Shock and spray: Electrostatic-spray ionization mass spectrometry
- Published: Sep 17, 2012
- Author: Jon Evans
- Channels: Detectors
No physical contact
A novel version of electrospray ionization (ESI) offers a whole host of advantages over the conventional kind, say French scientists. It’s more sensitive, can generate positive and negative ions, and doesn’t suffer any electrode fouling.
Perhaps most impressively, though, not only can it can ionize liquid samples exiting a capillary or channel, like ESI, but it can also ionize sample spots on a plate, just like matrix-assisted laser desorption/ionization (MALDI).
Originally developed by a team led by Graham Cooks at Purdue University in the US, practically the only difference between this novel version and conventional ESI is that there is no physical contact between the ionizing electrode and the sample. Instead, the electrode is kept close but physically separate from the sample, with a sufficiently high voltage pulse applied to the electrode to induce electrostatic charging. Because there is no direct contact, the electrode doesn’t suffer any fouling from chemical reactions taking place on its surface, as can happen with ESI.
Building on this work, a team led by Hubert Girault at the Ecole Polytechnique Fédérale de Lausanne in Switzerland has now conducted the first in-depth studies into the mechanism and capabilities of this novel version. They have also given it a name and an acronym: electrostatic-spray ionization (ESTASI).
These studies have revealed that the electrostatic charging results in the formation of a couple of connected capacitors. The first capacitor consists of the electrode and the liquid sample as the conductors, with the material separating the electrode and sample acting as the insulator. The second capacitor consists of the sample and the counter electrode, usually a mass spectrometer, as the conductors, with the air between the sample and the counter electrode as the insulator.
Applying a positive voltage to the electrode charges both of these capacities in turn, until the accumulated charge overwhelms the second capacitor. Once this happens, the system starts to operate like a normal ESI, with charged ion-containing droplets spraying from the sample into the mass spectrometer. Because the applied voltage is positive, this process initially generates positively-charged ions, but as these positive ions are sprayed into the mass spectrometer a negative charge begins to build up in the two capacitors
To remove this negative charge, the electrode is disconnected from the power supply and grounded. This causes the two capacitors to discharge once again, spraying negatively-charged ions into the mass spectrometer. By repeatedly applying pulses of positive voltage to the electrode and then grounding it, with each cycle taking around 1.5 seconds, ESTASI can transform the analytes in a sample into both positively- and negatively-charged ions, allowing a wider range of analytes to be detected by mass spectrometry.
Capillary, channel or plate
After deducing the mechanism, Girault then showed that ESTASI can work with both silica capillaries and the tiny channels on micofluidic chips. For the silica capillaries, Girault simply fitted a steel crocodile clip around the end of the capillary to act as the electrode, while in the microfluidic chip he deposited an electrode made from carbon ink around 2mm away from the end of the microchannel. In both cases, ESTASI caused both positively- and negatively-charged analytes to spray out the end of the capillary and channel.
This means that ESTASI can be used to ionize analytes separated by capillary-based techniques such as liquid chromatography and capillary electrophoresis. As a demonstration, Girualt and his team showed that ESTASI could be to used to detect proteins and peptides in a sample at concentrations as low as 5nM, making it more sensitive than conventional ESI.
But ESTASI can also operate in a way that ESI can’t, by analysing sample spots on a polymer plate, just like MALDI. In this case, the electrode is attached to the back of an insulating polymer plate, while the sample is deposited on the front. Applying voltage pulses causes ions to spray away from the spots and into a mass spectrometer. Using this set-up, Girault detected 15 different peptides in a mixture separated by capillary electrophoresis and then deposited as 18 spots on a plate.
So far, Girault has been using an ion-trap mass spectrometer with ESTASI, but he would like to test it with other types of mass spectrometer, as well as with a greater range of samples. ‘The idea now is really to show more examples of what can be done, because the applications are very large,’ he tells separationsNOW.
Analytical Chemistry, 2012, 84, 7422–7430: "Electrostatic-Spray Ionization Mass Spectrometry"
Article by Jon Evans
The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.