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Mass Spectrometry

Shrinking magnetic sectors

Analytical chemists are bringing these old-school mass analyzers into the high-tech, miniature realm

by Celia Henry Arnaud
May 28, 2018 | A version of this story appeared in Volume 96, Issue 22

Graphic showing ion trajectories for single-slit and multislit apertures on a magnetic-sector mass spectrometer.
Credit: Courtesy of Jason Amsden
When a single slit admits ions of a particular mass-to-charge ratio (m/z) to a cycloidal magnetic-sector mass spectrometer, they travel with a certain trajectory and hit the detector in a particular spot, generating a single peak in the spectrum. With multiple slits, there's a pattern of peaks for each m/z that reaches the detector. Scientists can mathematically extract strong, single peaks from those patterns by knowing the physics of the magnetic sector and the pattern of slits.

Magnetic sectors were the mass analyzers used in the early days of mass spectrometry. Those instruments were quite large, and today, other, more compact, types of mass analyzers capable of separating ions with wider mass ranges have superseded magnetic sectors for most applications.

So it may come as a surprise that people are working on miniaturizing magnetic-sector mass specs. While most scientists have focused on ion traps as the mass analyzers for miniature mass spectrometers, Jason J. Amsden, an assistant research professor in Jeffrey T. Glass’s lab at Duke University, is focusing on magnetic sectors.

“Magnetic sectors are better at some sorts of analyses than ion traps,” Amsden says. “They have better mass resolution and better mass accuracy. If you need those things, you would want to use a sector rather than something else.” In lab-based analyses, magnetic sectors live on primarily for isotope-ratio analyses in geological applications and in accelerator mass spectrometers. Ion traps have been attractive for their wide mass range, their ability to work at higher pressures, and their compact size.

In magnetic-sector instruments, ions travel through a magnetic field to a detector with trajectories that depend on their mass-to-charge ratios (m/z). Scanning the magnetic field changes which ions reach the detector. Multiple mass-to-charge ratios can be detected simultaneously by using an imaging detector or an array of detectors. Regardless of how many detectors are used, there’s a single peak for each mass-to-charge ratio.

In conventional magnetic-sector instruments, ions pass into the mass analyzer through a single, narrow slit whose width defines the instrument’s throughput and mass resolution. To help maintain throughput and resolution while making a smaller version, Amsden uses an array of slits, each one with a different width, instead of a single slit. “Instead of just one peak per mass-to-charge, we get a pattern” because of the slit array, he says. He knows the pattern of slits and the physics of the ions’ flight through the instrument, so he can mathematically extract the conventional mass spectrum. That approach allows him to put many more ions through the instrument without sacrificing resolution.

Amsden has another trick up his sleeve to shrink the magnetic sector. The most common type of magnetic-sector instrument has an electric sector in front of the magnetic sector. The magnetic sector performs the actual mass-to-charge separation, and the electric sector helps improve the focus of the ion beam by correcting its energy and angular dispersion.

But in Amsden’s miniature magnetic-sector instrument, instead of having physically separate magnetic and electric sectors, magnetic and electric fields are superimposed at right angles to each other in something called a cycloidal mass analyzer. The magnetic field moves the ions in a circle, and the electric field moves them in a straight line. The result is that ions stream through the array of slits and spiral toward the detector.

Superimposing the fields “makes the instrument much smaller,” Amsden says.

Amsden is focused on designing his miniature mass specs for applications, such as geological analysis or food authentication, for which miniature ion traps aren’t yet being used. He’s especially interested in low-mass applications, looking at isotope ratios of elements such as carbon, oxygen, and nitrogen. Various miniaturized ion traps have low-mass cutoffs of about m/z 50. Amsden’s current cycloidal mass analyzer is tuned to have a range of m/z 10 to 120.


“Now, the only way to do an isotope ratio measurement is to take a sample and send it to the lab,” Amsden says. His instrument could change that.


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