Reagent gas pressure

Note The initial kinetic energy of slow ions can be lost upon several collisions, thereby stopping their motion along the cell. [119,120] Under such conditions, the continuous ion current into the cell is the only impetus to push the ions through as a result of space-charge effects. The resulting dwell time of about 10 ms allows up to about 5000 (reactive) collisions to take place at some 5 Pa collision gas (or reagent gas) pressure in an octopole collision cell. [123]... 

Ionization in Cl is the result of one or several competing chemical reactions. Therefore, the sensitivity in Cl strongly depends on the conditions of the experiment. In addition to primary electron energy and electron current, the reagent gas, the reagent gas pressure, and the ion source temperature have to be stated with the sensitivity data to make a comparison. Modem magnetic sector instmments are specified to have a sensitivity of about 4 x 10" C pg for the [Mh-H] quasi-molecular ion of methylstearate, m/z 299, at / = 1000 in positive-ion Cl mode. This is approximately one order of magnitude less than for El. 

FTMS has been used to examine negative ion chemical ionization (Cl) reactions which can be used to differentiate isomeric compounds. A number of unusual anions formed in a conventional high pressure Cl source were successfully produced in the FTMS with much lower reagent gas pressures. Exact mass measurements were used to establish the elemental compositions of these anions. 

Chemical ionization with an ion trap mass spectrometer is possible at much lower-reagent gas pressures and longer reaction times than typical external sources. A preliminary ionization period is used to establish the reagent gas ion concentration, followed by a relatively long (10s of ms) reaction period at a slightly higher radio frequency potential, during which sample ions are formed and stored. The radio... 

These theoretical considerations of the thermochromatographic process presume that the adsorption entropy and enthalpy do not depend on the temperature. It was also postulated that the adsorbent was homogeneous, its surface was not saturated with the adsorbate (monolayer or less), and the carrier (reagent) gas was unsorbable. Diffusion in the solid phase (adsorbent) and surface diffusion were ignored. Furthermore, in the theoretical considerations the effect of the carrier (reagent) gas pressure on the substance transport was not taken into account, which, however, should be considered in the case of TC at reduced reactant gas pressures and vacuum TC or with densely filled columns. 

Negative ion spectra of flavonoid O- and C- glycosides have been recorded using desorption c.i.-m.s. at a low reagent gas pressure. In laser desorption m.s., cationization of saccharides is best achieved by introduction of cations in the +1 oxidation state rather than other oxidation states, and it was shown that sample preparation and the concentration of salt and saccharide (above a threshold level) had little effect. ... 

The principle of Cl was first described by Munson and Field (Munson and Field, 1966). Cl has now developed into a widely used technique for structural determination and quantitation in GC-MS (Stan and Kellner, 1981). Instead of an open, easily evacuated ion source, a closed ion volume is necessary for carrying out Cl. In the high vacuum environment of the ion source, a reagent gas pressure of about... 

This effect is seldom made use of in GC-MS analysis in contrast to being the prevailing reaction in ESI-LC-MS (electrospray ionization), but must be taken into account on evaluating Cl spectra. The enhanced formation of adducts is always observed with intentional protonation reactions where differences in the proton affinity of the participating species are small. High reagent gas pressure in the ion source favours the effect by stabilizing collisions. 

Ion Trap Instruments Ion trap mass spectrometers with internal ionization can be used for Cl without hardware conversion. Because of their mode of operation as storage mass spectrometers, only a very low reagent gas pressure is necessary for instruments with internal ionization. The pressure is adjusted by means of a special needle valve which is operated at low leak rates and maintains a partial pressure of only about 10 Torr in the analyser. The overall pressure of the ion trap analyser of about 10 -10 Torr remains unaffected by it. Cl conditions thus set up give rise to the term low pressure CL Compared to the conventional ion source used in high pressure Cl, in protonation reactions, for example, a clear dependence of the Cl reaction on the proton affinities of the reaction partners is observed. Collision stabilization of the products formed does not occur with low pressure Cl. This explains why high pressure Cl-typical adduct ions are not formed here, which would confirm the identification of the (quasi)molecular ion (e.g., with methane besides (M + H), also M + 29 and M +41 are expected). The determination of ECD-active substances by electron capture (NCI) is not possible with low pressure Cl (Yost, 1988). 

The spectrum of ions formed from the reagent gas for chemical ionization, monitored to adjust the correct reagent gas pressure in the ion source. 

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