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5Chemical mapping

The analysis of a sample like, e.g. a coating on a polymer surface for the chemical composition answering the question “which substances can be identified in the coating” sometimes leads to the additional question: “where are these substances located?. This means the desired information is the variation of the chemical composition at different points of the surface or inside the bulk material. Imagine a black coating that seems to be homogeneous on the first sight. But there are some strange features in the surface topography like Figure I.7 shows. This might lead to the question if the visible inhomogeneities are correlated with chemical properties!? The investigation of this question asks for a two-dimensional chemical analysis resolving the chemical composition with a high lateral resolution.

Two different approaches have to be distinguished with this kind of analysis: mapping and imaging.

Figure I.7: a) Optical microscope image (DIC) of a 2K high gloss polyurethane coating surface with unusual topographic features, b) infrared microscopy mapping analysis of the coating surface (marked area); false colour image displaying the peak intensity of the 2238 cm-1 peak of free (unreacted) isocyanate groups of the hardener

The so-called mapping can be done by scanning and probing the surface in two dimensions point by point or line by line acquiring a full spectrum for each point and resolving the chemical composition by extracting key features of the spectra. There are a few methods available to perform this kind of analysis which deliver different information: infrared microscopy imaging, Raman microscopy mapping, TOF-SIMS imaging and SEM mapping. Mapping means that a region of interest (ROI) is visually selected and (rather than taking a simple intensity spectrum of the whole area) rastered by the exciting beam point by point and line by line. At each point a whole spectrum is collected and stored. This can be achieved either by moving the sample on a computerized stage (like with an infrared microscope) and keeping the exciting beam fixed. Or the beam of the primary radiation is rastered while the sample is fixed on a stage like scanning electron microscopy (SEM) does. The whole set of spectra of the specified area of interest is the database of this measurement. In a second data analysis process after the measurement special features can be selected and extracted to produce images.

Imaging in contrast means taking all spectral intensities simultaneously from the whole ROI and filtering the information according to one selected part of the spectrum.

5.1Infrared microscopy mapping

The imaging mode of infrared microscopy analysis, which will be described in detail in Part IV Chapter 5, delivers molecular information with an information depth of 0.5 to 1 μm into the surface.

Figure I.7b shows a false colour 2D image of the coating surface of Figure I.7a. The colour codes the intensity of a specific signal of the NCO group of the isocyanate hardener between dark blue (no free NCO group) and white (high NCO content). This coloured image is the translation of a complex physic-chemical procedure (the excited vibration of the NCO group) into an easy-to-understand picture. The image demonstrates that the content of free isocyanate is not homogeneously distributed in the uppermost layer of the coating. This leads to the conclusion that there must have been something wrong with the mixing of the polyurethane components. Whereas the “back end” (ATR-FT-IR microscopy) is something you cannot understand without knowing vibrational theory, this “front end” picture can transport the key information in a glance.

5.2TOF-SIMS imaging

In contrast to the infrared microscopy mapping (IRMM) the TOF-SIMS imaging allows for a higher spatial resolution which is shown in Figure I.8. The cross section of a 25 μm three-layer primer system consisting of a phthalate primer in between two chlorine primer layers has been analysed by a primary ion beam of Ar+ ions in a TOF-SIMS V mass spectrometer.

Figure I.8: Optical microscopy image and TOF-SIMS image of a cross section through a three layer primer system displaying the secondary ion Cl - of chlorine compounds and (C6H5)COO- of phthalate anions

The false colour images of the Cl- ion and the (C6H5)COO- ion display the intensity of the two characteristic secondary ions in the spectrum of the negative secondary ions of the cross section and thus is an extract from the whole data set of the two dimensional measurement. Whereas IRMM detects vibrational transitions of characteristic bonds and displays the intensity of the absorptions, the peak intensity of a characteristic fragment like C6H5COO- is derived from the integration of the peak area. This is not molecular information because this fragment can originate from a lot of molecules that contain this specific group. However, knowing that the fragment C6H5COO- can be attributed to phthalate compounds and analysing further fragments associated with this secondary ion like e.g. 191 u for PET or 219 u for PBT, the molecular information can be deduced from the mass spectrometry data. The image shows, that there is a phthalate ester-based primer in between a “sandwich” of thin layers of chlorine containing primers.

5.3SEM-EDS mapping

Figure I.9: SEM-EDS mapping result of a paint defect (top left) sowing the false colour intensity images for calcium, silicon and iron (clockwise)

Another method for the visualization of lateral chemical differences which will be described in this book is the EDX (= energy dispersive X-ray analysis) or EDS (= energy dispersive spectroscopy). The sample is scanned point by point and line by line and excited by a beam of electrons between 1 keV and 30 keV. This results in the emission of characteristic X-rays for each element present in the target area. The intensity is detected and gathered by a detector and the result can be extracted from this so-called hypermap as 2d false colour image for each detected element. In contrast to the TOF-SIMS and the infrared microscopy mapping this technique (only) delivers elemental information. If, for example, calcium has been detected (=> Figure I.9) and the lateral distribution is displayed, it cannot be distinguished between calcium carbonate, calcium hydroxide or a calcium soap.