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An adventure in electrospray The question seems to be one that only a die-hard mass spectrometrist could love: how are ions formed from droplets during electrospray? In fact, one group of mass spectrometrists who wound up tackling the question really had other plans. Martin Jarrold and colleagues Sarah Mabbett, Josh Maze, Lloyd Zilch, and John Smith of Indiana University were designing a new method to measure the masses of large (106 –109 Da) ions, which is reported in the November 15 issue of Analytical Chemistry (pp 8431–8439). “What we were looking to do was to try and push the size range on which you could do mass spectrometry measurements,” says Jarrold. “When we first set this up, we never had any intention of doing droplets. Our original goal was to just jump straight into doing viruses and things like that.” In pursuit of that goal, the researchers had designed a new MS method, which they call pulsed-acceleration charge detection MS. In a related method, charge detection MS, particles are charged, are accelerated by a voltage change, and then travel into an image charge detector. A particle’s charge can be deduced from the equal-but-opposite charge it leaves on the cylinder of the image charge detector, and its m/z can be calculated from the time it takes the particle to move through the detector, provided the acceleration voltage is known. Charge detection MS works well for smaller particles, but as larger particles travel through the electrospray interface, their speed increases in an unpredictable way. “They’re already traveling quite rapidly before you accelerate them,” says Jarrold. “If you are looking at very heavy things, you cannot get an accurate m/z out of a time of flight, because you don’t know what their initial velocities were.” To measure the initial velocity of the large particles, the researchers added a second charge detector to their setup. This way, they could measure the initial 8826
Studying how electrosprayed droplets behave helps researchers optimize the process.
velocity and charge of a particle, pulseaccelerate it, measure its final velocity and charge, and calculate its m/z from that data. Thus, pulsed-acceleration charge detection MS was born. But during the course of their experiments, the researchers observed something odd in their negative-mode electrospray runs: fully 75% of the droplets they could see were positively charged. “This led us into an adventure where we were trying to figure out how it is possible that you can get positively charged droplets out of a negatively charged droplet,” says Jarrold. They landed smack in the midst of a heated debate that had been going on in the MS community for years: how electrosprayed droplets shrink down to the size where desolvated ions can be generated. One theory is that the relatively large, charged droplets generated by an electrosprayer gradually evaporate and shrink until they reach the Rayleigh limit, a point at which the electrostatic forces dominate the surface tension forces. At this point, the theory goes, the droplet discharges by shooting out a jet of small, highly charged droplets. Jarrold says that if this theory were correct, the droplets generated in their new setup would be charged at close to their Rayleigh limits. But the droplets held charges that were only ~10% of the limit—not enough to cause the
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jetting action. Instead, the researchers have proposed that the turbulence that occurs as the droplets are carried into the mass spectrometer may play a larger role in droplet breakup than previously thought. The turbulence theory also could explain why they were seeing positive charges in the negative-ion mode of the instrument. The researchers think that as the droplets encounter turbulent air, they deform from a spherical droplet into a more flattened state, which then swells up like a parachute catching wind. The “parachute” part of the droplet is stretched very thin and accumulates negative charge, while a thicker water ring forms around the bottom and attracts a positive charge. When the structure finally gives out and bursts, the water ring breaks up into the relatively large, positively charged droplets that the scientists can see, but the parachute is turned into thousands of tiny, undetectable, negatively charged droplets. The researchers are still refining this theory and hope to have more data soon. “We’re trying now to get photographs of droplets as they go into the interface so we can get a better idea of what’s going on,” says Jarrold. It looks like this sidetrack adventure may keep them busy for awhile. a —Jennifer Griffiths
