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PubMed1986

Liquid ionization mass spectrometry of phospholipids.

Several phosphatidylcholines (PC) and a phosphatidylethanolamine (PE) were subjected to liquid ionization (LI) mass spectrometry, in which a sample is ionized through energy transfer from metastable argon atoms under atmospheric pressure. Commercially available and synthesized, saturated or unsaturated fatty acid containing phospholipids and their mixtures were studied. A sample either as a concentrated chloroform-methanol solution or with glycerol (matrix) gave characteristic peaks such as MH+ and four fragment ions. One of the fragment ions (e.g., m/z 551 of PC 16:0, 16:0) containing both fatty acid residues has been commonly observed with other ionization methods such as CI, FD, and FAB, but the other fragment ions have not been observed in other mass spectra with one exception on desorption CI. Ions b and d (e.g., m/z 464 and 328, respectively, for PC 16:0, 16:0) contain one fatty acyl residue and the other ion containing the phosphorylcholine moiety appears at m/z 196 for PC. Thus the masses of the MH+ ion and these fragment ions provide useful structural information even in the case of a mixture. The ion b (e.g., m/z 488 of PC 18:0, 18:2) observed during an early period of heating was formed mainly by the loss of one acyl group at sn-1 of the glycerol backbone and thus may be used to differentiate the positional specificities of the constituent fatty acids. The temperature of the sample, however, should be controlled precisely, because it has a significant effect on the mass spectrum. The present method (LI) also provided useful information for a mixture of PC and PE.

Mass Spectrometry↗
PubMed1991

Kinetics of the hydrolysis of monodispersed dihexanoylphosphatidylcholine catalyzed by bovine pancreatic phospholipase A2: roles of Ca2+ binding and ionizations of amino acid residues in the active site.

Phospholipases A2 (PLA2s) are classified into two groups, I and II, according to differences in the polypeptide-chain length and intramolecular disulfide bondings. The hydrolysis of monodispersed 1,2-dihexanoyl-sn-glycero-3-phosphorylcholine (diC6PC), catalyzed by bovine pancreas PLA2 (Group I) was studied at 25 degrees C and ionic strength 0.2, and the initial velocity data were analyzed by means of the Michaelis-Menten equation. The Michaelis constant, Km, was found to be practically independent of Ca2+ concentration and also of pH value. The latter result indicates no participation of the ionizable groups in the active site in the substrate binding. The pH-dependence curve of the logarithm of the catalytic center activity, kcat, obtained in the presence of a practically saturating amount of Ca2+, showed three transitions ascribable to the participation of three ionizable groups with pK values of 5.00, 8.40, and 9.50. The respective groups were tentatively assigned to the catalytic group His 48, the N-terminal alpha-amino group, and invariant Tyr 52, which is located in close proximity to the imidazole ring of His 48. Deprotonation of His 48 and protonation of Tyr 52 were shown to be essential to the catalysis. The importance of the ionization state of the alpha-amino group was also indicated.(ABSTRACT TRUNCATED AT 250 WORDS)

Amino Acids↗
PubMed1975

Low plasma ionized calcium and response to calcium therapy in critically ill man.

Marked lowering of plasma ionized calcium concentrations [Ca++] occurred in eight patients (2 days to 54 years old) who required extensive pharmacologic support of the circulation. [Ca++]'s ranged from 0.21 to 0.53 mM. Only one patient survived. The hypocalcemia occurred in the absence of massive transfusion of citrated whole blood or well after such transfusions had been discontinued. These abnormally low concentrations of ionized calcium were not readily corrected by intravenous administration of calcium salts in doses generally recommended. The process responsible for inadequate hemodynamic function appeared to be associated with a severe disturbance in calcium metabolism. Contribution of the latter to the severity of hemodynamic deterioration is unclear, and little benefit from intravenous calcium therapy was found. In two patients, normal [Ca++] could not be restored by administration of CaCl2 alone, but [Ca++] rose to normal following continued calcium replacement therapy in conjunction with increased isoproterenol infusion. There was no predictable relationship between total and ionized plasma calcium concentrations. Thus, measurement of total calcium provided no indication of the level of the biologically active moiety. [Ca++] was low with both normal and low plasma pH values. The data suggest that a very high infusion rate of CaCl2 may required to restore [Ca++] to normal and that hypocalcemia occurring during low-flow states often cannot be corrected by calcium therapy alone. It is recommended that calcium replacement therapy be undertaken only with close monitoring of [Ca++].

