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PubMed2002

Chromatographic and ionization properties of polybrominated diphenyl ethers using GC/high-resolution MS with metastable atom bombardment and electron impact ionization.

The chromatographic and ionization properties of 35 polybrominated diphenyl ether (PBDE) congeners were investigated using GC/HRMS with metastable atom bombardment (MAB) and electron impact (EI) ionization. A multiple linear regression model based on bromine substitution patterns and MOPAC calculated physical properties was developed to predict relative GC retention times of individual PBDE congeners. Although five different sources of metastable rare gas atoms (He, N2, Ar, Xe, and Kr) were investigated with MAB ionization, only MAB-N2 provided adequate ionization efficiency and predictability. Because of reduced background noise to the MS detector, MAB-N2 had a lower limit of detection for tetra- and penta-BDEs than EI, despite having a lower sensitivity. Using MAB-N2, the molecular ion was always the base peak, with little fragmentation taking place. Conversely, using EI ionization, the [M - nBr]+ peak (where n = 1-4, depending on the number of Br substituents) was the dominant ion for all PBDE congeners. Multiple linear regression models representing the molecular ion response of PBDE congeners analyzed by GC/ HRMS with MAB-N2 and EI ionization were also developed using the number and type of Br substituents and ionization potentials. A significantly higher level of predictability was obtained for the MAB-N2 response model than for EI.

Electrons↗
PubMed2003

Ionization energies of hypervalent Li2F, Li2Cl and Na2Cl molecules obtained by surface ionization electron impact neutralization mass spectrometry.

Ionization energies of hypervalent Li(2)F, Li(2)Cl and Na(2)Cl molecules detected by surface ionization electron impact neutralization mass spectrometry are reported. The ionization energies were 3.78 +/- 0.2 eV for Li(2)F, 4.93 +/- 0.2 eV for Li(2)Cl, and 4.21 +/- 0.2 eV for Na(2)Cl. The ionization energies (IE) agree with theoretical ionization energies calculated by ab initio methods, supporting the theoretical prediction that Li(2)F has a hyperlithiated configuration in which the odd electron delocalizes over the two lithiums and with photoionization measurement. The first ionization energy of Na(2)Cl was experimentally confirmed earlier and for Li(2)Cl as well.8 We have developed and used this new approach for the problem--in the present work ions were first formed by surface ionization, followed by electron attachment (neutralization).

Journal Article↗
PubMed1983

The interaction of an ionizing ligand with enzymes having a single ionizing group. Implications for the reaction of folate analogues with dihydrofolate reductase.

Binding theory has been developed for the reaction of an ionizing enzyme with an ionizing ligand. Consideration has been given to the most general scheme in which all possible reactions and interconversions occur as well as to schemes in which certain interactions do not take place. Equations have been derived in terms of the variation of the apparent dissociation constant (Kiapp) as a function of pH. These equations indicate that plots of pKiapp against pH can be wave-, half-bell- or bell-shaped according to the reactions involved. A wave is obtained whenever there is formation of the enzyme-ligand complexes, ionized enzyme . ionized ligand and protonated enzyme . protonated ligand. The additional formation of singly protonated enzyme-ligand complexes does not affect the wave form of the plot, but can influence the shape of the overall curve. The formation of either ionized enzyme . ionized ligand or protonated enzyme . protonated ligand, with or without singly protonated enzyme-ligand species, gives rise to a half-bell-shaped plot. If only singly protonated enzyme-ligand complexes are formed the plots are bell-shaped, but it is not possible to deduce the ionic forms of the reactants that participate in complex formation. Depending on the reaction pathways, true values for the ionization and dissociation constants may or may not be determined.

Enzymes↗
PubMed1999

Effect of excited states on the ionization balance in plasmas via the enhancement of ionization and recombination rate coefficients.

