Heated nebulizer-atmospheric pressure chemical ionization

Atmospheric-pressure chemical ionization (APCI) is another of the techniques in which the stream of liquid emerging from an HPLC column is dispersed into small droplets, in this case by the combination of heat and a nebulizing gas, as shown in Figure 4.21. As such, APCI shares many common features with ESI and thermospray which have been discussed previously. The differences between the techniques are the methods used for droplet generation and the mechanism of subsequent ion formation. These differences affect the analytical capabilities, in particular the range of polarity of analyte which may be ionized and the liquid flow rates that may be accommodated. 

Atmospheric-pressure chemical ionization (APCI) An ionization memod in which a liquid stream is passed through a heated capillary and a concentric flow of a nebulizing gas. Ions are formed by ion-molecule reactions between me analyte and species derived from me HPLC mobile phase. 

Atmospheric Pressure Chemical Ionization. As its name reveals, APCI[16] is a Cl carried out at atmospheric pressure instead of under vacuum, as occurs for classical Cl. As for ESI, the sample must be in a solution that is continuously flowing into the APCI source (flow rate between 0.2 and 2 ml min ). The solution passes through a pneumatic nebulizer and is desolvated in a heated quartz tube or heating block, thus producing vaporization of solvent and analyte molecules (Figure 2.4). 

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]... 

Atmospheric pressure chemical ionization (APCI) is a gas phase ionization process based on ion-molecule reactions between a neutral molecule and reactant ions [31]. The method is very similar to chemical ionization with the difference that ionization occurs at atmospheric pressure. APCI requires that the liquid sample is completely evaporated (Fig. 1.12). Typical flow rates are in the range 200-1000 xL min , but low flow APCI has also been described. First, an aerosol is formed with the help of a pneumatic nebulizer using nitrogen. The aerosol is directly formed in a heated quartz or ceramic tube (typical temperatures 200-500 °C) where the mobile phase and the analytes are evaporated. The temperature of the nebulized mobile phase itself remains in the range 120-150 °C due to evapo-... 

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... 

Atmospheric pressure chemical ionization (APCI), is more efficient than ESI for nonpolar (hydrophobic) analytes, such as steroids, which do not readily form ions in solution [61], Since APCI requires use of a heated nebulizer, thermally labile compounds may decompose in the ionization source. 

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. 

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. 

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). 

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. 

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 (APCl) In an APCI sonrce, liquid effluent is nebulized into fine droplets by a high gas flow and carried into a heated ehamber where the solvent droplets are evaporated. 

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.)...

There are three types of API sources that are readily coupled on-line to LC atmospheric pressure chemical ionization (APCI), atmospheric pressure photoionization (APPI) and electrospray ionization (ESI). While the three techniques differ in the physico-chemical processes that lead to ion formation, they all readily accommodate liquid flow by making use of heating and/or nebulization aids, often involving the flow of inert gases to aid desolvation. As such the three sources have similar design features that also include optics for transferring ions from the atmospheric pressure ion source into the vacuum required for operation of the m z analyzer, a pressure drop by a factor 10 . 

Atmospheric pressure chemical ionization (APCI) uses a heated nebulizer to spray the HPLC mobile phase into an atmospheric pressure chamber. As the solvent evaporates, it is ionized by a corona discharge to form a reagent gas that, in turn, ionizes the analyte by chemical ionization. Van Breemen et al. (310) published the first application of atmospheric pressure chemical ionization for the LC-MS quantitative analysis of retinol and retinyl palmitate in human serum (Fig. 6). Retinyl acetate was used as an internal standard. Sample preparation consisted of hexane extraction of serum, which was followed by C30 reversed-phase HPLC separation with on-line positive ion APCI mass spectrometric quan-... 

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