Negative-ion mass spectrometry

Tobe and coworkers have extended their work to the pyridine-based cyclophynes 17 and 18 in efforts to detect the incorporation of heteroatoms into the fullerene structure [39]. Similar to the behaviour of the hydrocarbon 16, heterocyclic 17 and 18 show the successive loss of indane units and hydrogen under the conditions of LD mass spectrometry (negative ion mode) culminating in the observation in both cases of the formation of the anion C5gN2. The relative low intensity of the diazafullerene anion observed can be attributed to the kinetic and thermodynamic instability of the heterocage formed. 

Gas Chromatography-Mass Spectrometry Negative-ion chemical ionisation mass spectrom-etry is the best method to identify a wide range of synthetic corticosteroids in horse urine (E. Houghton et at, Biomed. Mass Spectrom., 1982, 9, 459-465). Method. Extract 10 ml of urine with 25 ml of methylene chloride, separate the solvent layer, dry over anhydrous sodium sulphate, and evaporate to dryness in a rotary film evaporator at 30° to 40°. Heat the residue with 100 p.1 of an 8% solution of methoxyamine hychochloride in dry pyridine in a reaction tube at 80° for 30 minutes, add 50 pi of trimeOiylsilylimidazole, [Pg.98]

Kirchhoff, J.F., Marble, D.K., Weathers, D.L., McDaniel, ED., Matteson, S., Anthony, J.M., Beavers, R.L., Bennett, T.J. (1994) Fabrication of silicon-based optical components for an ultraclean accelerator mass spectrometry negative ion source. Review of Scientific Instruments, 65, 1570-1574. 

Multiphoton Excitation in Mass Spectrometry Negative Ion Mass Spectrometry, Methods Neutralization-Reionization in Mass Spectrometry Photoelectron-Photoion Coincidence Methods in Mass Spectrometry (PEPICO)... 

Volkening, J., Koppe, M., and Heumann, K. G. (1991) Tungsten isotope ratio determinations by negative thermal ionization mass spectrometry. International Journal of Mass Spectrometry and Ion Processes, 107, 361-368. 

Fig. 10 Gas chromatography-electron-capture negative-ion high-resolution mass spectrometry multiple-ion chromatogram (sum of penta-octachlorobornanes) of a sediment sample from the Mississauga Basin of Lake Ontario (2-3 cm depth) showing predominance of B6-923 and B7-1001 [84]. Other congener identities are defined in ref [84]. [Reproduced with permission from SETAC]...

It is natural that mass spectrometric techniques should have been applied to species that are already charged, and a number of studies have been made of both negative and positive ion formation in chemical reactions. One of the earliest investigations by mass spectrometry of ions produced outside the spectrometer was that of Brasefield who produced ions in an electric discharge. Although such systems are of great importance in the study of ion-molecule reactions, they do not really fall within the scope of this chapter, and most of the results to be discussed concern chemi- or photo-ionization. 

In many cases the mere observation of a parent negative ion in mass spectrometry or ion mobility spectrometry is evidence of the positive electron affinity of a molecule. The ECD kinetic model is applicable to the ions observed in NICI experiments so the same quantities measured in the ECD can also be measured with this technique. There is a large body of NIMS data taken at two temperatures for compounds significant to those used in environmental chemistry that can be analyzed to obtain approximate electron affinities and activation energies [10]. 

Munson and Field reported in 1966 on a technique of ionizing molecules by gas phase ion-molecule reactions, which they called chemical ionization (Cl). In this way, break-up of the molecules can be greatly reduced or even avoided. Thus, measured ion currents can be correlated with the densities of the respective parent neutral compounds, allowing for on-line monitoring of rather complex gas mixtures. The fundamental principles of gas phase ion chemistry on which Cl is based, as well as the instrumentation for Cl, have been reviewed in great detail by Harrison." The wide variety of Cl methods that has been developed includes Medium Pressure Mass Spectrometry, Fourier Transform Mass Spectrometry, Quadrupole Ion Trap Mass Spectrometry, Pulsed Positive Ion-Negative Ion Chemical Ionization, and Atmospheric Pressure Ionization Mass Spectrometry (API-MS). Of these, API-MS has developed into a very reliable and widely used technique for analysis of VOCs in flavor release studies and human breath. A variety of API-MS applications in these fields of research has been described in a recent volume by Roberts and Taylor. ... 

See also Fragmentation in Mass Spectrometry Hydrogen Bonding and other Physicochemical Interactions Studied By IR and Raman Spectroscopy Ion Collision Theory Ion Energetics in Mass Spectrometry Ion Molecule Reactions in Mass Spectrometry Metastable Ions Negative Ion Mass Spectrometry, Methods Neutralization-Reionization in Mass Spectrometry Proton Affinities Sector Mass Spectrometers Stereochemistry Studied Using Mass Spectrometry. 

For a limited range of substances, negative radical anions (M ) can be formed rather than positive ions (Equation 3.3). Negative radical anions can be produced in abundance by methods other than electron ionization. However, since most El mass spectrometry is concerned with positive ions, only they are discussed here. 

