Repeatability of n-octanol/water partition coefficient values between liquid chromatography measurement methods

Area of Science:

• Environmental Chemistry
• Analytical Chemistry
• Chemical Risk Assessment

Background:

• The n-octanol/water partition coefficient (KOW) is crucial for environmental risk and exposure assessments, estimating properties like solubility, bioavailability, and toxicity.
• Accurate KOW values are essential for quantitative structure-activity relationships (QSARs) but can be challenging to obtain for certain hydrophobic compounds.
• This study focuses on pyrethroid insecticides (cypermethrin, bifenthrin) and polycyclic aromatic sulfur heterocycles (dibenzothiophene and its alkyl derivatives).

Purpose of the Study:

• To compare the KOW measurement outcomes from three common literature methods for six hydrophobic chemicals.
• To evaluate the reliability and reproducibility of KOW values obtained using different techniques.

Main Methods:

• KOW values were determined using two High-Performance Liquid Chromatography (HPLC) methods (isocratic and gradient programming) and the slow-stirring method.
• Methods followed OECD guidelines 117 (chromatographic) and 123 (slow-stirring).
• Measurements were performed on bifenthrin, cypermethrin, dibenzothiophene (DBT), methyl-DBT, dimethyl-DBT, and diethyl-DBT.

Main Results:

• Mean log KOW values varied significantly between the methods, although reproducibility within each method was high.
• For example, HPLC with gradient programming yielded log KOW values of 8.4 ± 0.1 for bifenthrin and 6.0 ± 0.3 for cypermethrin.
• Dibenzothiophene derivatives showed increasing log KOW values with alkyl substitution, ranging from 4.8 ± 0.0 for DBT to 6.8 ± 0.0 for diethyl-DBT.

Conclusions:

• Significant discrepancies in KOW values obtained by different methods were observed.
• The study suggests that KOW values are not always interchangeable between methods.
• Caution is advised when assessing chemical properties and environmental risks based solely on KOW values without considering the measurement methodology.