Atmospheric pressure chemical ionization heated nebulizer interface

Detector MS, PE Sciex API III, MS/MS, positive atmospheric pressure chemical ionization, heated nebulizer interface, m/z 256 to 44... 

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 8.5 Schematic representation of an API sonrce with a heated nebulizer interface for APCI. (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... 

In order to combine reversed-phase LC with atmospheric pressure chemical ionization (APCI)-MS (125), a commercially available heated nebulizer interface that can handle pure aqueous eluents at flow rates up to 2 ml/min in addition to nonvolatile buffers has been used (126). The heated nebulizer inlet probe consists... 

Complementary to ESP and ISP interfaces, with respect to the analyte polarity, is the atmospheric pressure chemical ionization (APCI) interface equipped with a heated nebulizer. This is a powerful interface for both structural confirmation and quantitative analysis. 

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. 

A comparison of various LC-MS systems for the analysis of complex mixtures of PAHs showed that (1) the moving belt interface was mechanically awkward and is compatible only with a limited range of mobile phases, (2) particle-beam interface had low sensitivity, and the response was nonlinear, (3) a heated nebulizer interface that uses atmospheric pressure chemical ionization (APCI) was the preferred procedure (Anacleto et al. 1995). 

Figure 9.7. Schematic diagram of a heated pneumatic nebulizer LC-MS interface and atmospheric pressure chemical ionization source with a cross-sectional view of the nebulizer probe.

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. 

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.)...

In atmospheric pressure chemical ionization (APCI) interfaces, nitrogen is introduced to nebulize the mobile phase producing an aerosol of nitrogen and solvent droplets, which are passed into a heated region. Desolvation occurs, and ionization is achieved by gas-phase ion-molecule reactions at atmospheric pressure, electrons and the primary ions being produced by a corona discharge. 

Next to ESI, APCI is an alternative, especially for less polar compounds. The APCI interface consists of a different solvent inlet probe, the so-called heated nebulizer, and a discharge electrode, but uses exactly the same atmospheric-pressure ion source and vacuum transition region as in ESI (see Figure 1). In a heated nebulizer, the LC column effluent is first pneumatically nebulized into an aerosol, which subsequently is evaporated in a heated vaporizer zone. Solvent and analyte vapor meet the discharge needle, where a potential of a few kilovolts initiates a continuous discharge that results in the ionization of solvent molecules, which in turn ionize the analyte molecules by gas-phase ion-molecule reactions (common chemical ionization processes). 

In a particle-beam interface (Figure 3A), the column effluent is pneumatically nebulized into an atmospheric-pressure desolvation chamber. This is connected to a momentum separator where the analytes are transferred to the MS ion source while the low molecular mass solvent molecules are efficiently pumped away. The analyte particles hit the heated source surface, evaporate and can be ionized by electron or chemical ionization. The evaporation step obviously limits the application range of the interface to not-too-polar analytes. 

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