Adolescent↗
PubMed1988

An in vitro evaluation of ionized calcium levels and clotting in red blood cells diluted with lactated Ringer's solution.

The addition of lactated Ringer's solution which contains calcium (RL) to citrated blood products poses a potential risk whenever the level of ionized calcium reaches a concentration capable of catalyzing the coagulation cascade. RL solution is used extensively as both a replacement and a maintenance fluid in the operating room. However, major surgical procedures often require replacement of blood components as well, during the operation. This study examines the in vitro propensity for coagulation when red blood cells (RBC) are diluted with RL. Seven mixtures with different ratios of RBC to RL were prepared from each of 23 units of RBC. These mixtures were analyzed for ionized calcium, total calcium, and pH, and were checked for any indication of coagulation. Nineteen additional RBC units were split into two parts which were mixed with equal volumes of normal saline (NS) or RL. These mixtures were filtered (40 micron) and the weight gain of the paired filters compared. From these studies, the authors identified a threshold value for ionized calcium (0.23 mM/L) below which the probability of clot formation is less than 0.01. This concentration is not reached if the RBC to RL volume ratio is 2:1 or greater. As much as 100 ml of RL can be added to a unit of RBC without exceeding the threshold value. Although not advocating the routine use of RL as a diluent for RBC, the authors conclude that, within the guidelines described, calcium containing salt solutions may be used to dilute blood products.

Blood Coagulation↗
PubMed1990

Ionization and hemodynamic effects of calcium chloride and calcium gluconate in the absence of hepatic function.

Serial serum ionized calcium concentrations were measured before and after administration of either calcium chloride or calcium gluconate during the anhepatic stage of liver transplantation in 15 patients to determine the release of ionized calcium in the absence of hepatic function. When hypocalcemia (Ca++ less than 0.8 mM) occurred during the anhepatic stage, patients were randomly assigned to treatment with chemically equivalent doses of either calcium chloride (10 mg/kg, n = 8) or calcium gluconate (30 mg/kg, n = 7). Serum concentrations of ionized calcium and citrate, hematocrit, arterial blood gas tensions, acid-base state, and hemodynamic profiles were determined before and up to 10 min after calcium therapy. In both groups of patients initial similar and rapid increases in Ca++ (0.98 +/- 0.14 mM in the calcium chloride group and 1.05 +/- 0.10 mM in the calcium gluconate group) were followed by gradual decreases over the next 10 min. Measured hemodynamic values were similar in the two groups, and neither group showed improvement in cardiovascular function after calcium therapy, possibly because of the decrease in preload that occurred during the anhepatic stage. Equally rapid increases in Ca++ after administration of calcium chloride and gluconate in the anhepatic state suggest that calcium gluconate does not require hepatic metabolism for the release of Ca++ and is as effective as calcium chloride in treating ionic hypocalcemia in the absence of hepatic function.

Adult↗
PubMed1999

Subpicosecond pulse laser absorption by an overdense plasma with variable ionization.

Transient ionization of an overdense plasma produced by a subpicosecond, p-polarized obliquely incident pulse laser of moderate intensity (10(16)-10(18) W/cm(2)) changes the plasma heat transfer via processes dominated by the return current and the absorption rate via ion acceleration. To explore the effect of variable ionization, a hybrid one-dimensional electro-magnetic particle-in-cell method that conforms to a direct solution of the Fokker-Planck-Landau equation is applied. A method that includes the Langevin equation to account for Coulomb collisions and the average ion model to calculate the nonlocal thermodynamic equilibrium ionization balance provides good agreement between the computed absorption and the measured results.

Journal Article↗
PubMed2002

Two-dimensional collision of probe photons with relativistic ionization fronts.

The collision of a probe laser pulse with a relativistic ionization front is analyzed via two-dimensional ray-tracing theory and simulations. It is shown that collisions in higher dimensions lead to new regimes for the frequency upshift of the probe photons; the frequency upshift can be considerably higher for particular collision angles that maximize the interaction length with the ionization front gradient. Finite ionization fronts also lead to angle-dependent frequency upshifts, thus acting as diffraction gratings.

