Solvent adduct ions

Use of Solvent Adduct Ions To Confirm Structure of Selected Herbicides with Thermospray Liquid Chromatography/Mass... 

In the work described here the utility of solvent adduct ions in TSP LC-MS which consist in the use of novel additives in the chromatographic eluent, such as ammonium formate or chloroacetonitrile, will be demonstrated for confirmation of structure of a variety of herbicides including triazines, phenylurea and chlorinated phenoxyacids. Complementary adduct ion information to the conventional TSP LC-MS mode of operation will be obtained. Because TSP LC-MS involves mainly a chemical ionization process where the vaporized eluent acts as chemical ionization gas, it will be of interest to compare the different adduct ions obtained here with those using other interfacing systems such as direct liquid introduction (DLI) (13-18). 

Shang et al. [7] studied the effect of different additives (NaAc, NaOH, NaCl, NH4Ac) on analyte signal intensity and they found that the relative intensity of NPEO adduct ions may be enhanced by all additives, but NaAc produced the most abundant adduct ions for the entire ethoxylate series with good reproducibility. Additionally, the intensity of adducts, especially for mono- and diethoxylates was found to depend on reaction time prior to LC-ESI-MS analysis and concentration of NaAc. They recommended 0.5 mM NaAc for normal-phase separation with solvent system toluene-MeOH-water. In reversed-phase systems the highest abundance of sodium adducts for NPEOs ( iEO = 1-10) was observed at concentrations higher than 10 xM, while any further increase in concentration had very low influence on signal intensity [10,11],... 

A az, s ) and solvent (k, s ) to a carbocation intermediate of solvolysis are first determined from the ratio of the yields the azide ion and solvent adducts (equation (1)). The value for may then be calculated from this product rate constant ratio M ), and az = 5 X 10 s for the diffusion limited... 

For standard MALDI sample preparation, the analyte should be soluble to about 0.1 mg ml in some solvent. If an analyte is completely insoluble, solvent-free sample preparation may alternatively be applied (Chap. 10.4.3). The analyte may be neutral or ionic. Solutions containing metal salts, e.g., from buffers or excess of non-complexated metals, may cause a confusingly large number of signals due to multiple proton/metal exchange and adduct ion formation even complete suppression of the analyte can occur. The mass range of MALDI is theoretically almost unlimited in practice, limits can be as low as 3000 u, e.g., with polyethylene, or as high as 300,000 u in case of antibodies. 

Protonated and cationized species are the most commonly detected ions using the ES process if a positive ionization mode is selected. Protonation is a result of the addition of proton(s) to a neutral molecule for every proton added, a net charge of +1 will result. Similarly, canonization is due to the addition of cation(s) to a neutral molecule. Detection of cationized ions can be useful in the determination of molecular mass of unknown analytes.If ESI is operated under negative ion mode, deprotonated ions will usually be the most dominant ions. Eor either operation mode, solvent adducts of the protonated/deprotonated ions are frequently detected in ESI/MS mass spectra. 

ESI and APCI produce predominantly pseudo-molecular ions [M + H] + species in a positive mode and [M — H] in negative ion mode, with variable fragmentation depending upon the cone voltage used and, in the case of APCI, on the temperamre used. Adduct ions are very often seen in ESI, and it is these that can cause most problems to inexperienced users, but they can be useful. Their formation depends on the coordinating properties, polarity and concentration of the analyte and to some extent on the solvent being used. 

Metal-Oxygen Compounds. The addition of metal-oxygen compounds to olefins is exemplified by the addition of mercuric acetate to olefins in hydroxylic solvents (83). Both cis and trans adducts may be obtained depending upon the structure of the olefin. Where trans addition is hindered by substitutents, cis addition appears to occur (3, 94). Ward and Henry have obtained kinetic evidence that the addition of mercuric acetate to ethylene in aqueous solution involves the prior formation of a 7r-ethylene complex. Presumably, trans addition results from attack by external solvent or ion, and cis addition results from ligand addition (101),... 

Sodium chloride may be added to the eluting solvent at a concentration of 0.01 M in order to accentuate sodium adduct ions in the MALDI-TOF mass spectrum and suppress the formation of potassium adduct ions. 

The proposition that locally excited triplet states can be formed from back electron transfer within a doublet-doublet radical ion pair has firm theoretical (88) and experimental support. For example, with time-resolved Resonance Raman spectroscopy, one can directly monitor the chemical fate of the exciplex, solvent separated ion pair, and doublet free radical ion pairs formed between stilbene and amines. As might be expected from the above discussion, adduct formation is observed from the exciplex or contact ion pairs, whereas enhanced intersystem crossing ensues from the solvent separated ion pairs, producing spectroscopically observable stilbene triplets. This back electron transfer process, eq. 30 (89),... 

Bhattacharyya, A. Banerjee, S. Mohapatra, P.K. Basu, S. Manchanda, V.K. Role of ligand structure and basicity on the extraction of uranyl isoxazolonate adducts, Solvent Extr. Ion Exch. 22 (2004) 13-29. 

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