The effect of excited states on the effective ionization and recombination rate coefficients for the ground states was investigated analytically and by computer simulation. The calculation was done for carbon ions. The results using carbon ions show (1) the contribution from excited states to ionization rate coefficients becomes significant even at an electron density as low as 10(15) cm(-3) and saturated from around N(e) approximately 10(20) cm(-3); (2) the lower the electron temperature, the larger the contribution; (3) in the case of recombination rate coefficients, there is still a non-negligible contribution from excited states even at a very low electron density of 10(10) cm(-3), where the contribution has been considered negligible; (4) this contribution to the recombination rate coefficients increases linearly with the electron density; (5) the enhancements of the ionization and recombination rate coefficients increase as N(e) increases and are saturated to the same value at higher densities; (6) there exists a region of temperature and density where the recombination is effectively hindered. Some of the behaviors of the ionization and recombination rate coefficients in the extreme regions of a very low and high electron density were analytically understood. The calculated ionization and recombination rate coefficients for carbon ions, including the effect of excited states, were used in a one-dimensional magnetohydrodynamic code for the calculation of the ionization balance of carbon ions in a Z-pinch carbon plasma and the gain of C VI H(alpha) (18.2 nm) line. The significant change in the evolution of the ionization balance was observed. The rapid depletion of C VII ions by the increased recombination rate reduces the gain significantly by a factor of approximately 3 compared to the case where the contribution from excited states was neglected. Such calculations can be done for other ions. The characteristics found for carbon ions are generic and applicable to other ions.

Journal Article↗
PubMed2000

Depth ionization curves for an unmodulated proton beam measured with different ionization chambers.

Differences in depth dose curves for a 78 MeV unmodulated proton beam were measured with four commercially available ionization chambers. Measurements were performed both in water and in a commercially available solid water phantom. A depth scaling factor (Cpl) was determined from the ratio of depths distal to the Bragg peak where the dose is reduced to 80% of the maximum dose in water and in the solid water phantom. This scaling factor provides good agreement between the ionization curves at all depths in water and in this solid water phantom. There is no significant difference in the value of the depth scaling factor between the ratios (R80wat/R80med) and (R50wat/R50med), or (R100wat/R100med) for 78 MeV unmodulated proton beams. The depth scaling factor for this commercially available solid water phantom is 1.023. An effective point of measurement for a cylindrical ionization chamber was found to be slightly greater than the 50% of the cavity radius proposed by the AAPM-TG25 dosimetry protocol for electron beams and amounts to 62.5% of the cavity radius of cylindrical ionization chambers. The ion collection efficiency, Pion, and the polarity correction factor, Ppol, for all the ionization chambers studied are within 1% and 0.4% of unity, respectively. Absolute doses measured with a parallel plate ionization chamber in water and in the solid water phantom show that the doses measured in the solid water phantom are 1.4% +/- 0.5% lower than in water. The dose rate dependent response of the beam line monitor chamber was also investigated. Agreement between all the chambers was within 1.5% at the dose rates studied but the results showed that all four ionization chambers are less dose rate dependent than the monitor chamber.

Algorithms↗
PubMed2005

[Polarity effects of commercial plane-parallel ionization chamber in therapeutic electron dosimetry-Results with C-134A ionization chamber.].

Plane-parallel ionization chambers that exhibit polar effects with low energy electron beams are recommended for therapeutic electron dosimetry. In this study, the polarity effects of a C-134A ionization chamber, a major commercially available plane-parallel ionization chamber in Japan, were characterized as a function of mean energy at various depths. Polarity effects were measured at representative depths along depth dose curves of nominal 4, 6, 9, 12 and 15 MeV electron beams, and were compared with previously reported results. Polarity errors for the ionization chamber studied were shown to monotonically increase with decreasing mean energy at a given depth and were maximal at about 1-2 MeV. It was also shown that polarity errors depended on the energy of the incident electron beam. The polarity error of the C-134A ionization chamber was larger than that of other previously investigated plane-parallel ionization chambers. Because the magnitude of polarity effects should be determined throughout the depth dose curve in therapeutic electron dosimetry, it is always necessary to measure ionization readings taken at both polarities.

Electrons↗
PubMed2001

Choosing between atmospheric pressure chemical ionization and electrospray ionization interfaces for the HPLC/MS analysis of pesticides.

