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The partition coefficient is the equilibrium distribution of an analyte between the sample phase and the gas phase.
Samples must be prepared to maximise the concentration of the volatile components in the headspace and minimise unwanted contamination from other compounds in the sample matrix. To help determine the concentration of an analyte in the headspace, you will need to calculate the partition coefficient (K).
Partition Coefficient (K) = Cs/Cg
where:
Cs is the concentration of analyte in sample phase;Cg is the concentration of analyte in gas phase
Compounds that have low K values will tend to partition more readily into the gas phase and have relatively high responses and low limits of detection.
An example of this would be hexane in water: at 40°C, hexane would have a K value of 0.14 in an air-water system.
Compounds that have high K values will tend to partition less readily into the gas phase and have relatively low responses and high limits of detection.
An example of this would be ethanol in water: at 40°C, ethanol has a K value of 1355 in an air-water system.
Partition coefficient values for other common compounds are shown in the table on the right.
Sensitivity is increased when K is minimised
Sensitivity is increased as the partition coefficient is decreased and volatiles can more readily enter the gas phase. This is illustrated by the graph on the left.
K can be lowered by changing the temperature at which the vial is equilibrated or by changing the composition of the sample matrix. In the case of ethanol, K can be lowered from 1355 to 328 by raising the temperature of the vial from 40°C to 80°C.
The partition coefficient may also be changed by adding salts or by changing the Phase Ratio. These will be examined in the next two sections.
