Plasma ionization energy

Atoms and ions are excited via collisions, probably mainly with electrons, and then emit light. Most elements with ionization energies less than 8 eV exist mainly as singly charged ions in the plasma. Therefore, spectral lines from ions are most intense for these elements, whereas elements with high ionization energies (such as B, Si, Se and As), as well as the easily ionized alkalis (Li, Na, K, Rb, and Cs), emit most strongly as atoms. 

Ion extraction. The aspirated or laser ablated sample is transported from the sample introduction system into the center of the torch by a 1 1/min flow of Ar carrier gas where it is immediately dissociated and ionized by energy transfer with the hot -6000 K temperature of the surrounding Ar plasma. Ionization efficiencies are >95% for U and Th (Jarvis et al., 1992). For laser ablation sampling, helium may be employed as the carrier... 

Doubly-charged ions exist because the potential of second ionization of many metals is relatively low with respect to the plasma thermal energy. For instance, 11.9 eV are needed to remove two electrons, in contrast with 6.1 eV for one electron, from a calcium atom. At 8000K, a little less than 0.1% of Ca would be in the Ca + form and overlap with Mg+ isotopes. Likewise, Ba is rather easily formed and overlaps with Zn+ isotopes. Isobaric interferences with doubly-charged ions are easily identified as odd-mass atoms will produce peaks at half masses, such as Ca at mass 21.5. 

The FAB plasma provides conditions that allow to ionize molecules by either loss or addition of an electron to form positive molecular ions, M" , [52,89] or negative molecular ions, M, respectively. Alternatively, protonation or deprotonation may result in [Mh-H]" or [M-H] quasimolecular ions. Their occurrence is determined by the respective basicity or acidity of analyte and matrix. Cationization, preferably with alkali metal ions, is also frequently observed. Often, [Mh-H]" ions are accompanied by [MH-Na]" and [Mh-K]" ions as already noted with FD-MS (Chap. 8.5.7). Furthermore, it is not unusual to observe and [Mh-H]" ions in the same FAB spectmm. [52] In case of simple aromatic amines, for example, the peak intensity ratio M 7[Mh-H] increases as the ionization energy of the substrate decreases, whereas 4-substituted benzophenones show preferential formation of [Mh-H]" ions, regardless of the nature of the substituents. [90] It can be assumed that protonation is initiated when the benzophenone carbonyl groups form hydrogen bonds with the matrix. 

Fig. 5.2. Mass spectra of the plasma effluent neutral species, obtained with an electron impact ionization energy 15 eV...

Emission spectroscopy utilizes the characteristic line emission from atoms as their electrons drop from the excited to the ground state. The earliest version of emission spectroscopy as applied to chemistry was the flame test, where samples of elements placed in a Bunsen burner will change the flame to different colors (sodium turns the flame yellow calcium turns it red, copper turns it green). The modem version of emission spectroscopy for the chemistry laboratory is ICP-AES. In this technique rocks are dissolved in acid or vaporized with a laser, and the sample liquid or gas is mixed with argon gas and turned into a plasma (ionized gas) by a radio frequency generator. The excited atoms in the plasma emit characteristic energies that are measured either sequentially with a monochromator and photomultiplier tube, or simultaneously with a polychrometer. The technique can analyze 60 elements in minutes. 

Figure 2.5 Dependence of ionization degree a on first ionization energy Et of chemical elements by inductively coupled plasma ionization.

The ionization energy of Ar is 15.8 electron volts (eV), which is higher than those of all elements except He, Ne, and F. In an Ar plasma, analyte elements can be ionized by collisions with Ar+, excited Ar atoms, or energetic electrons. In atomic emission spectroscopy, we usually observe the more abundant neutral atoms, M. However, the plasma can be directed into a mass spectrometer (Chapter 22), which separates and measures ions according to their mass-to-charge ratio.17 For the most accurate measurements of isotope ratios, the mass spectrometer has one detector for each desired isotope.18... 

Matrix effects on the yield of ions in the plasma are important, so calibration standards should be in the same matrix as the unknown. Alternatively, internal standards can be used if they have nearly the same ionization energy as the analyte. For example, Tm can be used as an internal standard for U. The ionization energies of these two elements are 5.81 and 6.08 eV, respectively, so they should ionize to nearly the same extent in different matrices. If possible, internal standards with just one major isotope should be selected for maximum response. 

As a consequence of the interaction of the two particles forming a bound state with the surrounding plasma, the effective ionization energy may become zero at a certain density nM for a given temperature T,... 

The plasma is created by an electrical field between both parallel plates (see figure), ionizing the gas volume inbetween. In a plasma, the energy is transferred by collisions between all particles. Due to their smaller mass, the energy of the electrons increases much faster than the energy of the heavier ions. This means that mainly the electrons are responsible for the ionization processes and the formation of reactive free radicals. The big energy difference between electrons and ions is reflected in the respective temperatures the electron temperature of a typical plasma is about... 

Reaction cells appear to be a much better way to reduce signals due to Ar-containing molecular ions and Ar+ itself than the use of cold plasma conditions. Because normal plasma conditions are used, elements with high ionization energies, such as Se and As, do not suffer from sensitivity losses, unlike cold plasma conditions. The severe chemical matrix effects that are typical of cold plasma conditions are prevented. The first commercial ICP-MS instrument to use this concept was introduced by Micromass UK Ltd. However, as noted, reaction product ions must be controlled or removed to prevent other (new) spectral overlaps. 

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