## 3 Results
### Alignment of SIMS Images  
In this study, SIMS images were obtained through a multiplane approach achieved via scanning the surface at X and Y with the primary Cs\({}^{+}\) beam. Each plane corresponded to sputtered slices along the Z-axis. In the first approach, the analyzed planes were stacked and summed to accumulate the secondary ion signal on a \"sum\" image compiling the data over the entire analyzed depth of the section.  
The \"sum\" image obtained via SIMS at a mass of \({}^{12}\)C\({}^{14}\)N (Figure 2A) represents the biological material rich in nitrogen and carbon.  
Figure 1: Characterization of TiO\({}_{2}\) NPs via TEM and DRX analysis. **(A)** Diffractogram at 26 angles with black line as the signal intensity, stearic acid (CH\({}_{3}\)(CH\({}_{2}\))\({}_{16}\)COOH) as a reference (the green vertical bar). Compared with anatase picks (the blue vertical bar), eleven picks (the red vertical bar) indicate a rutile structure. **(B)** TEM image of NPs: 1.4 \(\times\) 1.08 μm; 672 \(\times\) 518 pixels; scale bar: 250 nm.  
Figure 2: Comparison of NanoSMS50 ”sum” images at a \({}^{12}\)C\({}^{14}\)N mass of skin/NP samples without drift correction and with drift correction. The gray scales are the \({}^{12}\)C\({}^{14}\)N signal intensities in the number of counts: **(A)** without drift correction and **(B)** with drift correction. **(C)** Profiles of the \({}^{12}\)C\({}^{14}\)N signal intensities along the green line for **(A)** and the red dashed line for **(B)**. Dwell time: 2 ms/pixel; raster: 10 μm \(\times\) 10 μm, 256 \(\times\) 256 pixels, sum of 20 plans. The arrow indicates sunscreen on the skin’s surface.  
This image shows corneocytes (the outermost structures of the skin, characteristic of the peeling part) in light gray; they are clearly distinguishable from the sunscreen organic matter that is poorer in carbon and nitrogen (the arrow). In this \"sum\" image, the corneocytes appear blurred (SIMS signal spread) following the stacking of the 20 images acquired during the multiplane analysis (20 successive planes). This blur may result from either an acquisition drift occurring plane by plane or a genuine morphological difference, such as a change in diameter, among the corneocytes in each plane. However, at the end of 20 planes, the sputtered section thickness was about 7.4 nm. Thus, it was reasonable to assume that the diameter or thickness of the corneocytes, which were on the order of one micrometer, did not significantly vary within this sputtered thickness to account for the blur. The corneocytes could, therefore, be used as a reference to align the SIMS images. The resultant image (Figure 2B) exhibits reduced blurriness. Profiles (Figure 2C) were drawn at the same coordinates on the two images (the line) without alignment (Figure 2A) and with alignment (Figure 2B). They show that the width of the corneocytes (estimated via the width of the corresponding peaks) was reduced by 8 \(\pm\) 4% (the average calculated on five different profiles) via the alignment. The contrasts at the edges of the corneocytes (estimated using the derivatives) were more pronounced with alignment (a peak growth rate of 27 \(\pm\) 17%), and the maximum number of counts in the \"sum\" image with alignment was higher than in the \"sum\" image without alignment. These results confirm the reduction in blurring and the alignment efficiency. This alignment procedure was systematically applied to all multiplane analyses.