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Have you ever faced changes in elution order after modifying the column
temperature or the heating rate in the temperature program of the GC analy-
sis of complex samples? This so-called cross-over phenomenon, which can
lead to problems in peak identification, has been a well-known mystery in
GC for decades.1 But, so far, the physico-chemical background is still not well
understood.
The cross-over phenomenon is very common when separating compounds with different
functional groups on polar stationary phases. For example, we observed a reversal in the elu-
tion order for components like linalool and camphor on a polyethylene glycol column
(Carbowax 20M) after changing the column temperature programming rate: at 5°C/min.
linalool elutes before camphor but at 3°C/min. camphor will elute first. Effects like this are
often observed when essential oils are analyzed or, to be more precise, when the GC methods
are optimized. The reversal of the elution order is mainly explained as a result of the differ-
ent temperature-dependencies of the intermolecular interactions, which are responsible for
the retention: London-type dispersion forces and induction forces are independent of tem-
perature, whereas the orientation forces and hydrogen bridge bonds depend strongly on the
temperature (Figure 1).
However, this explanation is only half the truth and we should examine the influence of col-
umn temperature on retention in some more detail. It is generally known that the column
temperature is one of the two most important variables in GC (the other being of course the
nature of the stationary phase). In partition GC, the effect of temperature on the solute par-
tition coefficient K is given by the van´t Hoff relationship ln K = HS/RTc + C (with HS being
the molar heat of solution of solute). From this follows the fundamental correlation between
column temperature Tc and retention factors:
ln k´ = HS/RTc + C´ - ln ß
where k´ is the retention or capacity factor (k´ = t´R/t M) and ß the column phase ratio.
This equation indicates that the retention decreases logarithmically as the column temper-
ature increases.
the Restek Advantage
2007.02
IN THIS ISSUE
Editorial
Retention Cross-over Phenomenon in
Gas Chromatography–Can the Mystery
be Revealed? Part 1 . . . . . . . . . . . . . . . . . . . . . . . 2
Pharmaceutical
Explaining the Small
Particle Advantage . . . . . . . . . . . . . . . . . . . . . . 3
Revised USP 467 Residual
Solvent Method . . . . . . . . . . . . . . . . . . . . . . . . . 6
Clinical/Forensics
GC Inlet Liner Deactivations
for Basic Drug Analysis . . . . . . . . . . . . . . . . . . . 8
Foods, Flavors & Fragrances
Simple, Reliable HPLC Analyses
of Organic Acids . . . . . . . . . . . . . . . . . . . . . . . . 10
Environmental
Separate Explosives and
Propellant Residues . . . . . . . . . . . . . . . . . . . . 12
Chemical/Petrochemical
Fast, Accurate FAMEs Analyses
of Biodiesel Fuel. . . . . . . . . . . . . . . . . . . . . . . . 14
Restek Perfromance Coatings
Assure Accurate Sampling and
Reliable Sample Purity. . . . . . . . . . . . . . . . . . 16
GC Accessories
Peak Performers: Introduction
to Pressure Regulators. . . . . . . . . . . . . . . . . . 18
Tech Tips
Preventing Septum Problems . . . . . . . . . . 20
How Hot is Your Septum? . . . . . . . . . . . . . . . 22
Restek Trademarks
Allure, Crossbond,Cyclosplitter, IceBlue, Pinnacle, Rtx, Silcosteel,
Siltek, Stabilwax, Sulfinert,Thermolite,Trident, Restek logo.
Other Trademarks
BTO, CenterGuide (Chromatography Research Supplies, Inc.),
TRACE (Thermo Scientific), Carbowax (Union Carbide).
Retention Cross-over Phenomenon inGas
Chromatography–Can theMystery be
Revealed? Part 1
By Werner Engewald, Ph.D., Professor Emeritus, University of Leipzig, Institute of Analytical Chemistry,
Leipzig, Germany;
Linalool (bp.: 199°C) Camphor (bp.: 209°C)
Figure 1 Functional groups influence elution order.
Continued on page 23.