Mass spectrometry negative

Tr ifluoroacetate has been determined in seawater in amounts down to 32 ng/1 by extraction with methyl tert butyl ether and derivatised with pentafluorophenyl diazomethane prior to gas chromatography mass spectrometry (negative ionisation) [541],... 

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]

Figure 7 ESI-MS studies of tetramolecular quadruplexes. The TG5T oligonucleotide was incubated in 0.15 M pH 7.0 ammonium acetate buffer for various amounts of time, then analysed by electrospray mass spectrometry (negative mode). From the initial peak corresponding to the TG5T single-strand (SS), one can see the gradual conversion to a quadruplex (G4), as well as the presence of dimeric (Di) and trimeric (Tri) species...

Jimenez, J. J., Bernal, J. L., Del Nozal, M. J., and Martin, M. T., Use of a particle beam interface combined with mass spectrometry/negative chemical ionization to determine polar herbicide residues in soil by liquid chromatography, J. AOAC Int., 83, 756-761, 2000. 

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)... 

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.

GC/MS has been employed by Demeter et al. (1978) to quantitatively detect low-ppb levels of a- and P-endosulfan in human serum, urine, and liver. This technique could not separate a- and P-isomers, and limited sensitivity confined its use to toxicological analysis following exposures to high levels of endosulfan. More recently, Le Bel and Williams (1986) and Williams et al. (1988) employed GC/MS to confirm qualitatively the presence of a-endosulfan in adipose tissue previously analyzed quantitatively by GC/ECD. These studies indicate that GC/MS is not as sensitive as GC/ECD. Mariani et al. (1995) have used GC in conjunction with negative ion chemical ionization mass spectrometry to determine alpha- and beta-endosulfan in plasma and brain samples with limits of detection reported to be 5 ppb in each matrix. Details of commonly used analytical methods for several types of biological media are presented in Table 6-1. 

ECD = electron capture detector GC = gas chromatography HPLC = high-performance liquid chromatography MC = microcoulometric detector MS = mass spectrometry NICI = negative ion chemical ionization RSD = relative standard deviation SPE = solid phase extraction... 

Mass Spectrometry. Mass spectrometry holds great promise for low-level toxin detection. Previous studies employed electron impact (El), desorption chemical ionization (DCI), fast atom bombardment (FAB), and cesium ion liquid secondary ion mass spectrometry (LSIMS) to generate positive or negative ion mass spectra (15-17, 21-23). Firm detection limits have yet to be reported for the brevetoxins. Preliminary results from our laboratory demonstrated that levels as low as 500 ng PbTx-2 or PbTx-3 were detected by using ammonia DCI and scans of 500-1000 amu (unpublished data). We expect significant improvement by manipulation of the DCI conditions and selected monitoring of the molecular ion or the ammonia adduction. 

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