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F. Horak:
"Development of an ultra-sonic separation cell for on-line sample-pretreatment prior to ICP-OES Analysis";
Betreuer/in(nen): A. Limbeck, B. Lendl; Institut für Chemische Technologien und Analytik, 2015; Abschlussprüfung: 02.06.2015.

ICP-OES is a well established method for identification and quantification of elements in solid and aqueous samples but lacking information about the chemical nature of the element in the sample if not used in combination with a hyphenated separation technique.
Based on the approach presented by Bauer et al. [1], surface modified MCM-41 particles have been synthesized and applied on herbal tea extracts in order to separate the organic-complexed fractions from the free ionic fraction of selected analytes. As the particle size is of great importance when it comes to particle manipulation with an acoustic standing wave, alternatives to the standard MCM-41 particles used by Bauer et al. were investigated. The implementation of a separation cell using an acoustic standing wave removes the centrifugation step from the conventional DPE method, therefore making it automatable and suitable for on-line application.
To correctly quantify the amount of particles used in each measurement, the implementation of a dopant into the MCM-41 particle was also investigated, as the glassware involved in the measurement setup always created a background signal for Si. The addition of Lithium as internal standard to the buffer was found as a viable way to monitor measurement conditions.
Acoustic standing waves can be used for various kinds of particle manipulation. In this work the emphasis was put on trapping a set amount of particles within an indigenously created trapping cell and releasing them once the fraction of the sample which did not interact with the particles has left the cell. As the simulations carried out by colleagues associated with this project indicated that the acoustic force would not be sufficient to successfully trap particles of 1 µm or smaller, a different cell-design was developed and tested.
During this work, multiple ways for pumping the sample through the cell into the ICP were compared. As the commonly used peristaltic pump of the ICP instrument does not offer truly continuous flow, a syringe-pump was tested with success as an alternative but introduced a significant limitation on the measured volume. An alternative to both pumps has successfully been found in using the nebulizer of the ICP-OES as a "self-priming pump" and in controlling the flow-speed by adjusting the nebulizer-gas flow. Experiments indicated that this method allows continuous and stable measurements without the need for changing syringes, and without disturbing the flow within the separation-cell. To ensure reproducibility of the setup, a microcontroller assisted setup to control the timings of each step of the measurement was developed and successfully put to work.
It has been discovered that a minimal change in the resonance of the acoustic field (i.e., introducing a sample with a slightly different density) can lead to a collapse of the trapping force within the cell, limiting the viability of this method to acoustically closely matched samples and solvents.
Another discovery of this work was that seemingly inorganic standard solutions can elute organic complexing agents over time, therefore masking them from the SCX-groups on the particles.
As pointed out throughout this work, the size of the particle, which has to be manipulated, is of great importance. As this problem could not be completely solved during the course of this work, the viability of this method may improve greatly when paired with larger particles, as preliminary experiments with much larger particles have indicated.

Dispersed particle extraction, trace elements, fractionation analysis

Projektleitung Andreas Limbeck:
Nanoparticles for improved trace metal analysis