Shifting masses
z„ values of 0.730 and 0.714 cm, respectively, whereas the dihydrobenzofuran exShifts along the m/z axis can be a good perienced little or no chemical mass shift. tiling, particularly if they allow the sepaThe isobars naphthalene and rfr-nitrobenzene ration of seemingly unseparable ions. In differ in molecular mass by only 0.003 m/z, the October 15 issue of Analytical Chembut they were readily separated because, at a istry (p. 4448), Graham Cooks and his z,, of 0.707 cm, d-nitrobenzene had a chemical co-workers at Purdue University demass shift of 0.6 m/z. scribed experiments in which they could separate isobaric and isomeric comCooks emphasizes that their results do pounds in an ion trap mass spectrometer not mean that commercial ion trap mass simply by changing the distance between spectrometers produce mass spectra with the end-cap electrodes. Although this wrong m/z values. "You can set up the ineffect was first observed by George Stafstrument to run like any ion trap and give ford and John Syka of Finnigan in 1983, good mass-to-charge values. Or, you can Cooks and his co-workers are the first to set it up by adjusting the geometry so that examine these shifts systematically and the fields are unusual. In this experiment, to demonstrate isomeric ion separation. we didn't want the m/z to come out 'right'. We wanted the apparent mass to be influChanging the distance between the end-cap electrodes (the 2(> value) in an ion trap mass spectrometer introduces higher order electric fields. Certain compounds respond to these electric fields in ways that result in a "chemical mass shift". In this system, the end-cap electrodes were mounted on moving stages connected to a linear micrometer. The v, could be adjusted from 0.684 to 0.920 cm in ().(X)76-cm or smaller increments. At values greater than or equal to the z, value of commercial instruments (0.783 cm), the effects of the higher order fields cancel each other and there is no chemical mass shift With this system, Cooks and his coworkers separated the isobaric pairs of (/.-nitrobenzene/naphthalene and />-nitrotoluene/3-ethoxyaniline, as well as the isomers acetophenone and dihydrobenzofuran. For example, the chemical shift for Separating the structural isomers acetophenone acetophenone was 0.5 m/z and 0.9 m/z at shifts.
Electrospray can measurrrhe solution binding constants for noncovalent complexes of vancomycin.
Ka values using only the relative ion intensities in ESI-MS. This has allowed them to develop a fast, parallel method for high throughput screening of vancomycin-type compounds. In comparison, solution spectroscopic techniques only work on one receptorligand complex at a time. Heck and his colleagues tested their method on vancomycin itself and three peptide ligands, which represented the precursor peptides of the bacterial cell wall. They found the vancomycin-peptide complexes were transported intact from solution into the vacuum gas phase of the mass spectrometer. The equilibrium concentrations in solution were represented by the MS peak intensity of a particular complex
enced by something other than the mass of the molecule." The chemical mass shift is particularly pronounced for nitroaromatic compounds. Cooks' group wants to exploit these large shifts to detect nitroaromatic compounds in complex mixtures. "Hie driving force is explosives detection," says Cooks. "If you have a [feature] like a 0.7 m/z shift that you can use on top of any other feature in a complex mixture, it could be extremely valuable in detecting the presence of one class of compounds or eliminating another." If the term chemical mass shift seems vaguely familiar, it should. "The parallel with NMR is intentional," says Cooks. "We couldn't call them chemical shifts because the name was reserved." Celia Henry
and dihydrobenzofuran with chemical mass
relative to the summed intensities of all complexes and the free antibiotic. In addition, the team investigated the pH stability of the ristocetin-peptide complex using ESI-MS and compared the results with the conventional circular dichroism approach. Very similar results were produced with both, so Heck believes ESI-MS could be used not only to assay potential drug candidates but also to provide an estimate of their likely efficacy at different pH values. Heck adds that the ESI mediod should be applicable to a variety of other systems, such as enzyme inhibitors, because the technique is based solely on ion intensities. David Bradley
Analytical Chemistry News & Features, November 1, 1998 705 A
