Filament temperature stabilizer for a thermal ionization mass spectrometer

A major enhancement in temperature stabilization of a filament has recently been achieved by means of the Halas–Kami ski bridge, in which the reference resistance of one leg is directly proportional to the filament temperature. In this article we describe a novel version of this type of electronic circuit which feeds dc power voltage to the filament via a switching transistor. The switching frequency is regulated by means of a triangular waveform generator incorporated into the system. The duration of the heating peak (square wave) is automatically adjusted properly once at each break between two subsequently appearing heating peaks, which is the interval when the bridge imbalance signal is measured. At the end of each break the bridge imbalance signal is kept on the output of the sample-and-hold amplifier. This signal compared with the generator output voltage provides the square wave for the switching transistor. The application of this circuit for a thermal ionization mass spectrometry is simple and straightforward. The filament temperature is set by two decade low-ohm resistors. Long-term variance of the ion current is approximately 5 times lower than that obtained by use of a commercial voltage stabilizer for the filament supply. It is shown theoretically that stabilization of the filament resistance results in the lowest possible variance of temperature.     

Matrix-assisted laser desorption ionization with a magnetic mass spectrometer.

Matrix-assisted laser desorption ionization has been carried out with a high mass double-focusing magnetic mass spectrometer. The pulsed ion signal, generated by irradiation of the sample (substance P, ubiquitin, and cytochrome c) embedded in 2,5-dihydroxybenzoic acid with a XeF excimer laser (353 nm, 12 ns pulse, 10-160 Hz), was recorded with an integrating array detector. Good resolution of 2600 (full width at half maximum) and high sensitivity (a few pmol) were obtained. The loss of small neutral fragments was observed, supporting the notion that the peak broadening observed in the time-of-flight mass spectrometers commonly used for this ionization mode is due to such metastable decomposition.     

Laser-induced electron impact ionization in a reflectron time-of-flight mass spectrometer

Some details of the generation of electrons by impinging a laser beam on a metal surface are described. It is shown that highly efficient electron generation is observed only during the laser pulse. Therefore, this technique delivers intense pulses of electrons. The process is investigated and different ion source set-ups are discussed. In conjunction with a time-of-flight mass spectrometer this technique can be used to produce mass spectra of different samples ranging from simple organic molecules to peptides.     

Rapid determination of 5-hydroxymethylfurfural by DART ionization with time-of-flight mass spectrometry

DART (direct analysis in real time), a novel technique with wide potential for rapid screening analysis, coupled with high-resolution time-of-flight mass spectrometry (TOF-MS) has been used for quantitative analysis of 5-hydroxymethylfurfural (5-HMF), a typical temperature marker of food. The DART/TOF-MS method was optimised and validated. Quantification of 5-HMF was achieved by use of a stable isotope-labelled 5-HMF standard prepared from glucose. Formation of 5-HMF from saccharides, a potential source of overestimation of results, was evaluated. Forty-four real samples (honey and caramelised condensed sweetened milk) and 50 model samples of heated honey were analysed. The possibility of using DART for analysis of heated samples of honey was confirmed. HPLC and DART/TOF-MS methods for determination of 5-HMF were compared. The correlation equation between these methods was DART?=?1.0287HPLC?+?0.21340, R ²?=?0.9557. The DART/TOF-MS method has been proved to enable efficient and rapid determination of 5-HMF in a variety of food matrices, for example honey and caramel.     

A study of resonance electron capture ionization on a quadrupole tandem mass spectrometer

Procedures that allow the realization of resonance electron capture (REC) mode on a commercial triple-quadrupole mass spectrometer, after some simple modifications, are described. REC mass spectrometry (MS) and tandem mass spectrometry (MS/MS) experiments were performed and spectra for some compounds were recorded. In particular, the charge-remote fragmentation (CRF) spectra of [M - H](-) ions of docosanoic and docosenoic acids under low-energy collisionally activated dissociation (CAD) conditions were obtained, and showed that there were no significant differences for [M - H](-) ions produced at different resonances (i.e. for [M - H](-) ions with different structures). This observation was explained on the basis of results obtained from deuterium-labeled fatty acids, which showed that different CRF ions (but with the same m/z value in the absence of labels) could be produced by different mechanisms, and all of them were obviously realized under CAD conditions that made spectra practically indistinguishable. The other example, which compared the REC-MS/MS spectrum of [M - H](-) ions and EI-MS/MS spectrum of M(+.) ions of daidzein, demonstrated the potential of the REC-MS/MS technique for more complex structure elucidation.     

