Chapter 17
Characterization of Marine Toxins by Means of
Liquid Chromatography - Electrospray Ionization -
Mass Spectrometry
M. Vincenti . A. Irico
17.1
Introduction
In a previous review on application of mass spectrometric methods to the analysis of
natural and anthropogenic marine contaminants (Vincenti 1997), it was advanced that
rapid development and diffusion of electrospray ionization (ESI) interface for combining liquid chromatography with mass spectrometry (LC-MS) was going to have
strong impact on research and routine analysis of the biological substances produced
by marine microorganisms, among which marine toxins. This trend has been confirmed in the last two years. It is today clear that ESI represents one of the decisive
steps in the development of mass spectrometry. The dramatic improvement of
LC-MS effectiveness, as a consequence of ESI outbreak, has made the interfacing techniques previously developed obsolete. The present review will only consider the applications of marine toxin LC-MS determination in which an ESI interface has been
used.
The principles (Kebarle and Ho 1997) and instrumental design (Bruins 1997) of ESI
has been reviewed in several texts. In ESI, the ions initially present or formed in an
electrolytic solution are transferred to the gas phase, by means of an electrical field
applied to the tip of a capillary through which the solution is flowed. Upon formation
of a double layer on the meniscus of the solution by electrolyte ion separation, a spray
of charged droplets is released in the gas phase. The efficiency of this process depends
on a number of parameters, including the composition and electrolyte concentration
in solution, the flow rate, the voltage and polarity of the electrical field. To assist the
charged spray formation and expand the conditions (expecially the solution flow rate)
feasible to ESI, various devices have been alternatively added to the capillary including a coaxial sheath liquid, a nebulizing gas (pneumatically-assisted ESI or Ionspray),
an ultrasonic transducer (ultrasonic-assisted ESI or Ultraspray), and some form of
heating. Once the charged droplets are formed, solvent evaporation occurs, leading to
cycles of droplet shrinkage, increasing coulombic repulsion and droplet explosion. At
the end of this cyclic process, desolvated ions are transferred into the mass spectrometer and analysed. During spray formation and desolvation, several solution and gasphase ion-molecule reactions occur, at atmospheric pressure, possibly leading the formation of charged analyte species. In this sense, ESI is one of the ways leading to atmospheric pressure ionization (API). To assist the desolvation process, heating of the
interface walls or a counter current flow of dry nitrogen are frequently employed. Ion
transfer from the atmospheric pressure region into the high vacuum of the mass spectrometer takes place through a series of orifices and/or capillaries and a final skimmer plate located before the mass analyser. By regulating the skimmer voltage, incom-
Characterization of Marine Toxins by Means of
Liquid Chromatography - Electrospray Ionization -
Mass Spectrometry
M. Vincenti . A. Irico
17.1
Introduction
In a previous review on application of mass spectrometric methods to the analysis of
natural and anthropogenic marine contaminants (Vincenti 1997), it was advanced that
rapid development and diffusion of electrospray ionization (ESI) interface for combining liquid chromatography with mass spectrometry (LC-MS) was going to have
strong impact on research and routine analysis of the biological substances produced
by marine microorganisms, among which marine toxins. This trend has been confirmed in the last two years. It is today clear that ESI represents one of the decisive
steps in the development of mass spectrometry. The dramatic improvement of
LC-MS effectiveness, as a consequence of ESI outbreak, has made the interfacing techniques previously developed obsolete. The present review will only consider the applications of marine toxin LC-MS determination in which an ESI interface has been
used.
The principles (Kebarle and Ho 1997) and instrumental design (Bruins 1997) of ESI
has been reviewed in several texts. In ESI, the ions initially present or formed in an
electrolytic solution are transferred to the gas phase, by means of an electrical field
applied to the tip of a capillary through which the solution is flowed. Upon formation
of a double layer on the meniscus of the solution by electrolyte ion separation, a spray
of charged droplets is released in the gas phase. The efficiency of this process depends
on a number of parameters, including the composition and electrolyte concentration
in solution, the flow rate, the voltage and polarity of the electrical field. To assist the
charged spray formation and expand the conditions (expecially the solution flow rate)
feasible to ESI, various devices have been alternatively added to the capillary including a coaxial sheath liquid, a nebulizing gas (pneumatically-assisted ESI or Ionspray),
an ultrasonic transducer (ultrasonic-assisted ESI or Ultraspray), and some form of
heating. Once the charged droplets are formed, solvent evaporation occurs, leading to
cycles of droplet shrinkage, increasing coulombic repulsion and droplet explosion. At
the end of this cyclic process, desolvated ions are transferred into the mass spectrometer and analysed. During spray formation and desolvation, several solution and gasphase ion-molecule reactions occur, at atmospheric pressure, possibly leading the formation of charged analyte species. In this sense, ESI is one of the ways leading to atmospheric pressure ionization (API). To assist the desolvation process, heating of the
interface walls or a counter current flow of dry nitrogen are frequently employed. Ion
transfer from the atmospheric pressure region into the high vacuum of the mass spectrometer takes place through a series of orifices and/or capillaries and a final skimmer plate located before the mass analyser. By regulating the skimmer voltage, incom-