A big step forward came with the discovery that bombardment of a liquid target surface by abeam of fast atoms caused continuous desorption of ions that were characteristic of the liquid. Where this liquid consisted of a sample substance dissolved in a solvent of low volatility (a matrix), both positive and negative molecular or quasi-molecular ions characteristic of the sample were produced. The process quickly became known by the acronym FAB (fast-atom bombardment) and for its then-fabulous results on substances that had hitherto proved intractable. Later, it was found that a primary incident beam of fast ions could be used instead, and a more generally descriptive term, LSIMS (liquid secondary ion mass spectrometry) has come into use. However, note that purists still regard and refer to both FAB and LSIMS as simply facets of the original SIMS. In practice, any of the acronyms can be used, but FAB and LSIMS are more descriptive when referring to the primary atom or ion beam. 

Standard El spectra are obtained with an electron energy of 70 eV (electrons accelerated through 70 V). For most compounds, it is easier to produce positive ions than negative ones, and most El mass spectrometry is concerned with positive ions. 

A review pubHshed ia 1984 (79) discusses some of the methods employed for the determination of phenytoia ia biological fluids, including thermal methods, spectrophotometry, luminescence techniques, polarography, immunoassay, and chromatographic methods. More recent and sophisticated approaches iaclude positive and negative ion mass spectrometry (80), combiaed gas chromatography—mass spectrometry (81), and ftir immunoassay (82). 

In other articles in this section, a method of analysis is described called Secondary Ion Mass Spectrometry (SIMS), in which material is sputtered from a surface using an ion beam and the minor components that are ejected as positive or negative ions are analyzed by a mass spectrometer. Over the past few years, methods that post-ion-ize the major neutral components ejected from surfaces under ion-beam or laser bombardment have been introduced because of the improved quantitative aspects obtainable by analyzing the major ejected channel. These techniques include SALI, Sputter-Initiated Resonance Ionization Spectroscopy (SIRIS), and Sputtered Neutral Mass Spectrometry (SNMS) or electron-gas post-ionization. Post-ionization techniques for surface analysis have received widespread interest because of their increased sensitivity, compared to more traditional surface analysis techniques, such as X-Ray Photoelectron Spectroscopy (XPS) and Auger Electron Spectroscopy (AES), and their more reliable quantitation, compared to SIMS. 

In secondary ion mass spectrometry (SIMS), a beam of energetic primary ions is focused onto the surface of a solid. Some of the ions are reflected but most of the energy of the primary ions is dissipated in the surface by binary collisions that cause neutrals, excited neutrals, and ions (positive and negative) to be ejected or sputtered from the surface. The secondary ions can be analyzed by a mass spectrometer to provide information about the surface composition of the solid. 

S. Lacorte and D. Barcelo, Determination of parts per trillion levels of organophospho-rus pesticides in groundwater by automated on-line liquid- solid extraction followed by liquid chr omatography/atmospheric pressure chemical ionization mass spectrometry using positive and negative ion modes of operation . Anal. Chem. 68 2464- 2470 (1996). 

Carbocations are intermediates in several kinds of reactions. The more stable ones have been prepared in solution and in some cases even as solid salts, and X-ray crystallographic structures have been obtained in some cases. An isolable dioxa-stabilized pentadienylium ion was isolated and its structure was determined by h, C NMR, mass spectrometry (MS), and IR. A P-fluoro substituted 4-methoxy-phenethyl cation has been observed directly by laser flash photolysis. In solution, the carbocation may be free (this is more likely in polar solvents, in which it is solvated) or it may exist as an ion pair, which means that it is closely associated with a negative ion, called a counterion or gegenion. Ion pairs are more likely in nonpolar solvents. 

For many years, electron ionization, then more usually known as electron impact, was the only ionization method used in analytical mass spectrometry and the spectra encountered showed exclusively the positively charged species produced during this process. Electron ionization also produces negatively charged ions although these are not usually of interest as they have almost no structural significance. Other ionization techniques, such as Cl, FAB, thermospray, electrospray and APCI, however, can be made to yield negative ions which are of analytical utility. 

Electrospray is the softest mass spectrometry ionization technique and electrospray spectra therefore usually consist solely of molecular ions. Electrospray is unique, however, in that if the analyte contains more than one site at which protonation (in the positive-ion mode) or deprotonation (in the negative-ion mode) may occur, a number of molecular ions with a range of charge states is usually observed. For low-molecular-weight materials (< 1000 Da), the number of sites... 

Figure 5.62 Product-ion MS-MS spectra of the molecular ions from 8-hydroxy-2 -deoxyguanosine, obtained by (a) positive, and (b) negative ionization. Reprinted by permission of Elsevier Science from Comparison of negative- and positive-ion electrospray tandem mass spectrometry for the liquid chromatography-tandem mass spectrometry analysis of oxidized deoxynucleosides , by Hua, Y., Wainhaus, S. B., Yang, Y., Shen, L., Xiong, Y., Xu, X., Zhang, F. Bolton, J. L. and van Breemen, R. B., Journal of the American Society for Mass Spectrometry, Vol. 12, pp. 80-87, Copyright 2000 by the American Society for Mass Spectrometry.

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