Partial pressure of the reagent gas

Cl differs from what we have encountered in mass spectrometry so far because bimolecular processes are used to generate analyte ions. The occurrence of bimol-ecular reactions requires a sufficiently large number of ion-molecule collisions during the dwell time of the reactants in the ion source. This is achieved by significantly increasing the partial pressure of the reagent gas. Assuming reasonable... 

Cl is a gas-phase ion-molecule reaction in which the analyte (molecule) is ionized via a proton transfer process. (For more detailed description of the Cl processes and analytical applications, a classic book by Harrison [17] is recommended.) The formation of the reactive ions in this ion-molecule reaction process is triggered by El ionization of a reagent gas that is, most commonly, methane, isobutene, or ammonia. The partial pressure of the reagent gas (1-0.1 Torr) is much higher than that of the analyte (ca. lO" to 10 Torr), so the gas molecules can be considered as a protective shield for the analyte molecules to avoid direct El ionization. El ionization of methane results in the fragmentation of methane molecular ion and some of these ions react with neutral methane. The ionization of the analyte molecule occurs by proton transfer between reagent gas ions and the analyte, or to a less extent, by adduct formation. Some characteristic mechanistic steps for methane Cl can be summarized as follows ... 

Reaction between an absorbed solute and a reagent lowers the equilibrium partial pressure of the solute and thus increases the rate of mass transfer. The mass transfer coefficient likewise may be enhanced which contributes further to increased absorption rate. Three modes of contacting gas and liquid phases are possible The gas is dispersed as bubbles in the liquid, the liquid is dispersed as droplets, the two phases are contacted on a thin liquid film deposited over a packing or wall. The choice between these modes is an important practical problem. 

Gas electrodes are similar to ISEs and usually work on the same basic principles. They look much like a standard ISE except that they have a gas-permeable, water-impermeable membrane at the tip. Gas present in the environment passes through the membrane, reacts with reagents in the interior, and produces a voltage. This voltage is related to the partial pressure of the gas being measured. 

In this context, attention should be drawn to those procedures where formation of gases might take place in the FIA system itself. This will happen whenever the partial pressure of the gas exceeds that corresponding to the solubility of the gas under the prevailing conditions, thus leading to formation of microbubbles. Thus without protection from the ambient air, alkaline reagent solutions will absorb carbon dioxide, and if the re-... 

For Cl to be efficient, the reagent partial pressure in the source must be between 1 and 5 torr. With classic sources described earlier, it is necessary to replace the open ion volume used in El by a closed ion volume (see Figure 3.10). Indeed, an open ion volume does not allow attainment of the necessary partial pressure in the reagent because the gas is sucked in by the pumping system at the level of the source. The closed ion volume is not completely closed it is pierced with a hole to allow the... 

In ion trap mass spectrometers using internal ionization (see Chapter 4), the ions are trapped over periods approximately 100.000 times higher than the residency time of the ions in a quadrupole source. With an approximately equal probability of a meeting between ions and neutrals, Cl may be performed with a partial pressure in a reagent gas around 100.000 times inferior to that necessary for Cl in an external source. This has several beneficial consequences that are detailed below. 

For the description of gas-phase reactions, the activities in Eqn. (13) are substituted by the respective partial pressures of reagent and product components. In the case of... 

Figure 2. Schematic illustration of the subprocesses comprising a gas-aqueous reaction in a single cloud droplet. A represents aqueous phase reagent species transferred from the gas phase B, species in rapid equilibrium with A and C, product species, at the surface of the drop (a) or in the interior (r) PA represents gas-phase partial pressure of A at the surface of the drop (a) and at large distance from the drop (oo).

Conceptually, the Cl process is quite distinct from El. Cl is accomplished through gas-phase acid-base reactions between the sample molecules and the reagent gas ions. Three steps are involved in the Cl process. In the first step, the reagent gas, the partial pressure of which is 10 to 100 times greater than the sample pressure, is ionized at a pressure of 0.1 to 1 torr by bombardment with a beam of 200- to 500-eV electrons. In the second step, one or more stable reagent ions are produced by ion-molecule reactions. Finally, the sample molecules are ionized by gas-phase ion-molecule reactions with the stable reagent ions. 

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). 

Pressure measurement deviees sueh as a manometer are used without disturbing the system being monitored. Another type of reaeting system that ean be monitored involves one of the produets being quantitatively removed by a solid or liquid reagent that does not affeet the reaetion. An example is the removal of an aeid formed by reaetions in the gas phase using hydroxide solutions. From the reaetion stoiehiometry and measurements of the total pressure as a funetion of time, it is possible to determine the extent of the reaetion and the partial pressure or eoneentrations of the reaetant and produet speeies at the time of measurement. 

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