For proper elemental quantification and visualization of features in LA-ICP-MS maps one needs to be aware of image degradation via blur, in the form of “halo effects” (due to the physical size of the laser beam) and “smearing effects” (evoked by poor cell washout and transfer of LA generated particles), and noise (for real-world situations mostly related to flicker noise). To quantify image degradation, ablation targets with periodic gratings are required for the construction of a modulation transfer function (MTF) and subsequent determination of the lateral resolution as a function of image noise and contrast. Since ablation targets with suitable matrix composition are not readily available, computer-generated periodic gratings were virtually ablated via a computational process based on a two-step convolution procedure using empirical/experimental input data. This experimental-modeling procedure simulates LA-ICP-MS imaging based on two consecutive processes, viz. LA sampling (via ablation crater profiles [ACPs]) and aerosol washout / transfer (via single pulse responses [SPRs]). We will demonstrate that by random selection of experimental SPRs from a large database for each individual pulse during the simulation, the convolution procedure simulates an accurate elemental image map of the periodic gratings with realistic noise. In this way indirect retrieval of the experimental lateral resolution is facilitated without performing actual line scanning on periodic gratings. Although lateral resolution is normally used as a quality-criterion for imaging, we will show that linear artifacts with an angular aspect to the line scan direction may have a different spatial resolution, depending on the blur introduced as a result of “smearing effects” by the LA cell/ICP-MS interface. Via the computational approach described above, using a radial resolution target (Siemens star), we were able to visualize this angular resolution dependency. The latest generation of “ultrafast” LA cells, with FW0.01M values lower than 10 ms, minimizes “smearing effects” and as a result, the resolution becomes practically independent of the angular aspect between line scan and artifact, even for very fast scanning.
B.04 Guest lecture
COBISS.SI-ID: 6107162Calibration in laser ablation ICP-MS imaging is challenging due to the absence of suitable microanalytical certified reference materials that are composed of biological materials, especially posing quantification problems for imaging in disciplines such as biology, biomedicine, plant physiology, etc. So far pressed pellets of certified reference materials, spiked gelatine gels, spiked polymeric films, etc. have been used for calibration purposes. Nevertheless, homogeneity of these custom made materials is always doubtfull and sometimes lacking. In this work several calibration standards based on different materials and preparation procedures were rigorously tested for homogeneity. Gelatine gels, spiked with (multi)element solutions have by now been extensively used as calibration standards in LA-ICP-MS analysis and (bio)imaging. Either in their native (wet) form or after drying they seem very appealing as they are supposed to mimick protein matrices, and are used as quasi matrix-matched calibration standards in e.g. brain or other protein tissues LA-ICP-MS (bio)imaging work. For plant physiological imaging work polysaccharide hydrocolloid gels as calibration standards have not found much use, except maybe agarose gels, although a wide range of hydrogel forming polysaccharides exists, such as e.g. ethylcellulose, xanthan gum, carrageenan, alginate, guar gum and gellan. Other still unexplored materials with great potential are chelating media, such as for instance 3M Empore TM disks based on selective functional groups (iminodiacetate functionalized SDB cross-linked polymer). All these materials have the potential to be used as matrix-matched element standards in various fields and as such have been tested in this work for microanalytical calibration, i.e. based on pixel sizes associated with beam sizes as low as 5 µm.
B.03 Paper at an international scientific conference
COBISS.SI-ID: 6106906The accuracy of quantitative measurement in laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) is strongly related to the availability of matrix-matched standards. The lack of suitable microanalytical certified reference materials (CRM) for samples such as animal tissue implies that they have to be custom-prepared. Many different preparation approaches have been reported such as (1) spiking, homogenizing and cryo-cutting of animal tissues (brain), (2) spiking and drying of hydrocolloid gels, (3) making of pressed pellets based on CRMs powders. Gelatins have widely been used as the material for preparation of standards, but in our opinion not always very successful due to homogeneity issues on the microscale. Problems arise upon drying and setting of gelatin gels during the process of cooling. Molecular species, added as liquid standards to the dissolved heated gelatins, tend to migrate in the process of cooling. To make matters even more complex, different elemental species can migrate differently based on the charge of these species and the isoelectric point (IEP) of the gelatin. In this work we have systematically investigated the factors that affect the migration of elemental species in the gelatin upon setting/drying, to obtain the experimental conditions for preparation of highly homogeneous standards in lateral and vertical direction in dried gelatin films. The different conditions regarding gelatin type, bloom strength, gelatin concentration, acid content and the cooling/drying temperatures will be discussed.
B.03 Paper at an international scientific conference
COBISS.SI-ID: 6246426