tion behavior of a suite of model amines, including primary and secondary aliphatic, peptide, and protein-N. Using a postcolumn reaction with iodide, followed by amperometric detection, ,t is possible to detect submicromolar chloramine concentrations. In all cases, the iV-chloramine has greater retention because it has greater hydrophobicity than the parent amine. Chlorination/dechlorination experiments were performed with effluent from two wastewater treatment facilities. Residual chlorine levels, as
LC system for detection ofU-chloramines.
estimated by a modified iodometric titration and the LC method, exceeded EPA criteria levels. Kinetic studies show that a fraction of the organic iV-chloramines was slowly dechlorinated with sulfite (half lives of >20 min). Retention and kinetic behavior of the Akhloramines in the sewage samples is consistent with that of the model peptides and proteins. The results suggest that peptides and proteins contribute to the slow rate of dechlorination observed in sewage effluents. (Environ. Sci. Technol. 1198,32,3640-45)
SCIENCE To improve the sensitivity, Clemmer's group has added an ion trap between the ionization and the ion mobility instrument. Ions accumulate in the ion trap and are injected into the drift tube of the ion mobility spectrometer. The ion packets from the ion mobility instrument are then analyzed by TOFMS. Because of the different time scales of the measurements (milliseconds for ion mobility versus microseconds for TOF) miiny mass spec~ tra can be recorded for each time window in the ion mobility measurement The data sented as contour plots with the ion mobility information on the #-axis and the TOF information onthe^-axis
to observe this sort of effect to assign the charge states, the families don't exist as clearly," says Clemmer. "That's because the proteins unfold when the charge inElectrospray ionization has helped revolucreases. When you have drastic changes tionize the maas spectrometry of biomolein conformation, the cules. However, electrospray of mixtures technique won't sepacan result in spectra that are difficult to rate ions into charge interpret. David E. Clemmer and his costate families." workers at Indiana University combine ion mobility spectrometry and time-of-flight Clemmer has several (TOF) MS to rapidly and sensitively anavisions of where this lyze mixtures of proteins and peptides technique could lead. "I (Anal. Chem. 1197,69,728 A-735 A)A can imagine doing moThey described their work in the January bility separations cou15 issue of Analytical Chemistry (p. 291)9 pled with MS/MS, in which ions are fragThe combination results in more informented in order to obmation than either technique alone. Ion tain sequence informamobility provides a shape-to-charge ratio tion for components of measurement, whereas MS provides a very large mixtures," he mass-to-charge ratio measurement. "You says. "I can also imagcan see different isomers, conformations, ine coupling it with or sequence inversions," says Clemmer. other separation tech"There's a chance you could resolve those niques such 3.s LC" in the ion mobility distribution based on Contour plots of nested drift time The currently differences in shape, but you wouldn't be Clemmer and his (bottom) and flight time (left) data for able to in mass spectrometry because co-workers found that working on both of a mixture of peptide ions, which they would have identical mass-to-charge the peptides separate were formed by direct electrospray those projects ratios." into families according They are particularly of a tryptic digest of cytochrome c. to charge state along interested in developing According to Clemmer, ion mobility the ion mobility dimension. The mass a routine method of identifying and quantispectrometry is opening the door to gasspectral assignment is simplified because fying compllx unknown mixtures of biologphase separations to biomolecules. "We've ical molecules. "I think we're at the first converted what would be liquid chromatog- the charge state is already known. The step of being able to do that with this techraphy-based separations to a gas-phase sep- researchers demonstrate the method with peptides from tryptic digests of the nique," says Clemmer. "This technique is aration." He attributes that capability in in a very interesting time regime. The millipart to the way electrospray works. "Electro- proteins cytochrome c and myoglobin. spray allows you to take a mixture of mole- Results with mixtures of known composi- second drift times are intermediate betion indicate that the detection limits are tween the microsecond flight times associcules in solution, introduce them into the -0.5-3 pmol. ated with the m/z measurements and the gas phase, ionize them, and then deal with seconds-to-minutes time scales associated the separation. At that stage, you have a The technique works best for peptrue gas-phase separation technique for tides with low charge states. "In the case with condensed-phase separations." biological molecules." of proteins, where it would be interesting Celia Henry
Gas-phase separation of biomolecules
Analytical Chemistry News & Features, February 1, 1999 8 5 A
