Carrier Gases

Carrier Gases

. The GC determines the current column temperature and adjusts the column head pressure so that the desired average linear velocity is obtained.

Hydrogen offers many advantages for GC/MS. It will decrease the analysis time due to the higher linear velocities required to obtain optimum column performance. It will aslo keep your source cleaner and hence you will go for longer periods without having to vent and clean the source. It is quite pure as supplied and requires little or no scrubbing.

Disadvantages:

Dangerous...especially if there are leaks on the GC oven.(some vendors offer detectors to warn and shutdown gas supply if a leak is detected).

Carrier gas linear velocities (and flow rates) are dependent on column temperature. At a constant head pressure, carrier gas linear velocities decrease as column temperature increases. This means the average linear velocity needs to be set at the same temperature for a given method if reproducible results are desired. Setting the average linear velocity at a different column temperature results in retention and resolution changes. Since the average linear velocity is dependent on column temperature, the velocity decreases during a temperature program run. Electronic pressure controlled injectors can be programmed to maintain a constant average linear velocity or flow rate throughout the temperature program. Using this feature may result in better resolution of the later eluting peaks or shorter analysis times.

The effect of carrier gas average linear velocity on efficiency is best illustrated using a van Deemter curve or plot (Figure 18). By determining efficiencies (as H) for a range of average linear velocities and plotting the results, a van Deemter curve is generated. van Deemter curves show that there is an average linear velocity that provides maximum efficiency (uopt). This value is where the curve reaches the smallest value of H (highest efficiency) and is at the lowest point in the parabolic curve (Figure 18). van Deemter curves also show that using an average linear velocity that is too far from uopt results in a loss of efficiency. Most analyses are run at an average linear velocity values above uopt. This value is called the optimal practical gas velocity (OPGV) and corresponds to a value 1.5-2 times uopt. It is the average linear velocity where maximum efficiency per unit time is obtained. A small efficiency loss is tolerated for a much larger decrease in analysis time.

Figure 18. van Deemter Curve

Nitrogen, helium and hydrogen are the most common carrier gases used with capillary GC columns. The differences between the gases are evident when comparing their van Deemter curves (Figure 19). Nitrogen provides the best efficiency; however, its uopt is at a fairly low average linear velocity. The steepness of the nitrogen van Deemter curve means that small changes in the average linear velocity result in large changes in efficiency. The low average linear velocity at uopt and steepness of the van Deemter curve make nitrogen the least desirable carrier gas for capillary GC. Compared to nitrogen, helium’s uopt is at a higher average linear velocity but with slightly less efficiency. Analysis times with helium are about 1/2 the value when using nitrogen, and there is only a very small sacrifice in efficiency. The helium van Deemter curve is much flatter than the nitrogen curve, thus changes in the average linear velocity do not decrease efficiency by a large amount. Hydrogen’s uopt is the highest of the three common carrier gases, and the van Deemter curve is very flat. Hydrogen’s high uopt results in the shortest analysis times. Also, the wide range over which high efficiency is obtained makes hydrogen the best carrier gas for samples containing compounds that elute over a wide temperature range. Figure 20 shows the typical differences in analysis times and resolution for the three carrier gases.

Figure 19. van Deemter Curves for Nitrogen, Helium and Hydrogen

Contrary to common perception, hydrogen is a safe carrier gas. It is extremely diffusive in air and only explosive when air is present within a narrow concentration range. In other words, it is difficult to create a hydrogen explosion under GC conditions. Finally, safety features built into modern GCs greatly reduce any explosion possibilities and hazards.