Trace detection of inorganic oxidants using desorption electrospray ionization (DESI) mass spectrometry

Desorption electrospray ionization (DESI), an established ambient ionization method in mass spectrometry (MS) for the analysis of organic compounds, is applied here to trace detection of inorganic salts, including inorganic oxidants. In-situ surface analysis of targeted compounds, including nitrogen-, halogen- and sulfur-salts, down to sub-nanogram levels, was performed using DESI-MS. Successful experiments were carried out in both the negative and the positive ion modes; simple anions and cations as well as small cluster ions were observed. Various surfaces are examined and surface porosity effects were briefly explored. Absolute detection limits on porous polytetrafluoroethylene (PTFE) of 120 pg (surface concentration 0.07 ng mm⁻²) and 50 pg (surface concentration 0.03 ng mm⁻²), were achieved for sodium chlorate and sodium perchlorate, respectively. The compounds of interest were examined in the presence of a hydrocarbon mixture to assess matrix effects: only a two- or three-fold decrease in the target ion intensity was observed. Commercial fireworks were analyzed to determine perchlorate salts in complex mixtures. This work demonstrates the potential applicability of ambient ionization mass spectrometry to forensic investigations involving improvised explosives.      

Forensic analysis of inks by imaging desorption electrospray ionization (DESI) mass spectrometry

Desorption electrospray ionization mass spectrometry (DESI-MS) is employed in the forensic analysis of documents. Blue ballpoint pen inks applied to ordinary writing paper are examined under ambient conditions without any prior sample preparation. When coupled to an automated moving stage, two-dimensional molecular images are generated. Proof-of-principle experiments include characterization of a simulated forged number and examination of older written records. This application of DESI has advantages over extractive techniques in terms of speed and sample preservation. The effects of the desorbing solvent composition, in this case a mixture of methanol and water, and of flow rate, are evaluated. Results suggest that the solubility of the analyte (dyes Basic Blue 7, Basic Violet 3 and Solvent Blue 26) plays an important role in desorption from the paper surface.     

The development of fieldable laser-induced breakdown spectrometer: No limits on the horizon

In this review, new trends in the development of fieldable instrumentation based on laser-induced breakdown spectroscopy (LIBS) and its recent applications is presented. Depending on the LIBS configuration we will distinguish between portable, remote and stand-off instruments. Moreover, the development of portable systems gives greater flexibility and also increases the range of LIBS applications. In general, portable instruments are employed in close-contact applications like immovable artworks, contaminated soils and environmental diagnostic, while remote and stand-off instruments are normally used in analytical applications at distances where access to the sample is difficult or hazardous. Although remote and stand-off instruments are both used for chemical analysis at distances, the instrumental configurations are completely different. In remote analysis, an optical fiber is employed to deliver the laser energy a certain distance. This approach has been usually restricted to industrial applications, bulk analysis in water, geological measurements and chemical analysis on nuclear stations. In the case of stand-off applications, the laser beam and the returning plasma light are transmitted in an open-path configuration. In this article we also discuss the instrumental requirements in the design of remote and stand-off instruments.     

Coupling of liquid chromatography with mass spectrometry by desorption electrospray ionization (DESI)

A liquid chromatography/mass spectrometry (LC/MS) method using desorption electrospray ionization (DESI) as a versatile interface has been established, which allows a wide range of elution flow rates, online derivatization via reactive DESI and further combination with electrochemistry.     

An atmospheric pressure ionization source based on desorption electrospray ionization technology (DESI) for ion cyclotron resonance mass spectrometry

An atmospheric pressure ionization source based on desorption electrospray ionization technology for a bench-top hybrid FTICR mass spectrometer is described. The ion source was characterized using low-molecular-weight-weight pharmaceutical samples. The dependences of signal intensities on various experimental parameters (solvent composition, surface temperature, spray voltage, etc.) were studied. Based on the results obtained, plausible mechanisms of desorption electrospray ionization for the analytes under the study are discussed.     

Desorption ionization/tandem mass spectrometry with a caesium ion source and a triple quadrupole mass spectrometer

The coupling of a caesium ion source to a triple quadrupole mass spectrometer is described. The potential of this combination for examining thermally labile, non-volatile molecules, such as thiamine hydrochloride, is examined. Emphasis is placed on the advantages the various scanning modes of tandem mass spectrometry provide in ion structure elucidation and the investigation of desorption ionization mechanisms. Use of the caesium ion source for desorption of neutral molecules which are chemically ionized by an auxiliary gas is demonstrated. This procedure may be especially useful for examining non-volatile, non-ionic samples.     

