Isomer Identification

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Science  23 Sep 2005:
Vol. 309, Issue 5743, pp. 1969
DOI: 10.1126/science.309.5743.1969a

Although mass spectrometry is among the most sensitive methods used to identify molecules, it is ill-suited for distinguishing structural isomers, which are chemically distinct entities that have the same mass. Gas or liquid chromatography can be used to separate isomers before applying mass spectrometry, but this adds a relatively slow step.

In traditional mass spectrometry, analytes are ionized nonselectively by collisions with electron or atom beams, and the resulting ions are identified as a pattern of fragments on the basis of their mass-to-charge ratios in electric or magnetic fields. Dela Cruz et al.instead use phase-modulated ultrashort laser pulses to induce ionization. By first dispersing the pulses through a tunable liquid crystal array, they introduce wavelength-dependent phase shifts that subtly influence the excited state dynamics of the irradiated molecules. Through trial and error, they determine reproducible pulse shapes that induce different fragmentation patterns in different isomers. One well-shaped pulse, for example, causes p-xylene to break into methyl and tropylium fragments more than twice as efficiently as o-xylene. Once the pulse shape is determined, it can be used to quantify isomer mixtures in less than a second. Similar pulses were achieved for quantifying mixtures of isomers of cresol and nitrotoluene, and of several cis and trans olefin isomers. — JSY

J. Phys. Chem. A 10.1021/jp0539425 (2005).

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