An evaluation of over 75 pesticides by high-performance liquid chromatography/mass spectrometry (HPLC/MS) clearly shows that different classes of pesticides are more sensitive using either atmospheric pressure chemical ionization (APCI) or electrospray ionization (ESI). For example, neutral and basic pesticides (phenylureas, triazines) are more sensitive using APCI (especially positive ion). While cationic and anionic herbicides (bipyridylium ions, sulfonic acids) are more sensitive using ESI (especially negative ion). These data are expressed graphically in a figure called an ionization-continuum diagram, which shows that protonation in the gas phase (proton affinity) and polarity in solution, expressed as proton addition or subtraction (pKa), is useful in selecting APCI or ESI. Furthermore, sodium adduct formation commonly occurs using positive ion ESI but not using positive ion APCI, which reflects the different mechanisms of ionization and strengthens the usefulness of the ionization-continuum diagram. The data also show that the concept of "wrong-way around" ESI (the sensitivity of acidic pesticides in an acidic mobile phase) is a useful modification of simple PKa theory for mobile-phase selection. Finally, this finding is used to enhance the chromatographic separation of oxanilic and sulfonic acid herbicides while maintaining good sensitivity in LC/MS using ESI negative.

Atmospheric Pressure↗
PubMed1991

The response of parathyroid hormone to specific changes in either ionized calcium, ionized magnesium, or protein-bound calcium in humans.

Because the effects of specific changes in magnesium or protein-bound calcium on biologic processes in humans have not been clearly defined, we developed an experimental system to specifically lower either ionized calcium, protein-bound calcium, or ionized magnesium. As an indicator of the biological effect of these changes, we measured parathyroid hormone (PTH) in blood. To selectively dilute only one constituent, we infused one of four specially prepared solutions into each of six healthy blood donors following ordinary blood donation. Each donor received at random 450 mL of a different solution at four different blood donations. The respective concentrations of ionized calcium (mmol/L), total magnesium (mmol/L), and albumin (g/L) in these fluids were as follows: control fluid, 1.25, 1.20, 40; no calcium fluid, 0, 1.20, 40; no magnesium fluid, 1.25, 0, 40; no albumin fluid, 1.25, 1.00, 0. Using selective in vivo dilution of the volunteers' blood, we specifically lowered either (1) ionized calcium by 0.07 mmol/L (5.5%), (2) protein-bound calcium by 0.09 mmol/L (10%), or (3) ultrafiltrable magnesium by 0.03 mmol/L (4.8%). While measurements of intact PTH showed that concentrations of PTH in blood did not respond to either decreased ultrafiltrable magnesium or decreased protein-bound calcium, these measurements showed that PTH increased by 400% within 10 minutes in response to decreased ionized calcium.

Adult↗
PubMed1996

Selective determination of haloperidol in human serum: surface ionization mass spectrometry and gas chromatography with surface ionization detection.

Surface ionization organic mass spectrometry (SIOMS) has been performed on the clinically important drug haloperidol using quadrupole mass spectrometry in which the thermal ion source has a rhenium oxide emitter. The surface ionization (SI) mass spectrum is presented, interpreted in a purely empirical way by means of evidence from previous investigations, and then compared to results from conventional electron impact (EI) ionization. An approach to detection of this drug in serum by gas chromatography (GC) with a surface ionization detector (SID) and GC-SIOMS is described. This approach demonstrates that (a) haloperidol is efficiently surface-ionized, giving a unique SI mass spectrum, (b) experimental results rationalize the combined sensitivity and selectivity of the GC-SID for the examined drug, (c) the detection limit for haloperidol in serum is 1.1 ng/ml (S/N = 3) by GC-SID (the coefficients of variation of the assay are generally low, i.e., below 8.5%) and (d) the GC-SIOMS coupling can be used for sensitive and selective detection of haloperidol in serum.

Chromatography, Gas↗
PubMed2000

Mixed-mechanism ionization to enhance sensitivity in atmospheric pressure ionization LC/MS.

A novel dual-mechanism ionization technique for LC/MS/MS has been observed, characterized and applied to the quantitation of a tertiary amine-containing drug compound in dog plasma. This mixed-mechanism ionization approach can improve the sensitivity of the pneumatically assisted electrospray experiment. Under conditions of higher than normal chromatographic flow and lower electrospray voltage, approximately a 4-fold increase in sensitivity was realized. A detection limit of 16 pg (45 fmol) on-column, and inter-day imprecision and inaccuracy of < 11 and < 15%, respectively, were obtained. A trade-off in concentration sensitivity in favor of ease of sample preparation was made to increase sample throughput. Although results strongly suggest that mixed-mechanism ionization is in operation, and that pneumatically assisted electrospray is a partial contributor to the overall ionization process, the exact nature of the second mechanism of ionization is unclear at this time.

