Product ions

In the ideal case for REMPI, the efficiency of ion production is proportional to the line strength factors for 2-photon excitation [M], since the ionization step can be taken to have a wavelength- and state-mdependent efficiency. In actual practice, fragment ions can be produced upon absorption of a fouitli photon, or the ionization efficiency can be reduced tinough predissociation of the electronically excited state. It is advisable to employ experimentally measured ionization efficiency line strengdi factors to calibrate the detection sensitivity. With sufficient knowledge of the excited molecular electronic states, it is possible to understand the state dependence of these intensity factors [65]. 

An example of enhanced ion production. The chemical equilibrium exists in a solution of an amine (RNH2). With little or no acid present, the equilibrium lies well to the left, and there are few preformed protonated amine molecules (ions, RNH3+) the FAB mass spectrum (a) is typical. With more or stronger acid, the equilibrium shifts to the right, producing more protonated amine molecules. Thus, addition of acid to a solution of an amine subjected to FAB usually causes a large increase in the number of protonated amine species recorded (spectrum b). 

Reed, R.I., Ion Production by Electron Impact, Academic Press, New York, 1962. 

This last result describes the Donnan equilibrium condition as it applies to the system under consideration. Like other ionic equilibrium expressions, it requires the equality of ion products in equilibrium solutions. 

Ion Product. The ion product of water is the product of the molality of the hydrogen and hydroxide ions, oh - temperature... 

Ionic Equilibria.. The ion product constant of D2O (see Table 3) is an order of magnitude less than the value for H2O (24,31,32). The relationship pD = pH + 0.41 (molar scale 0.45 molal scale) for pD ia the range 2—9 as measured by a glass electrode standardized ia H2O has been established (33). For many phenomena strongly dependent on hydrogen ion activity, as is the case ia many biological contexts, the difference between pH and pD may have a large effect on the iaterpretation of experiments. 

Relative photoionization cross sections for molecules do not vary gready between each other in this wavelength region, and therefore the peak intensities in the raw data approximately correspond to the relative abundances of the molecular species. Improvement in quantification for both photoionizadon methods is straightforward with calibration. Sampling the majority neutral channel means much less stringent requirements for calibrants than that for direct ion production from surfaces by energetic particles this is especially important for the analysis of surfaces, interfaces, and unknown bulk materials. 

The LIMS technique is rarely used for quantitative elemental analysis, since other techniques such as EPMA, AES or SIMS are usually more accurate. The limitations of LIMS in this respect can be ascribed to the lack of a generally valid model to describe ion production from solids under very brief laser irradiation. Dynamic range limitations in the LIMS detection systems are also present, and will be discussed below. 

I. D. Kovalev et al. Int. J. Mass Spectrom. Ion Phys. 27, 101, 1978. Contains a discussion of laser-solid interactions and ion production under a variety of irradiation conditions. 

Indeed, because most hydrogen atoms in liquid water are hydrogen-bonded to a neighboring water molecule, this protonic hydration is an instantaneous process and the ion products of water are and OH ... 

The most common reaction of aldehydes and ketones is the nucleophilic addition reaction, in which a nucleophile, Nu , adds to the electrophilic carbon of the carbonyl group. Since the nucleophile uses an electron pair to form a new bond to carbon, two electrons from the carbon-oxygen double bond must move toward the electronegative oxygen atom to give an alkoxide anion. The carbonyl carbon rehybridizes from sp2 to sp3 during the reaction, and the alkoxide ion product therefore has tetrahedral geometry. 

These equations say that the Kh of an amine multiplied by the of the corresponding ammonium ion is equal to Kw, the ion-product constant for water (1.00 x 10 14). Thus, if we know Ka for an ammonium ion, we also know for the corresponding amine base because /stronger base has an ammonium ion with a larger p... 

If Q > K, the solution contains a higher concentration of ions than it can hold at equilibrium. A precipitate forms, decreasing the concentrations until the ion product becomes equal to Ksp and equilibrium is established. 

K. See Equilibrium constant Ka. See Acid equilibrium constant See Base equilibrium constant Kc. See Equilibrium constant Kf. See Formation equilibrium constant Kr See Equilibrium constant K,p. See Solubility product constant K . See Water ion product constant K-electron capture The natural radioactive process in which an inner electron (n = 1) enters the nucleus, converting a proton to a neutron, 514 Kelvin, Lord, 8... 

This time the trial product, 4 X 10-4, is greater than Klp = 2.4 X 10 s so a precipitate does form. Solid calcium sulfate, CaS04, will continue to form, lowering the concentrations [Ca+2] and [SO -2] until they are low enough that the ion product equals Kcp. Then equilibrium exists and no more precipitation occurs. 

A 50 ml volume of 0.04 M Ca(N03)2 solution is added to 150 ml of 0.008 M (NH4)2S04 solution. Show that a trial value of the calcium sulfate ion product is 6 X 10-6. Will a precipitate form ... 

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