Field ionization mass spectrometry of carbohydrates

It is shown that FI mass spectra can serve to elucidate some structural features and determine the molecular weights of mono- and oligosaccharides.     

Temperature controller for thermal ionization mass spectrometry.

We describe the use of a simple voltage stabilizer that controls the filament temperature (T(f)) in the ion source of a thermal ionization mass spectrometer. The filament voltage (V(f)) is measured by means of a separate pair of wires connected inside of the ion source in parallel to the wires supplying power. It has been demonstrated that V(f) is directly proportional to T(f) in a wide range of filament temperature. The T(f) value is solely controlled by the reference voltage (V(r)) that can be manually selected from a voltage divider or by means of a computer. Digital signals from the computer in the form of a series of pulses are transmitted opto-electronically and subsequently converted to analog signals. The temperature controller described here was successfully applied for analysis of potassium concentration by the isotope dilution method.     

Field ionization mass spectrometry of higher n-alkenes

In a strong electric field the molecular ions of n-alkenes ≤C-12 decompose via cleavage of the C? C bond β to the double bond to form the characteristic alkenyl ions that may be used for the identification of positional isomers. For 3-alkenes (>C-10), 4-, 5- and 6-alkenes the formation of the ions with m/e 54 via double β-cleavage is typical. The field ionization mass spectra of the cis and trans isomers are indistinguishable.     

A linear time-of-flight mass analyzer for thermal ionization cavity mass spectrometry

In this paper we report the basic design characteristics, typical operating parameters, and isotope ratio performance of an orthogonal acceleration linear thermal ionization cavity time-of-flight mass spectrometer (TIC-TOFMS). The present system is capable of mass resolution of 750–850 (FWHM) over a wide range of masses, and can generate and analyze multi-element spectra from sub-μg samples (solids and solution residues) in <30–45 min. The optimum precision (1σ) of isotope ratios determined from 60–80 spectra (each the average of 600 individual spectra) is 0.2–0.4% R.S.D., and is limited by the instrument drift, dead time and the data acquisition and processing capabilities of the 8-bit digital oscilloscope used to collect the data. Isotope ratio accuracy (1σ, per mass unit) for major isotopes is typically <±1.0%.     

Atmospheric-pressure laser ionization: a novel ionization method for liquid chromatography/mass spectrometry

We report on the development of a new laser-ionization (LI) source operating at atmospheric pressure (AP) for liquid chromatography/mass spectrometry (LC/MS) applications. APLI is introduced as a powerful addition to existing AP ionization techniques, in particular atmospheric-pressure chemical ionization (APCI), electrospray ionization (ESI), and atmospheric pressure photoionization (APPI). Replacing the one-step VUV approach in APPI with step-wise two-photon ionization strongly enhances the selectivity of the ionization process. Furthermore, the photon flux during an ionization event is drastically increased over that of APPI, leading to very low detection limits. In addition, the APLI mechanism generally operates primarily directly on the analyte. This allows for very efficient ionization even of non-polar compounds such as polycyclic aromatic hydrocarbons (PAHs). The APLI source was characterized with a MicroMass Q-Tof Ultima II analyzer. Both the effluent of an HPLC column containing a number of PAHs (benzo[a]pyrene, fluoranthene, anthracene, fluorene) and samples from direct syringe injection were analyzed with respect to selectivity and sensitivity of the overall system. The liquid phase was vaporized by a conventional APCI inlet (AP probe) with the corona needle removed. Ionization was performed through selective resonance-enhanced multi-photon ionization schemes using a high-repetition-rate fixed-frequency excimer laser operating at 248 nm. Detection limits well within the low-fmol regime are readily obtained for various aromatic hydrocarbons that exhibit long-lived electronic states at the energy level of the first photon. Only molecular ions are generated at the low laser fluxes employed ( approximately 1 MW/cm(2)). The design and performance of the laser-ionization source are presented along with results of the analysis of aromatic hydrocarbons.     