While average linear velocity is the important carrier gas measurement, the flow rate is not a completely unimportant value. Column flow rates are needed when setting split ratios, using headspace and purge & trap samplers, and GC/MS systems. Due to the low flow rates used with most capillary columns or measurement difficulties, flowmeters cannot provide an accurate and precise measure of carrier gas flow rates. Typical carrier gas flow rates are 1-10 mL/min for capillary columns which is at or below the lower range of most flowmeters. For small diameter columns, errors or differences as little as 0.1 mL/min can significantly affect retention and efficiency. Usually a flowmeter is connected to the detector to obtain the carrier gas flow rate. This requires that detector gases be turned off, and a proper seal be between the flowmeter and detector (not possible with GC/MS systems). This is inconvenient and subject to measurement errors. Average flow rates (F) can be calculated using Equation 11. This is a more reproducible and consistent method to obtain a carrier gas flow rate; however, it is an average flow rate. The injection of an unretained compound is required. As with average linear velocity, flow rates are dependent on column temperature, so a consistant measurement temperature is required.

Equation 11. Average Flow Rate

Figure 20. Comparison of Nitrogen, Helium and Hydrogen

Carrier Gas Purity

The use of high purity gases results in better sensitivity and longer column and gas trap life. Higher carrier gas purities become more important when the GC is operated near its sensitivity limit. If column life is not an issue or higher concentration samples are being analyzed, using the highest purity carrier gas is often not necessary. Since gas prices increase along with the purity level, use the gas purity required to meet sensitivity or column life requirements. Gas traps can be installed on the gas lines to reduce the levels of impurities.

The main gas impurities of concern are oxygen, water and hydrocarbons. While higher levels of all three species degrade sensitivity, oxygen and water accelerate stationary phase degradation, especially at higher column temperatures. The concentration of oxygen, water and hydrocarbons should be below 1-2 ppm each. The concentration of other impurities such as CO, CO2, Ar, N2, He and H2 is not as critical; however, total impurities including oxygen, water and hydrocarbons should be <10 ppm. Nitrogen, helium and hydrogen purities of 99.995-99.999% are recommended for most applications.

Compressed gas cylinders are usually certified only down to 10% of their original pressure. (e.g., 200 psi if the original pressure, was 2000 psi). At pressures below 10%, higher levels of water, organics or other impurities may be present. Guarantees on gas purity are not usually applicable below 10% of the original cylinder pressure. Besides any negative effect on GC sensitivity and column life, premature expiration of gas traps may occur due to the high level of impurities in the gas. Gas cylinders should be changed when they reach 10% of their original pressure. This also helps to prevent complete exhaustion of the cylinder and subsequent heating of the column without carrier gas flow. This results in rapid and severe column damage.

Tubing and Regulators

Do not use plastic or rubber tubing for any of the gas lines in a GC system. Teflon, nylon, polyethylene, polypropylene or polyvinyl chloride (PVC) contain contaminants that degrade gas purity. Plastics are permeable to oxygen and moisture, thus contaminating the carrier gas. Also, most plastic tubing cannot tolerate the pressures used in many GC systems, thus they are unsafe to use. Stainless steel or copper are the preferred tubing materials.

Only use gas regulators that have stainless steel diaphragms not neoprene or Buna-N. Like plastic tubing, these polymeric diaphragms are sources of contaminants and are permeable to oxygen and moisture.

Gas Traps
The purpose of gas traps is to remove detrimental impurities from the carrier and detector gases. Moisture (water), oxygen and hydrocarbon are the most common traps used with GC systems. A few combination traps are available which remove moisture, oxygen and/or organics with a single trap. Reducing impurity levels can prolong column life and may improve sensitivity. The amount of improvement depends on the initial quality of the gas. Little enhancement is obtained when using very high purity gases (e.g., ultra-high purity or similar grades) while obvious improvement is obtained with lower grades of gas.

Constant exposure of capillary columns to oxygen and moisture, especially at high temperatures, results in rapid and severe column damage. The use of oxygen and moisture traps for the carrier gas may extend column life. Since stationary phase degradation or damage is accelerated at higher temperatures, gas traps are more beneficial when columns are used near or at their upper temperature limits. Some column stationary phases are particularly susceptible to oxygen damage. Traps may provide a some protection if there is a leak at or around the gas cylinder. Any moisture or oxygen introduced into the gas stream due to the leak will be removed by the trap until it expires. This creates an opportunity to detect and fix the leak before column damage occurs.

