Agilent Technologies 7697A Advanced Operation Manual page 31

Analyte
Toluene
Toluene
Ethanol
Ethanol
Ethanol
Advanced Operation
Controlling K
When optimizing a headspace analysis, first consider the
partition coefficient of the solvent. The table below lists the
K values for several common solvents at 25 °C.
Solvent
Decane
Water
Decane
Water
Water, saturated with Na SO
2
At higher temperatures, K will decrease. At 40 °C, the K
value for ethanol in water is ~1350. At 80 °C, the K value
lowers to ~330.
As can be seen from the table, K also depends on both the
analyte and the matrix. Note the change in K for the
ethanol- water system compared to the similar system
saturated with Na
2
Therefore, to improve the concentration of analyte in the
headspace volume, heat the sample. If needed, consider
changing the solvent (if possible), or consider addition of an
inorganic salt to lower the solvent K value.
The other factor to manipulate to increase sensitivity is the
phase ratio, V /V . Recall the vapor phase concentration
G L
equation:
C
O
C =
G
V
G
K +
V
L
Where K is small, reducing the phase ratio will produce a
higher concentration of analyte in the headspace volume.
The 7697A can use a variety of sample vials. Select a sample
vial and sample amount to create a higher concentration of
analyte.
Where K is large, reducing the phase ratio results in less
gain.
K (25 °C)
~3000
~4
~60
~5000
~300
4
SO .
4
3
Method Development
31