20 STIBOREK, PREISLER, SHEKARGOFTAR, KANICKÝ, AND KELAR Table 1: Ablation parameter settings monitored in addi- tion to the other isotopes of interest. Isotope integration time 0.1 s Flow rate of the He carrier gas 1.0 l·min−1 Flow rate of the Ar auxiliary gas 0.6 l·min−1 Flow rate of the Ar plasma gas 15 l·min−1 Laser pulse repetition rate 10 Hz Laser-beam waist 250 µm Laser fluence ∼ 0.75 J·cm−2 ity of the modification are the voltage and frequency of the electromagnetic field, the composition of the gaseous atmosphere, the exposure time and the geometries of the sample and electrodes. 2. Experimental In the first experiment, a DCSBD device (RPS400- Roplass plasma system 400 W, Roplass, Czech Repub- lic) was used to increase the surface energy of the PET substrate (Bayer MaterialScience, Germany). The sub- strate surface was modified at 300 W for one, five and ten seconds. Subsequently, 1µl of 0.5 mM trisodium citrate (Sigma-Aldrich, USA) solution was deposited onto ten replicates of the PET substrate as a model biological sam- ple. The contact angles were measured by the instrument (see System E, Advex Instruments, Czech Republic) us- ing a technique called “sitting droplet on a solid foun- dation”. The device consists of a charge-coupled device (CCD) camera placed on a sliding table in front of which a substrate plate is positioned. Using a micropipette, a 1µl droplet of the sample is deposited onto the substrate and the See System software calculates the contact angle based on the interpolation of the height and width of the arc that describes the shape of the drop. The size of the contact angle was evaluated immediately after modifica- tion of the substrate surface as well as after one, seven and 31 days. Based on the previous experiment, where the aging of the modified surface was monitored by varying the con- tact angle and size of the stain, depending on the dropping time that elapsed from the surface modification, a com- plementary experiment was performed. 1mL of trisodium citrate sample was mixed with cadmium and indium stan- dards (aqueous calibration solutions, certified reference materials (CRM), Analytika, Czech Republic) so that the resulting solution contained 400 ng·l−1 of these stan- dards. Five replicates of the 200 nl droplet sample so- lutions prepared were immediately applied using a mi- cropipette to the surface of the modified PET substrate as well as one, three and six days after the modification. The Cd and In standards, chosen because of their sim- ilar atomic masses and first ionization energies, were used Figure 1: Dependence of the contact angle of droplets of 0.5 mM trisodium citrate solution on the PET sur- face modified by DCSBD after one, five and ten seconds; droplets were deposited immediately as well as one, seven and 31 days after the modification. to correct the intensity of the monitored signals. The pur- pose of this experiment was to demonstrate whether ag- ing of the substrate affects the intensities of the signals. At the same time, the size of the stains was measured. Ide- ally, the application conditions should be identical to the experiment where aging was observed by measuring con- tact angles. However, this was not possible due to differ- ent sample-volume requirements for the given measuring technique. 1 µl was required to measure contact angles and 0.2 µl for SALD ICP-MS. The plate with samples was then inserted into the ablation cell (UP213 model, New Wave Research, Inc., USA) equipped with a 213-nm pulsed Nd:YAG laser, 3D positioning system and built-in CCD camera for visual control of the ablated samples. The dry aerosol created during ablation was analysed by the ICP mass spectrom- eter (Agilent 7500ce ICP-MS, Agilent, USA). In order to determine the beginning and end of the ablation process, the 13C isotope was isotope was monitored in addition to the other isotopes of interest. For the ablation of the stains, a “zig-zag” ablation tra- jectory was selected with a 170 µm ablation line and a 160 µm·s−1 scanning speed. The remaining basic abla- tion parameters are shown in Table 1. 3. Results and discussion First, the contact angle formed by 1 µl droplets of 0.5 mM trisodium citrate onto a PET substrate surface was evaluated immediately after modification of the substrate surface as well as after one, seven and 31 days. Ten repli- cates of the substrate surface were modified for one, five and ten seconds. Fig. 1 shows that the droplets of 0.5 mM trisodium citrate solution that were applied to the substrate surface of PET modified by DCSBD for one second exhibited the greatest contact angle, hence the least wettability; i.e., a longer duration of DCSBD modification causes a higher degree of wettability. It is also evident from Fig. 1, how- Hungarian Journal of Industry and Chemistry 22 STIBOREK, PREISLER, SHEKARGOFTAR, KANICKÝ, AND KELAR strate surface. However, the disadvantage of this tech- nique is the aging of the treated surface, i.e. the return of the modified physicochemical properties of the sub- strate surface to their original state prior to the modifi- cation. The experiments showed that the integrated sig- nal intensities of the selected analytes, 111Cd and 115In, present in the desorbed spots of 0.5 mM trisodium cit- rate solution, that were deposited onto the substrate sur- face at various time intervals following its modification differed. Also, the repeatability of the analysis of individ- ual samples expressed as RSDs changes with deposition time. RSD values were approximately 30% for 0.5 mM trisodium citrate samples. When quantifying the ratio of integrated 115In/111Cd signals, it was found that the val- ues did not differ significantly from each other and could be used in quantitative analysis. Using internal standards, RSD values were reduced to less than 10% for trisodium citrate samples. Acknowledgement We gratefully acknowledge the financial support of the Czech Science Foundation (15-05387S and 18-16583S) and the Ministry of Education, Youth and Sports of the Czech Republic under the project CEITEC 2020 (LQ1601). 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