## 3 Results and Discussion
### Raman Analysis of Irradiated PE  
To assess radical-induced surfaces changes of the PE microplastics, analyses were conducted using Raman spectroscopy and bright field and dark field microscopy. When the irradiated PE microplastics were viewed under the stereomicroscope at 45\(\times\) magnification,  
Figure 3: Integrated areas from GCMS peaks for _n_-dodecane extracted (SPME) from: (**A**) 3.7 ppm dodecane in water; (**B**) original PE in water, \(\sim\)0.5 MGy; (**C**) original PE in water, \(\sim\)2 MGy; (**D**) original PE in saltwater, \(\sim\)0.5 MGy; (**E**) original PE in water, no irradiation. The vertical axis represents 10\({}^{9}\) integration values.  
many particles appeared to have crosslinked. Figure 4 shows representative examples of PE microplastics before and after solution gamma irradiation. Magnified PE particles observed under dark field microscopy for irradiations of 0.5-2.0 MGy suggest induced surface divets and/or crosslinked particles.  
Figure 5 shows representative Raman spectra for non-irradiated PE (black trace) and irradiated PE samples. The Raman signals, exemplifying chemical structures, are essentially the same for all of the samples. Further examination of the extended scan of the Raman fingerprint region out to the CH stretch bands indicates an increase in the fluorescence background for the irradiated PE samples. Such a fluorescence increase can indicate the formation of defects or other impurities on the PE surface. However, no impurities were identified. The absence of data showing chemical changes of the hydrocarbon backbone suggests that PE microplastics will persist, and possibly combine to form larger particles when in contact with other microplastics in low O\({}_{2}\) to anerobic aqueous environments. These data support the expected reactivity in Equations (3) and (5), crosslinking and fragmentation, predicted to primarily occur under the low O\({}_{2}\) conditions.  
For each irradiated PE sample, high spatial resolution data were collected using a 100\(\times\) objective, with an estimated sampling depth of 1.3 \(\upmu\)m, from twenty locations on a single particle. The CH\({}_{2}\) twisting band intensity at 1296 cm\({}^{-1}\) was used to normalize  
Figure 4: PE microplastic particles viewed using dark field microscopy. The left image is a non-irradiated PE particle, and the right image is post-high-dose irradiation.  
Figure 5: Raman spectra using 475 nm excitation of gamma-irradiated PE in fresh water (red trace), salt water (green trace), and Lake Michigan water (blue trace) show substantial fluorescence in comparison to the stock PE (black trace).  
each spectrum for comparison. Typical spectra are shown in Figure 6 and additional spectra are included in the Supplementary Information. Analysis of the CH\({}_{2}\) bending and twisting bands can be used to calculate the fractions of amorphous and crystalline phases. Crystalline-phase PE contributes to the CH\({}_{2}\) bending band at 1416 cm\({}^{-1}\) while the amorphous phase is indicated by a CH\({}_{2}\) bending band at 1440 cm\({}^{-1}\) and a shoulder on the CH\({}_{2}\) twisting band at 1303 cm\({}^{-1}\)[52]. Minimal changes were seen in the calculated fraction for the amorphous phase, suggesting that at these radiation doses in water media, the PE does not undergo any significant structural changes. This is consistent with the SPME headspace extraction results which did not find the formation of smaller molecules or PE degradation products at the total absorbed doses between 0.5-2.0 MGy.