Understanding the chemical properties of the compounds in your sample matrix, both the ones you want to detect and the ones you want to eliminate, is necessary for successful method development. Is the molecule acidic or basic? What functional groups are present, and can they be ionized? Is the molecule hydrophilic or hydrophobic? In this post, we discuss logP, and its role in sample preparation.
So, what exactly is logP?
LogP, or octanol-water partition coefficient, is a measure of how hydrophilic or hydrophobic a molecule is. It indicates how readily an analyte will partition between an aqueous and organic phase. A more polar, hydrophilic compound will have a lower logP (the value can even be negative) and prefer to “reside” in the aqueous phase. More non-polar, hydrophobic compounds will have a higher logP, and will partition into an organic phase. Typical values range from -3 (polar) to 7 (non-polar).
Why do we need sample prep?
The goal of sample preparation is to create cleaner samples, collecting the compounds of interest and eliminating interferences – that “junk” we don’t care about the can cause ion suppression and matrix effects that affect sensitivity, accuracy and precision. The ideal sample prep method removes all interfering compounds and produces 100% recovery of all analytes of interest. The problem is that many interfering compounds have properties that are similar to the compounds we need to detect and quantitate. It takes some skill and knowledge to develop a method that washes interfering compounds away and elutes the analytes of interest in a separate step. You need to make sure your washes don’t elute the compounds you care about, and you don’t want interfering junk eluting with your analytes.
We have prepared for you plenty of interesting resources on extraction techniques and method optimization.
What’s the role of logP in sample prep methods?
- Supported Liquid Extraction (SLE) methods:
LogP gives you an indication of how well the molecule will partition into the organic elution solvent. The higher the logP, the more likely the compound will partition from the SLE phase into the organic elution solvent like dichloromethane, ethyl acetate or MTBE. Compounds with a low or negative logP will not partition into the organic elution solvent as well. SLE works best with more hydrophobic compounds, but more polar compounds can be recovered by modifying the elution solution with 2-10% of a more polar solvent like 2-propanol or acetonitrile. Make sure the elution solvent is still immiscible with water. Acid or base pretreatment of the sample can also influence the partitioning and improve elution of more hydrophilic compounds or increase retention of interfering compounds.
- Solid Phase Extraction (SPE) methods:
As long as you don’t use an organic wash solvent that is strong enough to elute your compounds you can get cleaner samples using SPE than with SLE. If you’re a chromatographer, think about where your compounds elute on an LC column. If they elute late in a reverse phase gradient separation where the organic mobile phase content is high, you can probably use a stronger organic wash and still retain your compounds for subsequent elution. If they elute from the LC column using a mobile phase with low organic content, you may only be able to use a wash with 5% or 10% organic in water (v/v) or no organic wash at all. If the compounds can be ionized, ion exchange SPE is a good option. Another good choice for hydrophobic compounds is reverse phase SPE. Retention of the analytes is based on logP. Aqueous and organic washes can be chosen to remove interferences.
The take home message is: Know your compounds! Look up their logP to understand their hydrophobicity, reverse phase retention properties, and formulate your sample prep conditions from this information.
Do you want to know more about how to choose between SLE and SPE extraction methods?