Environmental Engineering Reference
In-Depth Information
based on their boiling points and vapor pressure, but are relatively water soluble.
The EPA recommends 80 C purge conditions along with a specific capillary
column (DB-WAX) for improved recoveries. As shown in Figure 7.14, P&T is often
coupled with GC and programmed to operate first in the purge mode (10-12 min)
and then in desorbing mode (1-2 min).
7.6.2 Static Headspace Extraction
In the static headspace extraction (SHE), two phases are in equilibrium in a
sealed vial. In Figure 7.14b, the gas phase (g) is commonly referred to as the
headspace and lies above the condensed sample phase (s). The sample phase
(solid or liquid) initially contains the compound(s) of interest. 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 equilibrium
as depicted by the arrows. The sample is then taken from the headspace either
manually or by an automated headspace sampler. The partition coefficient (K)is
defined as:
K ¼ C g =
C s
ð7
:
16Þ
where C g is the concentration of analyte in the gas phase and C s is the concentration
of analyte in sample phase. Compounds that have high K values 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 hexane in water; at 40 C, hexane has a K
value of 7.14 in an air-water system. 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 ethanol in water; at 40 C, ethanol
has a K value of 0.00075 in an air-water system. Another parameter related to the
partitioning is the phase ratio, defined by:
b ¼ V g =
V s
ð7
:
17Þ
where V s and V g are the volumes of sample phase and vapor phase, respectively.
Lower values of b (i.e. larger sample size) will yield higher responses for
volatile compounds. By using the mass balance equation (Eq. 7.18) and
combining the partition coefficient (K) and phase ratio (b), the final
concentration of volatile compounds (C g ) in the headspace of sample vials can
be determined.
C g
K V s þC g V g
C 0 V s ¼
ð7
:
19Þ
C 0
C g ¼
ð7
:
20Þ
1
=
K þb
where C g is the concentration of volatile analytes in the gas phase and C 0 is the
original concentration of volatile analytes in the sample. Since C g can be
 
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