From the observations above obtained with various configurations, we concluded that OH− was much more effective than Cl− or other anions in driving the extraction of not only anionic arsenic species but also neutral As(III) species. Therefore, 100 mM NaOH (pH 13) was chosen as the optimal acceptor phase for the enrichment of arsenic species using Aliquat 336 as a carrier in the organic phase.
3.2. Optimization of other extraction conditions
When the concentration of Aliquat 336 in the organic layer was increased, the EF values were increased but the drop became unstable due to the amphiphilic nature of Aliquat 336. We chose 50 mM of Aliquat 336 in octanol as optimal. In addition, a capillary of 25 μm id was chosen to improve the drop stability by increasing the end surface area for the drop attachment, as compared to the most widely used one of 50 μm id. Another advantage of a capillary with a smaller id combined with a longer length was the increased level of precision in controlling the drop geometry with a given pressure control precision level of a commercial CE instrument. However, one drawback was the relatively low detection sensitivity.
As the extraction time was increased from 5 to 15 min, the EF values increased linearly with the extraction time as shown in Fig. 7. When the time exceeded 15 min, however, the extraction results became unreliable due to the instability of the drop. We chose 15 min as the optimal extraction time.
From the observations above obtained with various configurations, we concluded that OH− was much more effective than Cl− or other anions in driving the extraction of not only anionic arsenic species but also neutral As(III) species. Therefore, 100 mM NaOH (pH 13) was chosen as the optimal acceptor phase for the enrichment of arsenic species using Aliquat 336 as a carrier in the organic phase.
3.2. Optimization of other extraction conditions
When the concentration of Aliquat 336 in the organic layer was increased, the EF values were increased but the drop became unstable due to the amphiphilic nature of Aliquat 336. We chose 50 mM of Aliquat 336 in octanol as optimal. In addition, a capillary of 25 μm id was chosen to improve the drop stability by increasing the end surface area for the drop attachment, as compared to the most widely used one of 50 μm id. Another advantage of a capillary with a smaller id combined with a longer length was the increased level of precision in controlling the drop geometry with a given pressure control precision level of a commercial CE instrument. However, one drawback was the relatively low detection sensitivity.
As the extraction time was increased from 5 to 15 min, the EF values increased linearly with the extraction time as shown in Fig. 7. When the time exceeded 15 min, however, the extraction results became unreliable due to the instability of the drop. We chose 15 min as the optimal extraction time.
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