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PubMed2006

Factors affecting primary ionization in dopant-assisted atmospheric pressure photoionization (DA-APPI) for LC/MS.

The sensitivity of dopant-assisted atmospheric pressure photoionization (DA-APPI) for LC/MS is generally reduced at higher solvent flow rates. Theory suggests that quenching of excited-state precursors to the dopant ions, via collisions with vaporized solvent molecules, may be one mechanism responsible for this trend. To ascertain if the primary rate of ionization is affected by quenching, experiments were performed utilizing an ionization detector to determine the primary ion current generated by irradiating vaporized mixtures of toluene dopant and methanol solvent. The results indicate that no loss of primary ion current occurs as the solvent flow is increased, provided the dopant-to-solvent ratio is held constant. Additional primary ion current can always be generated by increasing the dopant flow rate and/or the lamp power. Thus, quenching of excited-state precursors to the dopant ions, leading to a reduction in the primary rate of ionization, is not the mechanism responsible for the observed loss of sensitivity at higher liquid solvent flow rates.

Journal Article↗

PubMed2004

Determination of Delta9-tetrahydrocannabinol from rabbit plasma by gas chromatography-mass spectrometry using two ionization techniques.

The purpose of the study was to develop a gas chromatography-mass spectrometric (GC-MS) method for the identification and quantitation of Delta(9)-tetrahydrocannabinol (THC) in rabbit plasma. Two ionization techniques were utilized for GC-MS: electron impact ionization (EI) after i.v. administration and negative chemical ionization (NCI) after sublingual administration. THC was isolated from plasma by solid phase extraction and derivatized by either trimethylsilylation (EI) or trifuoroacetylation (NCI), with deuterated THC as an internal standard. The validity of analytical method was confirmed by investigating selectivity, limit of quantitation, linearity, accuracy, precision, recovery and stability of the analyte. The method proved to be selective, linear, accurate and precise over a range of 10-430 and 0.3-530 ng/ml of THC in plasma for EI and NCI, respectively. The extraction recovery was >81% for each concentration level studied, and the analyte was shown to be stable during storage and sample preparation. The method was applied successfully in analysing THC from rabbit plasma.

Absorption↗

PubMed2005

Multislice theory of fast electron scattering incorporating atomic inner-shell ionization.

It is demonstrated how atomic inner-shell ionization can be incorporated into a multislice theory of fast electron scattering. The resulting theory therefore accounts for both inelastic scattering due to inner-shell ionization and dynamical elastic scattering. The theory uses a description of the ionization process based on the angular momentum representation for both the initial and final states of the atomic electron. For energy losses near threshold, only a small number of independent states of the ejected atomic electron need to be considered, reducing demands on computing time, and eliminating the need for tabulated inelastic scattering factors. The theory is used to investigate the influence of the collection aperture size on the spatial origin of the silicon K-shell EELS signal generated by a STEM probe. The validity of a so-called local approximation is also considered.

Journal Article↗

PubMed1996

Electrostatic coupling between retinal isomerization and the ionization state of Glu-204: a general mechanism for proton release in bacteriorhodopsin.

The pKa values of ionizable groups that lie between the active site region of bacteriorhodopsin (bR) and the extracellular surface of the protein are reported. Glu-204 is found to have an elevated pKa in the resting state of bR, suggesting that it corresponds to the proton-releasing group in bR. Its elevated pKa is predicted to be due in part to strong repulsive interactions with Glu-9. Following trans-cis isomerization of the retinal chromophore and the transfer of a proton to Asp-85, polar groups on the protein are able to interact more strongly with the ionized state of Glu-204, leading to a substantial reduction of its pKa. This suggests a general mechanism for proton release in which isomerization and subsequent charge separation initially produce a new electrostatic balance in the active site of bR. Here it is proposed that those events in turn drives a conformational change in the protein in which the ionized state of Glu-204 can be stabilized through interactions with groups that were previously inaccessible. Whether these groups should be identified with polar moieties in the protein, bound waters, or Arg-82 is an important mechanistic question whose elucidation will require further study.

