Page 167 - Academic Press Encyclopedia of Physical Science and Technology 3rd Analytical Chemistry

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Encyclopedia of Physical Science and Technology EN006F-275 June 29, 2001 21:12

458 Gas Chromatography

A typical gas chromatograph has three independently Unlike some other chromatographic processes, the
controlled thermal zones: proper temperature of the in- physical interactions between the mobile phase and solute
jector zone ensures rapid volatilization of the introduced molecules in GC are, for all practical purposes, negligible.
sample; the column temperature is controlled to optimize Thus, the carrier gas serves only as means of molecular
the actual separation process; and the detector must also (solute) transport from the beginning to the end of a chro-
be at temperatures where the individual sample compo- matographic column. The component separation is then
nents are measured in the vapor phase. For certain GC primarily due to the interaction of solute molecules with
separations, it is advisable to program the temperature of those of the stationary phase. Since a variety of column
the chromatographic column. materials are available, various molecular intertactions
As shown in Fig. 2, different sample components appear can now be utilized to enhance the component separation.
at the column’s end at different times. The retention time Moreover, these interactions are temperature-dependent.
t R is the time elapsed between injection and the maximum For the mixture component with no affinity for the sta-
of a chromatographic peak. It is defined as tionary phase, the retention time t 0 serves merely as the
marker of gas linear velocity u (in cm/s) and is actually
t R = t 0 (1 + k), (1)
defined as
where t 0 is the retention time of a mixture component that L
has no interaction with the stationary phase (occasionally t 0 = u , (3)
referred to as dead time), and k is the capacity factor. The
where L is the column length. The gas velocity is, in turn,
capacity factor is further defined as
related to the volumetric flow rate F since
V s F
k = K , (2) u = , (4)
V M s
where K is the solute’s distribution coefficient (pertaining where s is the column cross-sectional area. The gas-flow
to a distribution between the stationary phase and the mo- rate is chiefly regulated by the inlet pressure value; the
bile phase), V s is the volume of the stationary phase, and higher the inlet pressure the greater the gas-flow rate (and
V M is the volume of the mobile phase in a chromatographic linear velocity) becomes, and consequently, the shorter t 0
column. The distribution coefficient K = C s /C M (where is. The retention time t R of a retained solute is also mod-
C s is the solute concentration in the stationary phase and ified accordingly. Correspondingly, fast GC separations
C M is the solute concentration in the mobile phase) is a are performed at high gas-inlet pressures. The so-called
thermodynamic quantity that depends on temperature as retention volume V R is a product of the retention time and
do all equilibrium constants. The molecular interactions volumetric gas-flow rate:
between the phases and the solutes under separation are
V R = t R F. (5)
strongly temperature-dependent. If, for example, a solid
adsorbent (column material) is brought into contact with Since the retention times are somewhat indicative of
a permanent (inorganic) gas and a defined concentration the solute’s nature, a means of their comparison must be
of organic (solute) molecules in the gas phase at a certain available. Within a given chemical laboratory, the relative
temperature, some solute molecules become adsorbed on retention times (the values relative to an arbitrarily chosen
the solid, and others remain in the permanent gas. When chromatographic peak) are frequently used:
we elevate the system temperature, less solute molecules
t R 2 V R 2 K 2
are adsorbed, and more of them join the permanent gas; α 2,1 = = = . (6)
the distribution (adsorption) coefficient, as defined above, t R 1 V R 1 K 1
changes correspondingly. Likewise, if the stationary phase This equation is also a straightforward consequence of
happens to be a liquid, the solute’s solubility in it decreases Eqs. (1) and (2). Because the relative retention represents
with increasing temperature, according to Henry’s law, re- the ratio of distribution coefficients for two different so-
sulting in a decrease of the distribution (partition) coeffi- lutes, it is frequently utilized (for the solutes of selected
cient. chemical structures) as a means to judge selectivity of the
According to Eqs. (1) and (2), the retention time in GC solute–column interactions.
depends on several variables: (a) the chemical nature of For interlaboratory comparisons, the retention index ap-
the phase system and its temperature, as reflected by the pears to provide the best method for documenting the GC
distribution coefficient; (b) the ratio of the phase volumes properties of any compound. The retention index system
in the column V s /V M ; and (c) the value of t 0 . In the practice compares retention of a given solute (on a logarithmic
of chromatography, these variables are used to maximize scale) with the retention characteristics of a set of stan-
the component separation and the speed of analysis. dard solutes that are the members of a homologous series: