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Nanoscale
molecular analysis:
combining SNOM with Mass Spectrometry
Materials
and structures on the nanometer scale have become of great
interest and relevance in modern science and engineering.
Although molecular or atomic resolution can nowadays routinely
be achieved with various forms of scanning probe microscopy
(SPM) methods, the majority of SPM techniques provides only
topographical analysis with little or no chemical contrast.
In 2001, our group demonstrated the combination of scanning
near-field optical microscopy (SNOM) with mass spectrometry
for the first time.
We are now developping a SNOM-MS instrument with an Ion Trap-TOF
MS analyzer built specially for this application to overcome
previous instrumental limitations. This will ultimately
allow chemical analysis with nanometer spatial resolution
at atmospheric pressure.
figure
1: setup
A SNOM tip, into which a pulsed UV laser is coupled, is
kept within 5-15 nm distance above the surface of the
sample. By keeping the tip in constant shear-force feedback,
a topographical image of the surface can be acquired while
moving the sample (or tip) using a piezo stage.
At a desired location, the laser is switched on for ablation
(spatial resolution of up to 70 nm FWHM). The ablated
analyte is “sucked” into the sampling capillary,
transferred into the IonTrap and ionized therein. The
ion trap serves as a storage and accumulation / pre-concentration
device and is pulsed into a time-of-flight (TOF) mass
analyzer.
SNOM
probes
The SNOM tips for nano laser-ablation are produced by
chemically etching an optical multimode fiber with hydrofluoric
acid.
This self-determinating process yields tips with a high
optical transmission and a tip apex of 70 - 200 nm. After
etching, the tips are metal coated to ensure light only
exits trough the front aperture. Further information about SNOM tip etching...
figure
2: SNOM tip
Laser
ablation on the nanometer scale
Laser ablation on the nanometer scale was performed using
a SNOM tip as shown above into which a pulsed UV laser was coupled.
The image shows the topography of an Rhodamine 6G pellet
surface after firing ns laser pulses. The holes in the
organic crystal show no rims or craters indicating
a true local vaporization of material on a scale of smaller
than 1000nm. This material can be collected by a vacuum sampling
device and analysed in the SNOM MS.
 
figure
3: Rhodamine 6G surface with ablation craters
Contact:
Thomas Schmitz, schmitz@org.chem.ethz.ch
Liang Zhu, zhu@org.chem.ethz.ch
Prof. Dr. Renato Zenobi, zenobi@org.chem.ethz.ch
Literature:
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Zhu L., Gamez G., Schimitz T. A., Krumeich F., & Zenobi R., Material ejection and redeposition following atmospheric pressure near-field laser ablation on molecular solids.Anal. Bioanal. Chem., 396, (2010), 163 -172. |
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Schmitz T A., Gamez G., Setz P D., Zhu L., & Zenobi, R., Towards Nanoscale Molecular Analysis at Atmospheric Pressure by a Near-Field Laser Ablation Ion Trap/Time-of-Flight Mass Spectrometer. Anal. Chem., 80, (2008), 6537–6544. |
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Gamez G, Zhu L., Schmitz T A., & Zenobi, R., Photoelectron Emission as an Alternative Electron Impact Ionization Source for Ion Trap Mass Spectrometry. Anal. Chem., 80, (2008), 6791–6795. |
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Schmid, T.; Schmitz, T. A.; Setz, P. D.; Yeo, B. S.; Zhang, W. H.; Zenobi, R., Methods for molecular nanoanalysis. Chimia 2006, 60, (11), A783-A788 |
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P. D. Setz, T. A. Schmitz, and R. Zenobi, Design and performance of an atmospheric pressure sampling interface for ion-trap/time-of-flight mass spectrometry, Rev. Sci. Instr., 77, 024101 (2006) |
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R. Stöckle, P. Setz, V. Deckert, T. Lippert, A. Wokaun, and R. Zenobi, Nanoscale Atmospheric Pressure Laser Ablation - Mass Spectrometry, Anal. Chem. 73, 1399 - 1402 (2001). |
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