Chemistry Reference
In-Depth Information
R
H
3
+
C8 chain
(a)
(b)
SO
3
-
NH
3
+
R
SO
3
-
H
O
O
O
O
O
Si
Si
Si
Si
OH
H
O
O
O
OH
OH
Si
Si
Si
Si
O
O
O
O
OH
(a) Electrostatic interaction
(b) Non-polar interaction
Figure 7.4
Retention mechanisms in a mixed mode SPE cartridge.
Syringe needle
Calibrated barrel
Plunger
Figure 7.5
Gas-tight syringe.
(Figure 7.5) or a solid phase microextraction (SPME) device (Figure 7.6) and
injected into the GC for subsequent analysis.
Headspace analysis is based on the principle of Henry's law, which states
that at a constant temperature, the amount of a given gas that dissolves in a
given type and volume of liquid is directly proportional to the partial pres-
sure of the gas in equilibrium with the liquid.
Mathematically, Henry's law can be expressed as
p = κ
H.
c
(7.1)
where p = partial pressure of the gas phase solute, c = concentration of the
solute and κ
H
= Henry's law constant (depends on solute, solvent and tem-
perature). Headspace GC analysis is used in the analysis of volatile organic
compounds, such as alcohol in blood.
Solid phase microextraction is a technique used for the extraction or
concentration of volatile or semivolatile compounds from a sample matrix.
It can be used for either headspace extraction or direct extraction from the
liquid phase; in this section, headspace SPME is discussed. The principle of
SPME is to adsorb the compounds of interest onto a silica-coated fibre; the
fibre is typically 1 cm in length.
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