Fig 1.
Scanning electron micrograph of Olyset netting illustrating the size of the fiber relative to a 50 μm x 50 μm analysis area.
Image obtained at 5 kV after sputter coating the specimen with gold palladium.
Fig 2.
Negative secondary ion high resolution mass spectra of permethrin and an Olyset fiber surface acquired by ToF-SIMS.
The strong peaks at m/z 35 and 37 correspond to the two naturally occurring stable isotopes of chlorine. The peak at m/z 37 may also include a contribution from C3H- ion. Note that the ratio of 35Cl- and 37Cl- appears to be higher than their naturally abundant 3:1 ratio in the spectrum. For pristine permethrin, this is due to saturated 35Cl- and 37Cl- ion counts to the detector. On pure Permethrin and as received Olyset fiber, 35Cl saturation is observed. However, this signal saturation does not occur for washed and incubated samples because the permethrin concentration is significantly lower. For the Olyset fiber surface, the contribution of C3H- at m/z 37 from HDPE is significant, but the instrument has sufficient mass resolution to separate 37Cl- and C3H-. Separation of 37Cl- and C3H- cannot be shown on this plot, they are too close in mass. This plot is intended to show a wider mass range. However, if we zoom in at the m/z 37 region, we clearly see two individual peaks. The C4H3- ion at m/z 51 was used in this study as a HDPE reference peak.
Fig 3.
Box plots showing the effects of sample pressing and gentle sputtering on the 35Cl-/C4H3- secondary ion intensity ratios from Olyset fibers obtained from ToF-SIMS negative ion high resolution mass spectra.
Each plot represents readings from 20 random locations on a single sample specimen. Analysis area of each location is 500 μm × 500 μm.
Table 1.
Statistical summary of 35Cl-/C4H3- secondary ion ratios for Olyset net as-received (unpressed), and pressed, with and without subsequent sputtering.
Fig 4.
ToF-SIMS negative ion high spatial resolution images showing 35Cl- distribution on Olyset fiber surfaces as received, after acetone wash, and after incubation for 4.5 days at 33°C.
Chlorine regions are shown in green. Analysis area of each location is 400 μm × 400 μm.
Fig 5.
Box plots showing the regeneration of 35Cl- at the surface of Olyset fibers from four specimens (A through D) after washing with acetone and subsequent incubation for 4 and 10 days at 30°C.
Initial readings were carried out after samples were pressed and sputtered. Analysis area of each location is 500 μm × 500 μm.
Table 2.
Statistical summary of 35Cl-/C4H3- intensity ratios for four samples washed and incubated at 30°C.
Fig 6.
ToF-SIMS Depth profiles at two points on a fiber from an unused Olyset net, and on fibers from two depleted nets recovered from household use in rural Kenya.
The fiber was sputtered with 20 nA 3keV Cs+ over 120 μm × 120 μm and the secondary ions were analyzed with 0.3 pA 25 keV Bi3+ after each cycle of sputtering over 50 μm × 50 μm from the center of crater area. Permethrin (PM) concentrations shown for the depleted nets were obtained using conventional gas chromatography.
Fig 7.
Contribution of all of the intensity regions to the overall concentration of chlorine in the sample.
Fig 8.
Cumulative distribution function and fit to the 3-parameter logistic curve.
Fig 9.
Two-dimensional map of data taken at 1.89 μm below the surface of the fiber at a location containing a high-chlorine domain.
Fig 10.
Three-dimensional stacked scatterplot of the points with high 35Cl- secondary ion emissions (Ix ≥ 50) in a 50 μm x 50 μm area of an acetone-washed fiber with the depth of each data point indicated by its darkness.
Data is plotted to simulate the view from the fiber surface into the direction of the core. Data points lying closest to the surface (depth < 0.5μm) are circled for clarity.