A study of the thermal denaturation of ribonuclease S by electrospray ionization mass spectrometry

The thermal stability of ribonuclease S (RNase S), an enzymatically active noncovalent complex composed of a 2166-u peptide (S-peptide) and a 11,534-u protein (S-protein), was investigated by electrospray ionization mass spectrometry (ESI-MS) and capillary electrophoresis ESI-MS (CE-ESI-MS). The intensities of peaks corresponding to the RNase S complex were inversely related to both the applied nozzle-skimmer (or capillary-skimmer) voltage bias in the atmosphere-vacuum interface and the temperature of the RNase S solution. By using a heated metal capillary-skimmer interface and a room temperature solution of RNase S, the intensities of RNase S molecular ion peaks were observed to decrease with increasing metal capillary temperature. Mass spectrometric studies with both the nozzle-skimmer and capillary-skimmer interface designs allowed determination of phenomenological enthalpies for dissociation of the RNase S complex in both solution and for the electrosprayed microdroplet-gas phase species. Intact RNase S complex could also be detected with CE-ESI-MS separations by using a 10-mM ammonium bicarbonate (pH 7.9) solution as the electrophoretic buffer. These studies provide new insights into the stability of multiply charged noncovalent complexes in the gas phase and the mass spectrometric conditions required for such studies, and suggest that information regarding solution properties can be obtained by ESI-MS.     

Field ionization mass spectrometry—III: Cardenolides

The electron impact (EI) and field ionization (FI) mass spectra of some representative underivatized cardenolides have been studied together for the first time, with the objective of assessing the structural and in particular the sequence information afforded by this promising analytical method. Two series of cardenolides have been examined, each consisting of a mono-, di- and trisaccharide glycoside. The first was based on the aglycone digitoxigenin and comprised neriifolin, thevebioside and cerberoside; the second was based on the genin strophantidin and included cymarin, K-strophantin-β and K-strophantoside. Other cardiac glycosides investigated and discussed include digitoxin and helveticoside. Important and structurally diagnostic fragmentations have been uncovered, especially in FI mode. In addition, the EI and FI spectra of individual components of the cardenolides have been recorded, e.g. the genins digitoxigenin and strophantidin, and the mono-saccharides L-thevetose, D-cymarose and D-glucose.     

Characterization of an improved thermal ionization cavity source for mass spectrometry

A new thermal ionization source for use with a quadrupole mass spectrometer has been designed and characterized. The new source provides significant advantages over the previously reported prototype source and traditional filament-type thermal ionization sources. The operational interface between the source and the quadrupole mass spectrometer has been redesigned. A vacuum interlock, a translational stage, and an adjustable insertion probe are added to improve the source performance. With these modifications, the source is easier to operate while maximizing sample throughput. In this work, the performance of the newly developed source is examined. The ionization efficiencies are measured with a quadrupole mass spectrometer. The efficiency values obtained with this system are comparable to those obtained from a large scale isotope separator. The relationships among the ionization potential, vapor pressure, and measured ionization efficiency results are discussed. The crucible lifetime has been quantitatively estimated by measuring the crucible sputtering rate. Diagnostic studies of the new source show that the crucible position is a crucial parameter for sensitivity and performance. Stability tests demonstrate that the source can be run several weeks at a fixed emission current without significant degradation.     

Determination of rubidium by isotope dilution—inductively-coupled plasma mass spectrometry as an alternative to thermal ionization mass spectrometry

The combined techniques of inductively coupled plasma mass spectrometry (ICP-MS) and isotope dilution yield as much as a three-fold improvement in precision for trace-level rubidium determinations in geological materials over conventional isotope dilution using thermal ionization mass spectrometry (TIMS). Rubidium determinations by TIMS, precise to 0.6% (1 s.d.), are hindered by uncorrectable fractionation effects, whereas fractionation can be monitored during ICP-MS determinations, providing results as precise as 0.17% (1 s.d.). Precise rubidium data are critical for high-precision RbSr geochronology.     