Development of a tabletop time-of-flight mass spectrometer with an ion attachment ionization technique

We report a new type of mass spectrometry based on a time-of-flight mass spectrometer combined with an ion attachment ionization technique (IA-TOF). In contrast to electron ionization mass spectra, IA-TOF mass spectra are not complicated by peaks due to fragmentation of the molecular ion; the adduct ion formed in IA does not fragment. We developed a tabletop IA-TOF system and evaluated its performance by analyzing specimens originally in the gas, liquid, and solid phases. We obtained fragment-free spectra covering a mass range up to m/z 3400 with a mass resolution of about 4700. Our IA-TOF system realizes accurate and versatile real-time mass spectrometry.     

High-resolution accurate mass measurements of biomolecules using a new electrospray ionization ion cyclotron resonance mass spectrometer

A novel electrospray ionization/Fourier transform ion cyclotron resonance mass spectrometer based on a 7-T superconducting magnet was developed for high-resolution accurate mass measurements of large biomolecules. Ions formed at atmospheric pressure using electrospray ionization (ESI) were transmitted (through six differential pumping stages) to the trapped ion cell maintained below 10^?9 torr. The increased pumping speed attainable with cryopumping (> 10⁵ L/s) allowed brief pressure excursions to above 10^?4 torr, with greatly enhanced trapping efficiencies and subsequent short pumpdown times, facilitating high-resolution mass measurements. A set of electromechanical shutters were also used to minimize the effect of the directed molecular beam produced by the ES1 source and were open only during ion injection. Coupled with the use of the pulsed-valve gas inlet, the trapped ion cell was generally filled to the space charge limit within 100 ms. The use of 10–25 ms ion injection times allowed mass spectra to be obtained from 4 fmol of bovine insulin (M_r 5734) and ubiquitin (M_r 8565, with resolution sufficient to easily resolve the isotopic envelopes and determine the charge states. The microheterogeneity of the glycoprotein ribonuclease B was examined, giving a measured mass of 14,898.74 Da for the most abundant peak in the isotopic envelope of the normally glycosylated protein (i.e., with five mannose and two N-acetylglucosamine residues (an error of approximately 2 ppm) and an average error of approximately 1 ppm for the higher glycosylated and various H₃PO₄ adducted forms of the protein. Time-domain signals lasting in excess of 80 s were obtained for smaller proteins, producing, for example, a mass resolution of more than 700,000 for the 4⁺ charge state (m/z 1434) of insulin.     

Field ionization studies of unstable organic compounds with a double focusing mass spectrometer

Zusammenfassung Die Feldionisation einer Reihe instabiler organischer Verbindungen wurde mit einem doppeltfocussierenden Massenspektrometer unter Benutzung einer neuen kombinierten Feldionisations/Elektronenstoß-Ionenquelle bei direkter Probenzuführung untersucht. Das maximale Auflösungsvermögen m/m betrug etwa 30000. Leistungsdaten der Ionenquelle und Analysenergebnisse werden mitgeteilt. Im Gegensatz zur Elektronenstoßionisation wurden bei der Feldionisation in allen Fällen Molekülionen gefunden. Die Ergebnisse zeigen, daß die Feldionisation vorteilhaft für Molekulargewichtsbestimmungen und Strukturuntersuchungen instabiler hochmolekularer Verbindungen eingesetzt werden kann.

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Summary Field ionization of several unstable compounds has been studied with a double focusing mass spectrometer of high resolving power using a combined field ionization/electron impact ion source in connection with a direct inlet probe. A maximum resolving power with field ionization of more than 30,000 was attained. The performance data of the ion source are given. The results show that field ionization indeed enables molecular weight determinations and structure investigations of unstable high molecular weight compounds.

     

In situ analysis of agrochemical residues on fruit using ambient ionization on a handheld mass spectrometer

We describe a rapid in situ method for detecting agrochemicals on the surface or in the tissue of fruit using a portable mass spectrometer equipped with an ambient ionization source. Two such ionization methods, low temperature plasma (LTP) and paper spray (PS), were employed in experiments performed at a local grocery store. LTP was used to detect diphenylamine (DPA) directly from the skin of apples in the store and those treated after harvest with DPA were recognized by MS and MS/MS. These data therefore allowed ready distinction between organic and non-organic apples. DPA was also found within the internal tissue of purchased apples and its distribution was mapped using LTP. Similarly, thiabendazole residues were detected on the skin of treated oranges in a grocery store experiment in which paper spray was performed by wiping the orange surface with a moist commercial lens wipe and then applying a high voltage to ionize the chemicals directly from the wipe. The handheld mass spectrometer used in these measurements is capable of performing several stages of tandem mass spectrometry (up to MS(5)); the compounds on the fruit were identified by their MS/MS fragmentation patterns. Protonated DPA (m/z 170) produced a characteristic MS(2) fragment ion at m/z 92, while thiabendazole was identified by MS(3) using precursor to fragment ion transitions m/z 202 →m/z 175 →m/z 131. These particular examples exemplify the power of in situ analysis of complex samples using ambient ionization and handheld mass spectrometers.     