Bacteriorhodopsins↗

PubMed1997

Ionization, partitioning, and dynamics of tryptophan octyl ester: implications for membrane-bound tryptophan residues.

The presence of tryptophan residues as intrinsic fluorophores in most proteins makes them an obvious choice for fluorescence spectroscopic analyses of such proteins. Membrane proteins have been reported to have a significantly higher tryptophan content than soluble proteins. The role of tryptophan residues in the structure and function of membrane proteins has attracted a lot of attention. Tryptophan residues in membrane proteins and peptides are believed to be distributed asymmetrically toward the interfacial region. Tryptophan octyl ester (TOE) is an important model for membrane-bound tryptophan residues. We have characterized this molecule as a fluorescent membrane probe in terms of its ionization, partitioning, and motional characteristics in unilamellar vesicles of dioleoylphosphatidylcholine. The ionization property of this molecule in model membranes has been studied by utilizing its pH-dependent fluorescence characteristics. Analysis of pH-dependent fluorescence intensity and emission maximum shows that deprotonation of the alpha-amino group of TOE occurs with an apparent pKa of approximately 7.5 in the membrane. The fluorescence lifetime of membrane-bound TOE also shows pH dependence. The fluorescence lifetimes of TOE have been interpreted by using the rotamer model for the fluorescence decay of tryptophan. Membrane/water partition coefficients of TOE were measured in both its protonated and deprotonated forms. No appreciable difference was found in its partitioning behavior with ionization. Analysis of fluorescence polarization of TOE as a function of pH showed that there is a decrease in polarization with increasing pH, implying more rotational freedom on deprotonation. This is further supported by pH-dependent red edge excitation shift and the apparent rotational correlation time of membrane-bound TOE. TOE should prove useful in monitoring the organization and dynamics of tryptophan residues incorporated into membranes.

Fluorescent Dyes↗

PubMed2006

[Modifications in intact parathyroid hormone, total serum calcium, and ionized calcium levels in the surgery of primary hyperparathyroidism for single adenoma].

INTRODUCTION: We evaluated total serum calcium (TSC) and ionized calcium (IC) and their correlation with intraoperative intact parathyroid hormone (iPTH) in the surgery of primary hyperparathyroidism in patients with a single adenoma. MATERIAL AND METHOD: We performed a prospective, blind trial with determination of iPHT, TSC and IC in a cohort of surgical patients (n = 279; 244 were valid for the study) who underwent surgery in the Department of Surgery, Hospital de Cruces, between October 1999 and April 2006. Total calcium, ionic calcium and iPTH were measured in the outpatient department, on admission and intraoperatively (at anesthesia induction and every 5 minutes after surgical excision). RESULTS: Levels of calcium and iPTH were corrected in 234 (95.9%) patients. iPTH decreased from abnormal preoperative values of 294.43 +/- 286.38 pg/ml to 97.89 +/- 121.01 mg/dl (minute 5), 58.58 +/- 58.37 pg/ml (minute 10), 44.62 +/- 54.77 pg/ml (minute 15), and 38.42 +/- 51.72 pg/ml (minute 20). TSC decreased from preoperative values of 10.93 +/- 1.04 mg/dl to 10.2 +/- 0.97 mg/dl (minute 5), 10.17 +/- 1.00 mg/dl (minute 10), 10.12 +/- 0.98 mg/ml (minute 15), and 10.09 +/- 1.03 mg/ml (minute 20). The results for ionized calcium were as follows: 4.90 +/- 0.63 mg/dl at induction, 4.84 +/- 0.61 mg/dl (minute 5), 4.84 +/- 0.66 mg/dl (minute 10), 4.82 +/- 0.63 mg/dl (minute 15), and 4.82 +/- 0.63 mg/dl (minute 20). Frozen samples were conclusive for parathyroid tissue (19.56 +/- 15.3 after excision). CONCLUSIONS: Intraoperative total calcium levels may help to predict adequate elimination of parathyroid tissue in primary hyperparathyroidism when intraoperative iPTH is not available. Ionized calcium levels did not show the same decrease.

Adenoma↗

PubMed1976

Identification of drugs using a gas chromatography-mass spectrometry system equipped with electron impact-chemical ionization and electron impact-field ionizationfield desorption combination sources.

