**3. Results and discussion**

## **3.1. Optimization of separation and preconcentration method**

#### *3.1.1. Selection of adsorbent and eluent type: univariate approach*

Selection of the adsorbent and eluent type was achieved by packing columns using the ion exchange adsorbent (Dowex 1x8). Each experiment was done in triplicates for each solvent type and adsorbent combination. The flow rates and the sample pH were fixed at 2 mL min−1

for both the sample loading and elution and 7, respectively. The results obtained are presented in **Figure 2**.

Selection of the adsorbent and eluent was done using the absorbance of benzophenone and sulisobenzone at the wavelengths of 260 and 220 nm, respectively. Since the absorbance is directly proportional to concentration, a higher absorbance can be related to higher concentration. Thus, from the univariate optimization, the most effective adsorbent and eluent combination was found to be Dowex 1x8 and methanol (**Figure 2**). This combination was further used for the optimization of the solid phase extraction procedure for both benzophenone (UV-01) and sulisobenzone (UV-02).

Due to the overall charge of adsorbents, the extraction and preconcentration of the analytes was possible by means of ionic interactions. Dowex 1x8 has an overall positive charge, thus the negatively charged analytes could interact with the positive charges of the adsorbent. This resulted in higher absorbances and Dowex 1x8 resin was selected as the suitable adsorbent for further studies.

**2.4. Solid phase extraction procedure**

**Table 1.** Factors and levels used in 23

water and used for validation experiments.

**3. Results and discussion**

**3.1. Optimization of separation and preconcentration method**

*3.1.1. Selection of adsorbent and eluent type: univariate approach*

The columns were prepared according to [30]. Briefly, polyethylene columns of diameter 1.0 cm and 6 cm in height were used for preconcentration. Slurries of 0.5 g of Dowex 1x8 in double distilled deionized water were prepared and packed to columns to heights of about 1 cm. A porous frit was placed at the bottom of the column and at the top of the packing material to hold and confine the adsorbent within the designated capacity/volume. The columns were washed with 6 mL of double distilled deionized water followed by conditioning with 3 mL organic solvent (methanol, ethanol, ethyl acetate or mixture of methanol and acetonitrile) and then 3 mL of double distilled deionized water. Due to the scarcity of reference materials for UV-filters, a commercial sunscreen lotion was used as a reference material for the validation of the SPE/UV method. An appropriate amount of the sunscreen lotion was accurately weighed and dissolved using a small volume of methanol (2 mL) and made to the mark with double distilled deionized

factorial design for extraction and preconcentration of UV filters.

**Factors Low level (−1) Central point (0) High level (+1)**

Sample pH 4 7 10 Sample flow rate (SFR) (mL min−1) 2 3.5 5 Eluent flow rate (EFR) (mL min−1) 1 2 3

48 Emerging Pollutants - Some Strategies for the Quality Preservation of Our Environment

An aliquot (10 mL) of model solution containing benzophenone and sulisobenzone at a concentration of 100 μg L−1 was passed through a packed column at a flow rate of 2–5 mL min−1. After percolating the synthetic samples through, the cartridges were washed with 3 mL double distilled deionised water. Then retained analytes were eluted with 2 mL organic solvents. The optimization of the solid phase extraction method was carried out using a 23 full factorial design involving three variables such as pH, sample flow rate (SFR) and eluent flow rate (EFR). Maximum, central point and minimum levels are presented in **Table 1**. The second step of the optimization strategy involved the application of a response surface methodology (RSM) based on a central composite design. All the experiments were carried out in random order and the experimental data was processed by using the Minitab 17 software program.

Selection of the adsorbent and eluent type was achieved by packing columns using the ion exchange adsorbent (Dowex 1x8). Each experiment was done in triplicates for each solvent type and adsorbent combination. The flow rates and the sample pH were fixed at 2 mL min−1

The polarity of the solvent was the contributing factor on the elution of the analytes, the more polar solvent resulted in better elution from the SPE column, and hence higher absorbances for methanol were observed when used in combination with Dowex 1x8. This was a consequence of the methanol having the ability to displace the analytes from the positively charged Dowex 1x8 adsorbent. The 1:1 mixture of acetonitrile and methanol also had promising results, but did not perform better than the methanol alone. Acetonitrile could not be used for this study even though it is more polar than methanol, there are dangers associated with the use of pure acetonitrile like the risk of cyanide poisoning as a result of its decomposition products.

Experimental conditions; mass of adsorbent 0.5 g, sample volume 10 mL at 100 μg L−1, eluent volume 2 mL, pH 7, flow rates 2 mL min−1. MeOH = methanol, ACN = acetonitrile, EtOH = ethanol.

**Figure 2.** Selection of suitable adsorbent and eluent combination in model aqueous solution.
