Presentation on theme: "GC. 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."— Presentation transcript:
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 Care
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 heating the column above its temperature limit heating it above 50 o C without carrier gas flow Causes: 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.
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.
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. Changing Columns
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. Gas Purification
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: 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 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.
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.