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Techniques & Tools Mass Spectrometry, Capillary Electrophoresis, Liquid Chromatography

Upward Mobility

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We’re exploring better, faster, more effective ways to characterize a wide range of biological systems, including those affecting human health and the environment. I’ve had a longstanding interest in combining different separation techniques with mass spectrometry, including LC, SCF, capillary electrophoresis and capillary LC. Right now, my group is continuing that interest by combining MS with very high-resolution ion mobility spectrometry (IMS).

IMS has a great deal of potential for analytical science, but lack of resolution has limited its use. My lecture at ISCC focused on a new approach for IMS-MS based upon what we call Structures for Lossless Ion Manipulations (SLIM) – a new form of ion optics. SLIM are constructed from electric fields generated by arrays of electrodes on evenly spaced planar surfaces, to which various RF and DC electric potentials can be applied, and used to enable a broad range of ion manipulations. We exploit the robustness and ruggedness of mature technology developed to support electronics, but instead of moving electrons around a circuit, we’re using electric fields to manipulate ions in the gas phase.

The lossless ion transmission provided by SLIM provides the basis for exceptionally high sensitivity and we use this along with the ability to create very long path ion mobility separations – long, serpentine paths that allow us to achieve very high resolution. The combination of high resolution, sensitivity and speed are very attractive for many measurements. We have been able to separate a lot of previously indistinguishable isomers; for example, peptides modified with a phosphate group at different sites. We are also developing an application to look at peptides that contain a D rather than an L amino acid – diastereomers or epimers. These molecules are biologically interesting, but hard to resolve with standard techniques. Another potential application is to separate peptide isomers containing leucine versus iso-leucine amino acid residues, which are almost always indistinguishable by mass spectrometry; when we can separate them, we can characterize them effectively. Essentially, we’re addressing blind spots in biological separations. The enhanced resolution with SLIM means we can pull apart things that have almost identical mass spectra and that are difficult or impossible to separate by LC. The separations are extremely fast, typically under a second, and the reproducibility that we get using ion mobility is rock-stable. All we need to do is control temperature and pressure very precisely to achieve very high reproducibility. It’s an important development for many practical applications.

I would say it’s a significant departure from the way things have been done. But it has its roots in some of the technology development we’ve done in the past, such as the ion funnel for aiding sensitivity in MS measurements.

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About the Author

Richard D. Smith

Richard D. Smith is Battelle Fellow at the Pacific Northwest National Laboratory.

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