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INSTRUMENT SUPPLIES
Technical Tips
Technical Tips for Avoiding Inlet Problems
Inlet issues can lead to poor chromatography and be a significant source of frustration in the lab. Many prob-
lems can be avoided by considering some basic principles. Here are some simple tips to help you improve chro-
matography and select the best liner for your application.
Consider Volume and Inner Diameter
Problems such as broad or tailing peaks, poor reproducibility, ghost peaks, and nonlinear response can result
if sample expansion volume and linear velocity are not considered when choosing liner dimensions.
Injection
Expansion Volume (µL)
Volume
(µL)
H
2
O C
2
S
CH
2
Cl
2
Hexane Isooctane
0.5
710
212
200
98
78
1.0
1420 423
401
195
155
2.0
2840 846
802
390
310
5.0
7100 2120
2000
975
775
Solvent expansion volumes based on an
injection port temperature of 250°C
and a 10 psig head-pressure.
For a straight 4mm ID x 78.5mm long
liner, the effective liner volume is
approximately 500µL.
indicates expansion volume exceeds effective liner volume.
Linear Velocity
Choosing a liner with a narrow inner diameter will
give a faster linear velocity (for a given flow rate),
which will move the sample onto the column quick-
ly, improving efficiency and helping keep peak
widths narrow. This is particularly important for
gaseous samples introduced via purge-and-trap or
static headspace techniques, or when 0.18 mm, 0.15
mm, and 0.10 mm ID columns are used.
Protect Analytes from Active Sites
Active sites in the inlet or liner can interact with reactive analytes, resulting in poor chromatography and
reduced response. Exposure of analytes to active sites can be minimized by carefully considering liner packing,
geometry, and deactivation.
Sample Expansion Volume
When a liquid sample is vaporized inside an inlet
liner, its volume expands considerably. Care should
be taken to match the effective liner volume and the
expanded volume of the injected sample. If the liner
volume is exceeded, the sample will be forced back
into the gas lines, causing irreproducible peak areas
and sample carryover.
Packing
Sample characteristics and injection technique will dictate whether packing is used. Analyses of high molecu-
lar weight analytes, especially in split mode, benefit from the use of packing. However, while wool improves
sample vaporization, protects the column from nonvolatile compounds, and permits larger volume injections,
it can also be a source of active sites that cause poor peak shape. When using a wool packed liner, highly inert
Sky™ liners are recommended for optimal performance. (See pages 206-209 for technical comparison.)
Specialized Uniliner® Geometry
Unliner® and Drilled Uniliner® inlet liners reduce analyte exposure by allowing a fused silica column to be
connected directly to the liner through a seal made between the inner glass wall of the liner and the polyimide
coating on the outside of the column. This configuration maximizes the amount of sample transferred to the
GC column, minimizes sample exposure to hot injection port parts, and is a good choice for trace analyses.
Use the Drilled Uniliner® inlet liner with the hole near the bottom for semivolatile analyses or when com-
pounds of interest could be affected by a tailing solvent
peak. Use the Drilled Uniliner® inlet liner with the hole
near the top for chlorinated pesticides analyses, aqueous
injections, and analyses in which the compounds of
interest elute away from the solvent peak. Note that
instruments equipped with electronic pressure control
must use Drilled Uniliner® liners, rather than standard
Uniliner® liners.
Drilled Uniliner® with hole near top
Drilled Uniliner® with hole near bottom
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TelNo : 03 9762 2034 . . . in AUSTRALIA
Mar 2011
