research advances
September 2009 research highlight
Mass spectrometers on a chip
PSI-SGKB [doi:10.1038/nmeth0809-555]
A prototype for a mass spectrometer with single-molecule sensitivity has prospects for single-cell proteomics.
Although many technologies used in biological research are now being applied to make observations at the single-cell level, mass spectrometry is not one of them. Typically, millions of cells growing in culture are required to be able to detect all but the most abundant proteins by mass spectrometry. With such ensemble-averaged measurements, information about cellular heterogeneity is completely lost.
With new work from Michael Roukes's group at the California Institute of Technology, however, this could potentially all change. Roukes and his colleagues recently reported a nanoelectromechanical system (NEMS)-based method that can be used to detect molecular mass with single-molecule sensitivity.
NEMS sensors are nanoscale devices that resonate at frequencies close to the microwave range. Roukes and his colleagues have been working with these devices since more than a decade ago, when they first discovered that NEMS resonators are extremely sensitive to very small amounts of added mass. “These devices are so small and so light that the addition of just a single molecule can shift their frequency,” explains Roukes. Frequency-shift measurements are extremely precise, but turning a NEMS resonator into a practical single-molecule detector also required the researchers to design a very stable device such that the signal can be distinguished above the noise.
The prototypical NEMS mass spectrometer. A simplified schematic (not to scale) shows the injection site for electrospray ionization and the hexapoles used to guide the analytes to the NEMS sensor. Image reprinted from Nature Nanotechnology.
In a typical mass spectrometer, an analyte mixture is ionized and separated according to charge-to-mass ratio in a mass analyzer, and then the delivery of species of different charge-to-mass ratios to the detector is registered. Roukes and his colleagues built a prototypical NEMS mass spectrometer by integrating the NEMS sensor, which serves as both mass analyzer and detector, with an electrospray ionization injection system. The way that NEMS mass spectrometry works is fundamentally different from traditional mass spectrometry. It does not measure the charge-to-mass ratio; instead, the jumps in frequencies that are registered by the NEMS sensor are directly proportional to the mass of the absorbed species. As Roukes puts it, “we don't need an analysis chamber because our detector is smart, so to speak, and figures out what has arrived.”
In their recent Nature Nanotechnology paper describing this work, Roukes and his colleagues recorded single-molecule adsorption events for gold nanoparticles and for the protein bovine serum albumin with their NEMS mass spectrometer. Though the current results are, of course, preliminary, they allude to the most tantalizing prospect for the further development of NEMS mass spectrometry: the possibility of obtaining proteomic information at the single-cell level.
Proteins that are expressed at very low copy numbers, such as signaling proteins or transcription factors, are very difficult to detect with traditional mass spectrometry but should theoretically be detected with single-molecule NEMS mass spectrometry. It also may be possible to obtain detailed measurements of protein expression, rather than just ensemble-averaged measurements, for cell populations. “The next question is, that's a lot of molecules that you need to measure one by one, and how the heck are you going to do that?” says Roukes. He envisions an elaborate microfluidics-based front-end separation system, which would distribute the contents of a single cell to a chip consisting of thousands of individual NEMS sensors, each one a tiny mass spectrometer.
Besides developing the microfluidics-based front end, Roukes is collaborating with researchers at CEA (French Atomic Energy Commission) Leti in Grenoble, to make such chips with thousands or even millions of NEMS sensors. Another challenge they must tackle is pushing the mass resolution to below a single dalton; their current mass resolution is about 1,000 daltons. “This will require us to scale down [the size of] the individual NEMS resonators,” says Roukes.
If this sounds like an awful lot of work for a single laboratory, that is because it is. “It's really a collective enterprise that will be required to do the barn-raising for this new technology,” says Roukes. Roukes is eager to engage in collaborations to help develop NEMS mass spectrometry to its promising potential, but he is also interested to hear from those with fundamental objections. “I hope it inspires a collective imagination of this coming new paradigm in how mass spectrometry can be done,” he says.