A new method for calibrating the current gain of 1013 Ω amplifiers in thermal ionization mass spectrometry

We report a new method for calibrating the current gain of 10¹³ Ω amplifiers in both positive and negative mode used in thermal ionisation mass spectrometry (TIMS). This method uses any isotopic standard or sample to calibrate the gain factor as long as it can produce a stable current signal. It is simpler and more flexible than that recommended by Thermo‐Fisher (the manufacture of the TIMS). In these analyses, the gains of five 10¹³ Ω amplifiers were assessed. The precision of gain factors was better than 100 ppm (2 RSD) in a day, and the long‐term reproducibility was better than 300 ppm (2 RSD) within 2 to 8 months. After a gain was calibrated, the ratio accuracy and precision in the positive mode for ⁸⁷Sr/⁸⁸Sr of NIST 987 Sr and ¹⁴³Nd/¹⁴⁴Nd of La Jolla Nd were 0.710242 ± 60 (2 SD, n = 14) and 0.511842 ± 10 (2 SD, n = 22), respectively, at intensities of ⁸⁸Sr 0.3 V and ¹⁴²Nd 0.4 V, while in the negative mode for ¹⁸⁷Os/¹⁸⁸Os of Merck Os was 0.120229 ± 34 (2 SD, n = 23) at an intensity of ¹⁸⁷OsO₃ 0.01 mV. In addition, a difference in the gain factors between the negative mode TIMS (NTIMS) and positive mode TIMS (PTIMS) has been recognized. The values of the gain factor for NTIMS and PTIMS show a deviation of 0.54% on the Triton and 0.31% on the Triton Plus TIMS in this study; therefore, gain calibration should be carried out on both NTIMS and PTIMS. Moreover, a bias of ~ 1.5 × 10⁻⁵ between H and L Faraday cups for the same 10¹³ Ω amplifier has been detected, hinting that the efficiency of different Faraday cups may affect the gain factors, which can be eliminated through the new method of “cross‐calibration” discribed in this study.     

A general soft ionization method for mass spectrometry: Resonance-enhanced multi-photon ionization of biomolecules

We show here that ‘double-barrelled’ laser desorption/resonance-enhanced multi-photon ionization mass spectrometry has many advantages over traditional single-step desorption/ionization techniques, particularly if combined with Reflectron-Time-of-Flight mass analysers. We demonstrate the effectiveness of this technique for large labile biomolecules, namely native chlorophylls, porphyrins and a peptide, as examples of structural and mass analysis.     

Photoelectron resonance capture ionization mass spectrometry: a soft ionization source for mass spectrometry of particle-phase organic compounds

Photoelectron resonance capture ionization (PERCI) is a soft and sensitive ionization method, based on the attachment of low-energy (<1 eV) photoelectrons to organic analyte molecules. PERCI has been developed in our laboratory for the real-time analysis of organic particles by mass spectrometry, and is employed here to monitor the heterogeneous reaction of ozone with oleic acid. Simplified identification of the reaction products is possible as a result of the soft nature of PERCI, giving predominantly the [M--H](-) ions. The major particle-phase products are identified as: 1-nonanal, nonanoic acid, 9-oxononanoic acid, and azelaic acid, consistent with proposed mechanisms. New insight into this well-studied heterogeneous reaction is gained as additional minor particle-phase products, consistent with the Criegee mechanism, are readily detected.     

A facile microdialysis interface for on-line desalting and identification of proteins by nano-electrospray ionization mass spectrometry

The adverse effect of salts, especially inorganic salts, on electrospray ionization mass spectrometry (ESI-MS) is one of the most serious obstacles that might limit its application. Among the numerous desalting approaches, the microdialysis technique is favorable for large molecules, such as proteins. In this work, employing a hollow fiber membrane of cellulose acetate (MWCO 3000 Da), a simple, facile and efficient microdialysis interface with the dead volume of less than 1 microL was constructed for the on-line desalting and identification of proteins dissolved in high salt concentration buffer by nano-ESI-MS. Furthermore, with counterflow added, the desalting procedure was accelerated, and could be finished within 1 min. This system was successfully applied to the analysis of myoglobin dissolved in either high concentration ammonium acetate or sodium chloride buffer. The experimental results showed that, by using such a microdialysis interface, the salt concentration, even as high as 1 M, could be decreased by at least 2 orders of magnitude, while sample loss was less than 10%, demonstrating the potential of such an interface in broadening the application of nano-ESI-MS in the analysis of large molecules.     

