Post field ionization

Charge states, desorption fields and post field ionization... 

Fig. 2.12 Average charge state in field evaporation of Mo as a function of the applied field, obtained by Kellogg.68 The field evaporation rate is kept nearly constant by adjusting the temperature of the pulsed-laser heating. Curves are theoretical results A from the image-hump model, B from the same calculation with post field ionization correction. Calculations are based on those of...

Fig. 2.13 Theoretical post field ionization probability for tungsten as a function of field obtained by Kingham.64 He has presented similar curves for most of the metals in the periodic table.

Fig. 2.14 (a) Ion energy distributions of the doubly charged Ir ions. The distribution width is very narrow without a low energy tail and therefore there is no evidence of post field ionization of field evaporated Ir2+ ions. Evaporation field calculations for low temperatures indicate that Ir will first field evaporate as Ir2+ ions when the field is gradually raised. 

Although the detected charge states in field evaporation of metals are now known from atom-probe studies, they are unknown for adatoms. In addition, many investigators now believe that the multiply charged ions observed in field evaporation are produced by post field ionization of field evaporated ions. Thus the value of n in eq. (4.69) is uncertain for adatoms also. 

The anodic or distributed arc sources are basically evaporation sources heated by low voltage, high current unfocused e-beams (Sec. 6.3.1). The e-beam can be bent by a magnetic field so that the emission source is out of hne of sight of the evaporation source, as shown in Figure 8.1 (c), or it can he in the hne of sight. The electrons can be made to spiral in a magnetic field so as to increase the post-vaporization ionization probabflity of the evaporated material. 

Post-vaporization ionization (PVD technology) The ionization of the vaporized (sputtered or evaporated) film atoms to form film ions that can be accelerated in an electric field. See also Film ions. 

In both electron post-ionization techniques mass analysis is performed by means of a quadrupole mass analyzer (Sect. 3.1.2.2), and pulse counting by means of a dynode multiplier. In contrast with a magnetic sector field, a quadrupole enables swift switching between mass settings, thus enabling continuous data acquisition for many elements even at high sputter rates within thin layers. 

Mass spectrometry is the method of choice for identification of proteins, quantitation of proteins expression at a given time but also for characterization of post-translational modifications. Indeed, allowing direct analysis of peptides and proteins, the MALDI and ES ionization modes have been revolutionary in the field of mass spectrometry applied to biology. 

With the advent of very sensitive ionization techniques such as matrix assisted laser desorption (MALDI) coupled with time-of-flight (TOF) mass analysis, measurement of the intact mass of peptides at sub-pmol levels has become a reality (2) of which we have taken advantage for the systematic screening of HPLC fractions. Partial sequence information can be obtained by carrying out enzymatic hydrolysis with exoproteinases (e.g. carboxypeptidases and aminopeptidases) (3, 4). More recently, MALDI has been used to measure metastable decomposition occurring in the first field free region of a reflectron TOF instrument (referred to as post source decay (PSD)) with only marginally more sample (5-7). 

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