Imaging chemical

One potentially powerfiil approach to chemical imaging of oxides is to capitalize on the tip-surface interactions caused by the surface charge induced under electrolyte solutions [189]. The sign and the amount of the charge induced on, for example, an oxide surface under an aqueous solution is detenuined by the pH and ionic strength of the solution, as well as by the isoelectric point (lEP) of the sample. At pH values above the lEP, the charge is negative below this value. 

Figure 4 Chemical images of a nickei TEM grid. Fieid of view is approximateiy 25 x 15 im, 50 X 50 pixeis. Analyticai conditions Ga sputtering, spot size about 0.2 pm, 24S-nm radiation, acquisition time 33 minutes.

Schaller, R. D., Johnson, J. C Wilson, K R., Lee, L. F., Haber, L. H. and Saykally, R. J. (2002) Nonlinear chemical imaging nanomicroscopy from second and third harmonic generation to multiplex (broad-bandwidth) sum frequency generation near-freld scanning optical microscopy. 

Bard AJ, Fan FRF, Pierce DT, Unwin PR, Wipf DO, Zhou FM. 1991. Chemical imaging of surfaces with the scanning electrochemical microscope. Science 254 68-74. 

Figure 5.46. Chemical image of the precipitate. (Reproduced with permission of Penisson and...

Figure 2 Data cube generation in mapping and imaging. The four-dimensional hyperspectral data cube contains the full spectral information, absorbance vs. wavenumber (v, cm ), for each x,y pixel from the imaged area, as is shown above. A horizontal slice through that cube contains a chemical image (e.g., band intensity at selected v for each x,y pixel of the image) as is shown below. The latter result could be obtained by Global Imaging (in which only the intensity distribution of a certain band over the imaged area would be recorded).

Near-infrared chemical imaging (NIR-CI) refers usually to the hyperspectral imaging of samples using typically wavelengths from 0.9 to 2.5 pm (NIR region) and an FPA detector. There are some major methodical and instrumental differences between NIR and IR imaging. 

F. Clarke, A. Whitley, S. Mamedov, F. Adar, N. Lewis and E. Lee, A comparison of Raman and EDXRF chemical imaging for use in formulation process development and quality control, Newsletters, Raman Update (Horiba Jobin Yvon), Spring, 2005. 

ToF-SIMS Study of Organic Materials in Cultural Heritage Identification and Chemical Imaging... 

Nevertheless, the introduction of time-of-flight (ToF) analysers for SIMS analyses at the beginning of the 1980s, as well as the recent development of liquid ion sources delivering cluster projectiles now permit the analysis of organic materials with high sensitivity and selectivity. Moreover, thanks to its excellent lateral resolution (in the order of micrometres), and its minimal sample preparation, ToF-SIMS has become the reference technique for chemical imaging by mass spectrometry. 

As the primary ion beam can be focused to less than 1 pm, ToF-SIMS is well suited to chemical imaging. For this purpose, the beam is rastered by electrostatic fields all over the surface, and a spectrum is recorded for each point. This allows the distribution of a specific ion all over the analysed surface to be mapped, and also to access a mass spectrum... 

The second requirement is related to chemical imaging applications. Very flat samples are required to avoid problems of depth of field. Even if this is common to every imaging technique, it is in this case coupled with surface pollution problems. Sample preparation must then lead to flat surfaces without surface pollution. For cultural heritage samples,... 

Mass spectra interpretation can be very difficult, especially in the case of a complex sample, so the possibility of chemical imaging can help reduce the complexity of the technique. Indeed, instead of using the spectra of the whole sample, specific mass spectra can be recalculated from a precise area giving more specific information. A general method for spectra examination is described in Figure 15.3 from the global spectrum,... 

Finally, the versatility of the technique and its use as a chemical imaging technique allow retrieval of the structural composition of a sample, in order to understand its complete recipe [Mazel et al. 2008]. The composition of a sample from the Dogon statuette 71.1935.105.169 has been studied. Proteins, polysaccharides, lipids and minerals have been found. The distribution of these different chemicals shows that the patina sample can be divided into four different layers (Figure 15.14). Layers 1 and 3 are mainly composed of proteins whereas layer 2 consists of lipids and polysaccharides. Minerals can be found at the interface of layers 1 and 2, and 2 and 3. Finally, layer 4 is the more complex because it contains all the types of compounds. One can suppose that it is in fact composed of different layers that do not appear clearly on the cross-section. 

The second advantage is the possibility of performing chemical imaging analyses. Even if other techniques, such as matrix-assisted laser desorption/ionization, also allow images to be acquired, ToF-SIMS is, for now, the mass spectrometric technique with the best spatial resolution performance. 

V. Mazel, P. Richardin, D. Touboul, A. Brunelle, P. Walter and O. Laprevote, Chemical imaging techniques for the analysis of complex mixtures new application to the characterization of ritual matters on African wooden statuettes, Analytica ChimicaActa, 570, 34 40 (2006). 

Electron-hole pairs are only produced in areas illuminated by the light and spatially resolved photocurrents can be achieved using a focused scanning beam. Therefore the main advantage of LAPS is that it can work as a chemical imaging sensor the light-addressability of the LAPS allows one to obtain a two-dimensional map of the distribution of the ion concentration. 

S. Inoue, M. Nakao, T. Yoshinobu, and H. lwasaki, Chemical-imaging sensor using enzyme. Sens. Actuators, B 32, 23-26 (1996). 

In the literature these studies are classified as imaging mass spectrometry (IMS) and defined as the investigation of the chemical profile of a sample surface with a submicron lateral resolution and chemical specificity. The main aim is to use the power of mass spectrometry techniques to create chemical images showing the distribution of compounds ranging in size from atomic ions and small molecules to large proteins. 

The electronic nose and electronic tongue will be described as systems able to give olfactory and chemical images, respectively, in a variety of applications fields, including medicine, environment, food and agriculture. 

In the last decade much effort has been oriented to the fabrication of artificial olfaction machines able to determine chemical images (also odor images) of complex volatile compounds. Today many different electronic noses and tongues are available for odor detection and classification and for the creation of chemical images of liquids. 

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