In a case of toxicological analysis by gas chromatography (GC)-mass spectrometry-data system, chemical ionization (CI), field ionization (FI) and electron impact data are shown to lead rapidly to assignments for all significant GC peaks. Valuable data include not only full spectra in the three ionization modes, but also exact mass measurements in the FI or CI mode. Such measurements, obtained by a dynamic peak-matching technique, lead to the elemental composition of the compounds of interest. This knowledge makes the assignment of key GC peaks unequivocal and provides an extremely high level of confidence regarding the identity of whole metabolite series. It is also shown that the nature of the FI information is very similar to that obtained from CI data.

Aminopyrine↗

PubMed1979

Determination of clonazepam in human plasma by gas chromatography--negative ion chemical ionization mass spectrometry.

Our previously reported gas chromatographic--mass spectrometric assay for clonazepam using positive ion chemical ionization has been modified to employ negative ion chemical ionization. The sensitivity using this technique (less than 0.1 ng/ml) is approximately twenty times better than the positive ion chemical ionization procedure with similar precision. The method was used to determine the clonazepam plasma concentration in one human for 96 h following a single 2-mg dose of clonazepam.

Benzodiazepinones↗

PubMed2001

Monitoring of antifouling agents in water samples by on-line solid-phase extraction-liquid chromatography-atmospheric pressure chemical ionization mass spectrometry.

An automatic method for determining diuron, irgarol 1051, folpet and dichlofluanid in seawater samples have been developed. This method is based on the on-line coupling of solid-phase extraction (SPE) with a highly crosslinked polymeric sorbent, LiChrolut EN, to liquid chromatography followed by atmospheric pressure chemical ionization (APCI) and mass spectrometry. The operational parameters affecting the APCI interface have been studied in both positive and negative ionization modes. The use of LiChrolut EN in the SPE produced recoveries of over 85% for all the compounds when 100 ml of seawater sample was preconcentrated. Calibration was carried out in both ionization modes and in full-scan and selected-ion monitoring (SIM). The method allowed all the analytes to be detected at 5 ng l(-1) in SIM acquisition mode except folpet, which, because of its low response, could only be detected at 250 ng l(-1). The method was used to analyse water samples taken from five different marina and fishing ports along the coast of Tarragona, Catalonia (Spain), over a 5-month period. Diuron and irgarol 1051 were detected and quantified in most samples at concentration levels ranging from 27 to 420 ng l(-1) for diuron and from 15 to 511 ng l(-1) for irgarol 1051.

Atmospheric Pressure↗

PubMed2001

Laser-enhanced ionization and laser-induced atomic fluorescence as element-specific detection methods for gas chromatography. Application to organotin analysis.

We have demonstrated that flame laser-enhanced ionization (LEI) and flame laser-induced atomic fluorescence (LIAF) techniques can be used as, alternative sensitive detectors for gas chromatographic (GC) analysis of organotin compounds. These two element-specific detection methods are free from interferences from the organic solvent. Two types of LEI schemes for Sn detection are employed. For the two-step LEI scheme (TLEI), the tin atoms in the flame were stepwise excited and then ionized collisionally. In contrast, in detection with the single-step LEI scheme (SLEI), only one dye laser is used. For the analysis of tetramethyltin and tetraethyltin, the GC-TLEI, GC-SLEI, and GC-LIAF systems yield linear dynamic ranges of 0.015-400, 0.39-600 and 0.5-600 ng, respectively. The corresponding detection limits reach 0.15, 3.9 and 5.0 mg/l, with absolute quantities corresponding to 15, 390 and 500 pg, respectively, for an injection volume of 0.1 microl. These detection methods prove to be more sensitive and selective than the conventional flame ionization detection, which achieves absolute detection limits of 800 and 1667 pg for tetraethyltin and tetramethyltin, respectively, under identical GC conditions.

Calibration↗

PubMed1989

Screening of steroids in horse urine and plasma by using electron impact and chemical ionization gas chromatography-mass spectrometry.

