Massive Attack
A new world record in mass spectrometry has been set by researchers at ETH Zurich. The team, led by Renato Zenobi, has observed the largest ever mass-to-charge (m/z) ratio of over 1 million Dalton (MDa). The record opens up new possibilities for studying large macromolecular species, such as
proteins and even viral particles.
Zenobi and his colleagues had to overcome two obstacles: how to vaporize and ionize their sample without it disintegrating and how to detect such a large particle. The realized they could use modern "soft" ionization methods to shift macromolecules, such as proteins and nucleic acids, into the gas phase. Indeed, they used desorption and ionization using a pulsed UV laser.
To separate the ions by m/z ratio, the team also used a standard mass spectrometric method - time-of-flight, which separates the ions on the basis of the length of time they take to drift through the evacuated flight tube of the MS machine. The ions with the largest m/z (basically the biggest ions) have the longest drift times, but are difficult or impossible to detect with conventional detectors, because of this small drift velocity.
The researchers knew that by producing multiply charged ions they could produce a more accessible m/z ratio range. However, they chose a more direct and elegant way, by carrying out their measurements on an instrument developed by Comet AG of Switzerland, in collaboration with Damian Twerenbold, then at the University of Neuchatel, Switzerland, equipped with a device known as a "superconducting tunnel junction detector".
The superconducting tunnel junction is a completely different detection principle to the conventional detectors in a time-of-flight mass spectrometer, borrowed from astrophysics. "Essentially, a superconducting tunnel junction (STJ) works because when the materials are held at superconducting temperatures the electrons in layers of niobium that are the active part of it become grouped into what is known as 'Cooper pairs'," explains Zenobi, "Impinging particles (in the case of mass spectrometry, ions) generate phonons - lattice vibrations - which in turn break Cooper pairs into free electrons. These free electrons can then tunnel through a thin oxide layer on top of the niobium where they are detected as excess current. The current is directly proportional to the
(kinetic) energy deposited by the original impact."
Using this approach, the team could obtain simple and directly interpretable mass spectra of immunoglobulin M (which has a molecular weight of about 1 MDa) and a group of proteins that play an important role in blood coagulation called von Willebrand factor. These compounds had signals at 0.5, 1, 1.5 and 2 MDa.
"One very promising application is the direct mass spectrometric analysis of antibodies (150,000 Da range) with one or two antigens bound," adds Zenobi, "Due to the new detection principle, this is now easy to do. There is an enormous potential to replace immunochemical methods by much more rapid, more accurate, and more quantitative mass spectrometric analyses."
Anal Chem, 2005, in press; http://dx.doi.org/10.1021/ac0482054
http://www.zenobi.ethz.ch/
http://www.comet.ch/
http://en.wikipedia.org/wiki/Mass_spectrometry
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