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Distinction of structural isomers by fs-LIMS

Some time ago we discovered that fs-LIMS may be isomer specific, in particular if the fs-laser pulses are appropriately shaped, e.g. by an LCD device. Fs-LIMS thus provides a valuable tool to distinguish isomers, which may be difficult to distinguish by conventional MS techniques.

In original work employing this concept we have been able to distinguish o-xylene and p-xylene (see Fig. 1) by applying a linear chirp to the fs-laser pulses. Clearly, for pronounced negative chirp there are conditions where one of the isomers almost does not contribute to a given signal of interest, in this case the ratio of C++ to parent ion. For more details see ref. [1].

Fig. 1    Measured ratios of C2+ ion and parent-ion yield of o- and p-xylene as a function of chirp. The pulse energy was 26 µJ, 35 µJ, and 80 µJ, respectively.

Over the years, we have further developed the technique. In 2019, we succeeded in performing not only a qualitative analysis of three structural isomers of ortho-, meta- and para-Fluorotoluene but also the quantitative analysis of a mixture of the three isomers [2]. Note, that no separation step is involved in such an analysis. This is a truly online-capable approach.

Fig. 2  Quantitative analysis of a mixture of o-, m- and p-fluorotoluenen. The green symbol marks the actual composition. The red circle marks the uncertainty of the experimental analysis.

References

[1]    G. Urbasch, H.G. Breunig, K.-M. Weitzel, ChemPhysChem, 8, 2185-2188, (2007), http://dx.doi.org/10.1002/cphc.200700498

[2]   V. Schäfer, K.-M. Weitzel, Analytical Chemistry, 92, 5492-5499, (2020)
https://dx.doi.org/10.1021/acs.analchem.0c00234