MS chiral awareness

25 June 2015 - Mass Specs

chiral MS Fanood 2015.PNGThe separation of enantiomeric mixtures is a big thing in analytical chemistry. Gas chromatography and liquid chromatography both know how to play the chiral game on a routine basis but mass spectroscopy thus far has been "chirally blind". A group of VU Amsterdam researchers have brought chirally aware MS a step closer by demonstrating successfully the resolution of a limonene / camphor sample. (Fanood et al in Nature Communications DOI)

The equipment used to pull of this feat (mass-selective photoelectron circular dichroism or MS-PECD ) is no doubt extremely complex and expensive. As a physics paper it is crammed full of jargon and Wikipedia is not helping at all. The closest thing to an article on this topic is photoelectron photoion coincidence spectroscopy.

As the article at hand explains a molecular beam containing the analyte is irradiated with circular polarised laser light at right angles. Ions and electrons then shoot away in opposite directions on the third axis. The ions continue their flight as in regular MS and are detected at detector 1 by conventional ToF. In the opposite direction the electrons are traced by velocity map imaging. The resulting MS diagram is nothing special and just show the two molecular ions of limonene and camphor. The electrons are a different story. They strike the detector at the same time as the ions so the signals are tied. With any analyte exactly 50% of the electrons are moving one way to hit the detector but with chiral molecules more electrons shoot away at one specific angle. The asymmetry can be 1 to 10 percent, enough to assess the enantiomeric ratio. And that is exactly what the article is about.

Bonus points! The article is published open access and under a creative Commons license. This means that for once I can use the image displayed on the left (not mine!) without having to worry about commercial publishing house lawyers raiding the office!