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PubMed2002

Ionization energies of multiply protonated polypeptides obtained by tandem ionization in Fourier transform mass spectrometers.

Ionization energies (IE) of [M + zH](z+) (z+) electrospray-produced polypeptides were determined by electron ionization in a Penning cell of 4.7 and 9.4 T Fourier transform mass spectrometers. For z = 1+ and substance P, the found IE value of 11.0 +/- 0.4 eV is in agreement with that obtained earlier for ions generated with matrix-assisted laser desorption/ionization. For higher z, the following values were found: 11.7 +/- 0.3 eV for 2+ of [Arg-8]-vasopressin, 11.1 +/- 0.6 eV for 2+ of substance P, 12.2 +/- 0.7 eV for 2+ of renin substrate, 13.3 +/- 0.4 eV for 3+ of B-chain of insulin and 14.6 +/- 0.6 eV for 4+ and 15.1 +/- 0.4 eV for 5+ of melittin. It was found that 90% of existing IE data on polypeptides in the 1.0-3.5 kDa mass range are described with <or=0.5 eV uncertainty by the empirical equation IE(z) = 9.8 + 1.1z. The average IE increase of 1.1 eV/charge is attributed to Coulombic repulsion. The deduced ionization energy of a neutral polypeptide molecule, 9.8 +/- 0.3 eV, is consistent with literature expectations.

Angiotensinogen↗
PubMed1994

Possible changes in luminal surface charge densities of small intestine membrane in the 4-7.4 pH range exhibit varied influence on the absorption rate constants of the ionized and un-ionized species of sulfadiazine in rats.

The overall apparent first-order rate constants (Kab) of small intestinal absorption of sulfadiazine were determined in rats in situ at various pHs (4.01-7.42) of the recirculation fluids at 32 degrees. The purpose of the study was to verify our hypothesis that the rate-determining step for the absorption of sulfonamides involves the formation of an "activated complex" consisting of a transient association of the sulfonamide molecules with the surface protein of the microvillus membrane. The Kab versus Fu (where Fu is the fraction of un-ionized sulfadiazine at a given pH) profile prepared according to the equation Kab = Ki+Fu(Ku-Ki), where Ki and Ku are the first-order rate constants of absorption of the ionized and un-ionized species, respectively, was clearly resolvable into two linear segments: one based on the data in the 4.01-6.43 pH range and the other based on the data in the 6.43-7.42 pH range. In view of the fact that the combined molar concentration of aspartic acid and glutamic acid in the microvillus membrane protein is about twice that of arginine and lysine, the resolution of the data into two linear segments suggested that the negative charge density of the microvillus surface membrane protein in the 6.43-7.42 pH range is greater than that in the 4.01-6.43 pH range. The Ku and Ki values of sulfadiazine were determined in each pH range. It was noted that Ku was greater than Ki in each pH range, and that Ki in the 4.01-6.43 pH range was greater than that in the 6.43-7.42 pH range.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1989

Comparison of amino acid sequence analysis by electron ionization and negative-ion chemical ionization mass spectrometry.

The positive electron ionization and negative chemical ionization mass spectra of 15 different derivatives of the tripeptide Phe-Ala-Leu have been compared. Total ion currents and ion currents of sequence-characterizing ions have been measured and compared. The negative-ion spectra, using 10% carbon dioxide in argon as moderator gas, proved to be simpler and contained more abundant sequence ions than the positive electron ionization spectra.

Amino Acid Sequence↗
PubMed2002

Retention of ionizable compounds on high-performance liquid chromatography XI. Global linear solvation energy relationships for neutral and ionizable compounds.

A global linear solvation energy relationship (LSER) that simultaneously models retention in reversed-phase liquid chromatography as a function of solute LSER descriptors and mobile phase pH and composition has been derived from both the local LSER model and the linear solvent strength theory. At most only 13 mobile phase parameters and seven solute parameters are required to establish the global LSER model for neutral and ionizable solutes. This model implies only one mobile phase and two solute parameters more than the model previously set for neutral solutes. The additional mobile phase and solute parameters account for the ionization of the solute. The model has been successfully tested for 30 solutes of different type (acids, bases and non ionizable compounds) at 10 different pH values in three different acetonitrile-water mobile phases.

Chromatography, High Pressure Liquid↗
PubMed1998

Identification of pesticides by liquid chromatography--particle beam mass spectrometry using electron ionization and chemical ionization.

