>> Update My Profile | Subscribe to Access Agilent | Article Directory

How can I make my column last longer? Part II

By Cikui Liang, Mitch Hastings, and Jason Ellis
Agilent J&W GC Column Team

Last month we talked about how column contamination from injected sample is the primary cause of column failure and provided performance tips to help maximize column life. This month, we will talk about how you can minimize the danger of column degradation from these three additional causes:

1. Oxygen damage at elevated temperatures
2. Thermal damage by exceeding maximum temperature limits
3. Chemical damage from inorganic bases, inorganic acids and salts

Oxygen damage begins at higher temperatures

A leak in the carrier gas flow path (e.g. gas lines, fittings, and injector) is the most common source of oxygen exposure. While the leaks don’t cause damage at or near ambient temperatures, severe stationary phase degradation appears at elevated temperatures with oxygen concentration as low as 10 ppm. In less severe cases, the column may function at a reduced level, but more severe cases can irreversibly damage the column. Symptoms include: premature onset of excessive column bleed; peak tailing for active compounds; and loss of resolution.

The best way to extend column life is to maintain an oxygen- and leak-free environment by taking these steps:

• Use ultra high purity carrier gases
• Install a moisture and oxygen trap in carrier gas lines
• Perform periodic leak checks of gas lines and regulators
• Change septa regularly
• Change gas cylinders before they are empty

Exceeding maximum temperature limit causes thermal damage

Exceeding a column’s upper temperature limit for a prolonged period of time or heating a column without carrier gas flow even slightly above the ambient temperature can result in degradation of the stationary phase and tubing surface damage. Worse, as polarity increases, the temperature at which this damage occurs gets lower. Thermal damage is greatly accelerated in the presence of oxygen. Overheating a column with a leak or high oxygen levels in the carrier gas can lead to rapid and permanent column damage. Symptoms of thermal damage are similar to those of oxidation and include poor peak shapes for active compounds, loss of retention, and elevated background signal.

When subjecting a column to elevated temperature (e.g. upper temperature limit) for a prolonged period of time, more stress is put onto the column. The column lifetime can be shortened compared to standard operation, although the mode of failure might be due to fatigue of fused silica tubing, rather than stationary phase degradation.

Tips to improve performance:

• You can maximize column life by setting the maximum GC oven temperature at or a few degrees below the manufacturer’s specified temperature limit.
• If a column is already damaged, condition it for 8 to 16 hours at its isothermal temperature limit. Remove 10-15 cm from the detector end and reinstall it. Then you can continue to use the column, although lifetime and performance may suffer.
• After column installation, make sure carrier gas flows through the column before it is heated by injecting a small amount of an unretained compound. A well formed peak with a retention time of 0.5-2 minutes should be obtained.

Inorganic bases, inorganic acids, and salts cause chemical damage

Inorganic bases such as KOH, NaOH, and NH₄OH, and inorganic acids such as HCl, H₂SO₄, H₃PO₄, and HNO₃ are particularly damaging to stationary phases, causing peak tailing or broadening, adsorption of active compounds, or rising baseline.

Tips to improve performance:

• Use a guard column to minimize column damage, but keep in mind that frequent trimming may be required.
• Chemical damage is usually limited to the front of a column, so trim or cut 0.5 to 1 meter – or 5 or more meters in severe cases – from the column front to eliminate most chromatographic problems.

Column life is subjective

Lifetime projections are based on method performance criteria and each analyst’s performance expectations, but how long you can actually continue to use a column is a subjective determination. A column that may last five years performing one method can be unusable for more demanding analyses after just one injection of a dirty sample or after one run at a high temperature with a leaky septum.

In regular use, much of what we inject will contain some amount of residue even for pure solvents or standards. It’s also likely to find a small amount of oxygen native to the carrier gas and part-per-billion leaks in fittings, regulators, injector bodies, and septa. That means that once a column leaves the factory, there is no way of knowing for certain what the actual life span of each column will be – making the care you take with your sample, and instrument the most important factor in determining usable life span of a column.

>> Update My Profile | Subscribe to Access Agilent | Article Directory