Wolfgang Schrepp

A new high-frequency high-sensitivity SAW device for NO2 gas detection in the sub-ppm range

Abstract A high-frequency SAW gas-sensing device has been developed for an operating frequency of 600 MHz. It contains three separate SAW delay lines, which are the frequency-determining elements of oscillator circuits. They can be coated with thin organic films as adsorbents for gas concentration measurement. Due to the adsorption process the mass of the film increases, causing a decrease of surface wave velocity, which will correspondingly cause a downward shift of the oscillation frequency. First measurements are made with two classes of NO 2 -sensitive films: evaporated lead phthalocyanine layers from 1 to 15 nm thick and Langmuir—Blodgett (L—B) films with molecules consisting of a soluble iron phthalocyanine derivative. The dependence of sensitivity on the thickness of the sublimated films and the type of interaction of NO 2 with these coatings are discussed.

Self-assembled organic films on gold and silver

Abstract The formation of self-assembled organic monolayers and multilayers on noble metals is reported. Self-assembly was achieved by chemisorption of disulfide compounds from solutions onto gold and silver substrates. The ultrathin films were examined by grazing incidence IR spectroscopy, wettability and electrochemical impedance spectroscopy. The molecules within the layers are preferentially oriented along the surface normal, tightly packed and essentially pin-hole free. They show thermal stability up to 150 °C. Wetting properties are mainly determined by the terminal groups of the molecules forming the monolayer. A three layer superlattice was built up by a reaction sequence.

Gas detection in the ppb-range with a high frequency, high sensitivity surface acoustic wave device

Abstract We have developed a high sensitivity surface acoustic wave gas sensing device for NO 2 with a detection limit in the ppb range. The sensor layers were phthalocyanines coated by sublimation, the Langmuir-Blodgett technique, and by chemical immobilization. Thickness ranged from one monolayer to 40 nm.

Coupling of Liquid Chromatography and MALDI-TOF Mass Spectrometry

From the very early stages of development of modern mass spectrometry, the value of its combination with chromatography was quickly recognized. The coupling of GC with MS was a natural evolution since they are both vapor phase techniques, and very quickly GC-MS has been accepted as a standard component of the organic analytical laboratory. It has taken considerably longer to achieve a satisfactory and all-purpose mode of HPLC-MS coupling. The difficulties with HPLC-MS were associated with the fact that vaporization of typically 1 ml/min from the HPLC translates into a vapor flow rate of approximately 500–1000 ml/min. Other difficulties related to the eluent composition as result of the frequent use of non-volatile modifiers, and the ionization of nonvolatile and thermally labile analytes. However, during the past few years commercial interfaces have been developed which have led to a broad applicability of HPLC-MS [1–3].

Identification of Polymers

The type of polymer can be identified by determining the mass number of the repeat unit (r.u.) in the mass range where single oligomer resolution is achieved. This mass number is characteristic for a certain chemical composition and in most cases assignment is unambiguous. Very frequently, the type of polymer is already known before running a MALDI-TOF experiment. In these cases, the experiment aims at the analysis of possible by-products and various endgroups, and the determination of molecular heterogeneity. An overview on polymeric systems investigated so far by MALDI-TOF MS is given in Table 4.1.

Fundamentals of MALDI-TOF Mass Spectrometry

In Chap. 2 the main mass spectroscopic instrumentation with relevance to the analysis of synthetic polymers has been described, demonstrating that there is no single means solving all problems. MALDI MS now adds another facet to the possibilities for mass spectroscopic characterization of polymers. The main features are the extremely high mass range in combination with a useful mass resolution as compared to all other MS techniques.

Recent Developments and Outlook

This chapter describes some recent developments and chances for the future of MALDI MS. As already pointed out, the actual main driving force for technical advancements is the area of proteomics [1]. The requirements of high-throughput screening systems lead to considerable progress in automation, sample preparation, and interpretation. Without doubt the past decade has also seen a vivid development of MALDI applications to synthetic polymers but also limitations especially concerning polymers with broad molar mass distributions. Nevertheless nearly all items of MALDI MS are still under development.

Molar Mass Determination

For polymeric materials, the molar mass or molecular size plays a critical role in determining the mechanical, bulk, and solution properties. These properties govern polymer processing, end-use performance, and stability. Therefore, determination of molar mass and molar mass distribution is of central interest in polymer analysis; see Chap. 1.

Mass Spectrometric Instrumentation

This chapter describes the basic principles of mass spectrometry to allow a better assessment of the MALDI-technique. The typical procedure in mass spectrometry is as follows. After introducing the sample into the mass spectrometer, in the first step ions characteristic for the sample under investigation have to be created, i.e., an ion source is needed. Then the ions have to be separated according to their mass-to-charge ratio (m/z) which is the task of the (mass) analyzer. In the end the separated ions have to be detected and the signal displayed in a convenient form. Thus a mass spectrometer consists of the building blocks shown in Fig. 2.1.

Analysis of Complex Polymers

The chemical structure of a complex polymer is characterized by its chemical composition, the sequence of the monomer units in the polymer chain, the functionality, and the molar mass. Typically, polymer analysis has to deal with the overlaying effects of different types of molecular heterogeneity in one macromolecular system. As for functional homopolymers (telechelics, macromonomers) the molar mass distribution is superimposed by a functionality type distribution. Random or segmented copolymers consist of a multitude of molecules with different chain lengths and different chemical compositions. In the ideal case, such complex systems should be separated into individual oligomers, which then may be analyzed with respect to chain lengths, functional groups, and chemical composition.