Liquid chromatography-mass spectrometry (LC-MS) with a particle beam (PB) interface is used to separate and identify a group of pesticides. The mass spectra obtained under the different ionization modes, electron ionization (EI) and positive and negative chemical ionization (PCI and NCI) are compared. The operating conditions under each mode, determined by studying the influence on the ion abundance of the ion source temperature of the EI mode, and the gas pressure and ion source temperature in the methane CI were optimized. EI was more sensitive than PCI and NCI and of the latter two modes, NCI gave higher responses, especially for organophosphorus compounds. When on-line solid-phase extraction-LC-PB-MS was applied to real samples, limits of detection in full scan mode were in the range of 0.5 and 10 micrograms l-1 for EI. The analysis of real samples by on-line solid-phase extraction-LC-PB-MS enabled EI detection of one of the pesticides studied and confirmation by PCI and NCI. The combined EI/CI information also enabled the detection of some non-target compounds.

Chromatography, Liquid↗
PubMed1999

Mass spectrometric and tandem mass spectrometric behavior of nitrocatechol glucuronides: a comparison of atmospheric pressure chemical ionization and electrospray ionization.

The mass spectrometric (MS) and tandem mass spectrometric (MS/MS) behavior of six nitrocatechol-type glucuronides using atmospheric pressure chemical ionization (APCI) and electrospray ionization (ESI) was systematically studied, and the effect of operation parameters on the fragmentations are presented. The positive ion APCI- and ESI-MS spectra showed an intense protonated molecule and the respective negative ion spectra a deprotonated molecule with minimal fragmentation. The main fragment ions in the MS/MS spectra of the protonated and deprotonated molecules were [M + H - Glu]+ and [M - H - Glu]-, respectively, formed by the loss of the glucuronide moiety. The measured limits of detection indicated that ESI is a significantly more efficient ionization method than APCI in the negative and positive ion modes for the compounds studied. MS/MS was found to be less sensitive, but more reliable and simple than MS due to the absence of chemical noise.

Algorithms↗
PubMed1998

Derivatization for electrospray ionization mass spectrometry. 3. Electrochemically ionizable derivatives.

In this paper, the use of ferrocene-based "electrochemically ionizable" derivatives to enhance ES-MS analysis of simple alcohols, sterols, and phenols is discussed. These derivatives are designed to take advantage of the electrolysis process inherent to operation of the ES ion source for selective ionization. Derivatization procedures, electrochemical character of the derivatives, and the ES-MS operational parameters necessary to maximize electrochemical ionization and to enhance gas-phase detection are presented with reference to ferrocenecarbamate ester derivatives of a variety of alcohol standards, as well as the ferroceneboronate derivative of the diol, pinacol. Tandem mass spectrometric analysis of the derivatives (precursor and product ion spectra) is shown to provide derivative confirmation, enhanced detection, and additional analyte structure information. The utility of this derivatization approach for the selective detection of alcohols in complicated mixtures is demonstrated using a saw palmetto (Serenoa repens) fruit extract known to contain a variety of alcohols at low levels.

Alcohols↗
PubMed1999

Quantitation of monosaccharide isotopic enrichment in physiologic fluids by electron ionization or negative chemical ionization GC/MS using di-O-isopropylidene derivatives.

The aldonitrile pentaacetate and other derivatives lack ions in the electron ionization (EI) spectra possessing an intact hexose structure and thus must be analyzed by chemical ionization GC/MS in order to study multiple isotopomers. We report methods for quantitation of hexose di-O-isopropylidene acetate (IPAc) or pentafluorobenzoyl (PFBz) esters. These were prepared in a two-step procedure using inexpensive reagents that do not adversely impact the isotopomer structure of the sugar. The acetate derivative possesses an abundant [M - CH3] ion in the EI spectrum which is suitable for quantitative analysis of isotopomers. The negative chemical ionization (NCI) spectrum of the corresponding pentafluorobenzoyl derivative has a dominant molecular anion. Moreover, the PFBz derivative is about 100-fold more sensitive than the acetate, which offers some advantages for analysis of minor hexoses found in plasma. Isotopic calibration curves of [U-13C]glucose are linear over the 0.1-60% tracer/tracee range tested. The useful range for isotopic tracer studies is 25-2500 pmol for EI analysis of the acetate derivative and 0.1-55 pmol for NCI analysis of PFBz derivative (sample amount injected). For most studies where sample size is not limited, EI-GC/MS analysis of the IPAc derivative is preferred. NCI-GC/MS analysis is reserved when sample size is limiting or when studies involve hexoses other than glucose that are normally present at low concentration.

