Field ionization measurements

In field ionization (or field desorption), application of a large electric potential to a surface of high curvature allows a very intense electric field to be generated. Such positive or negative fields lead to electrons being stripped from or added to molecules lying on the surface. The positive or negative molecular ions so produced are mass measured by the mass spectrometer. 

The m/z values of peptide ions are mathematically derived from the sine wave profile by the performance of a fast Fourier transform operation. Thus, the detection of ions by FTICR is distinct from results from other MS approaches because the peptide ions are detected by their oscillation near the detection plate rather than by collision with a detector. Consequently, masses are resolved only by cyclotron frequency and not in space (sector instruments) or time (TOF analyzers). The magnetic field strength measured in Tesla correlates with the performance properties of FTICR. The instruments are very powerful and provide exquisitely high mass accuracy, mass resolution, and sensitivity—desirable properties in the analysis of complex protein mixtures. FTICR instruments are especially compatible with ESI29 but may also be used with MALDI as an ionization source.30 FTICR requires sophisticated expertise. Nevertheless, this technique is increasingly employed successfully in proteomics studies. 

Example Secondary kinetic isotope effects on the a-cleavage of tertiary amine molecular ions occurred after deuterium labeling both adjacent to and remote from the bond cleaved (Chap. 6.2.5). They reduced the fragmentation rate relative to the nonlabeled chain by factors of 1.08-1.30 per D in case of metastable ion decompositions (Fig. 2.18), but the isotope effect vanished for ion source processes. [78] With the aid of field ionization kinetic measurements the reversal of these kinetic isotope effects for short-lived ions (lO -lO" s) could be demonstrated, i.e., then the deuterated species decomposed slightly faster than their nonlabeled isoto-pomers (Fig. 2.17). [66,76]... 

Several methods have been employed to measure the energy of this state in nonpolar liquids. The methods fall into three categories the change in work function of a metal when immersed in the liquid, photoionization, and field ionization. Of these, the latter, in which field ionization of high-lying Rydberg states is utilized to locate Fq, has in recent years provided what are considered to be the most accurate values of Fq in fluid Ar, Kr, and Xe. 

In a variation of the photoelectric method, Fq can be determined by measuring the emission of electrons from the liquid into the vacuum. Even when Fq is negative, electrons can penetrate this barrier and be collected in the gas phase [35,36,47]. Borghesani et al. [48] used this technique and from the time evolution of the current reaching the anode for a sample of liquid Ar at 87 K found Fq to be —0.126 eV. This is in excellent agreement with the value of —0.125 eV given by Eq. (6) (see below) for this density (2.09 x 10 cm ) using the field ionization technique. 

The density dependence of Vg in Kr was determined by field ionization of CH3I [62] and (0113)28 [63]. Whereas previous studies found a minimum in Vg at a density of 12 X 10 cm [66], the new study indicates that the minimum is at 14.4 x 10 cm (see Fig. 3). This is very close to the density of 14.1 x 10 cm at which the electron mobility reaches a maximum in krypton [67], a result that is consistent with the deformation potential model [68] which predicts the mobility maximum to occur at a density where Vg is a minimum. The use of (0113)28 permitted similar measurements of Vg in Xe because of its lower ionization potential. The results for Xe are also shown in Fig. 3 by the lower line. 

There are basically two kinds of experiments which can be used for studying the mechanisms of the field ionization process near a field ion emitter surface and also the field ion image formation process. They are the measurement of field ion current as functions of tip voltage, tip temperature, and other experimental parameters, and the measurement of the ion energy distribution. 

Fig. 2.3 Magnetic sector mass spectrometer combined with a retarding potential ion energy filter used by Ernst et al 6 for measuring ion energy distributions in field ionization and field evaporation.

An interesting question is how H3 is formed on the emitter surface and whether H3 molecules can exist on the surface. This question can be investigated with a measurement of the appearance energy of Hj ions. Jason etal.264 find Hj in field ionization of condensed layers of hydrogen, and measure the appearance energy to be 12.7 eV. This value is 2.9 eV smaller than that of H2. Ernst Block conclude265 from a similar measurement in field ionization mass spectrometry of hydrogen that an H 3 ion is formed at the moment when a chemisorbed H atom combines... 

When the pulsed-field ionization signal of a single excited Rydberg peak is measured as a function of the delay time between the extraction field pulse... 

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