Animals↗
PubMed2003

Combined electrospray ionization-atmospheric pressure chemical ionization source for use in high-throughput LC-MS applications.

Fast and accurate analytical methods are essential to keep pace with sample libraries produced from combinational chemistry and high-throughput biological screening. Many laboratories now use a combination of ionization techniques for the characterization of these samples, including atmospheric pressure chemical ionization (APCI), electrospray ionization (ESI), and photoionization (PI). Data are shown here from the analysis of a compound collection plate containing a variety of sample structures. ESI will normally analyze around 80% of these samples, necessitating a source change to analyze a further 10%. In this work, we have developed a new combined ESI-APCI source (ESCi) for use in on-line HPLC applications. The combined source allows alternate on-line ESI and APCI scans with polarity switching within a single analysis. The ESCi source has been designed to be a simple replacement for the existing mass spectrometer interfaces. Each ionization method is optimized independently using separate tuning parameters. Instrument electronics can readily switch between the two ionization methods and polarities within normal interscan time periods. The new source has reduced the analysis time of sample plates by eliminating the need for a source hardware change, source optimization, and repeat analyses.

Journal Article↗
PubMed2001

Triacylglycerol analysis of potential margarine base stocks by high-performance liquid chromatography with atmospheric pressure chemical ionization mass spectrometry and flame ionization detection.

Several margarine base stock candidates have previously been prepared for the purpose of finding better, more oxidatively stable food components: high-saturate vegetable oils, randomized vegetable oils, vegetable oil-hard stock blends, and interesterified vegetable oil-hard stock blends. Here are reported the triacylglycerol compositions of these products, determined using reverse-phase high-performance liquid chromatography (HPLC) coupled with a flame ionization detector or a quadrupole mass spectrometer with an atmospheric pressure chemical ionization source. Triacylglycerol percent composition results for samples of known composition (randomized and interesterified samples) exhibited less average error by HPLC coupled with a quadrupole mass spectrometer with an atmospheric pressure chemical ionization source, after application of response factors, than the results by HPLC coupled with a flame ionization detector. The fatty acid compositions calculated from the mass spectrometric data exhibited less average error than the fatty acid compositions resulting from the flame ionization detector data. The average error of the fatty acid compositions by the mass spectrometer was lowest for interesterified blend samples, next lowest for randomized samples, then followed by high-saturated fatty acid oils, normal oils, and blends. Analysis of the vegetable oil-hard stock blends by mass spectrometer required special treatment for calculation of response factors.

Chromatography, High Pressure Liquid↗
PubMed2005

Valence double ionization of O2 at photon energies below and above the molecular double ionization threshold.

A recently developed time-of-flight photoelectron-photoelectron coincidence spectroscopy technique, which gives complete two-dimensional e(-)-e(-) spectra in single photon double ionization, is applied to molecular oxygen at photon energies below and above the adiabatic double ionization threshold of O(2). Analysis of the two-dimensional coincidence maps reveals specific indirect pathways for the double ionization process. Dissociative ionization paths with subsequent autoionization of atomic oxygen are found to be the dominant processes for all chosen photon energies. Spectra of the photoelectrons coincident with the autoionization electrons show that intermediate O(2)(+) states are involved which do not autoionize to molecular O(2)(2+). In particular, the ground state of O(2)(2+) is vibrationally resolved and shows a regular progression which can be well described by direct Franck-Condon transitions at an internuclear distance R(e)(X (1)Sigma(g)(+))=1.054 A. Quantum yields of double ionization for O(2), of a form discussed in this paper, are determined.

Journal Article↗
PubMed1998

A rapid novel derivatization of amphetamine and methamphetamine using 2,2,2-trichloroethyl chloroformate for gas chromatography electron ionization and chemical ionization mass spectrometric analysis.

