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Two main types of inks coexisted in Early Medieval times: carbon inks and iron-gall inks. Many variations within these types existed, depending on the source of raw materials and their proportion. In addition to these, an intermediate mixed type of ink, with both iron and carbon as colouring agents, was sometimes used, especially in early Arab manuscripts.
Two projects are being conducted currently at the Bundesanstalt für Materialforschung und -prüfung (BAM) concerning the analysis of inks with Raman spectroscopy. The first one aims at comparing the spectra of different carbon inks from Antiquity to the Middle Ages with Raman spectroscopy, while the second one focuses on the identification and comparison of mixed inks.
For the first project, several carbon inks were made using different sources of carbon (pine soot, olive oil soot, date seeds charcoal etc.) and various binders (fish glue, gum Arabic, proteinic binder etc.), following antique recipes. These inks were then analysed with Raman spectroscopy and fitted with the “two-peaks” and the “three-peaks” models following the procedure described by Goler et al., to help spotting differences between the spectra, depending on the recipe. Some samples were then disposed in an ageing chamber at 80°C and 60% RH and analysed again in order to determine whether changed in the spectra are observable. The procedure proposed by Goler et al. to date inks with Raman spectroscopy was also evaluated during this process. Although several carbon inks could be distinguished from their spectra, no differences were seen between aged and fresh samples. Furthermore, experimental conditions were found to have such a significant influence that reliable dating of inks through Raman spectroscopy seems dubious.
The growing need to implement sensors such as NIR or Raman spectroscopy for the in-situ monitoring of bioprocesses which follows the standards of Quality by Design is either restricted by the impact of the huge water signal or by a disturbing fluorescence background originating from compounds in the culture media. Furthermore, the characterization of the bioprocess samples is challenging due to changing conditions in course of cultivation.
Here we evaluate two different process-suitable Raman spectroscopic approaches, namely time-gated Raman which bears the potential to extract the Raman signal from the fluorescence background, and cw- Raman with NIR excitation in combination with Surface Enhanced Raman Spectroscopy- (SERS) to investigate cell-free supernatants of Escherichia coli sampled over the course of a cultivation. A confocal Raman microscope was used as a reference for the process devices. The concentration of the analytes, glucose, acetate as well as metabolites such as cAMP, AMP and amino-acids were determined by offline by High-Performance Liquid Chromatography (HPLC) to serve as reference for the calibration of the Raman and SERS spectral data.
Multivariate evaluation of the Raman and SERS spectra by Partial Least Squares Regression (PLSR) yielded for most of the analytes robust correlations at each sampling point. Repeated investigation of the off-line samples over a larger experimental period suggested not only a high reliability of the Raman data in general but also a high repeatability of the SERS experiments. Similar spectral features in different quality and signal/noise ratios were measured with all three set-ups. Major results of the comparison of the different Raman spectroscopic approaches and their combination with SERS are summarized and conclusions are drawn on which approach provides the most accurate concentration data among the target analytes.
Acknowledgement
The authors kindly thank Mario Birkholz (IHP, Frankfurt (Oder), Germany) for the opportunity to use a confocal Raman microscope, Alex Bunker and Tapani Viitala (Division of Pharmaceutical Biosciences, Centre for Drug Research, University of Helsinki, Finland).
A process spectroscopic approach for the monitoring of microplastics in environmental samples
(2018)
The potential of Raman and NIR process-spectroscopic as a rapid approach for the estimation of microplastics (MP) in soil matrix were tested. For this purpose, artificial MP/soil mixture samples consisting of standard soils or sand with defined ratios of MP (0,5 – 10 mass% polymer) from polyethylene (PE), polypropylene (PP), polystyrene (PS) and polyethylene terephthalate (PET) were prepared. MP particles with diameters < 2 mm and < 125 µm were obtained from industrial polymer pellets after cryo-milling. Spectra of these mixtures were collected by (i) a process FT-NIR spectrometer equipped with a reflection probe, (ii) by a cw-process Raman spectrometer and (iii) by a time-gated Raman spectrometer using fiber-optic probes. The evaluation of process-spectra was performed by chemometric methods. Whereas MP could be detected by Raman spectroscopy in coastal sand at 0.5 mass%, in standard soil detection of MP was limited to 10 – 5 mass% with the large fraction, but samples containing particles of the 125µm mass-fraction yielded no positive result at all. One reason for the lacking sensitivity could be fluorescence by soil organic matter and thus, in a next test time-gated Raman spectroscopy was applied. However, although being indeed more sensitive to the small particles this method failed at MP < 5 mass% indicating that fluorescence was not the major problem. Finally, FT-NIR was tested. Depending on the polymer, MP contents of 0,5 or 1 mass%, respectively, could be detected in standard soils and polymers identified. Furthermore, this approach could be used for the investigation of mixtures of up to four polymers and in real-world samples from bio-waste fermenter residues.