Gas chromatography with chemical ionization mass spectrometry and selected-ion monitoring provided a sensitive method for the screening and confirmation of steroids in horse urine and plasma. Chemical ionization mass spectrometry was more sensitive than the electron impact ionization mass spectrometry for most of the steroids except for testosterone, prednisone-metabolite-2 and prednisolone-metabolite-2. The chromatographic conditions used in this study provided clean separation of different natural and synthetic steroids. Approximately 75-85% of the steroids added to plasma and approximately 65-70% of the steroids added to urine were recovered by the extraction procedure used in this study.

Animals↗

PubMed1989

Trace detection of modified DNA bases via moving-belt liquid chromatography-mass spectrometry using electrophoric derivatization and negative chemical ionization.

Electrophoric derivatives of 5-methylcytosine and 5-hydroxymethyluracil nucleobases are determined using high-performance liquid chromatography-mass spectrometry coupled via a moving-belt interface. Standards as well as samples derived from DNA are analysed. As little as 9.9 pg (signal-to-noise ratio 5) and 180 fg (signal-to-noise ratio 10) of the respective nucleobases are detected in the electron-capture negative chemical ionization mode, and linear responses are observed over a moderate dynamic range. In a comparison study, liquid chromatography-electron-capture negative chemical ionization mass spectrometry is found to have a sensitivity comparable to gas chromatography-electron-capture negative chemical ionization mass spectrometry for 5-hydroxymethyluracil. A detection limit of 60 fg (signal-to-noise ratio 5) by gas chromatography-mass spectrometry is only three-fold better than the amount detected by liquid chromatography-mass spectrometry using the same mass spectrometer.

5-Methylcytosine↗

PubMed2002

Characterization of triglycerides in vegetable oils by silver-ion packed-column supercritical fluid chromatography coupled to mass spectroscopy with atmospheric pressure chemical ionization and coordination ion spray.

Characterization of triglycerides in vegetable oils was achieved by silver-ion packed-column supercritical fluid chromatography (SI-pSFC) with mass spectrometric detection. Hyphenation was made using commercially available liquid chromatography-mass spectrometry (LC-MS) interfaces without any modification. A make-up fluid was delivered through a T-piece placed before or after the SFC restrictor by means of a high pressure pump. Atmospheric pressure chemical ionization (APCI) and coordination ion spray (CIS) with silver ions were used as ionization modes. Compared to UV detection, the sensitivity was increased by a factor of 100. Both ionization modes are generating similar structural information. Molecular ions [M-H]+ or [M-Ag(-)] are observed in the mass spectra with exception of the saturated triglycerides for which only CIS gives intense molecular ions. The position at which the fatty acids are esterified to the glycerol backbone can be elucidated by pSFC-APCI although it remains speculative whether this is valid for highly unsaturated triglycerides because reference compounds are not available to proof this.

Atmospheric Pressure↗

PubMed2003

Identification of acylated xanthone glycosides by liquid chromatography-atmospheric pressure chemical ionization mass spectrometry in positive and negative modes from the lichen Umbilicaria proboscidea.

The xanthoside composition of the crude extract of Umbilicaria proboscidea (L.) Schrader was characterized using LC-UV diode array detection and LC-atmospheric pressure chemical ionization (APCI) MS methods. The presence of acylated xanthone-O-glucosides was determined by both positive and negative ion LC-APCI-MS methods. Based on UV and MS spectral data and NMR spectroscopy, a total of 14 compounds (6-O-acylated umbilicaxanthosides A and B) were identified in U. proboscidea for the first time. In order to further develop the applicability of LC-MS techniques in phytochemical characterization, the effect of different ionization energy on fragmentation was studied using APCI. The optimal ionization conditions were achieved in positive ion APCI by using ammonium acetate buffer and in negative ion APCI by using formic acid (pH 4).

Acylation↗

PubMed1999

Detection of new sequences of peptaibol antibiotics trichotoxins A-40 by on-line liquid chromatography-electrospray ionization mass spectrometry.