Amphetamine and methamphetamine are commonly abused central nervous system stimulants. We describe a rapid new derivatization of amphetamine and methamphetamine using 2,2,2-trichloroethyl chloroformate for gas chromatography-mass spectrometric analysis. Amphetamine and methamphetamine, along with N-propyl amphetamine (internal standard), were extracted from urine using 1-chlorobutane. The derivatization with 2,2,2-trichloroethyl chloroformate can be achieved at room temperature in 10 minutes. The electron ionization mass spectrum of amphetamine 2,2,2-trichloroethyl carbamate showed two weak molecular ions at m/z 309 and 311, but showed diagnostic strong peaks at m/z 218, 220, and 222. In contrast, chemical ionization of the mass spectrum of amphetamine 2,2,2-trichloroethyl carbamate showed strong (M + 1) ions at m/z 310 and 312 and other strong diagnostic peaks at m/z 274 and 276. The major advantages of this derivative are the presence of a diagnostic cluster of peaks due to the isotopic effect of three chlorine atoms (isotopes 35 and 37) in the derivatized molecule and the relative ease of its preparation. We also observed strong molecular ions for derivatized methamphetamine in the chemical ionization mass spectrum, but the molecular ions were very weak in the electron ionization mass spectrum. We used the scan mode of mass spectrometry in all analyses. When using a urine standard containing 1,000 ng/mL of amphetamine (a 7.4-micromol/L concentration) and methamphetamine (a 6.7-micromol/L concentration), the within-run precisions were 4.8% for amphetamine and 3.6% for methamphetamine. The corresponding between-run precisions were 5.3% for amphetamine and 6.7% for methamphetamine. The assay was linear for amphetamine and methamphetamine concentrations of 250 to 5,000 ng/mL (amphetamine, 1.9-37.0 micromol/L; methamphetamine, 1.7-33.6 micromol/L). The detection limit was 100 ng/mL (amphetamine, 0.74 micromol/L; methamphetamine, 0.67 micromol/L) using the scan mode of electron ionization mass spectrometry. We observed good a correlation between the concentrations of amphetamine and methamphetamine in five urine specimens positive for amphetamines using the more conventional pentafluoropropionyl derivative and our new derivative using 2,2,2-trichloroethyl chloroformate.

Amphetamine↗
PubMedDate not supplied

Polarity effect in plane-parallel ionization chambers using air or a dielectric liquid as ionization medium.

A plane-parallel ionization chamber having a sensitive volume of 2 mm3 and using the dielectric liquid tetramethylsilane as the sensitive medium instead of air is described. In the design of the chamber special attention was given to the factors that can cause unwanted currents in the cable, stem, or the chamber dielectric material. The chamber has been tested with respect to the polarity effect in regions of radiation fields where ordinary plane-parallel ionization chambers will often fail. These regions are the build-up region in photon fields, and the region close to the practical range for electrons where nonelectronic equilibrium is significant. Experimental results show that, despite the extremely small ionization volume in the liquid ionization chamber, the polarity effect never exceeds a few tenths of a percent in field positions where well-known commercially available chambers with much less spatial resolution designed for measurements in radiation therapy fields can show polarity effects of 5% to 30%. The origin of spurious currents and how they must be minimized in the design of either a liquid- or gas-filled ionization chamber is discussed.

Electrons↗
PubMed2004

Comparison of electrospray ionization, atmospheric pressure chemical ionization, and atmospheric pressure photoionization for the analysis of dinitropyrene and aminonitropyrene LC-MS/MS.

The only relevant source for human exposure to dinitropyrenes is diesel engine emissions. Due to this specificity, dinitropyrenes may be used as biomarkers for monitoring human exposure to diesel engine emissions. Only few analytical methods have been described for the quantitation of dinitropyrenes and their metabolites, aminonitropyrenes, and diaminopyrenes. Therefore, for dinitropyrenes, aminonitropyrenes, and diaminopyrenes were selected as model compounds for the development of a sensitive HPLC-MS/MS method (high performance liquid chromatography coupled to triple quadrupole mass spectrometry) was to quantify polyaromatic amines and nitroarenes in biological matrices was developed optimal methods by comparing electrospray ionization (ESI), atmospheric pressure chemical ionization (APCI), and atmospheric pressure photoionization (APPI) sources. Dinitropyrene was not effectively ionized and diaminopyrene yielded mainly [M(.)](+) ions by electrospray ionization. With APCI and APPI, precursor ions of diaminopyrene and aminonitropyrene were [M + H](+) and [M(.)](-) for dinitropyrene. Precursor ions with [M - 30(.)](-) for dinitropyrene and [M - 30 + H](+) for aminonitropyrene were observed. Reversed and normal phase HPLC-MS/MS with ESI, APCI and APPI were optimized separately with respect to unequivocal analyte identification and sensitivity. Normal phase HPLC coupled to APPI-MS/MS gave the highest precision and sensitivity for aminonitropyrene (6%/0.2 pg on column) and dinitropyrene (9%/0.5 pg on column). The limit of detection in spiked rat plasma was 5 pg/100 microL for aminonitropyrene (accuracy 82%) and 10 pg/100 microL for dinitropyrene (accuracy 105%). In plasma of rats treated with dinitropyrene by oral administration, no detectable levels of dinitropyrene but higher aminonitropyrene levels compared with intratracheal instillation were observed. These findings clearly demonstrate that dinitropyrene was absorbed after oral and intratracheal application and that a reduction of nitro groups occurs to a high extent in the reductive environment of the intestine. To our knowledge, this is the first time that aminonitropyrene was observed in plasma after intratracheal or oral administration directly demonstrating the reductive metabolism of dinitropyrene in vivo.

