Why GC Columns Go Bad

By Reginald Bartram  |  Spring 2015 Issue

If your peaks are starting to shrink, merge or tail off, discover eight factors that might be damaging your column.

We expect to have a reasonable lifetime for our gas chromatography columns. Most columns, if run under reasonable conditions, will last at least a year. Those with very clean samples that run their columns at oven temperatures considerably below the columns’ maximum operational temperatures may get up to five years of operation. Those who have to push their columns to run under temperatures that exceed or meet their columns’ maximum temperature have learned to accept short column life to get their analysis accomplished. If you take reasonable care with your selection of carrier gas purity, perform maintenance on your lines to avoid leaks, and take care to clean up your samples, you should get reasonable column life.

Nevertheless, columns go bad for many reasons. Let’s look at some of the reasons why this can happen.

1. Running the column without carrier gas
2. Running the column to temperatures above the acceptable maximum for the columns phase
3. Injection of samples that contain chemicals detrimental to the phase, such as strong acids or bases
4. Running carrier gases that contain oxygen, or moisture above 1 ppm
5. Leaks in the system allowing room air to enter the column
6. Exposure to large volumes of oxygen or moisture during cylinder change out
7. Column contamination resulting from heavily-, semi- or non-volatile compounds in the sample
8. Septa, ferrule leaks, and fragments contaminating the liner

All of these may cause shortened column life. We replace a column because the old column has extra peaks, peaks that tail, peaks that are shorter or provide less area due to column absorption, erratic baselines, peaks no longer separate, and/or extra column bleed. Regrettably, most of these reasons for shorter column life give the same feedback in column performance, making it difficult to troubleshoot the reason for the reduced column life.

Down time in the lab when samples cannot be run is very costly time for the lab. Changing a column cannot be done quickly. The old column is removed and the instrument is typically cleaned by changing septa and liners. The new column is installed and checked for leaks. Then the column must be conditioned, which can take hours to days. After conditioning, the system needs to stabilize and then standards are run and the instrument is calibrated. If all is well and the separation is duplicated, samples can be run again. This entire process requires a lot of down time and extra time from lab personnel. Therefore, you want your columns to run as long as possible without the disruption of having to change columns.

Let’s look at what damages columns and causes these problems.

Running the column without carrier gas

If carrier gas is not flowing through the column, the sample will not transport through the column. There may be many reasons why this might happen. The first is turning off the carrier gas to change cylinders or to do maintenance on a gas generator (hydrogen or nitrogen). Cool down the oven and if the column goes into the inlet or detector then those may need to be cooled, a well. At temperatures of 50° C or lower, very little damage would accrue to most stationary phases and very little reaction with moisture or oxygen would happen.

If the reason the column is not getting flow is due to the column breaking, then the entire column might be lost. With capillary columns, depending on where the break occurred, you might still have a usable section of the column. For instance, if the break was at the detector, then most of the column still had carrier gas and the column can be re-connected to the detector. If the break is at the front of the column for sure, the column is probably bad. For breaks at other sections of the column unions can be used to fix the break but may result in reduced separation, bleed or other problems. Most often, this means the installation and conditioning of a new column.

Running the column to temperatures above the acceptable maximum for the columns phase

All columns have published maximum temperatures that, when met or exceeded, the phase becomes too volatile and may even break down. When temperature programming an analysis you often approach or temporarily exceed this maximum temperature. Best practices calls for you to stay about 10° C below the maximum temperature. Shorter column life can be expected for those running up to or over the stated maximum temperature of the phase. Most column manufacturers will not guarantee their columns if you exceed this temperature.

Injection of samples that contain chemicals detrimental to the phase such as strong acids or bases

Many samples may naturally have acids or basic compounds in them. Gas chromatography can analyze many of these organic acids and basic compounds, but some must be neutralized before analysis. Alternatively, you may have treated your sample with a silylation agent to treat the acid or base or with neutralizing agents to remove some compounds. Often these compounds are then reacted with other compounds to make them neutral to the column, thereby providing better column life. Nevertheless, occasionally these acids get into the column. Strong acids or bases such as hydrochloric (HCL), sulfuric (H₂SO₄), nitric (HNO₃), phosphoric (H₃PO₄), and chromic (CrO₃) and the basic compounds ammonium hydroxide (NH₄OH), potassium hydroxide (KOH), and sodium hydroxide (NaOH) can be very damaging to columns. For the most part, they are nonvolatile, lay down on the front of the column and damage the phase in that part of the column. They may also react with compounds injected later and cause peak distortion including phase bleed, peak tailing for active compounds and/or loss of efficiency or resolution of two peaks.

Other chemicals that sometimes get into columns and cause damage are silylation agents. These agents are used to neutralize compounds to make them acceptable to the GC analysis and phase. However, if excess silylation agent gets onto the phase, it might react with the phase or even break it down. These excess reagents will damage the column permanently.