Using high-performance liquid chromatography (HPLC) coupled to electrospray ionization mass spectrometry (ESI-MS) the sequences of the microheterogeneous peptide mixture of the 18-residue "peptaibol" antibiotics trichotoxins A-40, isolated from the mold Trichoderma viride strain NRRL 5242, were reinvestigated. The structures of two major and one minor component [J. Chromatogr., 296 (1984) 236] could be confirmed and hitherto not known sequences of a further major and two minor peptides could be determined. It is demonstrated that ESI-MS in the positive ionization mode is advantageously completed by applying negative ionization. The methods used make possible the sequence determination of components of peptaibols without previous isolation and allow, in certain cases, sequencing of peptides which are incompletely or not resolved by HPLC.

Amino Acid Sequence↗

PubMed1997

Mathematical description of synergistic interaction of hyperthermia and ionizing radiation.

A simple mathematical model of simultaneous combined action of ionizing radiation and hyperthermia has been proposed. The model suggests that the synergistic interaction of ionizing radiation and hyperthermia is expected to result from the additional lethal damage arising from the interaction of sublesions induced by both agents. These sublesions are considered nonlethal after each agent taken alone. The model was applied to the quantitative analysis of the simultaneous action of hyperthermia and ionizing radiation. It predicts the dependence of synergistic interaction of the ratio of lethal events produced by every agent used, as well as the maximal value of the synergistic effect, conditions at which the maximal interactive effect can be achieved, and the dependence of synergistic effect on dose rate. The predictions of the model have been tested with four experimental data sets reported in the literature. The theory appears to be appropriate and the conclusions valid.

Animals↗

PubMed2002

Optimization of the ionization conditions for the trace analysis of PCDD/PCDF with ion trap MS/MS.

Commercial ion trap mass spectrometer provides easy-to-operate MS/MS analysis for the determination of PCDD/ PCDF. The limit of quantification is appropriately low (0.2 pg for 23478-P5CDF) because all the stages are performed in the trap and sample losses associated with the ion transportation are minimized. However, if excessive ions are injected into the trap, its electrical fields are distorted and an overall reduction in performance arises. Ionization condition is an important parameter as it affects the amount of the total ions produced. If the amount of interfering compounds are negligible, such as standard solution or cleaned-up sample, lower ionization condition (e.g. electron energy: 30 eV. emission current (EC): 150 microA) is preferable. On the contrary, in case excessive interfering ions are coexisting with PCDD/PCDF, such as crude extract or semi-cleaned-up sample, the ionization condition should be high (e.g. electron energy: 90 eV, EC: 350 microA) for the reproductive quantification.

Benzofurans↗

PubMed1996

The effect of ionizing radiation on signal transduction: antibodies to EGF receptor sensitize A431 cells to radiation.

What determines the degree of cell-resistance or sensitivity to ionizing radiation is not yet known. As a corollary to the ability of ceramide to induce apoptosis, some questions arise as to whether malignant cells escape apoptosis because of their inability to mount a ceramide response to inducers of apoptosis. To shed more light on the molecular mechanisms of tumor cell response to radiation, we tested whether exposure to ionizing radiation (of 200-1000 cGy) is associated with changes in ceramide levels in A431 tumor epithelial cells and whether the ability of ceramide to induce apoptosis is inhibited by protein kinase C (PKC) activation. Our studies demonstrate an immediate decrease in cellular levels of ceramide in response to radiation, while sphingosine levels increase. Under the same conditions the cellular 1,2-diacylglycerol (DAG) levels decrease as well, being accompanied by the translocation of PKC alpha from the membrane to the cytoplasm. Elevation of membrane PKC levels by 12-O-tetradecanoylphorbol 13-acetate (TPA) treatment had no effect on cell survival after irradiation, while treatment with EGF during and after irradiation augmented cell survival. Moreover, monoclonal antibodies to the EGF receptor (EGFR) sensitize cells to radiation by facilitating radiation-induced apoptosis. It is thus plausible that in human Squamous carcinoma cells, radiation activates predominantly the EGFR to induce resistance, while both sphingomyelin and PKC signal transduction pathways are deactivated and demonstrate no significant role in the modulation of the sensitivity or the resistance of A431 cells to ionizing radiation.

Antibodies↗