Column Care
Columns have a finite life, but they will last much longer if the
gas chromatography system is operated with care.
The major causes of reduced column life are
breakage
thermal damage
chemical damage
contamination
Column Breakage
If the polyimide coating on the column is damaged, the
column will break very easily.
Weakened columns can often break due to the
frequent heating and cooling of the oven, vibrations from
the oven fan, or contact with identification tags,
or metal oven parts.
Short pieces of broken column can be discarded.
If the column breaks closer to the middle,
the break can be repaired using a column union.
Thermal Damage
Causes:
heating the column above its temperature limit
heating it above 50oC without carrier gas flow
The most common symptom of thermal damage is
excessive column bleed at the end of the temperature
program. Peak tailing, especially of more polar compounds
is also a common symptom of thermal damage.
Oxygen Damage
Oxygen damage is usually the result of long term exposure to oxygen.
Small exposure to oxygen during sample injection does not cause problems.
Leaks, and the use of low grades of carrier gas (without a gas purifier
or with gas purifier used up) are the common causes of oxygen damage.
The symptoms of oxygen damage are the same as for thermal damage,
-----excessive column bleed at higher temperature and tailing peaks.
Chemical Damage
Inorganic or mineral acids
Hydrochloric acid
Sulfuric acid
Phosphoric acid
Nitric acid
Chromic acid
Hydrofluoric acid
Perchloric acid
Inorganic or Mineral bases
Sodium hydroxide
Potassium hydroxide
Ammonium hydroxide
Perfluoro acids
Trifluoroacetic acid
Pentafluoropropanoic acid
Heptafluorobutanoic acid
Chemical Damage
Ion-exchange SPE cartridges can be used to remove
acids or bases before injection, but if it is important to
maintain sample pH, organic acids and bases should
be used if possible.
Guard columns can also be used to reduce damage to the
column. Guard columns are simply a short piece of
deactivated fused silica tubing that is attached to the
head of the column.
Chemical Damage
The most common symptom of chemical damage is distorted
peak shapes. Acids and bases can also produce active sites
that result in tailing of some active compounds.
Sizes of some peaks may be reduced due to adsorption
by the column.
Contamination
Caused by:
non-volatile or semi-volatile compounds in the sample,
residues from sample vials, caps, solvents, pipets, septa, etc.
Contamination
Non-volatile Compounds
--Tend to form a layer of residue at the head of the column
-interfere with proper formation of the sample band,
-reduce the efficiency of partitioning
-may interact with the sample.
Usually result in poor peak shape.
Generally do not produce baseline problems since
they do not elute from the column.
Contamination
Semi-volatile compounds
-----May elute hours or days after the injection
Can cause erratic baselines or ghost peaks.
May also cause peak shape disturbances for the same
reason non-volatile compounds do.
In addition to being injected with the sample, semi-volatile
compounds may arise from the breakdown of non-volatile
contaminants in the column.
Contamination
Contamination can also be reduced by the use of a guard
column or by using a packed injector liner.
Packing the injector will silylated glass wool
will block non-volatiles from entering the column,
but can also cause problems if not properly installed,
used and maintained.
Contamination
It is possible to remove contamination from the column by
rinsing it with solvent.
This requires removal of the column from the GC and washing
it repeatedly with liquid solvents of different polarity and strength.
Baking Out
Column is heated to near is temperature limit for several hours.
can shorten column life and should be avoided as a common practice.
Column Testing
These mixes are designed to test things such as adsorptive sites,
polarity, etc. The column should be evaluated with standard
test mixes after installation and on a periodic basis and a
record of these tests should be kept for comparison.
Column Testing
Alkanes (C10 to C16)
lack functional groups and are the standard to which other
peaks in the test mix are compared.
Poor peak shapes of alkanes is usually the result of carrier gas
flow problems in the injector or detector, solid particles
in the column, poor injection technique, or an extremely
contaminated or damaged column.
Column Testing
Alcohols, (C8 and C12)
The hydroxly group can interact with many materials
through hydrogen bonding. If the alcohol peaks tail,
it is in indication of contamination or breakdown of
the stationary phase.
Column Testing
Acidic and Basic Compounds
Stationary phases usually have some acidic or basic character.
Acids generally tail if the stationary phase is too basic.
Bases tail if the stationary phase is too acidic.
Column Testing
Fatty Acid Methyl Esters (FAME’s)
Like the alkanes they generally do not have much
activity for reactive sites on the column so their peaks
should be narrow with little tailing.
