4
(Ideal)
K
C
g
(Ideal)
Figure 3
Sensitivity is increasedwhenK
is minimized.
K and
β
Figure 5
LowerK and
β
result in higher C
g
and better sensitivity.
ammonium chloride
ammonium sulfate
sodium chloride
sodium citrate
sodium sulfate
potassium carbonate
Table II
Common salts used to decrease
matrix effects.
(Ideal)
ββ
C
g
Figure 4
Sensitivity is increasedwhen
β
is minimized.
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C
g
K can be lowered by changing the
temperature at which the vial is equili-
brated or by changing the composition of
the samplematrix. 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. It can be lowered even
further by introducing inorganic salt into
the aqueous samplematrix. High salt
concentrations in aqueous samples
decrease the solubility of polar organic
volatiles in the samplematrix and promote their transfer into the headspace,
resulting in lower K values. However, themagnitude of the salting-out effect onK is
not the same for all compounds. CompoundswithK values that are already rela-
tively lowwill experience very little change in partition coefficient after adding a
salt to an aqueous samplematrix. Generally, volatile polar compounds in polar
matrices (aqueous samples) will experience the largest shifts inK and have higher
responses after the addition of salt to the samplematrix.
Table II
listsmost of the
common salts used for salting-out procedures.
PhaseRatio
The phase ratio (
β
) is defined as the relative volume of the headspace compared to
volume of the sample in the sample vial (
Figure 2
). Lower values for
β
(i.e., larger
sample size) will yield higher responses for volatile compounds (
Figure 4
). How-
ever, decreasing the
β
valuewill not always yield the increase in response needed to
improve sensitivity.When
β
is decreased by increasing the sample size, compounds
with highK values partition less into the headspace compared to compoundswith
lowK values, and yield correspondingly smaller changes inC
g
. Samples that contain
compoundswith highK values need to be optimized to provide the lowest K value
before changes aremade in the phase ratio.
CombiningK and
β
Partition coefficients and phase ratios
work together to determine the final
concentration of volatile compounds in
the headspace of sample vials. The
concentration of volatile compounds in
the gas phase can be expressed as
C
g
=C
o
/(K+
β
) (whereC
g
is the concen-
tration of volatile analytes in the gas
phase andC
o
is the original concentra-
tion of volatile analytes in the sample).
Striving for the lowest values for both
K and
β
will result in higher concentra-
tions of volatile analytes in the gas
phase and, therefore, better sensitivity
(
Figure 5
).
