3
Figure 1
Phases of the headspace vial.
Equation 1
PartitionCoefficient (K) =C
s
/C
g
Equation 2
PhaseRatio (
β
) =V
g
/V
s
C
s
=concentration of analyte in sample phase
C
g
=concentration of analyte in gas phase
V
s
=volume of sample phase
V
g
=volume of gas phase
Figure 2
K and
β
are important variables in
headspace analysis.
Solvent
KValue
cyclohexane
0.077
n-hexane
0.14
tetrachloroethylene
1.48
1,1,1-trichloromethane
1.65
o-xylene
2.44
toluene
2.82
benzene
2.90
dichloromethane
5.65
n-butyl acetate
31.4
ethyl acetate
62.4
methyl ethyl ketone
139.5
n-butanol
647
isopropanol
825
ethanol
1355
dioxane
1618
Table I
KValues of Common Solvents inAir-
Water Systems at 40°C
Basic Principles of Headspace Analysis
Most consumer products and biological samples are composed of awide variety of
compounds that differ inmolecular weight, polarity, and volatility. For complex
samples like these, headspace sampling is the fastest and cleanestmethod for
analyzing volatile organic compounds.A headspace sample is normally prepared in
a vial containing the sample, the dilution solvent, amatrixmodifier, and the
headspace (see
Figure 1
).Volatile components from complex samplemixtures can
be extracted from non-volatile sample components and isolated in the headspace or
vapor portion of a sample vial.An aliquot of the vapor in the headspace is delivered
to aGC system for separation of all of the volatile components.
In order to achieve the best performancewhen using headspace/GC, careful atten-
tion should be used in sample preparation and instrument setup. Key issues to
addresswhen setting up headspace/GC systems includeminimizing system dead
volume, maintaining inert sample flow paths, and achieving efficient sample
transfer. These issues, aswell as other instrument setup-related topics, are addressed
later in the
SystemOptimization
section of this guide.
Samplesmust be prepared tomaximize the concentration of the volatile components
in the headspace, andminimize unwanted contamination from other compounds in
the samplematrix. To help determine the concentration of an analyte in the
headspace, youwill need to calculate the partition coefficient (K), which is defined
as the equilibrium distribution of an analyte between the sample phase and the gas
phase (
Figure 2
).
Partition Coefficient
Compounds that have lowK valueswill tend to partitionmore readily into the gas
phase, and have relatively high responses and low limits of detection (
Figure 3
).An
example of thiswould be hexane inwater: at 40°C, hexane has aK value of 0.14 in
an air-water system. Compounds that have highK valueswill tend to partition less
readily into the gas phase and have relatively low response and high limits of
detection.An example of thiswould be ethanol inwater: at 40°C, ethanol has aK
value of 1355 in an air-water system. Partition coefficient values for other common
compounds are shown in
Table I
.
volatile
analytes
sample, dilution
solvent, andmatrix
modifier
}
}
G
S
G=the gas phase (headspace).
The gas phase is commonly referred to as the headspace
and lies above the condensed sample phase.
S=the sample phase.
The sample phase contains the compound(s) of interest
and is usually in the form of a liquid or solid in combina-
tionwith a dilution solvent or amatrixmodifier.
Once the sample phase is introduced into the vial and the
vial is sealed, volatile components diffuse into the gas
phase until the headspace has reached a state of equilib-
rium as depicted by the arrows. The sample is then taken
from the headspace.
