Effect of ion source

Aminoanthracene, 1- and 2-, reactions, effects of ion source temperature, 185,187/ Analytical Fourier transform mass spectrometry, instrumentation and application examples, 81-98 Apodization, 27,28 Applications, future, 14,15 Aromatic amines, negative ion chemical ionization, 176... 

TABLE 7.6 Effect of Ion Source Pressure on Methane PICI Spectrum for Di-n-Butyl Phthalate"... 

TABLE 7.10 Effect of Ion Source Temperature on NICI Spectrum of p-Toluic Acid... 

TOF mass spectrometers are very robust and usable with a wide variety of ion sources and inlet systems. Having only simple electrostatic and no magnetic fields, their construction, maintenance, and calibration are usually straightforward. There is no upper theoretical mass limitation all ions can be made to proceed from source to detector. In practice, there is a mass limitation in that it becomes increasingly difficult to discriminate between times of arrival at the detector as the m/z value becomes large. This effect, coupled with the spread in arrival times for any one m/z value, means that discrimination between unit masses becomes difficult at about m/z 3000. At m/z 50,000, overlap of 50 mass units is more typical i.e., mass accuracy is no better than about 50-100 mass... 

The effectiveness of various sources of fluoride ion in the displacement of the trifluoromethanesulfonic group has been demonstrated while introducing a fluorine atom into the five-membered carbocyclic ring of a prostaglandin precursor Treat tnent of the corresponding triflyl derivative with potassium fluoride in acetonitrile or with cesium fluonde in refluxing diinethylformamide or hexamethylphosphoric... 

Note The detection of signals from previously measured samples in the mass spectrum of the actual analyte is usually termed memory or memory effect. It is caused by contaminations of ion source or sample introduction system. The best way to reduce memory effects is to use the lowest amount of sample necessary to produce good spectra, to keep ion source and ion source housing at elevated temperature, and to allow some minutes for pumping between subsequent measurements. 

Jorgensen, T. J. D., Hvelplund, P., Andersen, J. U., Roepstorff, P. Tandem mass spectrometry of specific vs. nonspecific noncovalent complexes of vancomycin antibiotics and peptide ligands. Int J Mass Spectrom 2002, 219, 659-670. Tahallah, N., Pinkse, M., Maier, C. S., Heck, A. J. The effect of the source pressure on the abundance of ions of noncovalent protein assemblies in an electrospray ionization orthogonal time-of-fiight instrument. Rapid... 

The state of the art has now advanced to the point that synchrotron sources are frequently not the hmiting factor in biological XAS. In particular, there is concern that sample measurement techniques have not kept pace with the development of new sources. Several new detection schemes are under development, however it may be several years before detectors will be available which are capable of fully utilizing the intensity available firom new sources. In addition, there is concern that very intense sources may compromise sample integrity. Taken together, these concerns suggest that it is not reasonable to extrapolate past experience when attempting to predict the effect of new sources on future concentrations limits. Metal ion concentrations below ca. 100 )tM are likely to remain inaccessible for many years. 

However, some obstacles still have to be removed before HPLC-MS is fully accepted as a routine technique in the forensic labs. First of all the cost of equipment is still high, despite the reduction trend in the last years, as compared to GC-MS. Second, some problems have to be tackled, i.e., the susceptibility of ion sources (and particularly ESI) to matrix effects on analyte s ionization efficiency (suppression or enhancement) and the scarce reproducibility of MS fragmentation. The third obstacle is actually the other side of the coin of versatility the wide choice of technical alternatives makes HPLC-MS still far from being a highly standardized, one-button technique as GC-MS. 

ToF mass spectrometers as dynamic instruments gained popularity with the introduction of matrix assisted laser desorption/ionization (MALDI) and electrospray ionization (ESI) as effective pulsed ion sources for the soft ionization of large biomolecules (up to 10s dalton) due to their high ion transmission.38 ToF mass spectrometers, quadrupole analyzers and/or magnetic sector fields can be combined in tandem mass spectrometers (MS/MS) for the analysis of organic compounds. 

