Reagent systems ionization

It appears that gas-phase basicity of nitro compounds has been studied only scarcely. Thus, only the values of the parent compounds, nitromethane (179.2 kcalmol-1) and nitrobenzene (193.4 kcalmoD1), are found in the comprehensive listing given in Reference 39. The rather high PA values for nitro compounds suggest protonation by common chemical ionization reagent systems, such as hydrogen (H3+) and methane (CH5+). 

Charge exchange (CE) or charge transfer ionization occurs when an ion-neutral reaction takes place in which the ionic charge is transferred to the neutral. [8] In principle, any of the reagent systems discussed so far is capable to effect CE because the respective reagent molecular ions X" are also present in the plasma ... 

Unfortunately, this reagent system is not suitable for the oxidation of substrates of higher ionization potential nor for oxygen-substituted compounds. 

Hydrocarbon Positive-Ion Chemical Ionization Reagent Systems... 

To a considerable extent, the usefulness of CIMS arises from the fact that a wide variety of reagent gases and, hence, reagent ions can be used to ionize the analyte often the reagent system can be tailored to the problem to be solved. Problems amenable to solution by Cl approaches include (a) molecular... 

Hydrogen peroxide may react directiy or after it has first ionized or dissociated into free radicals. Often, the reaction mechanism is extremely complex and may involve catalysis or be dependent on the environment. Enhancement of the relatively mild oxidizing action of hydrogen peroxide is accompHshed in the presence of certain metal catalysts (4). The redox system Fe(II)—Fe(III) is the most widely used catalyst, which, in combination with hydrogen peroxide, is known as Fenton s reagent (5). 

Cl is an efficient, and relatively mild, method of ionization which takes place at a relatively high pressure, when compared to other methods of ionization used in mass spectrometry. The kinetics of the ion-molecule reactions involved would suggest that ultimate sensitivity should be obtained when ionization takes place at atmospheric pressure. It is not possible, however, to use the conventional source of electrons, a heated metallic filament, to effect the initial ionization of a reagent gas at such pressures, and an alternative, such as Ni, a emitter, or a corona discharge, must be employed. The corona discharge is used in commercially available APCI systems as it gives greater sensitivity and is less hazardous than the alternative. 

Anions of weak acids can be problematic for detection in suppressed IEC because weak ionization results in low conductivity and poor sensitivity. Converting such acids back to the sodium salt form may overcome this limitation. Caliamanis et al. have described the use of a second micromembrane suppressor to do this, and have applied the approach to the boric acid/sodium borate system, using sodium salt solutions of EDTA.88 Varying the pH and EDTA concentration allowed optimal detection. Another approach for analysis of weak acids is indirect suppressed conductivity IEC, which chemically separates high- and low-conductance analytes. This technique has potential for detection of weak mono- and dianions as well as amino acids.89 As an alternative to conductivity detection, ultraviolet and fluorescence derivatization reagents have been explored 90 this approach offers a means of enhancing sensitivity (typically into the low femtomoles range) as well as selectivity. 

Resistance to physical shocks and vibration required careful attention to selection of rugged components and to securing electrical and vacuum systems, wiring, connectors, components, and boards. Chemical ionization (Cl) was used for the first time in a fieldable military detector because of the advent of rugged turbomolecular pumps capable of handling the gas load from the Cl reagent. 

Some Srj I reactions can take place in the dark withont a catalyst. For example, the interaction of freons with nncleophiles in DMF at 20°C proceeds withont photoirradiation. The chain process begins when the system pressnre reaches 2 atm, in other words, when the concentration of the gaseons reagent becomes snfficient (Waksehnan and Tordenx 1984 Scheme 7.69). There is a favorable difference between the ionization potential of the nncleophile (PhS ) and EA of the substrate (CFjBr) the expressed bromide fugacity is also a favorable factor. 

Figure 4.16 — Schematic diagram of a split-stream FI system used for the determination of glutamine in bioreactor media C de-ionized water carrier stream R buffer diluent reagent stream S sample injection point L delay coils CPG controlled pore glass enzyme reactor ISE ammonium ion-selective membrane electrode W waste. (Reproduced from [139] with permission of the American Chemical Society).

A new gas chromatography (GC) method was developed to characterize artemether 28a and its metabolites in body fluids. The extracts were derivatized and then separated on an optimized capillary GC system and identified by chemical ionization MS using ammonia as the reagent gas <1998JCH(B)101>. A sensitive, selective, and reproducible GC-MS-SIM method has also been developed for the determination of artemether 28a and dihydroartemisinin 29a in plasma, using artemisinin 9a as an internal standard <1999JCH(B)251>. 

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