FAQ's

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Harold McNair, Virginia Tech, USA

Abstract

This chapter discusses the most frequently asked questions on GC practice.

Level: Basic

General information on how to treat and how not to treat columns is given in the the troubleshooting section, in the articles on GC columns. Hereunder you will find FAQ's and their answers, as collected by me and several other experts in the field.

"How do I clean a GC column?"

Cut 20-50 cm from the front end. You may want also to cut 15 cm from the detector side.
Reverse the flow: Connect the column detector end to the inlet. Heat the column for an hour 20°C above your usual highest temperature.
You might consider to heat/condition overnight.

"How do I evaluate and reduce the activity of my system?"

There is a risk that surface activity from dirty samples causes adsorption or breakdown of samples.
Use clean, deactivated liners.

"How to maximize column lifetime?"
Good practice that will enhance lifetime:

Before heating a newly installed column, remove the air by purging with carrier gas for ten minutes.
Allow the column oven to cool to below 60oC before opening the oven door.
Remove oxygen from the carrier gas by using gas filters
Use injection liners that contain quartz wool if your samples contain non-volatiles.
Be aware of the maximum recommended temperatures for each column.
Avoid repeated rapid heating if possible, slow temperature ramps are less destructive to the film.
Always cool a column to 40C before disconnecting. Thermal shock and the introduction of air at high temperatures can cause oxidation.
Store unused columns in their original box with the ends capped with a septum to prevent oxygen to enter.

"How often should I change my liner?"
The liner plays an important role in the chromatography and a deteriorating liner will affect chromatographic results. Much more info on practice of liner problems en how to prevent problems are described in the troubleshooting GC section. The article called "An overview from practice" by Marinissen offers many details.

Lifetimes are mostly determined by contaminations. The cleaner the sample the better. A sample that is sensitive to decomposition may reduce the lifetime.
Use a control sample regularly to decide if a liner needs changing.
As part of method development it should be established how many injections of a typical sample can be done prior to liner failure.
Liners should be changed as a matter of routine after 80%-90% of this figure.

"How often should I change my septum?"

Septa are consumables. They perform a very important job in the GC and can deteriorate quickly depending on the syringe needle gauge, injector temperature, or whether it is used with an autosampler or manual injection. They are inexpensive items and should be changed before they fail rather than immediately afterward. More on this in the troubleshooting section.

Generally, with autosampler injectors, the septa should last around 100 injections, less for manual injection as physical integrity is lost sooner.

"How should I treat my column prior to use?"

Column treatment depends on the column phase. It is recommended that before any column is subjected to a thermal gradient, all oxygen has been removed as the presence of oxygen in the system can shorten the column lifetime.
Although all columns have been preconditioned, we recommend that they be
conditioned after installation.
Heat the column from 50°C to the MAOT (or 20 °C above the maximum temperature in the method) at 5°C/min and hold for 1 hour.
Remember to stay within the maximum temperature range for the column.
Monitor the detector signal during conditioning until a stable baseline is reached. Due to the factory preconditioning of the column, this should be achieved in around 1 hour. Conditioning may take longer in the case of thick films and polar phases.

"How should I use a guard column?"
A guard column can protect the main analytical column from contamination by non volatile components. A guard column can also help to refocus solvents if a small injection liner is in use or facilitate on-column injection in narrow columns. If you find the analytical column regularly needs changing, cleaning or trimming because of contamination, a guard column can be installed before the analytical column. This will reduce the cost per analysis. A short 2-10 m length of uncoated deactivated fused silica tubing can act as a reliable guard column.

What purity should the carrier gas be?"
For the best results, He and H2 need to be at least 99.995% pure. For a long column lifetime, all oxygen should be removed from the carrier gas.
Always use a high quality gas supply with specified impurity information. Additionally you should use a series of gas filters, together with an indicators to show when the main removal filter has expired. Change them regularly.

"What is the best capillary column for the analysis of
A) drugs
B) arson samples
C) pesticides?
The best column for almost anything is a fused silica capillary column of 15- or 25-m of 0.225 mm i.d. with a film thickness of 0.25 mm of dimethylpolysiloxane, for example: DB-1 or SE- 54 or OV-73. This column is so efficient (high N) and inert, it handles most samples.

"Will a thick film capillary column work for the analysis of natural gas? Will I need to couple it with cryogenic cooling on my GC in order to get resolution of methane and ethane? "
No, you do not need cryogenic cooling. You must use at least a 1-mm in order to separate methane and ethane since they are too volatile for thin films.

"If you have a sample composed of both polar and non-polar compounds which packed column would you use?"
Always try a non-polar dimethysilicone column first (OV-1, SP-2100, SE-54, etc). These are almost universal columns; if polar peaks elute with reasonable peak shape, they will elute faster than on a polar column. If selectivity is too low, then try a more polar column.

"Are there any other means of reconditioning a capillary column other than cutting off the end?"

Bake out for 2 hours (preferably one night), close to the maximum temperature of the stationary phase.
For bonded phase columns, remove column and flush it backwards with 2 mls of hexane, methylene chloride and methanol. Be careful, many so called “bonded” Carbowax columns are merely immobilized and not suitable for solvent flushing. After solvent flushing, setup high flow rate (no detector!) at room temperature for several minutes, then slowly program 1°C/minute up to 200°C to remove all solvent.