Contactless atmospheric pressure ionization

Figure 2.15 Schematic representation of C-API MS, with sample delivery enabled by capillary action. A short tapered silica capillary [length, 1 cm base o.d., 363 pm (or 323 pm without polyimide) tip o.d., 10 pm] was positioned vertically above an electrically isolated aluminum slide, with the outlet end placed orthogonal to the inlet of a metal capillary attached to the orifice of an ion trap mass spectrometer. The distance between the outlet of the silica capillary and the inlet of the metal capillary, attached to the MS orifice, was 1 mm. Before the measurements, the silica capillary was filled with a makeup solution [deionized water/acetonitrile (1 1, v/v)] by means of capillary action. The inlet end of the silica capillary was then dipped into a droplet of a sample (10///) put onto the surface of the aluminum slide. The inset provides an illustration of the hypothetical mechanism of C-API [109], Reproduced with permission from Hsieh, C.-H., Chang, C.-H., Urban, P.L, Chen, Y.-C. (201 i) Capillary Action-supported Contactless Atmospheric Pressure Ionization for the Combined Sampling and Mass Spectrometric Analysis of Biomolecules. Anal. Chem. 83 2866-2869. Copyright (2011) American Chemical Society. See colour plate section for colour figure...

Later, it was discovered that a short tapered capillary ( 1 cm), placed in proximity ( 1 mm) to the MS inlet, could also support electrospray - even in the absence of a power supply, electrical grounding, or ultrasound [109]. Since there was no direct electric contact at the sample capillary, this method was named contactless atmospheric pressure ionization (C-API) [109] or contactless ESI [110]. Figure 2.15 shows the putative mechanism of the ion formation in C-API. Rearrangement of electric charges at the end of the tapered capillary is induced by the electric field present near the MS inlet. 

Hsieh, C.-H., Chang, C.-H., Urban, Pi., Chen, Y.-C. (2011) Capillary Action-supported Contactless Atmospheric Pressure Ionization for the Combined Sampling and Mass Spectrometric Analysis of Biomolecules. Anal. Chem. 83 2866-2869. 

Contrary to V-EASI, in the case of polarization induced (PI)-ESI [e.g., contactless atmospheric pressure ionization (C-API) and ultrasonication-assisted spray ionization (UASl)] [39-41], one does not need to supply nebulizing gas, which makes the monitoring system even more simplistic. In C-API, a glass capillary ( 20 cm, inner diameter 50 pm) was inserted to the reaction vial to direct the reaction solution toward the mass spectrometer. The tapered end of this capillary was placed in proximity ( 1 mm) to the inlet of the mass spectrometer (Figure 11.2a). The flow of sample was mainly due to capillary action and the influence of the electric field near the mass spectrometer, which may induce some electroosmotic flow (EOF) (see Chapter 6). In this approach, the sample flow rate is low ( 100 nl min" ). The sampling time can be reduced by using a shorter capillary as the sampling tube. Similarly to V-EASI, the capillary combines... 

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