Ammonium isotopic analysis

With the application of FIA in the mixture analytical mode for the analysis of environmental samples and after a marginal sample pretreatment by SPE, matrix effects are a high probability. But, as cited previously [27—31], matrix effects were not only observed with FIA but also in LC-MS and MS—MS modes. Advice to overcome these problems by, e.g. an improved sample preparation, dilution of the analyte solution, application of stable isotopic modification of LC conditions [29] or even application of two-dimensional LC separations [27], postcolumn standard addition [29], addition of additives into the mobile phase (e.g. propionic acid, ammonium formate) [34,35] or even matrix compounds [32] were proposed and discussed. 

Preston et al. [698,699] have described novel approaches to 15N-isotope dilution determination of ammonium and of free amino acids in natural waters, incorporating chemical derivatization and conventional GC-MS analysis. 

Other aspects of the report (42) on [Fe3S2(NO)5] are surprising. Elemental analysis of the ammonium salt was reported to distinguish between iron(II) and iron(III) in [Fe3S2(NO)5] , but to find these two types of iron present in equal numbers is most unusual for a triiron complex. Second, the molecular weight of the potassium salt was measured as 420 by mass spectrometry. This value is close to the M/Z of 421 calculated for the most abundant isotopic form of the ion-pair cation [KFe3S2(NO)5] +. Finally, the ESR spectrum reported is that of a dini-trosyliron species, which bears a remarkable resemblance to that reported (22) for a complex formed from Fe(II) and nitric oxide in aqueous alkaline solution. 

The most sensitive method for CVAA has recently been reported by Wooten et al. (39) using solid-phase microextraction to concentrate the derivatized analyte. Urine, with added ammonium acetate buffer and PhAsO as an internal standard, was derivatized directly with 1,3-propanedithiol and the derivative concentrated on a poly(dimethylsiloxane) (PDMS) solid-phase microextraction (SPME) fiber. Analysis was by automated GC/MS using SIM of the isotopic MH+ ions. An impressive detection limit of 7.4pg/ml was reported, using a benchtop GC/MS system. The method was validated using spiked human urine. 

Gardner, W. S., Bootsma, H. A., Evans, C., and St.John, P. A. (1995). Improved chromatographic analysis ofratios in ammonium or nitrate for isotope addition experiments. Mar. Chem. 48, 271-282. 

Preston, T., Bury, S., Presing, M., Monciffe, G., and Slater, C. (1996b). Isotope dilution analysis of combined nitrogen in natural waters. I. Ammonium. Rapid Commun. Mass Spectrom. 10, 959—964. 

The performance of the FBI can be enhanced by the use of (volatile) additives, such as ammonium acetate, formate, or oxalate, to the mobile phase [92]. They are assumed to act as carriers. Similarly, the use of additives with structures related to the target analyte structures, e.g., phenoxyacetic acid in the analysis of chlorophenoxyacetic acids, was evaluated as well [93]. The carrier effects, exerted by either mobile-phase additives, coeluting compounds, and/or isotopically-labelled standards, is not really understood from a mechanistic point of view. It caimot be applied to consistently enhance the performance for some compounds it works fine, while for others no effects are observed. 

The thermospray LC/MS analysis of a major urinary sulfate ester metabolite of SK F 86466 is shown in Figure 7. A weak ammonium adduct was observed at m/z 295, 297 with an isotope distribution consistent with the proposed structure. The major fragment ion at m/z 182, 184 occurs from loss of the sulfate ester moiety. The position of sulfation at either C7 or C9 of the molecule was determined by proton NMR NOE difference studies (data not shown). No protonated molecular ion was observed for this sulfate ester this would be expected, based on the proton affinity of the highly acidic sulfate ester moiety. 

The use of amplification reactions plays an important role in improving the sensitivity of some reactions, for example, the increase of phosphorus determination by the reduction of a heteropolycompound, ammonium phos-phomolybidate. Another way to increase the sensitivity of a reaction from a qualitative and quantitative point of view is the use of radioactive isotopes in chemical analysis. However, this area of analytical chemistry was replaced by new, more sensitive, safer, and less hazardous techniques for qualitative and quantitative analysis (total analytical techniques), such as inductively... 

The primary motivation for these studies is the analysis of the reactivity patterns of organic compounds, when Ce(IV) is used as an oxidant. These patterns are determined for the most part by product analysis of selected series of organic compounds. The results obtained in two studies that bear more directly on the chemical behavior of Ce(IV) as an oxidant for hydrocarbons have been interpreted to indicate different mechanistic behavior of Ce(IV). In a product study of the oxidation of isodurene (1,2,3,5-tetramethyl benzene) by ceric ammonium nitrate compared to anodic oxidation, Eberson and Oberrauch (1979) concluded that the oxidation by Ce(IV) occurs via a H atom transfer from the alkylaromatic compound to Ce(IV). Badocchi et al. (1980) measured the variation of second-order rate constants for the oxidation of a series of alkylaromatic compounds with added Ce(III). These results along with those from the determination of kinetic deuterium isotope effect were dted to support a mechanism involving radical cations. The Ce(IV)/Ce(III) functions as an electron acceptor/donor in such a mechanism. 

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