Running carrier gases that contain oxygen, or moisture above 1 ppm

Both instrument manufacturers and column manufacturers recommend the purchase of carrier gases with purities of oxygen, moisture and hydrocarbons at or below 1 ppm. Both columns and some detectors can be harmed if these compounds are above 1 ppm. This exposure is constant and if above 1 ppm will continually cause damage. Samples often have these compounds in them and their short-term exposure to the column does not cause damage in the way that long-term exposure of these contaminants in the carrier gas can. Be sure to ask your gas supplier about the amount of these compounds in the gas supplied. Because the definitions of the terms chromatograph grade or research grade differ between manufacturers/suppliers of the gas, it is important to know exactly what levels your gas contains.

Leaks in the system allowing room air to enter the column

The best grade of gas will do no good if it is contaminated by leaks on the way to the column. Unions, compression fittings, regulators, purifiers and other fittings used to transport gas through the tubing and inlet of the GC to the column will leak with time. When gas leaks out of the system, contaminants can enter the gas stream. It is important to maintain the purity of the gas all the way to the column. Any connection that is moved, vibrates, opened and closed, or exposed to varying temperatures will, with time, leak. This happens particularly when there are large runs of tubing that then require the use of unions in areas that cycle in temperature from morning to night and are exposed to large temperature changes, such as above the ceilings of labs or fittings outside the lab. In addition, fittings and valves behind the GC that are exposed to the oven vent temperatures will leak. Periodically pressure and leak test manifold systems. Leak detector fluids are not recommended for this task as they may also enter the system and cause problems with the GC. The best choice is always an electronic leak detector as it will find both small leaks and will detect large leaks that a liquid leak detection fluid will not show.

Exposure to large volumes of oxygen or moisture during cylinder change out

Changing a cylinder or rack of cylinders is perhaps the most dangerous and potentially damaging time for columns. Many things can occur, such as not remembering to cool down the GC or believing that cylinders can be changed fast enough to avoid problems. In the past, purge devices were used to purge out air that got into the regulator and connecting tubing before re-pressurizing the system. This was either not done or was not done effectively and large volumes of gas entered the system. Most thought that the installation of large purifiers would remove contaminants before they could get to the GC when in fact, most contaminants would be forced through the purifiers so fast on startup that the purifiers would not have time to remove the contaminants and they would reach the GC. One would hope that contaminants would purge though the GC before it started heating up, but in most cases, the purge time of contaminants in these systems is very long and they do reach the heated columns, causing some damage.

The recent introduction of a new valve installed in the cylinder connector (CGA, DIN) has greatly reduced this problem. Only a small amount of air can enter the system — about 1 ml or less. The valve allows carrier gas to enter the line under pressure but when the pressure goes down during the changing process the valve turns off, trapping all the gas in the line but not allowing air to enter. If this device, called a check valve, is installed into the lines then very little damage will occur.

Check valve installed in CGA fitting

Column contamination resulting from heavily-, semi- or non-volatile compounds in the sample

Samples can contain semi-volatile or non-volatile compounds that contaminate the front of the column, resulting in several bad effects. When deposited on the column, these compounds may change the polarity of the column. They can also react with the sample, removing some compounds or causing tailing of others. It is best to try various sample prep techniques to try to remove these compounds. If not, it may be necessary to use pre-columns and periodically remove and replace the pre-columns. For those not using pre-columns, there are ways to solvent rinse the front of the columns. Though after repeated heating of the columns, these undesired compounds often cannot be removed by a solvent rinse. One other effect from the presence of semi-volatile compounds is the appearance of very late, eluting peaks. Sometimes these peaks elute so late that they only show up in later analysis. The only cure is the removal of the contaminant semi- or non-volatile compounds from the front of the column.

Septa, ferrule leaks, and fragments

Most of the problems discussed with air getting into the system have been external to the GC but leaks do happen inside of the GC. Septa can leak after repeated injections, allowing air into the system as well as loss of carrier gas. Poor connection of the column to the inlet and detector of the GC can cause leaks into and out of the carrier gas lines. Take off panels to check the connections inside of the GC in and around flow and pressure controllers and other connections.

Septa and ferrule fragments can also interact with the sample, causing problems. They will accumulate in the glass liners of the injection port and may become reactive to samples. Always clean these liners to avoid problems.

Conclusion

There are several proactive steps to take to get a reasonable life time out of your column. Keep a tight carrier gas system leak free of oxygen and moisture. Treat samples to avoid contaminants that will not pass thought the column easily or might react with the column phase. Be careful changing cylinders to avoid air getting into the system and use connectors to the cylinders that have check valves. Do routine maintenance to avoid problems. At some point, the column will see an end due to natural decay caused by phase bleed. Always be ready with a spare column.