New mechanism proposed for corona-ion-dependent signal enhancement
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Kaveh Jorabchi, Michael Westphall, and Lloyd Smith at the University of Wisconsin Madison recently proposed a new mechanism to explain why the analyte ion signal in UV laser desorption (LD) MS is enhanced when sample droplets are in the presence of corona ions. The pulsing electronics in the group’s new apparatus decoupled three parameters: corona cation generation, electric field generation in the droplet region, and laser firing. The authors concluded that droplet charging rather than gas-phase ion–neutral reactions is responsible for the increase in signal. Accordingly, they call the new mechanism charge-assisted LD/ionization (CALDI).
Because the majority of gaseous molecules formed by LD are neutral, Smith’s group was studying the role of corona discharge in LD MS to improve ionization efficiency. Corona discharge is the ionization of the gaseous molecules near the tip of a pointed conductor when a strong electric field is applied to the conductor.
The results from the team’s pulsed experiments indicated that the generation of analyte ions is independent of the presence and density of corona ions, so they deduced that gas-phase corona ion–neutral reactions were not responsible for ionization of the analytes. The authors propose that in CALDI, the electrically isolated droplet adsorbs corona ions, becomes charged, and then forms an asymmetric charge distribution in the presence of an external electric field. Then the side of the droplet closest to the MS inlet sees an enhanced electric field, and ion formation is facilitated during the laser pulse. The researchers are currently trying to understand the role of gas-phase charge separation and direct desorption of charged droplets and clusters. (J. Am. Soc. Mass Spectrom. 2008, DOI 10.1016/j.jasms.2008.02.012)
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