Figure2
Fundamental headspace relationship.
K +
β
β=
K=
C
O
C
G
=
α
A
P
i
0
-
γ
i
1
α
K
C
S
/C
G
PartitionCoefficient
PhaseRatio
Concentration
dependent
Volumedependent
VaporPressure (
P
i
0
)
ActivityCoefficient
(
γ
i
)
Affectedby salting-out
Affectedby foreign solvent
Affectedbyderivitization
AffectedbyTemperature
V
G
/V
S
TheChemistryof StaticHeadspaceGasChromatography
ImproveMethodPerformancewithFundamentals
Organic volatile impurities (OVIs), commonly referred to as residual solvents,
are trace level chemical residues indrug substances anddrug products that are
byproducts ofmanufacturingor that formduringpackaging and storage.Drug
manufacturersmust ensure that these residues are removed, or arepresent only
in limited concentrations. The International Conference on Harmonization
(ICH) Q3C guideline lists the acceptable amounts of solvent residues that can
be present. Methodology, both independently developed and compendial,
should strive to coincidewith this guideline. In this guide,wewill take a com-
prehensive look at residual solvent analysis, in both theory and practice, and
illustrate options for the practicing chromatographer.
The analysis of residual solvents is commonlyperformedusing staticheadspace
gas chromatography (HS/GC). The basic premise behind headspace analysis
beginswith the additionof an exact, known volume orweight of sample into a
closed, sealed vial. This creates twodistinct phases in the vial—a sample phase
and a gaseous phase, or “headspace”. Volatile components inside the sample
phase, whether a solid or solution, can be extracted, or partitioned, from the
samplephase into theheadspace. Analiquotof theheadspacecan thenbe taken
anddelivered into aGC system for separation anddetection. If we look at the
anatomy of a headspace vial (Figure 1), we can begin to see the relationship of
the vial components andhowwe can control theseparameters to create analyt-
icalmethods.
Residual solvent analysis by staticHS/GC canbe enhancedby careful consider-
ation of two basic concepts—partition coefficient (K) and phase ratio (
β
).
Partition coefficients andphase ratioswork together todetermine the final con-
centrationof volatilecompounds in theheadspaceof samplevials.Volatilecom-
ponents partition from the sample phase and equilibrate in the vial headspace.
Striving for the lowest values for both K and
β
when preparing samples will
result in higher concentrations of volatile analytes in the gas phase and, there-
fore, better sensitivity (Figure 2).
Controlling thePartitionCoefficient
The partition coefficient (K) is defined as the equilibrium distribution of an
analytebetween the sample and gas phases.Compounds that have lowK values
will tend to partitionmore readily into the gas phase, and have relatively high
responses and low limits of detection. K can be further described as a relation-
ship between analyte vapor pressure (p
i
0
) and activity coefficient (
γ
i
). In prac-
tice,K canbe loweredby increasing the temperature at which the vial is equili-
brated (vapor pressure) or by changing the composition of the samplematrix
(activity coefficient) by adding an inorganic salt or a solvent of lesser solubility,
often referred to as a foreign solvent.High salt concentrations and foreign sol-
ventsdecreaseanalyte solubility in the samplephase (decreaseactivity)andpro-
mote transfer into the headspace, thus resulting in lower K values. Themagni-
tudeof this effect onK is not the same for all analytes.Compoundswith inher-
ent lowK values in thematrixwill experience little change in partition coeffi-
cient in response to the addition of a salt and temperature, while volatile com-
pounds in amatrixof similar polaritywill show the largest responses.
Adjusting thePhaseRatio
The phase ratio (
β
) is defined as the volume of the headspace over the volume
of the sample in the vial. Lower values for
β
(i.e., larger sample sizes)will yield
higher responses for compounds with inherently low K values. However,
decreasing
β
will not always yield the increase in response needed to improve
sensitivity.When
β
isdecreasedby increasing sample size, compoundswithhigh
Kvalueswillpartition less into theheadspacecompared tocompoundswith low
K values and yield correspondingly smaller changes in sensitivity.
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Figure1
Volatilecomponentspartition into
gaseousphaseuntil equilibrium is reached.
GaseousPhase
"Headspace"
Final
Gaseous Phase
Concentration
Initial
SamplePhase
Concentration
VolatileAnalyteMolecule
SolventMolecule
Sample
phase
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.
Δ
Temp.
Δ
Time
C
G
C
O
V
S
V
G
V
V
Where:
A= area
V
G
= volume of gas phase
V
S
= volume of sample phase
V
V
= total vial volume
C
O
= initial analyte concentration in sample
C
G
= analyte concentration in gas phase
C
S
= analyte concentration in sample phase
P
i
0
= analyte vapor pressure
γ
i
= activity coefficient
