200 / 413
the
Restek
Advantage
2008.01
IN THIS ISSUE
Editorial
Using Guard Columns and
Retention Gaps in GC (Part 2)
. . . . . . . . . . . .
2
Environmental
Accurately Quantify PAHs
Down to 5pg On-Column
. . . . . . . . . . . . . . .
3
13 Minute Chlorophenoxyacid
Herbicides Analysis
. . . . . . . . . . . . . . . . . . . . . .
6
Enhancing Air Monitoring Methods
with Thermal Desorption
. . . . . . . . . . . . . . . .
8
Chemical/Petrochemical
Selecting a GC Column for
Glycerin in Biodiesel . . . . . . . . . . . . . . . . . . . . 10
Stable Sulfur & Mercury
Sampling in Refineries . . . . . . . . . . . . . . . . . . 12
Foods, Flavors & Fragrances
High Sensitivity Melamine
|GC/MS Analysis of Cat Food
. . . . . . . . . . . .
14
Sample Preparation
Fast, Simple Sample Cleanup
. . . . . . . . . . .
16
Pharmaceutical
Multi-task with an Ultra IBD Column
. . . .
18
Clinical/Forensics
Fast, Sensitive Analysis of
Benzodiazepines by LC/MS/MS
. . . . . . . . .
20
Accurate, Reproducible
Amphetamines Analysis
. . . . . . . . . . . . . . . .
22
Tech Tip
Selecting the Right HPLC
Guard Column
. . . . . . . . . . . . . . . . . . . . . . . . . .
26
HPLC Accessories
Waste Overflow Indicator
for HPLC Systems
. . . . . . . . . . . . . . . . . . . . . . .
27
GC Accessories
The Forgotten Septum
. . . . . . . . . . . . . . . . .
24
Peak Performers
FID Detector Replacement Parts
. . . . . . . .
28
Electron Multipliers for
Mass Spectrometry
. . . . . . . . . . . . . . . . . . . . .
30
Restek Trademarks
Allure, Alumaseal, Crossbond, Integra-Gap, Integra-Guard, MXT,
Press-Tight, Rtx, Rxi, SeCure, Silcosteel, Siltek, Sulfinert, Uniliner,
Restek logo.
Other Trademarks
Dacthal (Amvac Chemical Corp.), API 3200, Cliquid, QTrap
(Applied Biosystems/MDS SCIEX Instruments), Snap Seal (J.G.
Finneran Associates, Inc.), SecureTD-Q (Markes International,
UK), Parker (Parker Intangibles LCC Ltd.), Swagelok (Swagelok
Company), Upchurch Scientific (Upchurch Scientific, Inc.), Valco
(Valco Instruments Company, Inc.).
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. In Part 1 of this article we reviewed retention gaps, which mainly
are used for focusing the sample components when introducing a large
(liquid) sample directly onto the column. In contrast, guard columns are
used to protect the analytical column from contamination. 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 ana-
lytical column tubing. By applying a “segment” coating technology the stationary phase can
be deposited only in a certain part of the column allowing a deactivated section at the begin-
ning. Column coupling is not required and maintenance is greatly simplified. Here we will
review guard columns and discuss the new segment coating technology.
Use of guard columns
The purpose of using guard columns is to protect the analytical column from contamina-
tion since the sample that is introduced is not always pure. Although the best chromatogra-
phy is obtained with “clean” samples, the practical situation is that sample clean-up proce-
dures are minimized and relative “dirty” samples are introduced onto the column. Samples
can contain particulates, heavy components, derivatization reagents, ionic residues, acids,
bases… all these compounds can interfere with the stationary phase and they will influence
the separation process. Usually the degradation of column performance is a slow process but
it will happen.
Most of the time the impurities accumulate in the first meter(s) of the column and by cutting
off this section adequate separation is restored. Many users choose to connect a guard col-
umn in front of the analytical column. Such a guard column is deactivated and can be
trimmed when contaminated and eventually replaced. Depending on the application, guard
columns have a lifetime of 1 week up to 6 months. One has different choices for guard
columns; a guard column can consist only of deactivated capillary, or it can be a coated capillary.
Deactivated capillary tubing:
Deactivated fused silica tubing can be purchased by the meter
and then a defined length can be coupled in front of the analytical column. Upon contam-
ination, a section of the guard column is removed. When the whole guard is “consumed” a
new guard column can be coupled. The disadvantage of cutting parts off of the guard col-
umn is that the column becomes shorter and this may affect retention times. However, if a
similar length is always cut from the guard column, the change in retention time becomes
very predictable. A deactivated guard column will also result in band focusing. If the injec-
tion is not optimal, there will be a focusing effect similar to that of a retention gap.
Coated capillary tubing:
As the guard column needs to prevent contamination of the ana-
lytical column, a coated guard column can help as it has both the surface deactivation and
also the stationary phase layer. The easiest and most economical way of using coated guard
columns (or precolumns) is to buy two analytical columns. One we will use as a separation
column and the second one will be used to make coated guard columns. From this second
column we will cut 2m sections and couple a section in front of the analytical separation col-
umn. We can run our samples until contamination affects peak shape/response and then we
can replace the guard with a new 2m section.
The system we have created will produce reproducible retention times as we always will
replace the entire 2m coated guard column. Since the stationary phase is the same on the
guard as on the analytical column, there will be no surprises. The coated guard column also
will allow more aggressive samples/more contamination before it will give up. Lastly, we are
able to cut 15 coated guard columns from a full 30m analytical column...that’s also eco-
nomical! However, if using a coated guard column, there will be no focusing effects.
UsingGuard Columns and
RetentionGaps inGC (Part 2)
Jaap de Zeeuw, International GC Consumables Specialist, Restek Corporation
Continued on page 31.