Needle, corona

The in-line source depicted in Figure 8.7 was designed by Bruins et al. to be mounted on a PE-Sciex triple quadrupole and it was derived from the standard heated nebulizer of their APCI source. The corona needle is replaced by a discharge lamp. Nitrogen is used as the nebulizing and the lamp gas, while air is used as the auxiliary gas. A dopant improves the efficiency of ionization and it is supplied through the auxiliary gas line and vaporized together with the solvent in the heated nebulizer. 

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... 

Surface voltage decay was measured with a Trek Model 341 high voltage electrostatic voltmeter (ESVM). The ESVM drives the potential of a probe body to the same potential as the film surface voltage. This device measures the voltage independent of distance within 5 mm of the film surface without causing current flow, which would modify the data. After poling, the corona needle was removed and the probe of the ESVM was placed 3 mm from the film surface. 

Fig. 8 The left picture shows the corona setup. In the insert the corona needle and the grid electrode inside the corona chamber are shown. The picture on the right shows a polymer film sample mounted on the aluminum electrode with conductive, double-sided adhesive tape...

The main difference between ESI and APCI is that, in APCI, the ions are formed when the analyte molecules in the gas phase interact with electrons discharged from a corona needle (see Figure 5.18). As with ESI, the eluent is sprayed through a heated vaporiser at atmospheric pressure. The heat from the vaporiser causes the liquid to vaporise and the resultant solvent molecules, now in the gas phase, to be ionised by the electrons from the corona needle. These ions then transfer charge to the analyte molecules through chemical reaction (hence the name atmospheric chemical ionisation ). These ions, as in ESI, pass through a capillary and pass into the mass analyser. 

Figure 3.32 A cascade of reactions caused by ionization of nitrogen at the corona needle produces both positively and negatively charged molecular ions in APCi.

Detector MS, PE Sciex API 3000, nebulizer 375°, nebulizer gas nitrogen at 75 psi, corona needle - 3 mA, auxiliary gas nitrogen at 10 units, curtain gas nitrogen at 8 units, negative ion mode, collision gas nitrogen at 5 units, collision energy 60 eV, m/z 338.2-78.1... 

Note The significant enhancement of primary ion formation by a corona discharge as compared to a Ni source has been implemented in early API sources, a context in which a basic APCI process has already been described (Eqs. 12.1-12.6 in Chap. 12.1.1) [174,175]. The corona needle employed to expand the acceptance range of TSP basically represents an APCI m e for these sources, too [28]. 

A portable instrument needs to be battery operated, simple in operation and robust in use. We have stripped down a standard 41/min sampling pump (Rotheroe and Mitchell L2SF) and fitted it with associated electronics for fibre precipitation and sensing into a hand-held unit running from rechargeable nickel-cadmium cells. The instrument top block (see Fig. 1) is cast from epoxy resin in which the corona needles, the precipitator electrode, and the detectors are set. The baseplate on which the block seals is made from aluminium. 

Generally the poling field is applied in one of two ways, referred to as two-electrode poling or corona poling (Fig. 4.25). As the names indicate, either two electrodes are placed on the film (and a field applied) or a base electrode is used with a corona needle probe (or array of probes). For a detailed discussion of poling see Chapter 5. 

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