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the
Restek
Advantage
2007.04
IN THIS ISSUE
Editorial
Using Guard Columns and
Retention Gaps in GC (Part 1)
. . . . . . . . . . . .
2
Environmental
Fast, Accurate Semivolatiles Analysis!
. . . .
3
Chemical/Petrochemical
Complete Resolution of Benzene from
Ethanol in Spark Ignition Fuels
. . . . . . . . . . .
6
Foods, Flavors & Fragrances
Rapid Characterization of Garlic Volatiles
No Sample Prep Required!
. . . . . . . . . . . . . . .
7
Clinical/Forensics
Simplify and Speed Up
Opiates Analysis
. . . . . . . . . . . . . . . . . . . . . . . . .
8
Pharmaceutical
Easy Transfer of HPLC
Methods to UHPLC
. . . . . . . . . . . . . . . . . . . . .
10
Industial Hygeine
Complete Resolution of 13 Carbonyls
as DNPH Derivatives
. . . . . . . . . . . . . . . . . . . .
12
HPLC Accessories
Capillary Stainless Steel
Tubing Assemblies
. . . . . . . . . . . . . . . . . . . . .
13
Restek Performance Coatings
Sulfinert® Treated Systems Preserve ppb
Levels of Active Sulfur Compounds
. . . . .
14
Air Monitoring
Performance Testing VOC Audit
Sample for Air Toxics
. . . . . . . . . . . . . . . . . . .
16
Tech Tip
Affected by the Helium Shortage?
. . . . . .
17
Warm Up Before You Run
. . . . . . . . . . . . . . .
20
GC Accessories
Parker Balston® Hydrogen Generators
. . .
18
Dual Vespel® Ring Inlet Seals
. . . . . . . . . . . .
22
Erratum
The heading of Figure 1 on page 8 of the 2007.03 issue of the Restek
Advantage incorrectly describes the column internal diameter as
0.18mm.The correct internal diameter is 0.32mm.
Restek Trademarks
Allure, MegaMix, Pinnacle, Rtx, Rxi, Siltek, Sulfinert, Uniliner,
Restek logo.
Other Trademarks
Kel-F (3M Co.), API 3200 (Applied Biosystems), Vespel (E.I. du
Pont de Nemours & Co., Inc.),TrueTube (O’Brien Corp.), Balston
(Parker Intangibles LLC), Super-Clean (SGT Middleburg BV),
Swagelok (Swagelok Co.).
Guard columns and retention gaps are used widely in gas chromatography
(GC). Many users have difficulty understanding the difference between
these two products, even though there is a significant difference in appli-
cation. Retention gaps mainly are used for focusing the sample components
when introducing a large (liquid) sample directly onto the column. Guard
columns are used to protect the analytical column from contamination.
When using a retention gap system, the retention gap will also act as a guard column, but its
primary function is to create a focusing effect.
Guard columns and retention gaps both must be coupled to the analytical column, and this
connection introduces a potential point of risk. A new approach is to integrate the retention
gap directly into the analytical column. By applying a “segment” coating technology, the sta-
tionary phase can be deposited in a certain part of the column allowing a deactivated section
at the beginning. Column coupling is not required, and maintenance is greatly simplified.
In Part 1 of this article, we will explore retention gaps and build a foundation for a compar-
ison to guard columns. In Part 2, we will review guard columns and discuss the new segment
coating technology.
Use of retention gaps
In today’s laboratory, GC methods must be simple, fast, and low detection limits are
required. Besides that, sufficient precision must also be obtained. It all starts by introducing
the sample in the smallest possible injection band and making the band migrate through the
capillary with minimal loss of the target components. With on-column injection, a liquid
sample is directly introduced into the capillary column as a liquid while the capillary column
is kept at a temperature 10-15°C below the boiling point of the solvent. During this process,
the sample components are spread in an unreproducible way over the first 20-100cm of cap-
illary while the solvent is evaporating. Parameters like injection speed, carrier gas flow, tem-
perature of solvent and column, type of solvent and pressure all will affect the injection band
width. Additionally, when nonbonded stationary phases are used, the direct contact with liquids
will result in a distortion of the stationary phase film and very short column lifetime. The
majority of today’s stationary phases, like the Rtx® and Rxi® phases, are immobilized by
cross- and surface bonding techniques.
For proper application of the on-column injection technique, the use of retention gaps is
essential.
1,2
The retention gap consists of a 1-3m length of deactivated capillary that is posi-
tioned in front of the analytical column. All the processes described will still take place, but
now the components are distributed over the retention gap. When the oven temperature is
UsingGuard Columns and
RetentionGaps inGC (Part 1)
Jaap de Zeeuw, International GC Consumables Specialist, Restek Corporation
Continued on page 23.
Figure 1
Retention gaps are used to focus components in a tight band at the
beginning of the analytical column
a)
Sample introduction: liquid film of
solvent and sample are deposited in
the first length of capillary.
b)
Oven temperature is increased
(temp. program run): solvent and target
compounds are vaporized and travel
unretained through retention gap.
c)
When target compunds come in contact
with the stationary phase, they are refo-
cused on the analytical column, resulting
in a narrow initial band width.