Imaging mass spectrometry of polymeric materials

Abstract

Imaging mass spectrometry (MS) is a technique that makes images of molecular distributions at surfaces based on mass spectral information. At a range (typically a raster) of positions, mass spectra are measured from the surface giving a characteristic fingerprint for the material that is present at the position analyzed. By selecting a peak from the spectrum that is specific for a certain compound, a distribution map of this compound can be made. A range of analytical techniques and sample preparation techniques is available for imaging MS. The first part of this thesis focuses on these methods and techniques and gives insight in the important aspects for polymer analysis. Instrumental developments focus on the use of stigmatic ion microscopy, as described in chapters 4 and 5. By using ion optics that make a direct projection of the sample onto a position sensitive detector, an imaging MS experiment can be performed using large-diameter, unfocussed ionization beam (typically an ion or laser beam). In chapter 5, a delay line detector is introduced for ion microscopy and shown to offer both resolution and acquisition-time benefits for secondary ion mass spectrometry experiments with fullerene primary ions. Over the past century, the application of polymeric materials has increased to virtually any type of consumer products, medical devices, pharmaceuticals, adhesives and coatings. In the development and engineering of increasingly complex polymer systems, it is of great importance to understand both their spatial and molecular properties. Imaging MS is a technique that provides spatial and chemical information based on the molecular weight of materials. Because polymers are built up from a chain of monomers, which leads to a variety of polymer chain lengths, polymers show up in a mass spectrum as a collection of peaks with a slightly different mass. A single peak of this distribution can be so small that it is hardly seen in the total image spectrum, especially when only a small part of the imaged area contains polymer. Statistical analysis tools like principal component analysis find the natural correlation between these peaks. By doing so, a molecular weight distribution can be obtained and the distribution of the polymer can be visualized with sufficient image intensity. The biomedical engineering field is an important and fast-growing application field for polymeric materials. In chapter 7, a biodegradable polymer designed for drug delivery, is implanted under the capsule of a rat kidney to obtain a better understanding of the foreign-body response and polymer biodegradation. This provides molecular cartography of the polymer implant as well as the cellular signature of the implantation environment in one single experiment, showing the power of MS imaging to chemically analyze diverse samples. Cellular infiltration into the polymer is visualized based on cellular molecular signatures. Because macrophages are known to play an important role in the foreign body response, the molecular signatures are compared with macrophage standards cultured in different polarization environments. Based on this comparison, an assessment can be made on which type of macrophages localize in which region around the implantation site.