Air Ionization↗
PubMed2005

Comparison of the effects of ionization mechanism, analyte concentration, and ion "cool-times" on the internal energies of peptide ions produced by electrospray and atmospheric pressure matrix-assisted laser desorption ionization.

The propensities of a series of peptide ions produced by both electrospray and atmospheric pressure matrix assisted laser desorption ionization (AP-MALDI) to fragment in an ion trap mass spectrometer under various conditions were studied in detail by measuring the extent of fragmentation of precursor ions by collision induced dissociation (CID) as a function of applied resonance excitation RF voltage. For the most basic peptides, the energy required to fragment MH+ ions generated by electrospray exceeded that required to fragment equivalent AP-MALDI ions under identical instrumental conditions; the reverse was observed for a peptide incorporating no basic residues, while peptides of intermediate basicity showed little difference between the ionization methods. This correlation between peptide basicity and the difference in the energy required to induce fragmentation of MH+ ions generated by AP-MALDI and electrospray is attributed primarily to a trend in the internal energies of the ions generated by AP-MALDI (the greater the difference in gas-phase basicities between the matrix and the analyte the greater the internal energy of the analyte ions produced). Furthermore the internal energies of ions produced by AP-MALDI, but not the equivalent ions formed by electrospray, were observed to decrease with decreasing analyte concentration. We attribute this finding to the cooling effect of endothermic dissociation of analyte ion/matrix molecule clusters following the matrix assisted laser desorption step. Time-resolved analyses (measurement of extent of fragmentation of precursor ions by CID as a function of pre-CID "cool times") revealed that cooling periods in excess of 250 ms were required to achieve internal energy equilibrium through cooling collisions with the helium buffer gas. Furthermore, these analyses demonstrated that, even after these extended cooling times, equivalent ions formed by the two ionization techniques showed different propensities to fragment. We conclude that the two different ionization techniques produce ion populations that may differ in their three-dimensional structure.

Atmospheric Pressure↗
PubMed2005

Comparison of atmospheric pressure chemical ionization, electrospray ionization, and atmospheric pressure photoionization for the determination of cyclosporin A in rat plasma.

Atmospheric pressure chemical ionization was compared with electrospray ionization and atmospheric pressure photoionization (APPI) as an interface of high-performance liquid chromatography (HPLC)-tandem mass spectrometry (MS/MS) for the determination of cyclosporin A (CsA) in biological fluids in support of in vivo pharmacodynamic studies. These ion sources were investigated in terms of their suitability and sensitivity for the detection of CsA. The effects of the eluent flow rate and composition as well as the nebulizer temperatures on the photoionization efficiency of CsA in the positive ion mode under normal-phase HPLC conditions were explored. The ionization mechanism in the APPI environment with and without the use of the dopant was studied using two test compounds and a few solvent systems employed for normal-phase chromatography. The test compounds were observed to be ionized mainly by proton transfer with the self-protonated solvent molecules produced through photon irradiation. Furthermore, ion suppression due to sample matrix interference in the normal-phase HPLC-APPI-MS/MS system was monitored by the postcolumn infusion technique. The applicability of these proposed HPLC-API-MS/MS approaches for the determination of CsA at low nanogram per milliliter levels in rat plasma was examined. These proposed methods were then compared with respect to specificity, linearity, detection limit, and accuracy.

Air Ionization↗