Transcript Slide 1

How to Recognize and Eliminate Ghost- and Deformed Peaks in Gas Chromatography
Jaap De Zeeuw, Restek Corporation, Middelburg, The Netherlands
Summary
Decomposition of
components in an active inlet.
If components degrade in a hot
inlet, extra peaks will be
observed in the chromatogram.
The chromatogram is like a fingerprint. If you can read the
chromatogram by looking at peak shapes, retention, base line and by
comparing with “normal” situation, you have a good chance to solve
problems and improve the analysis.
A ghost peak is a peak that is showing up, but is not supposed to be
there. Sometimes it is referred as a “system” peak. Ghost peaks can
be created in many ways. It’s a component that is added/created
somewhere in the system, it is injected/trapped/focused onto the
column, and will elute. Problems will escalate if a ghost peak
interferes with an analyte that has to be quantified. Sources for
ghost peaks can be sample vials, gloves, syringes, reagents, carrier
gas, tubing, the injection port, operation, memory effects and even the
column-phase itself.
What can cause a ghost or a
deformed peak?
“A ghost peak is a peak that appears at a position where we do not expect a
peak”. It is a component that shows up in the system and it may show up
everywhere in the chromatogram. Sometimes the peak is a sharp peak,
sometimes it’s a broad deformed peak or a “hump”, sometime sit is a rising
base line..
Some of the key origins for ghost or deformed peaks are:
•
•
•
•
•
•
•
•
•
The reactivity can be reduced by
using highly inert liners (for
instance the latest sky-liners).
Use lowest possible injection
temperature; Often a good
injection is possible at a lower
liner temperature.
Contamination via Gloves. In our aim
to work as clean as possible, we
sometimes overlook the obvious. In one
of our trace analysis methods, we saw
systematically a set of ghost peaks
appear.
After analysis a small piece of the
glove, we found the cause..
Think about it: If you can “smell”
something, it’s basically a head space,
which just needs concentration to be
visible..
Do not use wool;
With splitless use a pressure
pulse to make transfer faster;
Septa is a known source for ghost peaks. Septa is mostly 100% siloxane
polymer. When heated it will form bleed products. These products are cyclic
siloxanes that show as peaks. Intensity depends on septum type, age and
temperature. To reduce: always use septum purge, use low injection
temperatures, replace septa regularly. You can also consider to use a septumless system like a Merlin Seal.
Consider to use a different
technique that is “milder” for
your components: This can be a
PTV or the cold-on-column
The purity of the carrier gas
Using non-GC grade carrier gas tubing
Memory effects due to back flash
Contamination of injection port
Injection port reactivity
Septum, O-rings
Analytical column bleed
Column installation
Sample contamination from syringe, vials, vial-septa, gloves
Septa particles in the liner can be a big problem. As they are at high
temperature, they form siloxanes that are injected on the column. Such particles
will also “delay” sample transfer as the siloxane will also retain components.
Liner condition upon injection is very important as here the chromatography
starts. Use needles with more friendly (tapered/bended) top. Also a septum
with a center guide, like the BTO, can be used. Replace liners systematically. If
the precision liner is used the septa particles will remain on the top of the wool.
Extra peaks showing up as large noise produced by the stationary phase
upon fast cooling. High bleed in a column can focus when the oven is cooled
down. Because oven will always cool more on one side, there will be a
focusing effect for bleed products. In every column winding, bleed will focus.
Next run, all these focused bleed-peaks, will elute. We see this especially with
high bleed (cyano) phases.
Other sources for Ghost peaks are shown above: Using the MS makes the
identification easier. Besides siloxanes, septa also can contain phthalates.
(m/z 149,167 and 279)
Solution is to use temperature controlled cool down. The right signal above
was obtained with a -10ºC/min cool program. This effect will depend strongly
on the GC type and the temperature gradient in the oven upon cooling.
The m/z 277 is indicator for a contaminant that is often released from the Oring. Carry over of PAH is caused by contamination due to a previous
injection, probably to back-flash.
Dirty tubing will release residual hydrocarbons, which will be trapped on the
column and show up as “ghost” peaks. Use only GC grade copper tubing. Trace
1 in the figure above, shows the materials that elute from a “clean” batch of
copper tubing. Specific cleaning procedures eliminate this risk.
When a system is contaminated, a base line like above can develop. Here
hydrocarbon oils were back-flashed in the gas lines. This was confirmed by the
MS spectrum. Make sure that you do not inject more then the liner volume.
Note that each solvent has a different expansion volume. To prevent, use
larger diameter liners, inject less and know the liner and expansion volume of
your solvent.
Contamination of Split line can
cause broad “blobby” peaks.
When sample is splitted, the
lower outlet temperature makes
sample condense. This can built
up a contamination and will be a
source for ghost peaks. On the
right: A: split line nearly blocked;
B: after cleaning
Regularly check the split line and clean it. There are also split line filters.
Check also flow of split outlet with a flow meter.
In column decomposition: Thermo labile components will start to
decompose when the temperature reached a certain value. when the
decomposition product has less retention, a typical peaks shape is observed as
above. Here a brominated diphenylether, BDE-209 is measured. Important is
to reduce elution temperature and column activity; this is realized by:
1 Use high flow rate; 2 Slow temperature program; 3 Use thin films; 4 Use
short columns; 5 Use inert surface (Rtx-1614);
Sample contamination in the vial: Just by injecting 2 times through the
vial – seal, all the peaks above were introduced. Ghost peaks can be caused
by cross-contamination via the wash vial,. Also when the needle punctures
through a vial septum, each time the outside will take material of the
septum.
Want to learn Chromatography,Tips & Tricks
and troubleshooting?
Select sealing materials that are OK with your solvent. Also clean rinse vials
regularly or use multiple rinse vials.
Visit ChromaBLOGraphy at www.Restek.com