Atmospheric pressure chemical ionization corona discharge

Thus, either the emitted light or the ions formed can be used to examine samples. For example, the mass spectrometric ionization technique of atmospheric-pressure chemical ionization (APCI) utilizes a corona discharge to enhance the number of ions formed. Carbon arc discharges have been used to generate ions of otherwise analytically intractable inorganic substances, with the ions being examined by mass spectrometry. 

Atmospheric pressure chemical ionization (APCI) was introduced in 1973 by Horning et al. [38, 42, 43] and coupled to GC. This is also the introduction of atmospheric pressure ionization (API) in general. The next year corona discharge was introduced for ion generation as well as successful coupling to LC [44, 45]. In APCI of a liquid, a pneumatic nebulizer induces the flow of liquid to form a spray at atmospheric pressure. The spray droplets pass a corona discharge electrode situated close to the orifice, which... 

In atmospheric pressure chemical ionization (APCI) ion-molecule reactions occurring at atmospheric pressure are employed to generate the ions, i.e., it represents a high-pressure version of conventional chemical ionization (Cl, Chap. 7). The Cl plasma is maintained by a corona discharge between a needle and the spray chamber serving as the counter electrode. The ions are transferred into the mass analyzer by use of the same type of vacuum interface as employed in ESI. Therefore, ESI ion sources can easily be switched to APCI instead of an ESI sprayer, a unit comprising a heated pneumatic nebulizer and the spray chamber with the needle electrode are put in front of the orifice, while the atmospheric pressure-to-vacuum interface remains unchanged. [48,138]... 

FIGURE 8.6 Corona discharge ionization cascade in positive (a) and negative (b) ion modes. (Reproduced from Raffaelli, A., Atmospheric pressure chemical ionization (APCI), in Cappiello, A. (ed), Advances in LC-MS Instrumentation, vol. 72 (Journal of Chromatography Library), Elsevier, Amsterdam, the Netherlands, 2007, 11-25. Copyright 2007. With permission from Elsevier.)... 

In IC-MS systems, the core of the equipment is the interface. In fact, inside the interface evaporation of the liquid, ionization of neutral species to charged species and removal of a huge amount of mobile phase to keep the vacuum conditions required from the mass analyzer take place. Two main interfaces are used coupled to IC, namely electrospray ionization (ESI) and atmospheric pressure chemical ionization (APCI). In the ESI mode, ions are produced by evaporation of charged droplets obtained through spraying and an electrical field, whilst in the APCI mode the spray created by a pneumatic nebulizer is directed towards a heated region (400°C-550°C) in which desolvation and vaporization take place. The eluent vapors are ionized by the corona effect (the partial discharge... 

Atmospheric pressure chemical ionization APCI is a method closely related to electrospray ionization. It uses ion-molecule reactions to produce ions from analyte molecules. The sample is electrohydrodynamically sprayed into the source (Figure 14.3). The evaporation of the solvent is often supported by a heated gas at temperatures between 80 and 400°C. Within the source, a plasma is created using a corona discharge needle that is placed close to the end of the metal capillary. In this plasma, proton transfer reactions occur, leading to the ionization of the analyte, mainly by the formation of [M+H]+ ions. Compared to ESI MS, APCI MS is very well suited for the analysis of less-polar components and can therefore... 

Figure 14.3 Principle of atmospheric pressure chemical ionization. The dissolved analyte is sprayed through a capillary. Evaporation of the solvent is supported by a heated gas stream. Within the source, a plasma is formed by a Corona discharge needle, which creates the charged reagent gas (here HgO+j. The ionization of the analyte (M) is performed by the transfer of the charge (proton) via ion-molecule reactions.

In atmospheric pressure chemical ionization (APCI) a similar interface to that used for ESI is used. A corona discharge is used to ionize the analyte in the atmospheric pressure region. The gas-phase ionization in APCI is more effective than ESI for analyzing less polar species. Both ESI and APCI are complementary methods that are well-suited for LC/MS techniques. 

Figure 22-19 (a) Atmospheric pressure chemical ionization interface between a liquid chromatography column and a mass spectrometer. A fine aerosol Is produced by the nebulizing gas flow and the heater. The electric discharge from the corona needle creates gaseous ions from the analyte. [Adapted from E. C. Huang, T. Wachs, J. J. Conboy, and J. D. Henion, Atmospheric Pressure Ionization Mass Spectrometry," Anal. Chem. 1990,62,713A ] (b) Atmospheric pressure chemical ionization probe. [Courtesy Shimadzu Scientific Instruments, Columbia, MD.J... 

