Source region corona discharge

At the end of the APCI probe is a high-voltage (2.5—3.0 kV) metal needle to produce a corona discharge, causing solvent molecules eluting into the source to be ionized. Sample molecules that elute and pass through this region of solvent ions... 

Figure 8 Drift tube design for a corona discharge ion source where sample was excluded from the source region and ions extracted from the source region were mixed with sample in a volume outside of the source. In this design, a clean supporting atmosphere of the ion source is preserved rendering stable response without fouling by sample.

Similar interface to that used for ESI. In APCI, a corona discharge is used to ionize the analyte in the atmospheric pressure region. Ions are formed by charge transfer from the solvent as the solution passes through a heated nebulizer into the APCI source... 

A heated pneumatic nebuliser is used to produce the aerosol in APCI and the ions are produced by ion-molecule reactions initiated by corona discharges in the ion source region. White et al. (1998) found atmospheric pressure ionisation MS and LC-ICP-MS to be complementary techniques. 

Addition of a scavenger gas such as SF6 or a chlorinated solvent to the source region will prevent corona discharge [48,49], but is generally less practical than simply lowering the capillary voltage. 

Figure 11.18 Schematic diagram of an APCI source. The anaiyte species are desoivated from the mobiie phase in a stream of nitrogen gas in a heated quartz tube. The anaiyte species undergo a compiex gas-phase chemicai reaction produced by a gas-phase ionized environment (a piasma) generated by the high voitage of the corona discharge needie. The produced anaiyte ions can then enter the high vacuum region of the MS for further anaiysis.

Atmospheric pressure chemical ionization uses an atmospheric pressure ionization interface. The interface is similar to that used for ESI, but a corona discharge is used to ionize the analyte in an atmospheric pressure region. (Note This is different than the chemical ionization source described in Chapter 20 for GC-MS.) The gas-phase ionization of less polar analytes is more efficient than with ESI, but the mass range is hmited to about 2000 daltons. ESI and APCI are complementary. APCI has largely replaced thermal ionization as an interface in commercial instraments. 

The ion chemistry and electrical stability of a point-to-plane corona discharge (CD) ion source was described using a tandem mass spectrometer equipped with an atmospheric pressure source. This source was a continuous current-regulated discharge with a direct current (DC) power supply and danonstrated the regions of stability with potential and distance between the needle and plane. A sch atic overview of the mechanism of formation of ions in a CD is shown in Figure 4.1. 

Fig. 12.40. APCI source. The liquid flow is pneumatically sprayed into a heated vaporizer where ionization is initiated by a corona discharge at atmospheric pressure. The sampling orifice and interface region behind remain unaltered by ESI.

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