Helium nanodroplets host ion analysis

28 September 2010

Chemists have developed a sensitive new infrared (IR) spectroscopy method that analyses molecular ions by capturing them in nanosized bubbles of freezing helium. By slowing molecular vibration and rotation in this way, the technique can give useful information about important biomolecular ions.

IR spectroscopy works by passing a beam of infrared light through a sample and measuring how much energy is absorbed at each wavelength. Chemical bonds vibrate at characteristic frequencies, so specific covalent bonds can easily be identified.

'But when you study at normal temperatures, you see so many absorptions of light that it is difficult to get detailed information,' says Marcel Drabbels, who led the research at the Swiss Federal Institute of Technology. 'To solve this, we take the molecules and put them into small droplets of helium at 0.4° Kelvin to stop all vibration.'

At temperatures this close to absolute zero, the molecules are unable to vibrate or rotate. UV light can be used to ionise the molecules - and IR light can excite them to start vibrating again. 'This process gives us greatly simplified and clean spectra, which contains a lot of useful structural information,' Drabbels adds. The team tried the approach to record IR spectra of aniline ions, and found the method to be more sensitive by almost two orders of magnitude compared to other techniques used.

Using superfluid helium nanodroplets to record IR spectra of cold molecular ions. Ions that are ejected from the helium droplets following vibrational excitation can be detected and spectra recorded
© J. Am. Chem. Soc

But although the technique is simple, it may not be straightforward to commercialise, Drabbels notes. One challenge is that the nozzle used to create the helium droplets is so small - only around 5um - that it can be blocked by a single dust particle, meaning that an especially clean gas supply and lines are needed.

As well as studying the ionic forms of important biomolecules, larger clusters or aggregates might also be studied with this method. Interestingly, since the experiment cools molecules down to temperatures where superconductivity can be observed, the team hope that this process will give new insights into the phenomenon.

'This is a remarkable experimental result that opens up a new approach to the spectroscopy of complex molecules,' says Kevin Lehmann, a spectroscopy expert at the University of Virginia.

'There is a rapidly growing field of spectroscopy on cold biological molecules, and this work produces spectra of higher quality and resolution. If this method will work for almost any ion, I expect this will be an extremely attractive approach.'

Lewis Brindley


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