Equation for Electric Field Strength. To include electron-impact source terms in continuity equations for neutral species and to include the effect of ion bombardment on the rate of surface reactions, equations that predict electron and ion densities, momentums, and energy profiles are required. The profiles require an equation for the electric field strength. In general equations for electron and ion continuities (these equations yield electron density and ion density, respectively), electron and ion momentums (for electron and ion net or directed velocity), and electron and ion energies (for electron and ion random or thermal energy) must be solved. Finally, the electric field profile is obtained from Poisson s equation. 

The effective Time Dependent Kohn-Sham (TDKS) potential vks p (r>0 is decomposed into several pieces. The external source field vext(r,0 characterizes the excitation mechanism, namely the electromagentic pulse as delivered by a by passing ion or a laser pulse. The term vlon(r,/) accounts for the effect of ions on electrons (the time dependence reflects here the fact that ions are allowed to move). Finally, appear the Coulomb (direct part) potential of the total electron density p, and the exchange correlation potential vxc[p](r,/). The latter xc potential is expressed as a functional of the electronic density, which is at the heart of the DFT description. In practice, the functional form of the potential has to be approximated. The simplest choice consists in the Time Dependent Local Density Approximation (TDLDA). This latter approximation approximation to express vxc[p(r, /)]... 

Banner J. L., Musgrove M., and Capo R. C. (1994) Tracing ground-water evolution in a limestone aquifer using Sr isotopes effects of multiple sources of dissolved ions and mineral-solution reactions. Geology 22, 687—690. 

Uncertainties in the above values come from the dependence of the number and sizes of the inclusions in the matrix and non-linearities in the dependence of the Li ion yield with concentration. The main contributor to the large disparity between the Li concentrations found for samples DNA 1 and 2 is regarded as the local variation in the number and size of the inclusions. To reduce the uncertainty in the results a larger number of inclusions over a wider area would need to be sampled but even then the natural variation in the microstructure may still give a difference between the samples for practicable numbers of measurements. The calibration was for Li on a Fe substrate whereas for actual samples the Li was associated with A1 inclusions. This may result in a different RSF than used in the quantification of the Li to matrix ion intensity and so a variation from the reported Li concentration. The combined effect of the sources of uncertainty are compatible with the ten-fold difference between the measured results so that the present data are currently regarded as contributing to a best estimate of magnitude. 

These alkaline earth elements are also derived from other minerals, including aluminosilicates, which reduces the overall correlations. Where there has been a substantial inundation of seawater in the past, some Mg may remain from this source for every 8.3 mg of Na that is derived from residual seawater, there will be 1 mg L" Mg along with small amounts ofCa (0.3 mg L ), Sr (6 pg L" ) and Ba (2 pg L" ). The effects of ion exchange can alter these simple relationships. 

Figure 20 Delayed extraction applied to a TOF/MS ionization source, (a) Initial positions of ions A, B, and C. (b) Position of ions A, B, and C after translational equilibration based on initial ion momentum, (c) Effect of ion momentum on flight time.

Because of the widespread use of LC/MS/MS for drug discovery bioanalysis, there is currently less of a necessity for finely tuned solid-phase extractions than there once was in this area. Instead, generic solid-phase extraction conditions that can accommodate many different analyte structures using the same extraction conditions have become more interesting. To make a solid-phase sample preparation useful for LC/MS/MS it must remove as much of the sample salts as possible in order to reduce the effects of ion suppression [47,51—52] and it must remove as many nonvolatile matrix components as possible so that the instrument ion source is not quickly fouled. Because the LC/MS/MS instrument is inherently so selective, added assay selectivity, per se, is no longer an objective of solid-phase extraction. 

On the basis of precipitation chemistry data, Selezneva (1972) was also able to estimate that, over rural areas of the Soviet Union 30-40 % of the contamination found in precipitation is due to the background level of trace constituents, while 60-70 % of the concentration reflects the effects of local sources. About half of this latter fraction is provided by pollutants. In this important work she also showed that about 20-30 % of the quantity of ions is anthropogenic even over such clean areas as Siberia. 

Ion size plays an important role in determining the structure and stability of ionic solids, the properties of ions in aqueous solution, and the biologic effects of ions. As with atoms, it is impossible to define precisely the sizes of ions. Most often, ionic radii are determined from the measured distances between ion centers in ionic compounds. This method, of course, involves an assumption about how the distance should be divided up between the two ions. Thus you will note considerable disagreement among ionic sizes given in various sources. Here we are mainly interested in trends and will be less concerned with absolute ion sizes. 

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