Alkenes↗
PubMed2006

Ionization and double ionization of small water clusters.

The valence ionization and double ionization spectra of the water molecule, of the water dimer, and the cyclic water clusters (H2O)3 and (H2O)4 are calculated by ab initio Green's function methods and discussed in some detail. Particular attention is paid to the analysis of the development of the spectra with increasing cluster size. Electronic decay following inner valence ionization is addressed and a crude estimate for the kinetic energy spectrum of the secondary electrons is given for the clusters.

Journal Article↗
PubMed2005

Ionized magnesium levels and the ratio of ionized calcium to magnesium in asthma patients before and after treatment with magnesium.

OBJECTIVE: Prior studies have been equivocal about the efficacy of magnesium therapy in acute asthma exacerbations. We hypothesize that pretreatment ionized magnesium (Mg(2+)) levels and/or the ratio of ionized calcium to ionized magnesium (Ca(2+)/Mg(2+)) may have been confounding variables in these previous studies. Here, we report on the incidence of abnormal divalent ion levels in our asthma population. MATERIAL AND METHODS: The study was designed as a randomized, double-blind, placebo-controlled trial of intravenous magnesium. Inclusion criteria were: age >18 years, percentage predicted forced expiratory volume (FEV(1)) <75 % after an initial beta-agonist. African-American patients (AA) at an urban university hospital were randomized to 2 g IV Mg or placebo. Mg(2+) and Ca(2+)/Mg(2+) levels were measured pre- and post-infusion. Data were reported as means+/-SD. Student's t-test and Fisher's exact test were used where appropriate (alpha = 0.05, two tailed). RESULTS: Fifty-five AA patients (mean age of 42.7 years+/-15.6 years, range 18-75 years) were studied. A significantly (p<0.05) lower level of Mg(2+) was found in asthma (AS) patients compared with that in the AA group, by 0.03 mmol/L (95 % CI, 0.007-0.053 mmol/L). The AS group had a mean increase in Ca(2+)/Mg(2+) ratios over the AA group, of 0.27 (95 % CI, 0.16-0.38); 100 % of patients with abnormal divalent ion levels were corrected with IV magnesium. CONCLUSIONS: We identified a subgroup of asthmatic patients with significant abnormalities in their divalent ion concentrations, which was corrected with IV magnesium.

Adolescent↗
PubMed2001

Ionizational instability in an ionized inert gas flowing in a magnetic field.

We discuss the evolution of ionization instability, which gives rise to the formation of regular structures in plasma. The experiment was conducted in a disk magnetohydrodynamic channel in Xe. The evolution of luminous inhomogeneities, and changes in the luminosities of individual inhomogeneities, are traced. Electron concentrations and temperatures in fluctuations are measured. Average values of the electron concentration and temperature, plasma conductivity, and the Hall parameter are estimated. The average conductivity and Hall parameter are compared with the effective values. The influence of positive and negative fluctuations in plasma parameters on the enhancement of ionization oscillations and average current is analyzed. Based on experimental data, a mechanism is suggested that is responsible for the increase in the effective conductivity of ionizationally unstable inert gas plasma with increasing magnetic induction.

Journal Article↗
PubMed2004

Trace analysis of organics in air by corona discharge atmospheric pressure ionization using an electrospray ionization interface.

A corona discharge ion source operating at atmospheric pressure in the point-to-plane configuration was constructed by reconfiguring the ion source of a commercial electrospray ionization (ESI) quadrupole mass spectrometer. This new source allows direct air analysis without modification to the mass spectrometer. Detection and quantitation of semi-volatile compounds in air is demonstrated. The analytical performance of the system was established using the chemical warfare agent simulants methyl salicylate and dimethyl methylphosphonate. Limits of detection are 60 pptr in the negative-ion mode and 800 pptr in the positive-ion mode for methyl salicylate and 800 pptr in the negative-ion mode and 3.6 ppb in the positive-ion mode for dimethyl methylphosphonate. A linear response was observed from 60 pptr to 8 ppb for methyl salicylate in air in the negative-ionization mode. Cluster ion formation versus production of analyte ions was investigated and it was found that dry air or an elevated capillary interface temperature (130 degrees C) was needed to avoid extensive clustering, mostly of water. Reagent gases are not needed as proton sources, as is usually the case for atmospheric pressure chemical ionization, and this, together with the simplicity, sensitivity and speed of the technique, makes it promising for miniaturization and future field studies.

