Fractionation in Ionization Processes

Figure 1 Schematic of DC glow-discharge atomization and ionization processes. The sample is the cathode for a DC discharge in 1 Torr Ar. Ions accelerated across the cathode dark space onto the sample sputter surface atoms into the plasma (a). Atoms are ionized in collisions with metastable plasma atoms and with energetic plasma electrons. Atoms sputtered from the sample (cathode) diffuse through the plasma (b). Atoms ionized in the region of the cell exit aperture and passing through are taken into the mass spectrometer for analysis. The largest fraction condenses on the discharge cell (anode) wall.

It is particularly difficult to study charge transfer reactions by the usual internal ionization method since the secondary ions produced will always coincide with ions produced in primary ionization processes. Indeed these primary ions frequently constitute the major fraction of the total ion current, and the small intensity changes originating from charge transfer reactions are difficult to detect. For example, Field and Franklin (5) were unable to detect any charge transfer between Xe + and CH4 by the internal ionization method although such reactions have been observed using other techniques (3, 9,22). 

The ionization process can be described as a one-particle event, when only one term dominates in summation (3), yielding a pole strength close unity. In this case, 1 - Fu gives an estimate of the fraction of photoemission intensity dispersed in many-body effects. On the contrary, small pole strengths are indicative (18-20) of a breakdown of the one-particle picture of ionization. 

Surface enhanced laser desorption/ionization (SELDI) is a distinctive form of laser desorption ionization where the target plays an active role in the sample preparation procedure and ionization process [49]. Depending on the chemical or biochemical treatment, the SELDI surface acts as solid phase extraction or an affinity probe. Chromatographic surface is used for sample fractionation and purification of biological samples prior to direct analysis by laser desorption/ ionization. SELDI is mainly applied for protein profiling and in biomarker discovery by comparing protein profiles from control and patient groups. 

There are three models for mass fractionation of isotopes in the ionization processes a linear law, a power law and an exponential law (Wasserburg et al., 1981 Lee et al., 2001). 

A serious problem in LA-ICP-MS described in the literature on many occasions is the time-dependent elemental fraction (so-called ablation fractionation ) occurring during laser ablation and the transport process of ablated material, or during atomization and ionization processes in the inductively coupled plasma.20-22 Numerous papers focus on the study of fraction effects in LA-ICP-MS as a function of experimental parameters applied during laser ablation (such as laser energy, laser power density, laser pulse duration, wave length of laser beam, ablation spot size,... 

The success of API is due, at least in part, to the fact that both ion-spray and heated-nebulizer involve soft ionization processes that provide an abundance of molecular ions. This results in simple, easy-to-interpret spectra affording molecular weight information. Because of these features, this interface is perfectly suited to natural products chemistry. We use LC-MS on the material in those tubes identified as having biological activity. The combination of molecular weight and UV-visible data of biological active fractions is used for identification. 

In addition to those ions formed during or soon after primary ion impact, as in the processes just described, other ions arise through subsequent events. Unimolecular reactions of ions, akin to metastable decompositions in magnetic sector mass spectrometry, occur in the free vacuum. The resulting fragment ions have intensities which contradict the notion that SIMS is a "soft" ionization technique, although some fraction of the ion production events can be so characterized. 

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