Self-powered ionisation: Kelvin spray ion source

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  • Published: Oct 17, 2013
  • Author: Steve Down
  • Channels: Base Peak
thumbnail image: Self-powered ionisation: Kelvin spray ion source

Kelvin water dropper

A new type of dual-spray ion source for soft ionisation based on the Kelvin water dropper produces positive and negative ions simultaneously and does not require a power supply.

An electrostatic generator developed by Lord Kelvin in the 1860s has been used as the template for a new type of self-powered ion source that relies on the static electricity that it produces. The Kelvin spray source, as it has been named, was developed by Chung-Hsuan Chen, Jung-Lee Lin and Kent Gillig from the Genomics Research Center, Taipei, Taiwan, and Abdil Özdemir from Sakarya University, Turkey.

In the original Kelvin water dropper, two streams of water droplets were generated from a single reservoir, each passing through its own conducting ring to fall into its own metal container. The rings and containers were electrically insulated from each other and from ground, as were the rings. But the containers were electrically connected to the opposite conducting ring.

A small charge on either container, which can come from the air or by the falling water droplets, initiates the charging process. If one container has a slight negative charge, it will transfer some of that charge to the opposite conducting ring to which it is connected. That negative charge will begin to attract positive ions from the water. As the droplets fall into their own container they will carry the charge with them.

Since this container is attached to the opposite ring which is above the first container, it will transfer positive charge to that ring so that negative ions are preferentially collected in that container. The charges in the containers build up, generating static electricity which can be discharged in a number of ways, such as by sparking across the rings or containers.

The ion source

The same principles were applied to the novel ion source, except that the charged species were not collected in a container but were directed towards the inlet of a mass spectrometer. The liquid was sprayed from a syringe and split into two. Each flow passed through a pair of copper plates that were electrically connected the same way as in the Kelvin water dropper. A Styrofoam spacer between the plates insulated them from each other and had a cone-shaped orifice through which the droplets passed.

A flow of nitrogen gas was also introduced to aid droplet formation and stabilise the sprays. With this system, charging began within one minute, faster than the conventional water dropper, producing streams of charged species. Electrostatic potentials up to 4 kV were produced during typical operation.

In principle, positive species should be produced through one pair of copper plates and negative species through the other. However, in practice, both polarities were produced at the same time when the exit orifice was smaller than 3 mm, with one polarity dominating. When the copper plate was negatively charged and the mass spectrometer was operated in positive-ion mode, very good spectra were recorded that were similar to electrospray mass spectra.

Switching the mass spectrometer to negative-ion mode during the same droplet flow allowed negative-ion spectra to be recorded. However, the peak intensities of the negative ion spectra are about an order of magnitude smaller than those produced when the copper plate is positively charged.

Switching polarity from positive to negative was easy. It could be accomplished by touching one side of the copper plate, since the human body is electrically charged, or by touching it with a negatively charged object like Teflon. The system could be restarted easily as long as the plates are not touched, simply by turning on the spray and the gas flow.

Kelvin spray ionisation in practice

The positive- and negative-ion mass spectra of a series of peptide and protein standards like glutathione, histidine and myoglobin displayed good peak deconvolution. The charge-state distributions differed from those produced by electrospray ionisation but the signal intensities were similar.

The ionisation process is gentle and the internal energy transferred to the droplets can be reduced by lowering the solvent flow rate or the nebulising gas pressure. This enables non-covalent complexes to be analysed intact, as illustrated by the observation of glutathione-histidine and glutathione-L-glutamine complexes. They were also analysed by tandem mass spectrometry, in which protonated glutathione molecules were easily detected after dissociation of the complexes.

The self-powered nature of Kelvin spray ionisation is unique. Combined with the ability to produce positive and negative ions simultaneously and the gentle nature of the process, it holds great potential for the ionisation of labile compounds and intact non-covalent complexes.

Related Links

Analyst 2013, 138, 6913-6923: "Kelvin spray ionization"

Article by Steve Down

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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