ADV06-15_1-412

256 / 413

the Restek Advantage
2007.03
IN THIS ISSUE
Editorial
Retention Cross-over Phenomenon in
Gas Chromatography–Can the Mystery
be Revealed? Part 2 . . . . . . . . . . . . . . . . . . . . . 2
Chemical/Petrochemical
New MXT®-Biodiesel TG Column Line . . . . .3
Pharmaceutical
Optimize Selectivity & Efficiency
in UHPLC Separations . . . . . . . . . . . . . . . . . . . 6
Environmental
Faster Organochlorine Pesticide
Sample Throughput . . . . . . . . . . . . . . . . . . . . . 8
Resolving the Benzo(j)fluoranthene
Challenge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Foods, Flavors & Fragrances
Analysis of Nitrofurans in Honey . . . . . . . . 11
Clinical/Forensics
Why Derivatize? . . . . . . . . . . . . . . . . . . . . . . . . 12
Sample Preparation
Superior Fractionation of Extractable
Petroleum Hydrocarbons . . . . . . . . . . . . . . . 14
Restek Performance Coatings
Prevent Mercury Loss During
Transport and Storage . . . . . . . . . . . . . . . . . . 16
Protect Sample Integrity and Prolong
Sampling System Lifetime . . . . . . . . . . . . . . 17
HPLC Accessories
Hub-Cap Mobile Phase Accessories . . . . .18
GC Accessories
Cool Tools
(Fused Silica Column Cutters) . . . . . . . . . . 19
Peak Performers
(Column-to-Column Connections) . . . . . . 20
Tech Tip
Get Connected! . . . . . . . . . . . . . . . . . . . . . . . . 22
Restek Trademarks
Carbo-Prep, Crossbond, Hydroguard, Integra-Gap, MX, Pinnacle,
Press-Tight, Resprep, Rtx, Rxi, SeCure, Silcosteel, Siltek, Sulfinert,
Uniliner, Vu-Union, Vu2 Union
Other Trademarks
Teflon (E.I. du Pont de Nemours & Co., Inc.), Opti-Cap (Jour
Research), Auto SYS (PerkinElmer),Tygon (Saint-Gobain
Performance Plastics Corp.), Florisil (U.S. Silica Co.)
In the last issue of the Restek Advantage (2007.02), I showed some examples of
the cross-over phenomenon on polar (polyethyleneglycol) columns. Here in
Part 2, we will examine the cross-over phenomenon on nonpolar columns.
It is known to a lesser extent that changes in peak elution order also occur on
nonpolar or weakly polar stationary phases for hydrocarbons that differ only
in their carbon skeleton, e.g. aliphatic versus cyclic compounds or cyclic
compounds differing in their ring number. The terpenes sabinene, β-pinene and myrcene
are given as an example in Figure 1. The cross-over effect was observed on a polydimethyl-
siloxane phase with 5% phenyl (60m, 0.25mm ID, 1µm film thickness) as well as on a 100%
polydimethylsiloxane phase (60m, 0.32mm ID, 0.5µm film thickness). The column tempera-
ture was increased from 90°C to 160°C using isothermal mode. The elution order changed
from sabinene, β-pinene, myrcene at 90°C to myrcene, sabinene, β-pinene at 160°C. What
could be the reason for this effect? A closer look at the molecular structure shows that
sabinene and β-pinene are double ring systems whereas myrcene is an aliphatic hydrocarbon.
Other interesting analyte pairs prone to cross-over on methylsiloxane phases at different
column temperatures are o-xylene/ n-nonane, naphthalene/dodecane, as well as 1,2,3-
trimethylbenzene/ n-decane. In the latter case we also observe coelution and cross-over at
different temperature programming rates. At a heating rate of 2°C/min., n-decane elutes
before 1,2,3-trimethylbenzene, at 5°C/min. coelution occurs, and at 20°C the aromatic
hydrocarbon is the first peak (100% PDMS column, 12m, 0.2mm ID, 0.33µm film thickness,
starting temperature 35°C). It seems obvious that the geometry of the molecule, e.g. cyclic
versus open chain, contributes to the cross-over phenomenon.
Nevertheless, I have this long-standing friendly discussion with a former student of mine,
who persistently points out that the examples we have been looking at so far are always pairs
of conjugated versus nonconjugated compounds and that π interactions, specifically with
phenyl modified phases, should be taken into account.
Let’s, therefore, go back to the structure of substances presented in Figure 2: they are exclu-
sively saturated aliphatic and alicyclic hydrocarbons. The data in Figure 2 are from Hively
and Hinton (1968) and in that paper the relative retention and retention indices of approxi-
Retention Cross-over Phenomenon inGas
Chromatography–Can theMystery be
Revealed? Part 2
By Werner Engewald, Ph.D., Professor Emeritus, University of Leipzig, Institute of Analytical Chemistry,
Leipzig, Germany;
Continued on page 23.
1. α -Pinene 2. Sabinene 3. β-Pinene 4. Myrcene
bp.: 155°C bp.: 162-166°C bp.: 163-164°C bp.: 167°C
90°C 103°C
115°C 132°C
160°C
Figure 1 Elution order on a 100% PDMS column at various temperatures
(isothermal GC)