New chemical-analysis method promises fast results

The analytical chemists have most recently demonstrated how the technology, called desorption electrospray ionization, or DESI, rapidly detects the boundaries of cancerous tumors, information that could help ensure that surgeons remove the entire tumor.

"I wouldn't be surprised if pathologists are using this in operating rooms within two years," said R. Graham Cooks, the Henry Bohn Hass Distinguished Professor of Analytical Chemistry in Purdue's College of Science.

The technology has made it possible to speed up and simplify the use of a mass spectrometer, an analytical device that in its conventional form has been long established in modern laboratories. But while ordinary mass spectrometry is both time- and labor-intensive, the Purdue group has modified the technology to make it faster, more versatile and more portable.

"The theme in our lab is 'Don't mess with chemicals,' meaning we don't undertake the usual chemical separations and manipulations needed for conventional mass spectrometry," said Cooks, who has developed a wandlike probe that can quickly gather chemical information from samples in the environment.

A review paper about DESI and related techniques, which enable the direct chemical analysis of objects in an ordinary environment, will appear in the Friday (March 17) issue of the journal Science. The paper was written by Cooks, associate research scientist Zheng Ouyang, visiting scholar Zoltán Takáts and doctoral student Justin Wiseman, all in Purdue's Department of Chemistry. Several technical papers have been published about DESI experiments since the method was announced by the same laboratory in Science less than two years ago, but the new Science paper provides the first overall review of DESI and related techniques.

Mass spectrometry works by first turning molecules into ions, or electrically charged versions of themselves, which can be detected and analyzed.

"Having a charge enables you to not only detect molecules but also to measure their masses, which you can't do with neutral molecules," Cooks said.

Conventional mass spectrometers analyze samples that are specially prepared and placed in a vacuum chamber. The key DESI innovation is performing the ionization step in the air or directly on surfaces outside of the mass spectrometer's vacuum chamber.

In addition to DESI, Cooks' research group has designed and built a portable instrument that is roughly the size of a shoebox and weighs about 10 kilograms (22 pounds), compared to about 30 times that weight for a conventional mass spectrometer. The lightweight instrument can run on batteries, which means it can be carried anywhere. The focus of the Science paper, however, is the ionization method.

The procedure involves spraying water in the presence of an electric field, causing water molecules to become positively charged "hydronium ions," which contain an extra proton. When the positively charged droplets hit the surface of the sample being tested, the hydronium ions transfer their extra proton to molecules in the sample, turning them into ions. The ionized molecules are then vacuumed through a tube and into the mass spectrometer, where the masses of the ions are measured and the material analyzed.

DESI has been used to accurately detect cancer in human livers and pinpoint the boundaries of tumors. The researchers used the device to analyze slices of liver and other tissues removed in biopsies.

"We can show that there are differences in the profiles of chemicals that come from the tissue, and these tissue profiles can be used to diagnose a particular disease or determine how far the disease has progressed," Wiseman said. "We reported in a previous paper, and it is highlighted in the current article, that there are differences in profiles of chemical species from the tissue, indicating a diseased area and also the margin at which that diseased area is separated from the non-diseased area of the tissue."

The traditional assessment method now used in operating rooms is based on the trained eye of a pathologist, who views stained tissue slices under a microscope. The assessment is used to help determine which tissue to remove during surgery.

"In this traditional approach, there is no chemical information being transferred in any way, " Wiseman said.

DESI promises to be an important "high-throughput" tool to collect large amounts of data used in "metabolomics," a field in which researchers search for chemical compounds called biomarkers. These biomarkers are early warnings of disease, but they can be difficult to spot among the hundreds of distinct chemicals normally present in the urine, blood or serum of healthy people. The DESI experiment allows testing to be done without separating the compounds of interest from biological fluids, Cooks said.

"For example, we can place a small drop of urine on filter paper and then test the samples, recording a mass spectrum, which gives all the components of the urine," Cooks said. "Then we identify the individual components in another quick step.

"This two-step procedure, which is known as tandem mass spectrometry, allows us to rapidly confirm the presence of hundreds of components in a urine sample. If, for example, you were conducting medical research with animals and you were studying mouse urine samples, all the mice with a certain disease would show certain compounds in their urine - biomarkers - that signal that particular disease state, and the collection of those biomarkers together would represent a common feature of a particular disease."

Currently, metabolomics is carried out using a technique called nuclear magnetic resonance. DESI, however, could be used to more quickly and accurately analyze a person's urine for the presence of diseases revealed by biomarkers, Cooks said.

The researchers also have used DESI to detect residues from explosives, and the paper includes a table showing various explosives the scientists have detected with DESI.

"We really don't know of any explosives that we can't detect," Wiseman said.

Cooks' team is associated with several research centers at, or affiliated with, Purdue, including the Bindley Bioscience Center, the Indiana Instrumentation Institute, Inproteo LLC (formerly the Indiana Proteomics Consortium) and the Center for Sensing Science and Technology. Much of the research funding was provided by the National Science Foundation, Office of Naval Research and by Inproteo and Prosolia Inc., through the Indiana 21st Century Research and Technology Fund.

DESI is being commercialized by Inproteo and Prosolia, a spin-off subsidiary of Inproteo, which has been shipping DESI products since November and is adding scientific staff, said Prosolia's chairman Peter Kissinger.

"This technology shows great promise for the life sciences as well as environmental, forensic and homeland security applications," Kissinger said. "This is one of a long series of mass spectrometry innovations coming from the Cooks laboratory. We now have quite a number of mass spectrometry assets on the ground in Indiana, and we are ready to nurture this in the state."

Source: Purdue University