Negative ions formation

Almost all neutral substances are able to yield positive ions, whereas negative ions require the presence of acidic groups or electronegative elements to produce them. This allows some selectivity for their detection in mixtures. Negative ions can be produced by capture of thermal electrons by the analyte molecule or by ion-molecule reactions between analyte and ions present in the reagent plasma. 

All Cl plasmas contain electrons with low energies, issued either directly from the filament but deactivated through collisions, or mostly from primary ionization reactions, which produce two low-energy electrons through the ionization reaction. The interaction of electrons with molecules leads to negative ion production by three different mechanisms [5]  

These electrons can be captured by a molecule. The process can be associative or dissociative. The associative resonance capture that leads to the formation of negative molecular 

GC/MS TIC traces of butyl methacrylate dissolved in C11-C12 saturated hydrocarbon, (a) EL The peak corresponding to butyl methacrylate is marked by a dot. The peaks following are Cll saturated hydrocarbons. (b) Same trace obtained by Cl usig methane as reagent gas. Butyl methacrylate (dot) is still well detected, but the trace of the hydrocarbons is aten-uated. (c) Cl using isobutane as reagent gas. Butyl methacrylate is well detected, while the hydrocarbons are almost not detected. 

The associative resonance capture is favoured for molecules with several electronegative atoms or with possibilities to stabilize ions by resonance. The energy to remove an electron from the molecular anion by autodetachment is generally very low. Consequently, any excess of energy from the negative molecular ion as it is formed must be removed by collision. Thus, in Cl conditions, the reagent gas serves not only for producing thermal electrons but also as a source of molecules for collisions to stabilize the formed ions. 

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Chantry P J 1982 Negative ion formation in gas lasers Applied Atomic Collision Physics Vol 3, Gas Lasers ed FI S W Massey, E W MoDaniel, B Bederson and W L Nighan (New York Aoademio)... 

Cole, R.B. Harrata, A.K. Solvent Effect on Analyte Charge State, Signal Intensity, and Stability in Negative Ion ESI-MS Implications for the Mechanism of Negative Ion Formation. J. Am. Soc. Mass Spectrom. 1993,4,546-556. 

Figure 5. Mechanisms for negative ion formation in a chemical ionization source...

The three extreme cases of bonding which can be considered, correspond to the adsorption of hydrogen as H" " ions, covalently bonded H atoms or H ions. Dowden (109) has considered, from a theoretical standpoint, the factors favoring each of these types of adsorption. The criteria for negative ion formation are opposite to and readily distinguishable from the other two the distinction between the criteria for the formation of covalent bonds... 

Negative ions are also produced by electron bombardment through a variety of electron attachment or dissociative capture processes (Dillard, 1973). They may also be generated indirectly as the result of an ion-molecule reaction. The cross-sections for negative ion formation are at least an order of magnitude less than those for positive ions, a fact which accounts for the much weaker ion currents that are produced. Some of the common precursors for well-known negative ions are shown in (6)—(13). For several of these... 

Negative ion formation will be favored by (i) a low exit work function, (ii) a large negative value of Fermi surface, and... 

From various sources Dowden (27) has accumulated data referring to the density of electron levels in the transition metals and finds an increase from chromium to iron. The density is approximately the same from a-iron to /3-cobalt there is a sharp rise between the solid solution iron-nickel (15 85) and nickel, and a rapid fall between nickel-copper (40 60) and nickel-copper (20 80). From Equation (2), the rates of reaction can be expected to follow these trends of electron densities if positive ion formation controls the rates. On the other hand, both trends will be inversely related if the rates are controlled by negative ion formation. Where the rate is controlled by covalent bond formation, singly occupied atomic orbitals are deemed necessary at the surface to form strong bonds. In the transition metals where atomic orbitals are available, the activity dependence will be similar to that given for positive ion formation. In copper-rich alloys of the transition elements the activity will be greatly reduced, since there are no unpaired atomic d-orbitals, and for covalent bond formation only a fraction of the metallic bonding orbitals are available. 

Dowden and Reynolds observed that the rates of decomposition of hydrogen peroxide decreased from pure copper to copper-nickel alloys, thus suggesting that negative ion formation takes place in the heterogeneous catalytic reaction, in agreement with the Haber and Weiss mechanism based on catalysis in solution. 

Nitrogen attracts three additional electrons and is thus able to form three covalent bonds, as occurs in ammonia, NH3, shown in Figure 6.16. Likewise, a carbon atom can attract four additional electrons and is thus able to form four covalent bonds, as occurs in methane, CH4. Note that the number of covalent bonds formed by these and other nonmetallic elements parallels the type of negative ions they tend to form (see Figure 6.6). This makes sense, because covalent bond formation and negative ion formation are both applications of the same concept nonmetallic atoms tend to gain electrons until their valence shells are filled. 

In 1967 POF3 was reported to react with cesium fluoride to give Cs+[P02F2] and Cs+[PF6] (86). More recent work by Selig and Aminadav (59) has shown that reaction with alkali metal and nitrosyl fluorides in a 1 1 ratio yields [PF6] as the only fluorine-containing species. Primary and secondary negative-ion formation observed in the... 

Source conditions for both positive and negative ion mass spectrometry are usually discussed in terms of source pressure and style of ionization. The source conditions that have been used for negative ion formation are in Table I. 

Figure 2. The responses due to negative ion formation from benzil are (EC) the GC-electron capture response (EC-API) the corresponding response from the API source and (API-SIM) the selected ion response for the M ion of benzil (A), (benzil, CeH5-CO-CO-C6Hs, Ar/CH, 240°C, PZ179 column)...

Adduct ions are formed under some circumstances. Polyhalides will add Cl- to give negative ions, for example this mode of negative ion formation has been used for analytical purposes by Dougherty et al (9,10,11). 

Penning ionization is a dominant reaction when nitrogen or neon is used in the DART source. Nitrogen or neon ions are effectively removed by electrostatic lenses and are not observed in the background mass spectrum. When helium is used, the dominant positive-ion formation mechanism involves the formation of ionized water clusters followed by proton transfer reactions. Negative-ion formation occurs by production of electrons by Penning ionization or by surface Penning ionization ... 

Figure 2.2c presents a capture process leading to temporary negative-ion formation that is distinctly different from that in Figure 2.2d since the potential curve of AB- does not cross the Franck-Condon region. The mechanism can be described as vibrational excitation of the neutral molecule and subsequent capture of the incident electron. 

An asterisk indicates potential electronic excitation, which may include dissociation in the case of molecules. Asterisks could have also been applied to the charge transfer and ionizing collisions, but are left out for clarity. The processes can be summarized as excitation processes (23, 26, 29), charge transfer processes (27, 30, and negative ion formation processes, 24 and 25), and collisional ionization (28, 31, and free electron formation processes 24 and 25). 

Fig. 17. One-electron energy level diagram for negative ion formation near a metal surface. The dashed and solid curves represent the potential energy for an electron when the atom is at infinite and close distances to the surface, respectively. The affinity level for the neutral atom shifts downward and broadens as the atom approaches the surface.

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