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International Collaboration in Chemistry: Novel Instrumentation for Modern Elemental Speciation Analysis

Fachliche Zuordnung Analytische Chemie
Förderung Förderung von 2008 bis 2013
Projektkennung Deutsche Forschungsgemeinschaft (DFG) - Projektnummer 84887454
 
Erstellungsjahr 2012

Zusammenfassung der Projektergebnisse

The US/German research consortium jointly developed an 8-cm discharge chamber for the generation of a homogenous radio frequency glow discharge (RFGD) plasma that can be pulsed at kilohertz frequency and maintained at excitation powers of > 1 kW. After its construction, first investigations and applications at the Indiana University in Bloomington, IN, USA, (G.M. Hieftje et al.) the new chamber was shipped to project partner IFW Dresden (V. Hoffmann et al.) who further improved the 8-cm pulsed (p)RFGD source and finally installed the whole system in the laboratories of the third project partner University of Münster (W. Buscher et al.) for further investigation and analytical application. The Dresden group developed an Acousto-Optical Spectrometer (AOS) for imaging analysis of the pulsed RF glow discharge, while the Münster group already installed and investigated a Monochromatic Imaging Spectrometer (MIS) for this purpose. The MIS was equipped with a high performance iCCD camera (PI MAX 3) to achieve highest possible light sensitivity of the imaging system. The construction of the plasma chamber could be further improved in view of the stability of the plasma discharge. Pressure and power limits could both be increased for the discharge chamber to reach better sensitivity and spatial resolution for imaging spectrometric analyses. Finally, based on the practical experiences with the 8-cm pRFGD source, a new modular construction of a 4-cm diameter pRFGD source was developed and created by the Dresden group and has shown further improved results. This system is subject to current multiplication for the project partners in Bloomington and Münster. The electrical properties of the different pRFGD sources were extensively investigated. New electrical probes for RF voltages and currents were constructed in cooperation of the Dresden group and RF electronics specialists who had the necessary experience for such challenging RF measurements. Acoustic effects in the pulsed RF discharges could be discovered and investigated in detail. Generated pressure waves were caused by gas heating with subsequent gas expansion resulting in unwanted acoustic effects. A computational simulation has revealed that this effect could also explain the electrical pre-peak in pGD. Based on these findings, the new 4-cm GD source construction included elements to suppress such acoustical standing waves. The properties of the AOS were investigated in detail. A satisfactory spectral, spatial (approx. 150 μm) and time (< 5 ns) resolution together with fast (within 200 µs) switching between different spectral lines could be demonstrated satisfactorily. Some problems could be identified and are already or can be easily solved by specially developed software: the background image and lateral shift of the images at different wavelengths. The AOS should be technically updated to remove a discovered etaloning effect and known bugs. Also the synchronization capabilities of the AOS should be extended. The spatial resolution of images measured with the 8-cm pRF glow discharge chamber turned out to be limited by the plasma conditions rather than the optical properties of the optical systems applied (AOS and MIS). This is related to a still too low power density on the cathode surface. Additionally, the pressure (1-2 hPa) and the minimal pulse length (about 10 μs) in the pulsed RF discharge prevented the achievement of better lateral resolution. This could be confirmed by much better images detected in a Grimm-type and the 4-cm source. The lateral resolution of about 300 μm in an 8-mm Grimm type source could be achieved. The development of the new discharge chamber shall be continued. First quantitative experiments were carried out using the 4-cm pRFGD source. For both imaging detection systems, the AOS and the MIS, detection limits for Ag were determined in the order of 10^-9 g/mm2, which is close to the needs in gel electrophoretic applications, and can still be improved. In the AOS system, for instance, special care must be taken to reduce the high background signal. In view of element selective imaging analysis of proteins in electrophoresis, typical gels and membranes (polyvinylidene fluoride, PVDF, and nitrocellulose, NC) have been investigated. The membranes could be exposed to pRFGD for several minutes, indicating that this should be sufficient to sputter the protein-constituting elements for sensitive and element selective analysis as long as the plasma discharge remains homogenous, which is a prerequisite for quantitative analysis anyway.

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