Variable

Level

−1

0

+1

Concentration of sodium fluoride, %w/v

1% w/v

2% w/v

3% w/v

Concentration of PVP, %w/v

0

1

2

Volume of sample solution, mL

10 mL

20 mL

30 mL

Drying surface area, cm²

38 cm²

79 cm²

143 cm²