The retention indices of FAME’s relative to the alkanes
is a measure of stationary phase selectivity and polarity.
Changing Columns
For storage, the ends of the column should be sealed,
commonly by inserting the ends in a used septa.
Prior to storage, the history of the use of the column
should be recorded.
Non-polar stationary phases can be stored for 5 years or more,
and even many polar columns can be stored for years without
much loss of performance.
Gas Purification
Pressurized gasses are available in a number of different qualities
and the appropriate one should be selected.
Carrier gasses are usually passed through
traps to remove oxygen and water,
both of which are reactive substances
that can affect chromatography or
degrade the stationary phase of the
column. Generally these traps contain
an indicator that changes color when
the traps need to be replaced.
Injector Liners
With use, especially when dirty samples are
analyzed, non volatile compounds will build
up in the injector.
These compounds provide surfaces for analytes
to bind to and may also contain chemically
reactive sites that may react with analytes.
The injector must therefore be cleaned on a
regular schedule, based on the amount of use
and the composition of the samples.
Injector Liners
Not only does the insert need to be
cleaned, its surface needs to be
deactivated (silylated) to remove
reactive –OH groups.
In many cases, it may be easier to
replace the insert with a new one,
although they are fairly expensive.
Liner Problems
Causes:
liner not deactivated, needle hitting and breaking
liner packing, column end poorly cut, broken or chipped liner
Liner Problems
Cause: Sample decomposition,
-- Clean or replace inlet liner
Liner Problems
Cause: Sample decomposition,
-- Clean or replace inlet liner
Liner Problems
Cause: Column and inlet linger is misaligned
--Check column position and adjust as needed.
Liner Problems
Causes: Contamination of Injector Liner
(or contamination of the head of the column)
--Clean or replace liner
--Remove 1-2 feet of column
Septa
Must maintain a leak free barrier between the GC and the outside atmosphere,
while still providing easy access for sample injection.
They are usually made of special high-temperature, low bleed silicone rubber.
Generally septa designed for lower temperature are softer, seal better, and can
withstand more injections then those designed for higher temperature operation.
Septa Bleed
Septa contain small amounts of volatile compounds originating in the
manufacturing process. These compounds are released from the septa during
heating of the injector. Very low amounts of septa bleed can build up in the
column over long periods of time when the GC is not being used and the
oven is cool.
This buildup of septa bleed
materials will then elute with
the sample during the next
temperature ramp.
Septa Problems
Broad humps in the chromatogram usually in the first
chromatogram after a period of non use.
Septa Problems
Baseline shift (up or down) after a large peak (can be caused
by leakage at the septum during injection and a short time thereafter)
Septa Problems
Retention times increase ( can be caused by leakage of carrier
gas at the septa, causing reduced flow in the column).
Ferrules
Ferrules are used to seal the connections between the column ends
and the injector and the detector. Ferrules should provide a
leak-proof seal, that will stay sealed during temperature cycling.
Ferrules are most commonly
made of:
graphite
vespel
graphite/vespel
Ferrules
Graphite ferrules are soft and easy to seal, but small flakes
of graphite can be produced during tightening. These small
flakes can contaminate the injector, column or detector.
Graphite generally should not be used on the detector fittings
for GC/MS or ECD detectors because these detectors are
difficult to clean, and in the case of ECD detectors, they
must be returned to the manufacturer for repair. Graphite
ferrules can be reused as many as 15 times provided they
are not damaged by over-tightening. Graphite ferrules
can be used at higher temperatures than graphite/vespel ferrules.
Ferrules
Graphite/vespel ferrules are much harder than plain graphite
ferrules, which often makes it harder to form a leak-free seal.
Graphite/vespel ferrules may also leak due to temperature
changes in the system. Polymer bleed from these ferrules
may also cause problems with ECD or NPD detectors.
Ferrules
Positioning the Column
In addition to providing a seal, ferrules hold the column in the
proper position in the injector or detector. Placing the column
correctly is one of the most difficult parts of column
installation since you can’t see the end of the column inside
the injector or detector.
The column end is
usually positioned
improperly by
measuring the correct
distance and marking
a reference point on
the column relative
to the outside of the column nut. The reference point is then
held at its correct position while the column nut is tightened.
Positioning the Column
If the column is inserted too far or not far enough into
the injector it can cause tailing of the solvent or discrimination
among peak heights.