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Reconsideration of sample pH adjustment in bioanalytical liquid–liquid extraction of ionisable compounds

Liquid–liquid extraction (LLE) is widely used as a simple and robust sample preparation technique in bioanalytical sample preparation. When extracting ionisable compounds, most bioanalysts adjust the pH of the sample to achieve fully unionized compounds. Usually, a generally accepted rule is applied...

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Published in:Journal of chromatography. B, Analytical technologies in the biomedical and life sciences Analytical technologies in the biomedical and life sciences, 2007-06, Vol.853 (1-2), p.234-241
Main Authors: Hendriks, G., Uges, D.R.A., Franke, J.P.
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description Liquid–liquid extraction (LLE) is widely used as a simple and robust sample preparation technique in bioanalytical sample preparation. When extracting ionisable compounds, most bioanalysts adjust the pH of the sample to achieve fully unionized compounds. Usually, a generally accepted rule is applied to adjust the pH of the aqueous phase, known as the pKa±2 rule, depending on the acid/base characteristics of the analyte. By taking a closer look at the general equations that describe the extraction behaviour of ionisable compounds, we extended this pH adjustment rule by taking the distribution ratio and the volume of both liquid phases into account. By choosing an extraction pH based on this extended rule, the selectivity of the extraction can be influenced without loss of recovery. As a measure of this selectivity, two equations were proposed to indicate the ability of the extraction system to discriminate between two compounds. Also, milder extraction pH can be used for pH labile analytes. To use this new rule quantitatively, a new calculation method for the determination of the distribution ratio was derived. These calculations were based on normalized recoveries making this method less susceptible to errors in absolute recovery determination. The proposed equations were supported by demonstrating that careful pH adjustment can lead to higher selectivity. The main conclusion was that a closer look at the extraction pH in bioanalytical methods extends the possibilities of obtaining a higher selectivity or the possibilities of extracting pH labile analytes at milder pH conditions without loss of recovery.
doi_str_mv 10.1016/j.jchromb.2007.03.017
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subjects Algorithms
Analysis
Analytical, structural and metabolic biochemistry
Bioanalysis
Biological and medical sciences
Chemistry Techniques, Analytical - methods
Chromatography, High Pressure Liquid - methods
Fundamental and applied biological sciences. Psychology
General pharmacology
Hydrogen-Ion Concentration
Kinetics
Liquid–liquid extraction
Medical sciences
Modelling
pH adjustment
Pharmacology. Drug treatments
Recovery
Selectivity
Solvents - chemistry
title Reconsideration of sample pH adjustment in bioanalytical liquid–liquid extraction of ionisable compounds
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