Electrodeposition of technetium on platinum for thermal ionization mass spectrometry (TIMS)

A novel device has been fabricated for the electrodeposition of technetium metal onto platinum filaments for thermal ionization mass spectrometric (TIMS) measurements. The ability of the device to focus the deposition to diameters of hundreds of micrometers on pre-mounted TIMS filaments coupled with the ease of use and simplicity of design permit for an extremely sensitive yet economical TIMS filament loading technique. Electrodeposition parameters were varied in order to maximize deposition efficiency. X-ray photoelectron spectroscopy (XPS) was used to confirm and characterize the technetium deposit. The technetium is deposited in the metallic state, although surface oxides in the 4+ and 7+ state form readily. Initial TIMS measurements of the electrodeposited technetium in the presence of a barium sulfate ionization enhancer show potential for excellent sensitivity.     

Elucidation study of thermal decomposition products of human serum albumin using liquid ionization mass spectrometry

The origin of the intense but unknown peaks at m/z 235 observed in liquid ionization (LI) mass spectra of middle ear effusion and serum was investigated by using related standard compounds and the collision induced dissociation techniques. The ions were observed as the base peaks in mass spectra of the aqueous fractions of middle ear effusion and serum after chloroform extraction and in those of authentic human serum albumin (HSA) too. The ions commonly observed in serous fluids could be estimated as tyrosil-valil interchain immonium ions arising from thermal decomposition of HSA during the measurement. Such thermally stable interchain immonium ions, also observed in some oligopeptides having Val-Tyr sequence as their fragment ions, are likely to be characteristic ions for large protein molecules.     

Determination of boron isotopic variations in aquatic systems with negative thermal ionization mass spectrometry as a tracer for anthropogenic influences

A technique for precise boron isotope ratio measurements with a high detection power has been developed by negative thermal ionization mass spectrometry (NTIMS). Relative standard deviations in the range of 0.03–0.3% have been obtained for the determination of the ¹¹B/¹⁰B isotope ratio using nanogram amounts of boron. Ba(OH)₂ has been applied as ionization promoter for the formation of negative thermal ions. By adding MgCl₂ better reproducibilities of the measurement have been achieved. A possible interference of BO^-₂ ions at mass number 42 by CNO^- could be excluded by the sample preparation technique used. Contrary to other NTI techniques no dependence of the measured isotope ratio on the boron amount used has been observed. Anthropogenic and natural saline influences in ground water have been successfully identified by boron isotope ratio determinations with this NTIMS method, due to the different isotopic composition of boron in natural and anthropogenic substances. In sewage, the boron isotope ratio is substantially influenced by washing powder, which contains low ¹¹B/¹⁰B ratios (expressed in ¹¹B values normalized to the standard reference material NIST SRM 951). In contaminated ground water, low ¹¹B values are normally correlated with high boron and high chloride concentrations. On the other hand, ¹¹B shifts to higher values in less contaminated samples. For ground water with saline influences, only the ¹¹B determination, and not the boron or chloride content, allowed the correct identification of this natural source of contamination.     

Determination of boron isotopic variations in aquatic systems with negative thermal ionization mass spectrometry as a tracer for anthropogenic influences

A technique for precise boron isotope ratio measurements with a high detection power has been developed by negative thermal ionization mass spectrometry (NTIMS). Relative standard deviations in the range of 0.03-0.3% have been obtained for the determination of the (11)B/(10)B isotope ratio using nanogram amounts of boron. Ba(OH)(2) has been applied as ionization promoter for the formation of negative thermal ions. By adding MgCl(2) better reproducibilities of the measurement have been achieved. A possible interference of BO(-)(2) ions at mass number 42 by CNO(-) could be excluded by the sample preparation technique used. Contrary to other NTI techniques no dependence of the measured isotope ratio on the boron amount used has been observed. Anthropogenic and natural saline influences in ground water have been successfully identified by boron isotope ratio determinations with this NTIMS method, due to the different isotopic composition of boron in natural and anthropogenic substances. In sewage, the boron isotope ratio is substantially influenced by washing powder, which contains low (11)B/(10)B ratios (expressed in delta(11)B values normalized to the standard reference material NIST SRM 951). In contaminated ground water, low delta(11)B values are normally correlated with high boron and high chloride concentrations. On the other hand, delta(11)B shifts to higher values in less contaminated samples. For ground water with saline influences, only the delta(11)B determination, and not the boron or chloride content, allowed the correct identification of this natural source of contamination.