Journal Article↗
PubMed2002

Stopping power of nonideal, partially ionized plasmas.

The stopping power of strongly coupled, partially ionized plasmas is investigated for charged beam particles with arbitrary velocities. Our approach is based on kinetic equations of the Boltzmann type that are suitably generalized to describe three-particle collisions. In this way, we consider elastic collisions between the beam and free plasma particles as well as the ionization and excitation of composite plasma particles by beam particle impact. Explicit expressions for both contributions are given in terms of the momentum transfer cross section that has been generalized for three-particle collisions. For fast beam particles, we obtain a generalized Bethe formula that includes correction terms due to the nonideality of the target plasma. Results are shown for hydrogen, carbon, and argon plasmas. Considerable modifications compared to the ideal behavior arise for strongly coupled plasmas. In particular, we are able to describe the Mott transition in the stopping power of dense, partially ionized plasmas.

Journal Article↗
PubMed2002

Wavelength-dependent collective effects in the multiphoton ionization of atomic deuterium.

This paper presents the results of an experimental investigation into collective effects in the transient plasma formed by multiphoton ionization of atomic deuterium with a pulsed laser. The laser wavelength is varied in a narrow range around 243 nm, so that the photoionization is resonant with the metastable 2S(1/2) state. The ion yield, the ion time-of-flight spectra, and the yield of Lyman-alpha photons have been measured as a function of laser intensity (from 1 to 340 MW/cm(2)) and laser detuning around the 1S(1/2)-2S(1/2) two-photon resonance. During and shortly after the laser pulse, collective effects resulting from the mutual interaction of the photoelectrons and the ions affect the spatial and temporal distribution of the ions. Because of the near-degeneracy of the 2S(1/2), 2P(1/2), and 2P(3/2) states, the resonant multiphoton ionization is affected by the Stark mixing of these states in the collective field. As a result, the time-dependent yields of ions and of Lyman-alpha photons are modulated by the interplay of the multiphoton ionization of the atoms and the collective effects in the plasma. From the measurements it is deduced that collective effects are important above a critical charge density of 3x10(8) ions/cm(3). An asymmetry is observed in the line profile of the total ion yield as a function of laser detuning. This asymmetry is interpreted to be due to the effect of the collective field upon the intermediate resonant 2S(1/2) state of the photoionization process.

Journal Article↗
PubMed2003

Relativistic electron acceleration in focused laser fields after above-threshold ionization.

Electrons produced as a result of above-threshold ionization of high-Z atoms can be accelerated by currently producible laser pulses up to GeV energies, as shown recently by Hu and Starace [Phys. Rev. Lett. 88, 245003 (2002)]. To describe electron acceleration by general focused laser fields, we employ an analytical model based on a Hamiltonian, fully relativistic, ponderomotive approach. Though the above-threshold ionization represents an abrupt process compared to laser oscillations, the ponderomotive approach can still adequately predict the resulting energy gain if the proper initial conditions are introduced for the particle drift following the ionization event. Analytical expressions for electron energy gain are derived and the applicability conditions of the ponderomotive formulation are studied both analytically and numerically. The theoretical predictions are supported by numerical computations.

Journal Article↗
PubMed2005

Effects of delayed Kerr nonlinearity and ionization on the filamentary ultrashort laser pulses in air.

We present a systematic study of filamentary ultrashort laser pulses in air, through numerical solutions of the nonlinear Schrödinger equation for various contributions of the delayed Kerr nonlinearity. The results show that a relatively larger contribution of the delayed Kerr nonlinearity will lead to a longer stable filament. This is explained from the transfer of the nonlinear contributions from the frontier part to the back of the pulse and the counterbalanced action of the negative plasma induced nonlinearity by the delayed Kerr nonlinearity in the trailing part of the pulses. Furthermore, effect of ionization on the stability of the filament is investigated. Two formulas are used to generate the data of the ionization, i.e., the Perelemov, Popov, and Terent'ev (PPT) and the Ammosov, Delone, and Krainov formula. It is found that simulation with higher ionization rate (PPT) could generate a more stable and longer filament.

Journal Article↗
PubMed2005

Equation of state for partially ionized carbon at high temperatures.