Exact mass measurement on an electrospray ionization time-of-flight mass spectrometer: error distribution and selective averaging

An automated, accurate and reliable way of acquiring and processing flow injection data for exact mass measurement using a bench-top electrospray ionization time-of-flight (ESI-TOF) mass spectrometer is described. Using Visual Basic programs, individual scans were selected objectively with restrictions on ion counts per second for both the compound of interest and the mass reference peaks. The selected "good scans" were then subjected to two different data-processing schemes ("combine-then-center" and "center-then-average"), and the results were compared at various ion count limit settings. It was found that, in general, the average of mass values from individual scans is more accurate than the centroid mass value of the combined (same) scans. In order to acquire a large number of good scans in one injection (to increase the sampling size for statistically valid averaging), an on-line dilution chamber was added to slow down the typically rapid mass chromatographic peak decay in flow-injection analysis. This simple addition worked well in automation without the need for manual sample dilution. In addition, by dissolving the reference compound directly into the mobile phase, manual syringe filling can be eliminated. Twenty-seven samples were analyzed with the new acquisition and process routines in positive electrospray ionization mode. For the best method found, the percentage of samples with RMS error less than 5 ppm was 100% with repetitive injection data (6 injections per sample), and 95% with single injection data. Afterwards, 31 other test samples were run (with MW ranging from 310 to 3493 Da, 21 samples in ESI+ and 10 in ESI- mode) and processed with similar parameters and 100% of them were mass-calculated to RMS error less than 5 ppm also.     

Desorption electrospray ionization using an Orbitrap mass spectrometer: Exact mass measurements on drugs and peptides

Desorption electrospray ionization (DESI) is implemented on an Orbitrap mass spectrometer. The ion source is described and applications which utilize the high-resolution capabilities of the Orbitrap are emphasized, including the characterization of peptides and active ingredients in pharmaceutical tablets. Measurements are made in less than 1 s at a resolution of 60,000. The implications of the data for the mechanisms of DESI are discussed.     

Negative chemical ionization and accurate mass measurement applications for a linked gas chromatography/Fourier transform infrared spectrometer/Fourier transform mass spectrometer

Summary The analytical utility of a linked GC/FTIR/ FTMS system operating in accurate mass and negative CI modes is demonstrated. In addition to the complementary information derived from IR and MS search results, accurate mass measurement data (average error of 4.2 ppm) is employed in the analysis of a seven component mixture. A post search filter uses accurate mass data to eliminate spectral search results with incorrect elemental compositions. An alternate positive ion EI and negative ion CI GC/IR/MS analysis is also performed on a mixture of substituted phenyl aldehydes and ketones. The reagent gas, ammonia, is introduced by pulsed valve thus minimizing crossover spectral contamination while maximizing spectral resolution. Proton abstraction by the amide produces an intense (M-1)^– for all compounds in the mixture. Molecular weight information from the negative CI data is used in a post search filter to improve the reliability of both IR and MS search results.

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Anwendung der negativen chemischen Ionisation und genauer Massenmessungen für ein gekoppeltes GC/FTIR/FTMS-System

Zusammenfassung Die analytische Anwendbarkeit eines gekoppelten GC/FTIR/FTMS-Systems mit akkurater Massenmessung und NCI-Betrieb wird dargestellt. Zusätzlich zu den komplementären Informationen aus IR- und MS- Bibliothekssuchen werden Daten aus akkuraten Massenmessungen (mittlerer Fehler 4.2 ppm) bei der Analyse einer Mischung aus sieben Komponenten mit herangezogen. Im Anschluß an die Bibliothekssuche eliminiert ein Filter mit Hilfe der akkuraten Massen Ergebnisse mit fehlerhafter Elementarzusammensetzung. Von einer Mischung aus substituierten Phenylaldehyden und -ketonen wird eine Analyse mit alternierender Registrierung von positiven EI- und negativen CI-Spektren durch ein GC/IR/-MS-System ausgeführt. Das Reaktandgas Ammoniak wird über ein gepulstes Ventil zugegeben, um bei maximaler spektraler Auflösung minimale Störung durch Verunreinigung zu erhalten. Durch Abstraktion eines Protons von der Amidfunktion werden von allen Komponenten der Mischung intensive (M-1)-Ionen gebildet. Molekulargewichtsinformationen aus dem NCI-Experiment werden als Filter verwendet, mit dem die Zuverlässigkeit des Suchens in IR- und MS-Datenbanken erhöht werden kann.