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Levels of independent variables for PBD design.
| Factors | Levels | |
|---|---|---|
| Low (−1) | High (+1) | |
| Volume of sample (mL) | 5 | 10 |
| Ionic strength | 0 | 10 |
| pH | 5 | 8 |
| Extraction time (min) | 30 | 60 |
| Extraction speed (rpm) | 500 | 1000 |
| Elution time (min) | 5 | 15 |
| Derivatization time (s) | 30 | 60 |
