Direct Additive Detection in Polymer Films via Platinum-Assisted SALDI Mass Spectrometry Imaging

Abstract

In this study, we employed platinum-assisted surface-assisted laser desorption/ionization mass spectrometry imaging (MSI) (Pt-SALDI-MSI) to detect and visualize the spatial distribution of antioxidant additives and organic dyes in polystyrene films undergoing photodegradation. In traditional matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS), matrix-derived ion peaks often obscure signals from low-molecular-weight analytes. Pt-SALDI-MSI, which utilizes inorganic nanoparticles instead of an organic matrix, enables the interference-free analysis of low-molecular-weight compounds, thereby addressing the limitation of traditional MALDI-MS. Using Pt-SALDI-MSI, we observed the degradation and distribution of Irganox 1098 (an antioxidant) and crystal violet (an organic dye) following ultraviolet irradiation. This method effectively captures the photodegradation process, providing valuable insights into the environmental breakdown of plastics and the formation of microplastics.

Keywords: SALDI-MS; additives; mass spectrometry imaging; platinum vapor deposition; polymer.

Fig. 1. Pt-SALDI-MS mass spectra of polystyrene films containing various concentrations of Irganox 1098 with (A) 5 wt%, (B) 1 wt%, and (C) 0.1 wt%. Pt-SALDI-MS, platinum-deposited surface-assisted laser desorption/ionization mass spectrometry.

Fig. 2. Pt-SALDI-MS spectra of a polystyrene film containing 1 wt% Irganox 1098 and 1 wt% Irgafos 168 (A) before and (B) after UV irradiation. (C) Ion images of the polystyrene film containing 1 wt% Irganox 1098 and 1 wt% Irgafos 168 before and after UV irradiation using Pt-SALDI-MSI (the Na⁺ adduct of Irganox 1098 at m/z 660 ± 3.3, laser interval: 100 μm). Pt-SALDI-MS, platinum-deposited surface-assisted laser desorption/ionization mass spectrometry; Pt-SALDI-MSI, platinum-deposited surface-assisted laser desorption/ionization imaging mass spectrometry; UV, ultraviolet.

Fig. 3. Pt-SALDI-MS spectra of a polystyrene film containing 0.5 wt% CV dye before irradiation (No UV) and after 10, 20, and 30 minutes of UV exposure. Peaks from degradation products of the dye after UV exposure at m/z ②358, ③344, ④330, and ⑤316, besides the molecular ion at ①m/z 372. CV, crystal violet; Pt-SALDI-MS, platinum-deposited surface-assisted laser desorption/ionization mass spectrometry; UV, ultraviolet.

Fig. 4. Ion images obtained by Pt-SALDI-MSI (laser interval: 300 μm) for the polystyrene film containing 0.5 wt% CV after UV irradiation for 10 min (B)–(F). (B) m/z 372 ± 0.9, (C) m/z 358 ± 0.9, (D) m/z 344 ± 0.9, (E) m/z 330 ± 0.8, and (F) m/z 316 ± 0.8. The photograph of polystyrene film is also shown in (A). The contrast of the image inside the frame in (A) differs from that outside the frame because the image within the frame is a photograph of the sample, adjusted to enhance its visibility. The width of the slit is 2.0 mm. The scale bar is 4 mm. CV, crystal violet; Pt-SALDI-MSI, platinum-deposited surface-assisted laser desorption/ionization imaging mass spectrometry; UV, ultraviolet.

Fig. 5. Ion images obtained by Pt-SALDI-MSI (laser interval: 300 μm) for the polystyrene film containing 0.5 wt% CV after UV irradiation for 20 min (B)–(F). (B) m/z 372 ± 0. 9, (C) m/z 358 ± 0. 9, (D) m/z 344 ± 0. 9, (E) m/z 330 ± 0. 8, and (F) m/z 316 ± 0.8. The photograph of polystyrene film is also shown in (A). The contrast of the image inside the frame in (A) differs from that outside the frame because the image within the frame is a photograph of the sample, adjusted to enhance its visibility. The width of the slit is 2.0 mm, The scale bar is 4 mm. CV, crystal violet; Pt-SALDI-MSI, platinum-deposited surface-assisted laser desorption/ionization imaging mass spectrometry; UV, ultraviolet.