The mayor part of the Berlin collection `Manuscripta Americana´ consists of documents compiled by Alexander von Humboldt. The diversity of the written fragments and their shape presume few accordance concerning purpose, place or time of production. Even before the papers came to Berlin, manuscripts were copied, divided and collected by famous scholars also in Mexico. Back in Europe, Humboldt visited further collections and published the detected hieroglyphic writings altogether with cultural objects and landscapes in the book `Vues des Cordillères et Monuments des Peuples Indigènes de l`Amerique, voyage de Humboldt et Bonpland´, labelled corresponding to the cultural background.
By scientific and culture historical analyses, the Humboldt codices are revised and related to each other and others. In especial, the compilation of fragments shown on plate 36 in the book is looked at more closely. Whereas most of the plates represent only one sheet of painted amate paper, here segments with details of seven fragments are put together. It is tried to comprehend why these pieces are on one plate.
To find out a common feature of the documents, their content and represented form are compared. A correlation between some pieces is obvious and thus is also expected in the manufacture of the codices, in the kind of material used. For this the inks and colors are determined by scientific analysis including XRF-, Raman-, FTIR- and VIS- spectroscopy. Special material is detected and related to results of former analysis of codices written during the colonial period of America. Herewith a clear cultural assignment of the manuscripts is performed.
A relation of the fragments represented on plate 36 among each other and to other codices of the collection Manuscripta Americana in Berlin is discussed. Also, if the manuscripts are unique `originals´ or represent fragments of documents, whose corresponding pieces maybe found- in the best case- incorporated in other collections. We detected, that at least two of them can be put into relation to similar manuscripts deposited in Mexico. Further material analysis should clarify, if the corresponding pieces are identical- from one document, or if one of them is a copy. Herewith an approximation to authenticity features and history is issued.
Raman spectroscopy for online monitoring of a homogeneous hydroformylation process in microemulsion
(2021)
An important industrial reaction is hydro¬formylation for the production of aldehydes from alkenes and syngas on the basis of homogeneous catalysis. The main cost factors of the processes currently used are product selectivity and the loss of the catalysts used. Therefore, various concepts for the hydroformylation of long-chain olefins have been developed, including hydroformylation in microemulsions, which is being investigated on a mini-plant scale at the Technical University of Berlin [1]. In this study, online Raman spectroscopy of the reaction of 1-dodecene to 1-tri¬decanal in a microemulsion was performed [2]. First, an experimental design was used to obtain a good representation of the operating range in the mini plant with respect to the concentrations of five reactants in a laboratory setup [3]. Based on the Raman spectra, Partial Least Squares (PLS) models for the prediction of 1-dodecene and 1-tride-decanal were calibrated and with these the reactions were predicted on a laboratory scale. In the next step, the PLS models were applied to online spectra from a mini-plant. This resulted in promising estimates of 1-tridecanal and acceptable predictions of 1-dodecene mass fractions. The predictive power of PLS models in this particular case was limited by unexpected by-product formation which, however, can easily be compensated by an extended calibration. Hence, Raman spectroscopy is a promising technique for process analysis in microemulsions.
In the present talk the basics of the Raman spectroscopy and particularly of Raman microscopy are explained. Advantages and disadvantages of the method are highlighted through selected case studies. In the second part of the lecture examples of correlative imaging with electron, X-ray, ion and optical microscopies from micro- to the nanoscale are highlighted.