Film-like residue preparation by drop on demand picoliter printing for TXRF analysis

An accurate sample preparation is substantial for obtaining reliable results in total reflection X-ray fluorescence analysis (TXRF). Pipetting of sample solutions in the microliter range often leads to “coffee rings” where the specimen dries inhomogeneously with a mixed morphology (film-like in the center and particle-like at the ring). The specimen morphology can be controlled in a more reliable manner by picoliter printing achieving particle-like residues. The preparation of film-like residues is in general more challenging. It requires the optimization of the printing procedure, e.g. the deposited mass, additives as well as the elimination of static charges. Influences of different additives on the film formation were experimentally probed. The aim of this study was to develop a reliable method to improve the film formation of nickel on quartz reflectors, which are commonly used as sample carriers in TXRF. Nickel was chosen because it has a good sensitivity in TXRF and is often used as reference when studying the TXRF instrument performance. Grazing incidence X-ray fluorescence analysis (GIXRF) was used evaluating the morphology of the residues. The use of additives and variation of printing parameters improved the film formation slightly. The elimination of static charges was the most critical parameter in order to achieve films. The combination of Triton X-100 as a non-ionic surfactant with the static elimination yielded near-perfect films (1 ng Ni). The film morphology was investigated by atomic force microscopy, however the results were inconclusive.