Gas emerging from a capillary gas chromatography column can go directly into the ion source of a well-pumped mass spectrometer to provide qualitative and quantitative information about the components of a mixture. For liquid chromatography, atmospheric pressure chemical ionization utilizes a corona discharge needle to create a variety of gaseous ions. Alternatively, electrospray employs high voltage at the exit of the column, combined... 

Complementary to ESP and ISP interfaces is the APCI interface equipped with a heated nebulizer. The nebulized liquid effluent is swept through the heated tube by an additional gas flow, which circumvents the nebulizer. The heated mixture of solvent and vapor is then introduced into the ionization source where a corona discharge electrode initiates APCI. The spectra and chromatograms from APCI are somewhat similar to those from TSP, but the technique is more robust, especially with gradient LC, and it is often more sensitive. Atmospheric pressure chemical ionization is particularly useful for heat labile compounds and for low-mass, as well as high-mass, compounds. In contrast to the TSP interface, no extensive temperature optimization is needed with APCI. 

In atmospheric-pressure chemical ionization (APCI), a nebulized stream of droplets leaves the sample inlet tube and travels toward the entrance to the mass analyzer. During this passage, ions are produced, but the yield is rather small. By introducing electrodes across the flow of sample material at atmospheric pressure, a discharge can be started, which is essentially of a corona type. 

Atmospheric pressure chemical ionization (APCI) [15] involves the primary formation of ions by corona discharge in a solvent spray under atmospheric pressure. A solution of the sample is nebulized pneumatically and the droplets formed are vaporized by heating (120 °C). The resulting sample gas is chemically ionized by the transfer of protons from the primary reactant ions.The APCI technique is widely used in the analysis of drugs or metabolic studies. Detection is limited to a molecular mass of about 1000 Da. 

Mass spectrometry (MS) has become one of the most important analytical tools employed in the analysis of pharmaceuticals. This can most likely be attributed to the availability of new instrumentation and ionization techniques that can be used to help solve difficult bioanalytical problems associated with this field (1-8). Perhaps the best illustration of this occurrence is the development of electrospray (ESI) and related atmospheric-pressure ionization (API) techniques, ion-spray (nebulizer-assisted API), turbo ionspray (thermally assisted API), and atmospheric pressure chemical ionization (APCI nebulization coupled with corona discharge), for use in drug disposition studies. The terms ESI and ionspray tend to be used interchangeably in the literature. For the purpose of this review, the term API will be used to describe both ESI and ionspray. In recent years there has been an unprecedented explosion in the use of instrumentation dedicated to API/MS (4,6,8-14). API-based ionization techniques have now become the method of choice for the analysis of pharmaceuticals and their metabolites. This has made thermospray (TSP), the predominant LC/MS technique during the 1980s, obsolete (15). Numerous reports describing the utility of API/MS for pharmaceutical analysis have appeared in the literature over the last decade (7). The... 

FIGURE 3.22 Schematic diagram of an atmospheric pressure chemical ionization source (a) effluent from the column (b) nebulizing gas (c) heater (d) corona discharge needle (e) skimmer cones. 

Liquid introduction in atmospheric pressure chemical ionization resembles ionspray except that a heated capillary is used in place of the charged capillary. Figure 9.7. The combination of heat and gas flow desolvates the nebulized droplets producing fine particles of solvent and analyte molecules. The solvent molecules are then ionized by a corona discharge and the solvent ions formed ionize the analyte molecules by atmospheric pressure chemical ionization. 

Atmospheric pressure chemical ionization (APCI) is a simple and robust technique routinely used to interface the eluent from a high-performance liquid chromatography (FIPLC) to a mass spectrometer. The liquid stream passes through a heated nebulizer into a corona discharge region. Analyte molecules are ionized and extracted into the mass analyzer. 

Atmospheric pressure chemical ionization (APCI) APCI is another variation of electrospray where, in a similar manner to thermospray, the incorporation of a corona discharge into the spray increases the ion yield by electron stripping from both sample and solvent molecules. Much of the ion formation thus occurs by Cl processes. 

Figure 4 Atmospheric pressure chemical ionization source with corona discharge needle and heated nebulizer interface for combined liquid chromatography-mass spectrometry. (Reprinted with permission from Trends in Analytical Chemistry 13 (1994), 81 Elsevier.)...

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