Moisture (Water) Traps

There are several different adsorbents and indicating materials used in moisture traps. Indicating moisture traps are available in plastic or glass bodies. Glass body traps are used when potential contaminants from plastic trap bodies are a concern. Glass traps are normally encased in a protective, plastic shrink wrap or a high impact plastic shield (outer trap body). Glass and plastic bodied traps are usually pressure tested at 150 psi, thus safe for use at the typical pressures required by the GC.

Oxygen Traps

Oxygen traps usually contain a metal containing, inert support reagent. Most oxygen traps reduce the oxygen concentration to below 15-20 ppb. The capacity of a standard oxygen trap is approximately 30 mg of oxygen per 100 cc of trap volume. Oxygen traps can remove some small organics and sulfur compounds from gas streams. Table 9 lists some of the compounds that are removed from the gas stream and some of the gases in which oxygen can be removed (i.e., oxygen is trapped and the gas stream passes through untrapped).

Metal (usually aluminum) trap bodies are recommended for GC analyses. Some plastics are permeable to air and contain contaminants that can degrade gas quality. In addition, many of the metal bodied oxygen traps can withstand high pressures (up to 2000 psi). Some oxygen traps also remove moisture from the gas stream without affecting the oxygen removal capability.

Indicating oxygen traps change color when oxygen is present in the gas at harmful levels. Indicating traps are not intended to be the primary oxygen removal trap, but to show when a high capacity oxygen trap is expired and needs to be changed. They are installed after the oxygen trap in the gas line. Expired oxygen traps need to be immediately changed since they can contaminate the gas in addition to failing to remove oxygen. Larger size indicating traps do have a moderate capacity for oxygen, but using a high capacity oxygen trap is still recommended as the primary oxygen removal device.

Hydrocarbon Traps

Hydrocarbon traps remove organics, and only hydrocarbons, from the gas stream. The adsorbent is usually activated carbon or an impregnated carbon filter media. Carbon removes organic solvents from the gas stream including the typical solvents used in nearly every lab. Hydro-moisture traps are also available which remove water in addition to organics. Capillary grade hydrocarbons traps are purged with ultra-high purity helium and packed with a very efficient activated carbon material. Metal trap bodies are used to prevent any contaminants in plastic trap bodies from contaminating the carbon adsorbent. Most hydrocarbon traps can be refilled by the end user.

Installing Gas Traps

Depending on the number and location of the GCs, there are two options concerning the placement of gas traps (Figure 21). One is to install the traps on the main line (after the gas source) but before individual gas lines branch off to each GC. Each GC fed by that gas source share the same traps. The other method is to install a set of traps at each GC, thus each instrument has its own set of traps.

Figure 21. Gas Trap Installation Locations

With multiple GCs, it may not be cost or labor effective to install a set of traps for each GC. In these cases, traps are installed on the main gas line that services the multiple GCs. Use large capacity traps to minimize the frequency of trap replacement. When traps are installed on a main gas line, all of the connected GCs are taken off-line whenever a trap needs to be changed. A majority of the cost of traps is the hardware, thus larger volume traps cost less per unit of volume. The traps should be installed in the vertical position. If multiple traps are used, the order of the traps starting from the gas source should be 1) moisture, 2) hydrocarbon, 3) high capacity oxygen and 4) indicating oxygen. Depending on the situation, one or two of these traps may not be needed.

When installing a set of traps for each GC, the traps should be installed as close to the GC as reasonably possible. A point of operation panel system is the most convenient for this type of installation. These are small panels with multiple attachments for a cartridge style of trap. The cartridges are installed with a simple, hand tightened fitting. The panels can be set on the benchtop or mounted on a nearby wall. The cartridges are interchangeable, thus multiple combinations of adsorbents are possible.

If available, use a trap that is purged and filled with the gas to be purified. Some traps only use ultra-high purity helium since this is the most commonly used carrier gas in many parts of the world. Some traps require several days before all traces of the fill gas is removed from the adsorbent. Detectors that respond to a change in the gas composition or respond to the fill gas itself are the most affected. A drifting or unstable baseline is the typical symptom of an unequilibrated trap.