A simple alternative for ionization

Raquel Harper

Organic salts include molecules such as proteins and lipids, which are critical for human health. Structural information on these molecules helps reveal more about the body’s function and aids in developing appropriate drug treatments. Mass spectrometry is used to study the structure and composition of these molecules, but it requires that they first be ionized.

Ionization involves splitting them into their negatively and positively charged components. This has been done using electron beams or lasers in a vacuum in techniques such as secondary-ion mass spectrometry, fast atom bombardment, plasma desorption, laser desorption and field desorption. Researchers from Purdue University in West Lafayette, Ind., however, have developed a relatively simple alternative for the ionization of various nonvolatile materials, such as organic salts.

A simple, alternative method of ionizing molecules at atmospheric pressure has been devised. In the image, a pyrylium salt is heated in a nitrogen gas stream, and the resulting cations impact on solid lysine, reacting with it and carrying the derivatized amino acid ions to a distant mass spectrometer. Reprinted with permission of Angewandte Chemie.

Hao Chen, Zheng Ouyang and R. Graham Cooks of the department of chemistry call the method atmospheric-pressure thermal desorption ionization. It involves heating the molecular material on a hot stage and passing a stream of inert gas such as nitrogen over the sample. The heat ionizes the molecules, and the gas carries the ions into the interface of the mass spectrometer. The nonionized supporting gas then is pumped away, and the ions are analyzed in vacuum using electric and magnetic fields.

The researchers experimented with low-molecular-weight organic salts and found that the compounds provided intact anions and cations, most without significant dissociation. Molecules with low molecular weight require only low temperatures to separate their charges, so the scientists would like to experiment with larger biomolecules at higher temperatures and pressures.

“This ambient ionization method allows you to do ionization without preparing the sample, which is sometimes a very lengthy process,” Cooks said. He explained that the method should allow scientists to move rapidly from sample to sample because the experiment is conducted at atmospheric pressure rather than in a vacuum system.

The researchers are using the method to ionize peptides to try and figure out which ones are functional. They also are experimenting with subsequently heating ions to see if they can cause them to fragment in a controlled way for further study of their structure.

Angewandte Chemie, published online April 26, 2006.