Negative ionization

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. 

These arise either by an analogous process to that described above for Cl, i.e. the adduction of a negatively charged species such as Cl-, and the abstraction of a proton to generate an (M — H)- ion, or by electron attachment to generate an M- ion. The ions observed in the mass spectrum are dependent on the species generated by the reagent gas and the relative reactivities of these with each other and with the analyte molecule. 

Negative-ion Cl tends to be a very selective method of ionization which generates reduced levels of background, thus allowing sensitive analyses to be developed for appropriate analytes. 

Using MS as a detector in GC and especially in capillary GC is relatively simple. The most common mobile phases do not interfere, and the analytes are volatile and already in gas phase so that El and Cl can be used. The only problem is the pressure difference between the GC and MS units. The outlet of the column is commonly at atmospheric 

The analytes are transferred to the mass spectrometer without loss if the carrier gas flow rate at the column outlet is equal to the flow rate to the ion source. If the inlet flow rate is lower, less of the analyte is transferred, and if the carrier gas flow rate is lower, dilution of the analyte occurs. 

especially capillary GC-MS, has become a widely used method, and is becoming very important also in routine analyses. Quadmpole mass spectrometers (Section 3.6.2.6) are most common in GC-MS. The quadmpole MS instmment has a mass range that covers the molecular masses of compounds which can be chromatographed by GC. 

In addition to the type of MS, the minimum detectables (MDs) obtainable depend on both the mode of ionization and the mode of operation. Typical values for MD amount in full scan and selected ion monitoring (SIM) mode may be 10 and 1 pg, respectively, with El. 

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Image Formation and Stabilization. The sequence of reactions responsible for image formation and stabilization begins as alkaU in the reagent permeates the layers of the negative, ionizing each of the three dye developers (eq. 8) and the auxiUary developer (eq. 9), which may be present in one or more layers of the negative. 

Fig. 6. Orbital correlation diagram for the photoelectron spectra of 1,2-dithietes (the orbital energies given are the negative ionization energies, -/ j in eV).

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.

Negative ionization The production of negative ions of analytical signiflcance from the analyte of interest. 

A triple-quadrupole mass spectrometer with an electrospray interface is recommended for achieving the best sensitivity and selectivity in the quantitative determination of sulfonylurea herbicides. Ion trap mass spectrometers may also be used, but reduced sensitivity may be observed, in addition to more severe matrix suppression due to the increased need for sample concentration or to the space charge effect. Also, we have observed that two parent to daughter transitions cannot be obtained for some of the sulfonylurea compounds when ion traps are used in the MS/MS mode. Most electrospray LC/MS and LC/MS/MS analyses of sulfonylureas have been done in the positive ion mode with acidic HPLC mobile phases. The formation of (M - - H)+ ions in solution and in the gas phase under these conditions is favorable, and fragmentation or formation of undesirable adducts can easily be minimized. Owing to the acid-base nature of these molecules, negative ionization can also be used, with the formation of (M - H) ions at mobile phase pH values of approximately 5-7, but the sensitivity is often reduced as compared with the positive ion mode. 

Liquid chromatography, coupled to the different ionization sources, is generally the technique most used to characterize the phenolic profile in fruit and vegetable products. With regard to the source ionization, it seems that ESI is used more frequently than other sources, such as APCI or APPI. Another important aspect of this technique is the ionization of phenolic compounds. Negative ionization seems to be more suitable... 

Sample preparation requires only dissolution of the sample to a suitable concentration in a mixture of water and organic solvent, commonly methanol, isopropanol, or acetonitrile. A trace of formic acid or acetic acid is often added to aid protonation of the analyte molecules in the positive ionization mode. In negative ionization mode ammonia solution or a volatile amine is added to aid deprotonation of the analyte molecules. 

R.B. Desai, M.S. Schwartz and B.K. Matuszewski, The identification of three human metabolites of a peptide-doxorubicin conjugate using HLPC-MS-MS in positive and negative ionization modes. J. Chromatogr. Sci. 42 (2004) 317-322. 

In this section, we shaU outline a many-electron treatment of charge transfer, similar in spirit to that of Tully, which enables different charge-exchange mechanisms to be incorporated in the formalism simultaneously. Although we shall concentrate on the TDAN model of resonant neutralization and negative ionization, we shall indicate how other neutralization processes can be included, and the approach for the reverse process of positive ionization will be fairly apparent. 

The process o - d>(o - is, in effect, an excitation of an electron from Xi to X. In a similar way, the negatively ionized species, represented by (70), can lose one of its electrons (or possibly both, though we shall ignore this) into an unoccupied substrate state, giving a wavefunction of the form either of (69) or... 

Note MS analysis was carried out in negative ionization mode due to the moderate acidity of glucuroconjugates. The mobile phase ionic strength was adjusted to 5 mM ammonium acetate at pH 5.0 in order to facilitate in-liquid ionization under established... 

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