samples were implanted at room temperature with Yb ions at dose 4x1015 cm-2 and energy of 150 keV.The implanted samples were annealed at 1000 ℃ in N2 at atmospheric pressure to recover implantation damages.The photoluminescence (PL),PL excitation(PLE),and PL kinetics have been studied with continuous and pulse photo-excitations in 360-1100 nm spectral range at different temperatures.The characteristic Yb3+ion emission spectra were observed in the spectral range between 970-1050 nm.Theoretical fittings of the experimental PL temperature and PL kinetics data suggest that Yb3+ions are involved in at least two major luminescence centers.The PLE spectra indicate that excitation of the Yb3+ion occurs via electron-hole pair generation and complex processes.Magnetization versus magnetic field curves shows an enhancement of magnetic order for Yb-implanted samples in 5 K to 300 K temperature range.The Yb-implanted GaN sample showing weak ferromagnetic behavior was compared with the ferromagnetic in situ doped GaYbN material.     

Exploring infrared wavelength matrix-assisted laser desorption/ionization of proteins with delayed-extraction time-of-flight mass spectrometry

We report a study of the application of delayed extraction (DE) to infrared-wavelength matrix-assisted time-of-flight mass spectrometry (IR-MALDI-TOF-MS) of proteins. The shapes of the spectral peaks obtained with DE-IR-MALDI-MS are compared with those obtained from the same samples and matrix using continuous extraction (CE) IR-MALDI-MS. Application of DE results in significant improvements in the peak resolution, revealing spectral features (in proteins with molecular masses < 12 kDa) that were not resolved in the corresponding CE-IR-Maldi mass spectra. Particularly significant is a series of peaks on the high mass side of the protonated protein peaks that arise through replacement of protons by adventitious sodium ions in the sample. We deduced that these sodium replacement species are a significant contributor to the broad tails (and resulting peak asymmetries) that are a feature of the DE-IR-MALDI mass spectra of proteins with molecular masses > or = 17 kDa. The peak width reduction observed in IR-MALDI by DE suggests that, as in UV-MALDI, the initial velocity distribution for ions produced in the MALDI process contributes to the peak broadness in the CE mass spectra. In a systematic comparison between DE UV-MALDI and DE IR-MALDI, we determined that photochemical matrix adduction is present in UV-MALDI but absent in IR-MALDI. In addition, we find that protein ions produced by IR irradiation are less internally excited (i.e., cooler), exhibiting less fragmentation, more Na+ replacement and/or unspecified noncovalent adduction, and more heme adduction with apomyoglobin. Thus, IR-MALDI appears to be a softer means for producing gas-phase protein ions than is UV-MALDI. It will be of considerable practical interest to determine whether large protein ions produced by IR-MALDI are sufficiently cool to survive transport through reflecting TOF mass spectrometers (without loss of small neutral species such as H2O, NH3, and CO2) and the extended time periods required for detection by quadrupole ion trap and Fourier transform ion cyclotron resonance mass analyzers.     

Eu（Ⅲ） complexes involving 1,3,5-triazine diphosphine oxides

The 1,3,5-triazine diphosphine oxide ligands with donor-acceptor properties formed strong complexes with europium(III) ion in acetonitrile. Spectrophotometric titrations and mass spectra indicated that two ligands coordinated to one europium ion. The stability constants varied from 11.64 to 14.60 (log β). Binary complexes exhibited rather weak luminescence in solution. 1,3,5-triazine diphosphine oxides engaged as co-ligands in Eu(III) (2-thenoyltrifluoroacetonate)₃ complex contributed to the overall photoluminescence and allowed for excitation with longer wavelengths than the parent complex.     

Procedures for tandem mass spectrometry on an ion trap storage/reflectron time-of-flight mass spectrometer

In this work, we demonstrate tandem mass spectrometry on an ion trap storage-reflectron time-of-flight mass spectrometer (IT/reTOFMS). Ion isolation and activation were achieved by resonant excitation using multi- and single-frequency waveforms generated from an analog circuit. Product-ion spectra of small polypeptides are obtained, which are comparable in fragmentation to those acquired on sector or hybrid mass spectrometers. Several important parameters governing the tandem mass spectrometry process, including the activation tickle voltage, type of collision gas, activation period and cooling period after the fragmentation were optimized using leucine-enkephalin as a model. Although the limited energy deposition from collisional activation in our experiments does not allow efficient fragmentation of large singly charged polypeptides with m/z higher than 1000, the problem may be partially solved by taking advantage of fragmenting the multiply charged ions produced from the electrospray ionization source as demonstrated for a synthetic polypeptide of molecular weight 2782. Compared to the singly charged form, the reduced m/z of multiply charged forms experience a greater trapping force as described by the pseudopotential well-depth model. Increased pseudopotential well-depths for multiply charged species permit the use of greater fragmentation energy at lower RF potentials. These conditions facilitate the fragmentation of large polypeptides, yet are suitable for trapping singly charged fragments. These experiments indicate that the high efficiency associated with ion dissociation and fragment-ion collection in the trap and the storage capability for detection of ions using the non-scanning mode of the IT/ reTOF analyzer may provide an alternative means for acquiring sequence-specific information of polypeptides at low picomol levels of sensitivity.     

Rapid peptide mapping by reversed-phase liquid chromatography on nonporous silica with on-line electrospray time-of-flight mass spectrometry

Reversed-phase liquid chromatography (LC) using a nonporous silica support has been combined with electrospray (ES) time-of-flight (TOF) mass spectrometry (MS) for the fast separation and mass detection of peptides. Using this LC method, the resolution of a peptide mixture can be completed is less than 35 s. The resulting chromatographic peak widths are less than 1 s wide. Because of the unique nature of a TOF mass analyzer, complete mass spectra can be acquired at a rate which is sufficient to sample these narrow peaks. When compared with conventional LC, the same separation takes nearly 20 min to complete, and the signal-to-noise ratio observed in the total ion chromatogram is dramatically lower due to the influence of increased background noise in the mass spectra. The limit of detection for a low molecular weight peptide, Val-Pro-Leu, was found to be 6 pmol with the total ion chromatogram and 500 fmol with the reconstructed ion chromatogram. A peptide map of horse heart myoglobin, completed in 3.5 min, is shown as an example of the results which can be obtained from combining this fast LC method with fast ES/TOF/MS detection capability.