MEETING NEWS
Katie Cottingham reports from the American Society for Mass Spectrometry 54th Conference on Mass Spectrometry—Seattle
developing a different way to tackle this problem. According to Smith, they are “milking the droplets for all they’re worth.” In their approach, the researchers use a piezoelectric dispenser to create a droplet that is trapped by single-axis
New DESI-like ionization sources
molecular-weight nonpolar molecules, such as steroids and explosives. Takáts says that DAPCI can be advantageous when a researcher wants to specifically target these types of molecules. To obtain in-depth tissue analyses, the researchers developed jet desorption ionization (JeDI). In JeDI, a highvelocity, electrically charged water jet is directed onto a sample. Charged droplets are generated upon the impact of the water jet, and these droplets produce sample ions that are analyzed by MS. “DESI is a surface analysis technique, [but in] jet desorption, the high-velocity water intrudes into the sample,” says Takáts. Because the water jet is confined to a small point, researchers also can obtain much better spatial resolution with JeDI than with DESI. Takáts explains that these features should make 3D imaging with
RYAN HILGER AND LLOYD SMITH
droplets are of the same polarity, then the incoming droplets will simply hover near the trapped droplet. But if the incoming droplets are of the opposite charge, then they will be attracted to the levitating droplet, and the droplets Overcoming ionization will fuse. This fusion process initially suppression in electro neutralizes the charge of the spray trapped droplet, and additional Ionization of complex biological incoming droplets increase its mixtures by ESI is a bit like musicharge. “Eventually, the levitating cal chairs. The trick to the game droplet is so charged that the reis that for n participants, only pulsive force prevents the incomn – 1 chairs are available; whoeving droplets from adding again,” er doesn’t get a chair is out. Likesays Hilger. The researchers allow wise, there may not be enough the trapped droplet to “desolvate charge to go around when drop until the surface charge builds lets of complex mixtures are up, and then it will [undergo] electrosprayed. “Competition fission,” he explains. They then [can exist] between the analytes recharge the trapped droplet and in solution for the amount of repeat the process until all of the available charge” in ESI, explains analytes have been ejected. Ryan Hilger. Analyte molecules Now that the researchers have at the surface of a droplet are the charging and recharging more likely to be ionized than steps working, they must devise molecules buried deep within a way to direct the resulting anaStuck in the middle. Researchers trap, charge, then the droplet. lyte droplets to a mass spectromrecharge a droplet to overcome ionization suppression and To overcome this ionization eter. “We think we should be able analyze all of its molecules. suppression, many researchers to use an electric field to kick the incorporate separation steps in smaller progeny droplets out of their protocols so that fewer analytes acoustic levitation. They levitate the the trap [because] the acoustic force on are present in each droplet to compete droplet between a planar emitter and them is small,” says Hilger. After that, for the charge. But Hilger, Mike Westa curved reflector, then bombard it they plan to apply the method to the phall, Lloyd Smith, and colleagues with charged solvent droplets. If the study of complex biological mixtures, at the University of Wisconsin are levitating droplet and the incoming such as serum.
Variations on desorption electrospray ionization (DESI) have been developed by Zoltán Takáts, Graham Cooks, and colleagues at Purdue University and Semmelweis University (Hungary). The three new ambient methods are ideal for different types of molecules and matrixes. In desorption atmospheric-pressure chemical ionization (DAPCI), ions are produced by corona discharge of a nebulizing gas and a reagent vapor. The ions hit a solid target containing a sample, which undergoes ion–molecule reactions with the reagent ions. Sample ions are desorbed, then analyzed by MS. Liquid is not sprayed onto the sample in DAPCI, unlike in DESI. Also, whereas DESI ionizes many types of analytes, DAPCI ionizes only low-
1826 Journal of Proteome Research • Vol. 5, No. 8, 2006
JeDI MS a real possibility. Almost any molecule that can be ionized by ESI can be ionized with this method. The researchers plan to use JeDI MS to analyze tumors in vivo. The newest of the three techniques, extractive electrospray ionization (EESI), is ideal for studying complex liquid samples. “Usually, liquid-phase samples are a problem for DESI because when you put [a liquid sample] on a surface, the high-velocity nitrogen from the DESI just blows it away,” explains Takáts. In EESI, however, charged solvent droplets are sprayed into the path of sample droplets. According to Takáts, the droplets probably fuse before continuing on their journey to a mass spectrometer. He says that, so far, only small molecules and those molecules that readily undergo ESI can be analyzed by EESI MS.