Equation of state for partially ionized carbon at temperatures T approximately > or = 10(5) K is calculated in a wide range of densities, using the method of free energy minimization in the framework of the chemical picture of plasmas. The free energy model includes the internal partition functions of bound species. The latter are calculated by a self-consistent treatment of each ionization stage in the plasma environment taking into account pressure ionization. The long-range Coulomb interactions between ions and screening of the ions by free electrons are included using our previously published analytical model.

Journal Article↗
PubMed2000

Collective multielectron tunneling ionization in strong fields

We investigate the quantum mechanical process of two-electron tunneling in strong external electric fields. Numerical solution of a two-electron s-wave model reveals the existence of collective tunneling ionization in a mode where both electrons stay at equal distance from the nucleus. Otherwise the lagging electron is immediately recaptured. The corresponding double ionization rate fails to explain nonsequential multiple ionization in strong-field laser experiments. However, an empirically modified version of the analytical one-electron tunneling rate of Ammosov, Delone, and Krainov agrees with the experiments to a surprising accuracy. The reason for this agreement is presently unknown.

Journal Article↗
PubMed2000

Multiple ionization and fragmentation of negatively charged fullerene ions by electron impact

Cross sections for the electron-impact multiple ionization and fragmentation of negatively charged fullerene ions C(-)(n) ( n = 60, 70) to C(q+)(n-m) ( q = 1,2,3 and m = 0,2,4) have been measured for electron energies up to 1 keV. In the case of pure ionization all threshold energies are about 10 eV higher than the values expected. This shift, however, is not observed for the fragment ions. The experimental data indicate that there is no strong electron-electron interaction between the incident electron and the attached electron. A novel ionization mechanism is proposed which can be expected to be valid for all negatively charged molecular or cluster ions which are able to shield the attached electron from the incident electron.

Journal Article↗
PubMed2000

Suppressed molecular ionization for a class of diatomics in intense femtosecond laser fields

It is shown that the puzzling experimental observation of suppressed molecular ionization, in intense laser fields, of O2, and its absence in N2, is a symmetry induced dynamical effect. More generally, it is predicted that the ionization signal of the class of homonuclear diatomic molecules having valence orbitals with an antibonding symmetry (e.g., sigma(u), pi(g)) would be suppressed, but not those with a bonding symmetry (e.g., sigma(g), pi(u)). The suppression effect can be visualized as due to an effective destructive interference between the two subwaves of the ionizing electron emerging from the two atomic centers.

Journal Article↗
PubMed2000

Experimental evidence of threshold effects in the energy loss of protons in carbon and aluminum due to inner shell ionization

We explore the contributions of inner shell ionization to the energy loss of 7 to 270 keV protons in C and Al foils under experimental conditions such that the product of the observation angle and the projectile energy is kept constant. By normalizing these energy loss measurements to the energy loss in the forward direction we observe a pronounced rising behavior with increasing energy. This effect appears in the same range of energies where the respective K- and L-shell ionization cross sections of these elements show a similar threshold behavior. Based also on various theoretical considerations we interpret these results as clear evidence of the inner shell ionization contribution to the energy loss.

Journal Article↗
PubMed2000

Intense-field double ionization of helium: identifying the mechanism

We present quantum mechanical calculations of the electron and ion momentum distributions following double ionization of a one-dimensional helium atom by ultrashort laser pulses (780 nm) at various intensities. The two-electron momentum distributions exhibit a clear transition from nonsequential to sequential double ionization. We provide strong evidence that rescattering is responsible for nonsequential ionization by calculating the momentum spectrum of the He2+ recoil ions-which we find in excellent agreement with recent experiments-and by analyzing the electronic center-of-mass motion via Wigner transforms.

Journal Article↗
PubMed2001

Strong correlations in the He ground state momentum wave function observed in the fully differential momentum distributions for the p + He transfer ionization process.

The four-particle process of proton-helium transfer ionization has been studied using cold target recoil ion momentum spectroscopy to measure the momenta of all three particles in the final state. Most of the electrons are emitted in the H0 scattering plane and in the backward direction. The final state momentum distributions show discrete structures very different from those expected for uncorrelated capture and ionization. The measured momentum pattern is interpreted to be due to a new transfer ionization reaction channel which results from strong correlations in the initial He ground state momentum wave function.

Journal Article↗