It's a mass spectrometer, but not as we know it

Skip to Navigation

Ezine

  • Published: Jul 20, 2009
  • Author: Jon Evans
  • Channels: Detectors
thumbnail image: It's a mass spectrometer, but not as we know it

At the front, it's very similar to a conventional mass spectrometer (MS), utilising electrospray ionization (ESI) to transform analytes into ions and fire them down a tube. But everything else looks very different and soon even the ESI could disappear.

Together with colleagues, Michael Roukes, a professor of physics, applied physics and bioengineering at the California Institute of Technology and co-director of the Kavli Nanoscience Institute, is busy developing the world's first nanomechanical MS.

This comprises a tiny bridge made of silicon carbide covered with a metallic layer - 100nm wide and 1000nm long - linking two much larger silicon-based structures (see image). It takes advantage of the fact that a thin strip of silicon will naturally vibrate at a characteristic microwave frequency, known as its resonant frequency.

'The frequency of these devices precipitously jumps downward a small amount each time a protein molecule lands - is adsorbed - on the device,' explains Roukes. 'We have developed control electronics that allow the vibrational frequency to be continuously monitored with precision in the tens of parts per billion range. We thus observe, one-by-one, the arrival and adsorption of molecules onto the device.'

At the moment, after the analyte molecules are ionized using ESI, they are guided to the nanomechanical MS by the electric fields generated by ion guides. The precise magnitude of the frequency change depends on both the size of the ion and where it lands on the silicon bridge.

Similar devices that detect changes in the frequency of a vibrating strip of silicon have been developed before (see Good vibrations), but they have tended to be constructed on larger scales. This is the closest anyone has got to producing a working nanomechanical MS.

There are two main ways to utilise this nanomechanical MS to determine the masses of molecules. The simplest way is to monitor the frequency changes and just ignore where the molecule actually lands. But this requires making repeated measurements of the frequency change produced by the same molecule, in order to get over the fact that this change will be slightly different depending on where the molecule lands.

Roukes has showed that this method works for gold nanoparticles and the protein bovine serum albumin, allowing him to determine their masses accurately. But, in each case, it required over 500 separate measurements. As such, this method couldn't be used to determine the masses of a complex mixture of analytes.

To do that will require a technique for monitoring both the frequency change and determining exactly where each analyte lands, and this is what Rouke and his colleagues have now developed. 'We are currently perfecting this [technique] to allow mass spectrometry from arbitrarily complex mixtures,' Roukes told separationsNOW.

They are also improving the nanomechanical MS in other ways, such as by removing the need for the ESI. 'We are developing alternate techniques for injection and delivery of neutral species to our detectors,' he says. 'This will eliminate the ionization step and enable "whole-protein" mass spectrometry - even for very large species that would fragment after ionization in a conventional mass spectrometry system.'

These improvements should open up a whole range of intriguing possibilities for the nanomechanical MS. For a start, a large number of these devices could fit onto a single microfluidic chip. Roukes calculates that 1000 of these devices working in parallel could measures the masses of 10 million molecules in just 100 seconds.

As such, they could make ideal on-chip detectors for microscale chromatography and electrophoresis systems, says Roukes. 'The beauty of this is that the nanomechanical MS is intrinsically compatible. [It is] at the same volume scale as state-of-the-art chip-based microfluidic realizations of such preparatory protocols.'


The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

Nanoscale mass spectrometer

A scanning electron micrograph image of the metal-coated silicon bridge that forms the basis of the nanomechanical MS.

Photo courtesy of the Kavli Nanoscience Institute/Caltech.

Social Links

Share This Links

Bookmark and Share

Microsites

Suppliers Selection
Societies Selection

Banner Ad

Click here to see
all job opportunities

Most Viewed

Copyright Information

Interested in spectroscopy? Visit our sister site spectroscopyNOW.com

Copyright © 2017 John Wiley & Sons, Inc. All Rights Reserved