Journal Article↗
PubMed2004

Liquid injection field desorption ionization: a new tool for soft ionization of samples including air sensitive catalysts and non-polar hydrocarbons.

Mass analysis of air sensitive samples like organometallic catalysts requires inert sample preparation to avoid degradation of such reactive molecules. Non-polar samples like hydrocarbons are stable but nonetheless need soft ionization to reduce congestion of fragment peaks for analysis of complex mixtures. This paper describes a novel type of probe that combines the advantages of field ionization and field desorption (FI/FD) with an efficient liquid inlet. The new method is called Liquid Injection Field Desorption Ionization (LIFDI). Sample solutions are delivered to the emitter wire inside of the ion source without breaking the vacuum. Sample preparation is reduced to dipping the LIFDI transfer capillary into another sample vial. In case of air sensitive samples kept under inert gas, the preparation is inert without special experimental effort. The new tool provides for a significantly raised sample throughput at excellent sensitivity.

Journal Article↗
PubMed2006

Phase-dependent excitation and ionization in the multiphoton ionization regime.

We theoretically study the dependence of atomic excitation and ionization on the carrier envelope phase of few-cycle laser pulses in the multiphoton ionization regime. Our theoretical results for the hydrogen atom based on the solution of the 3D time-dependent Schrödinger equation show that the strong phase dependence can be seen in not only total ionization, but also bound-state population under the weak laser intensity regime.

Journal Article↗
PubMed2001

RNA-RNA noncovalent interactions investigated by microspray ionization mass spectrometry.

Electrospray ionization mass spectrometry is playing an increasing role in the study of noncovalent interactions involving biomolecules. RNA-RNA complexes are important in many areas of biology, including RNA catalysis, RNA splicing, ribosome function, and gene regulation. Here, microelectrospray mass spectrometry (microESI-MS) is used to study noncovalent base-pairing interactions between RNA oligonucleotides, an area not previously explored by this technique. Using a set of complementary RNA oligonucleotides, we demonstrate the formation of the expected double-helical RNA complexes composed of three distinct oligonucleotides. The ability to study specific RNA noncovalent interactions by microESI-MS has the potential to provide a unique method by which to analyze and assign precise molecular masses to RNA-RNA complexes.

Base Sequence↗
PubMed2002

Atmospheric pressure laser desorption/chemical ionization mass spectrometry: a new ionization method based on existing themes.

We have designed and constructed an atmospheric pressure laser desorption/chemical ionization (AP-LD/CI) source that utilizes a laser pulse to desorb intact neutral molecules, followed by chemical ionization via reagent ions produced by a corona discharge. This source employs a heated capillary atmospheric pressure inlet coupled to a quadrupole ion trap mass spectrometer and allows sampling under normal ambient air conditions. Preliminary results demonstrate that this technique provides approximately 150-fold increase in analyte ions compared to the ion population generated by atmospheric pressure infrared matrix-assisted laser desorption/ionization (AP-IR-MALDI).

Air Pressure↗
PubMed2002

Matrix-assisted ultraviolet laser-desorption ionization and electrospray-ionization time-of-flight mass spectrometry of sulfated neocarrabiose oligosaccharides.

Several commercial sulfated neocarrabiose oligosaccharides were analyzed by matrix-assisted ultraviolet laser-desorption ionization time-of-flight mass spectrometry (UV-MALDI-TOF-MS). UV-MALDI-TOF-MS was carried out in the linear and reflectron modes and, as routine, in both the positive- and negative-ion modes. 2,5-Dihydroxybenzoic acid and nor-harmane were used as matrices. In the positive- and negative-ion modes, with both matrices, peaks corresponding to (M+Na)(+) and (M-Na)(-) ions, respectively, were obtained, with only some signals due to glycosidic linkage cleavages (prompt fragmentation). With 2,5-dihydroxybenzoic acid abundant matrix signals were observed; nor-harmane afforded very few matrix signals in both ion modes, but more desulfation (prompt fragmentation) of the compounds occurred. When the desorption/ionization process was highly efficient, the post-source decay (PSD) fragmentation patterns were also investigated; most of the fragments detected derived from glycosidic linkage cleavages. Electrospray-ionization time-of-flight mass spectrometry (ESI-TOF-MS) in the negative-ion mode confirmed, with the observation of the (M-Na)(-) and the multiply charged anions, the identity and the purity of the samples.

Carbohydrate Sequence↗