Time Of Flight Secondary Ion Mass Spectrometry (TOF-SIMS)
TOF-SIMS is a very sensitive and surface-selective technique. Sampling depths are typically less than 20Å. Surface contamination (particularly with PDMS from gloves, oils, etc) is a big problem, so cleanliness is critical.
Mass spectra are produced for negative and positive ions. An example of the positive ion spectra of an mPEG-pyridyl disulfide SAM shows the various fragmentation patterns:
Data Analysis
Each point on the surface generates a complete mass spectrum, which may contain hundreds of individual peaks. These peaks correspond to fragments of surface molecules, and so may be correlated with many other peaks (i.e. those of other fragments from the same species). Data reduction is nearly always necessary to extract useful information from the spectra.
The most common method of data reduction is Principal Component Analysis (PCA). This is a statistical technique that generates several “principal components” (PC’s), which are linear combinations of the variables in the sample. The PC’s can be thought of as new variables which maximize the variance in the data set. The first few components describe the majority of the variation between samples, and can be used for imaging and qualitative/quantitative analysis.
We have a collection of papers and tutorials related to performing the Principal Component Analysis . This may be of assistance in analyzing ToF-SIMS and other multivariate data.
Research led by Drs. Lara Gamble, Dave Castner and Buddy Ratner at NESAC/Bio (U. Wash.) has developed much of the methodology of reducing single spectra and images. A selection of useful/relevant literature references is found below:
- Sanni, O.D., Wagner, M.S., Briggs, D., Castner, D.G., Vickerman, J.C. Classification of adsorbed protein static ToF-SIMS spectra by principal component analysis and neural networks.
Surface and Interface Analysis (2002), 33, 715-728.
- Wagner, M.S., McArthur, S.L., Shen, M., Horbett, T.A., Castner, D.G. Limits of detection for time of flight secondary ion mass spectrometry (ToF-SIMS) and X-ray photoelectron spectroscopy (XPS): detection of low amounts of adsorbed protein .
Journal Of Biomaterials Science: Polymer Edition (2002), 13(4), 407-428.
- Wagner, M.S., Shen, M., Horbett, T.A., Castner, D.G. Quantitative analysis of binary adsorbed protein films by time of flight secondary ion mass spectrometry.
Journal of Biomedical Materials Research A (2002), 64A(1), 1-11.
- Belu, A.M., Graham, D.J., Castner, D.G. Time-of-flight secondaryion mass spectrometry: techniques and applications for the characterization of biomaterial surfaces.
Biomaterials (2003), 24, 3635-3653.
- Wagner, M.S., Castner, D.G. Analysis of adsorbed proteins by static time-of-flight secondary ion mass spectrometry.
Applied Surface Science (2004), 231-232, 366-376.
- Graham, D.J., Wagner, M.S., Castner, D.G. Information from complexity: Challenges of TOF-SIMS data interpretation.
Applied Surface Science (2006), 252, 6860-6868.
- Wagner, M.S., Graham, D.J., Castner, D.G. Simplifying the interpretation of ToF-SIMS spectra and images using careful application of multivariate analysis.
Applied Surface Science (2006), 252(19), 6575-6581.
- Dubey, M., Emoto, K., Cheng, F., Gamble, L.J., Takahashi, H., Grainger, D.W., Castner, D.G. Surface analysis of photolithographic patterns using ToF-SIMS and PCA.
Surface and Interface Analysis (2009), 41(8), 645-652.
