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Imaging spectrometry reveals sub-cellular world

19 Oct 2006

A new mass spectrometry technique could enable biologists to analyse dynamic processes in cells, where many features of interest are smaller than the resolution limit of optical microscopes.

Researchers in the US have developed the prototype for a new nanometer-level imaging system that makes it possible to produce surface analytical images and to make quantitative measurements of metabolic activity in cells (Journal of Biology 5 20).

The new technique, known as multi-isotope imaging mass spectrometry (MIMS), can both image and quantify stable isotopic labels that are attached to molecules in cells in order to trace their movement. The imaging system incorporates sophisticated ion optics, such as ion beam focussing lenses, to achieve a resolution of around 30 nm.

"This is a new methodology which can be applied to many domains of biomedical research. We use stable isotope labels to follow the fate of molecules in sub-cellular compartments and to measure the amount of material going from one place to another," Claude Lechene, based at Harvard's National Resource for Imaging Mass Spectrometry, told optics.org. "Using this technique we can study the behaviour of proteins, sugars, lipids, nucleotides, and even drugs."

The approach used by the Harvard team involved feeding organisms with the target molecules, which were labelled with stable isotopes such as carbon-13 or nitrogen-15. The researchers then used a primary caesium ion beam 30 nm in diameter to scan the surface of the samples, causing secondary ions to be ejected as a result of sputtering.

By measuring the mass:charge ratios of these secondary ejected ions using spectrometry techniques, the researchers were then able to develop isotope ratio images that could identify areas where there had been high metabolic activity.

According to the researchers, MIMS technology represents a significant leap forward from its predecessor, secondary ion-mass spectrometry (SIMS), which is used for investigating the isotopic composition in the chemical and materials sciences -- but not for biological samples.

"The first major improvement on SIMS is the higher spatial resolution, a result of our smaller beam-size. The second is the ability to measure multiple isotopes simultaneously from the same sputtered volume. And the third improvement on SIMS is the higher mass resolution -- this is the ability to separate two masses with the same mass number but separated by a fraction of an atomic mass unit," team member Greg McMahon told optics.org.

Like SIMS, the new technique destroys 90% of the top cellular monolayer, but McMahon says that the removal of this layer has negligible effect on the subsequent analysis. "We can look at the same cell and do multiple analyses on it over several hours. In practice, it is not as if we have five seconds due to the layer removal," said McMahon.

According to the researchers, the next steps include applying the technique to measuring the transport of fatty acids and monitoring the turnover of proteins and nucleotides in the cell. In addition, the team is aiming to label cells permanently and analyse their long-term fate.

The team also has plans to exploit the destructive nature of the technique. "We are doing experiments where we take thousands of images of the single cell, slowly 'shaving' off layers, and then reconstruct them to create a 3D representation of the entire cell," said Lechene.

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